Environmental influence on linear optical spectra and relaxation dynamics in carbon nanotubes - Malic, Ermin; Hirtschulz, Matthias; Maultzsch, Janina; Reich, Stephanie; Knorr, Andreas (physica status solidi (b), 2009)
We present theoretical investigations on linear optical spectra and the temporal non-equilibrium dynamics of single-walled carbon nanotubes. In particular we discuss the importance of the surrounding medium. Our approach is based on the density matrix theory, which in combination with tight-binding wave functions allows the calculation of linear and nonlinear optical properties of arbitrary nanotubes. Its strength lies in the straightforward way to describe non-equilibrium properties, such as Coulomb driven ultrafast intrasubband carrier relaxation. We find a strong influence of the surrounding dielectric medium on the position of the peaks, the excitonic binding energy, and the carrier relaxation time. Furthermore, simple scaling laws describing the dependence on the dielectric background constant epsilon(bg) are shown. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Epitaxial Cobalt Oxide Films with Wurtzite Structure on Au(111) - Ammon, Maximilian Michael; Baumann, Sara; Kißlinger, Tilman; Rieger, Janek; Fauster, Thomas; Redinger, Josef; Hammer, Lutz; Schneider, M. Alexander (Physica Status Solidi-Rapid Research Letters, 2021)
Several-nanometer-thick, closed, and epitaxial cobalt(II) oxide films with wurtzite crystal structure (w-CoO) are grown on Au(111) and their structural and electronic properties analyzed. The structural quality of the (Formula presented.) oriented, oxygen-terminated, and unreconstructed films allow the application of surface-science methods to unravel the properties of this unusual polymorph of CoO and may pave the way for future thin-film applications. An experimental structural analysis by low-energy electron diffraction (LEED-IV) is presented with an excellent agreement between measured and calculated intensity spectra expressed by a Pendry R-factor of (Formula presented.) and few-picometer error bounds in the parameter values. Using scanning tunneling spectroscopy (STS) the bandgap of the semiconducting films is found to be 1.4 ± 0.2 eV. Ultraviolet photoelectron spectroscopy (UPS) confirms the presence of a gap and the position of the Fermi level (E F). The structural results of density functional theory calculations using (hybrid) functionals to treat electron correlations and van der Waals forces agree well with the experimentally determined structure of the antiferromagnetic w-CoO films. In contrast to generalized gradient approximation (GGA)+U calculations, the Heyd–Scuseria–Ernzerhof hybrid functional reproduces the semiconducting nature correctly and predicts surface states in the gap which might pin E F in agreement with STS and UPS.
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Keywords: bandgaps; cobalt oxide; electronic properties; polymorphism; structural analyses

Epitaxial graphene as an electrode material: a transistor testbed for organic and all-carbon semiconductors - Bayaya, Emmanuel; Waldmann, Daniel; Krieger, Michael; Weber, Heiko B. (RSC Advances, 2014)
We present a transistor testbed for novel organic and all-carbon electronic materials. Epitaxial graphene on silicon carbide (SiC) is used as source and drain electrodes. The gate is implemented as a bottom gate. Due to Fermi level pinning at the graphene/SiC interface, the gate is effective in the channel area only. The semiconductor above source and drain contacts and the graphene itself are unaffected by the bottom gate. This leads to a clear distinction of electronic properties in the sharply separated channel and contact areas. As an example, we investigate P3HT and fullerene films, representing standard p-type and n-type semiconducting materials. In particular, for P3HT an ohmic contact to graphene was observed, and an accurate and consistent determination of transistor parameters was achieved. The fullerene transistor showed a high on/off ratio of 3 × 103. The testbed offers the opportunity to determine semiconductor parameters of novel materials under very well-defined conditions. This journal is © the Partner Organisations 2014.
UnivIS-Import:2015-04-14:Pub.2014.nat.dphy.IAP.LAP.epitax

Epitaxial Graphene on silicon carbide as a tailorable metal-semiconductor interface - Krieger, Michael; Weber, Heiko B. (Wide Bandgap Semiconductors for Power Electronics, 2021)
In order to access semiconductor functionality, the metallic contact is a key issue. Silicon carbide (SiC) is unique, because it inherently provides a monolithic metallic contact with unprecedented properties: epitaxial graphene that can be grown out of its (0001) facet. It is an atomically thin metal that is ultrarobust and nearly fully transparent. The contact is atomically at, epitaxially defined and can be tailored to be either ohmic or of Schottky type. This chapter first describes fabrication and processing as well as structural and electronic properties of epitaxial graphene contacts to SiC. Then, unique properties of this combined material system are presented that include a local definition of the charge carrier type in graphene and a concept that employs this for fully functional analog and digital electronics. Moreover, this monolithic material allows unprecedented insight into the speed limits of Schottky rectification. Finally, epitaxial graphene/SiC contacts open up opportunities in the realm of quantum technology. As transparent electrodes they can be used for Stark tuning and spectral matching of luminescent point defects in SiC, a prerequisite for truly indistinguishable quantum systems.

Epitaxial Graphenes on Silicon Carbide - Seyller, Thomas; et al. (Mrs Bulletin, 2010)
The materials science of graphene grown epitaxially on the hexagonal basal planes of SiC crystals is reviewed. We show that the growth of epitaxial graphene on Si-terminated SiC(0001) is much different than growth on the C-terminated SiC(000 -1) surface, and discuss the physical structure of these graphenes. The unique electronic structure and transport properties of each type of epitaxial graphene is described, as well as progress toward the development of epitaxial graphene devices. This materials system is rich in subtleties, and graphene grown on the two polar faces differs in important ways, but all of the salient features of ideal graphene are found in these epitaxial graphenes, and wafer-scale fabrication of multi-GHz devices already has been achieved.
EAM Import::2019-03-14

Epitaxial inorganic metal-halide perovskite films with controlled surface terminations - Rieger, Janek; Kißlinger, Tilman; Raabgrund, Andreas; Hauner, Jonas; Niesner, Daniel; Schneider, M. Alexander; Fauster, Thomas (Physical Review Materials, 2023)
Metal-halide perovskite (MHP) thin films for next-generation solar cells are typically fabricated by wet-chemical synthesis in which surface properties such as the orientation of surface facets and their termination cannot be controlled. MHP device efficiencies depend critically on those surface properties. We demonstrate the epitaxial growth of several nanometer thick purely (001)-orientated films of CsPbBr3 and CsSnBr 3 MHPs on Au(001) by molecular beam epitaxy. The epitaxial films are in a cubic phase aligned with the substrate. Their surfaces are singly terminated and can be modified for the first time. While the CsBr and PbBr2 surface terminations differ in terms of their atomic structure and defect content, the films are intrinsic semiconductors irrespective of the termination. The work function of the PbBr2-terminated surface is increased by 0.7eV, which has drastic implications for the level alignment at MHP interface

Epitaxial Metal Halide Perovskites by Inkjet-Printing on Various Substrates - Sytnyk, Mykhailo; Yousefi-Amin, Amir Abbas; Freund, Tim; Prihoda, Annemarie; Götz, Klaus; Unruh, Tobias; Harreiß, Christina; Will, Johannes; Spiecker, Erdmann; Levchuk, Jevgen; Osvet, Andres; Brabec, Christoph; et al. (Advanced Functional Materials, 2020)
Metal-halide-perovskites revolutionized the field of thin-film semiconductor technology, due to their favorable optoelectronic properties and facile solution processing. Further improvements of perovskite thin-film devices require structural coherence on the atomic scale. Such perfection is achieved by epitaxial growth, a method that is based on the use of high-end deposition chambers. Here epitaxial growth is enabled via a approximate to 1000 times cheaper device, a single nozzle inkjet printer. By printing, single-crystal micro- and nanostructure arrays and crystalline coherent thin films are obtained on selected substrates. The hetero-epitaxial structures of methylammonium PbBr(3)grown on lattice matching substrates exhibit similar luminescence as bulk single crystals, but the crystals phase transitions are shifted to lower temperatures, indicating a structural stabilization due to interfacial lattice anchoring by the substrates. Thus, the inkjet-printing of metal-halide perovskites provides improved material characteristics in a highly economical way, as a future cheap competitor to the high-end semiconductor growth technologies.
CRIS-Team Scopus Importer:2020-09-11
Keywords: epitaxial growth; inkjet printing; metal-halide-perovskites; single-crystal-microstructures

Epithelial RAC1-dependent cytoskeleton dynamics controls cell mechanics, cell shedding and barrier integrity in intestinal inflammation - Martínez Sánchez, Luz del Carmen; Phuong Anh Ngo,; Pradhan, Rashmita; Becker, Lukas S; Böhringer, David; Soteriou, Despina; Kubankova, Marketa; Schweitzer, Christine; Koch, Tatyana; Thonn, Veronika; Erkert, Lena; Stolzer, Iris; et al. (Gut, 2022)
Objective Increased apoptotic shedding has been linked to intestinal barrier dysfunction and development of inflammatory bowel diseases (IBD). In contrast, physiological cell shedding allows the renewal of the epithelial monolayer without compromising the barrier function. Here, we investigated the role of live cell extrusion in epithelial barrier alterations in IBD. Design Taking advantage of conditional GGTase and RAC1 knockout mice in intestinal epithelial cells (Pggt1b (i Delta IEC) and Rac1 (i Delta IEC) mice), intravital microscopy, immunostaining, mechanobiology, organoid techniques and RNA sequencing, we analysed cell shedding alterations within the intestinal epithelium. Moreover, we examined human gut tissue and intestinal organoids from patients with IBD for cell shedding alterations and RAC1 function. Results Epithelial Pggt1b deletion led to cytoskeleton rearrangement and tight junction redistribution, causing cell overcrowding due to arresting of cell shedding that finally resulted in epithelial leakage and spontaneous mucosal inflammation in the small and to a lesser extent in the large intestine. Both in vivo and in vitro studies (knockout mice, organoids) identified RAC1 as a GGTase target critically involved in prenylation-dependent cytoskeleton dynamics, cell mechanics and epithelial cell shedding. Moreover, inflamed areas of gut tissue from patients with IBD exhibited funnel-like structures, signs of arrested cell shedding and impaired RAC1 function. RAC1 inhibition in human intestinal organoids caused actin alterations compatible with arresting of cell shedding. Conclusion Impaired epithelial RAC1 function causes cell overcrowding and epithelial leakage thus inducing chronic intestinal inflammation. Epithelial RAC1 emerges as key regulator of cytoskeletal dynamics, cell mechanics and intestinal cell shedding. Modulation of RAC1 might be exploited for restoration of epithelial integrity in the gut of patients with IBD.
CRIS-Team WoS Importer:2022-03-25

Ergebnisse der Sicca-Forschungsförderung 2016 - Musayeva, A; Gericke, A; Jäger, Franziska; Paulsen, Friedrich; Braun, M; Fabry, Ben; Braun, R; Pauly, Diana; Holtmann, C; Geerling, Gerd; Goldblum, David; Horwath-Winter, Jutta; et al. (Ophthalmologe, 2020)
The Sicca-Förderpreis (Dry Eye Award) supports the development of scientific research on the pathogenesis, diagnostics, and treatment of dry eye and ocular surface diseases. It is awarded after a limited call for proposals in German-speaking countries, written application and selection of the award winner after evaluation by a jury of ophthalmologists working in basic and clinical science. In this article examples of the results of funded projects of the Sicca-Förderpreis 2016 are cursorily described, which were presented at the Ophthalmological Academy of Germany 2019 (Augenärztliche Akademie Deutschland 2019) and therefore provide an insight into current scientific developments. The role of muscarinic receptors and those of urea in the pathogenesis of dry eye as well as the (missing) correlation of tear film stability, viscosity and surface tension are highlighted. A project on the early detection of ocular involvement in graft versus host disease and the idea of treating meibomian gland dysfunction with eyelid surgery techniques are also groundbreaking. The outlined projects represent the potential for further substantial developments in the understanding, diagnostics and treatment of dry eye; however, their long-term clinical relevance still needs to be established.
CRIS-Team Scopus Importer:2020-07-24
Keywords: Eyelid tension; Graft versus host disease; Muscarinic receptor; Tear film stability; Urea

Estimating the 3D pore size distribution of biopolymer networks from directionally biased data. - Lang, Nadine; Münster, Stefan; Metzner, Claus; Krauß, Patrick; Schürmann, Sebastian; Lange, Janina; Aifantis, Katerina E.; Friedrich, Oliver; Fabry, Ben (Biophysical Journal, 2013)
The pore size of biopolymer networks governs their mechanical properties and strongly impacts the behavior of embedded cells. Confocal reflection microscopy and second harmonic generation microscopy are widely used to image biopolymer networks; however, both techniques fail to resolve vertically oriented fibers. Here, we describe how such directionally biased data can be used to estimate the network pore size. We first determine the distribution of distances from random points in the fluid phase to the nearest fiber. This distribution follows a Rayleigh distribution, regardless of isotropy and data bias, and is fully described by a single parameter–the characteristic pore size of the network. The bias of the pore size estimate due to the missing fibers can be corrected by multiplication with the square root of the visible network fraction. We experimentally verify the validity of this approach by comparing our estimates with data obtained using confocal fluorescence microscopy, which represents the full structure of the network. As an important application, we investigate the pore size dependence of collagen and fibrin networks on protein concentration. We find that the pore size decreases with the square root of the concentration, consistent with a total fiber length that scales linearly with concentration.

Estimating the spatial distribution of vocalizing animals from ambient sound spectra using widely spaced recorder arrays and inverse modelling - Menze, Sebastian; Paranhos Zitterbart, Daniel; Biuw, Martin; Boebel, Olaf (Journal of the Acoustical Society of America, 2019)
The sound energy from marine mammal populations vocalizing over extended periods of time adds up to quasi-continuous "choruses," which create characteristic peaks in marine sound spectra. An approach to estimate animal distribution is presented, which uses chorus recordings from very sparse unsynchronized arrays in ocean areas that are too large or remote to survey with traditional methods. To solve this under-determined inverse problem, simulated annealing is used to estimate the distribution of vocalizing animals on a geodesic grid. This includes calculating a transmission loss (TL) matrix, which connects all grid nodes and recorders. Geometrical spreading and the ray trace model BELLHOP [Porter (1987). J. Acoust. Soc. Am. 82(4), 1349-1359] were implemented. The robustness of the proposed method was tested with simulated marine mammal distributions in the Atlantic sector of the Southern Ocean using both drifting acoustic recorders [Argo (2018). SEANOE] and a moored array as acoustic receivers. The results show that inversion accuracy mainly depends on the number and location of the recorders, and can be predicted using the entropy and range of the estimated source distributions. Tests with different TL models indicated that inversion accuracy is affected only slightly by inevitable inaccuracies in TL models. The presented method could also be applied to bird, crustacean, and insect choruses.
CRIS-Team Scopus Importer:2020-02-21

Evaluation of 4H-SiC MOSFET transfer characteristics using machine-learning techniques - Lehmeyer, Johannes; Citak, Timon; Weber, Heiko B.; Krieger, Michael (Electronics Letters, 2023)

Evaluation of electrospun poly(ε-caprolactone)/gelatin nanofiber mats containing clove essential oil for antibacterial wound dressing - Ünalan, Irem; Endlein, Stefan J.; Slavik, Benedikt; Büttner, Andrea; Goldmann, Wolfgang; Detsch, Rainer; Boccaccini, Aldo R. (Pharmaceutics, 2019)
The objective of this study was to produce antibacterial poly(ε-caprolactone) (PCL)-gelatin (GEL) electrospun nanofiber mats containing clove essential oil (CLV) using glacial acetic acid (GAA) as a “benign” (non-toxic) solvent. The addition of CLV increased the fiber diameter from 241 ± 96 to 305 ± 82 nm. Aside from this, the wettability of PCL-GEL nanofiber mats was increased by the addition of CLV. Fourier-transform infrared spectroscopy (FTIR) analysis confirmed the presence of CLV, and the actual content of CLV was determined by gas chromatography–mass spectrometry (GC-MS). Our investigations showed that CLV-loaded PCL-GEL nanofiber mats did not have cytotoxic effects on normal human dermal fibroblast (NHDF) cells. On the other hand, the fibers exhibited antibacterial activity against Staphylococcus aureus and Escherichia coli. Consequently, PCL-GEL/CLV nanofiber mats are potential candidates for antibiotic-free wound healing applications.
CRIS-Team Scopus Importer:2019-11-15
Keywords: Antibacterial; Biocompatibility; Clove essential oil; Electrospinning; Gelatin; PCL

Evaluation of lamina cribrosa and peripapillary sclera stiffness in pseudoexfoliation and normal eyes by atomic force microscopy - Braunsmann, Christoph; Hammer, Christian M.; Rheinlaender, Johannes; Kruse, Friedrich; Schäffer, Tilman; Schlötzer-Schrehardt, Ursula (Investigative Ophthalmology & Visual Science, 2012)
Pseudoexfoliation (PEX) syndrome is a systemic disorder of the elastic fiber system that can lead to PEX glaucoma. Elastotic alterations in the lamina cribrosa (LC) of PEX eyes suggested biomechanical implications predisposing to pressure-induced optic nerve damage. In this pilot study, the stiffness of LC and peripapillary sclera (ppSC) in eyes with and without PEX syndrome were analyzed by atomic force microscopy (AFM) nanoindentation.Unfixed cryosections (5-?m thick) were prepared from the optic nerve heads (ONH) of three donor eyes with PEX syndrome and three age-matched control eyes. AFM force mapping was performed in selected regions of the central, midperipheral, and peripheral LC and the ppSC using a spherical cantilever tip. To determine the local Young's modulus of elasticity (YME) as a measure of tissue stiffness, force curves were acquired and analyzed using the spherical Hertz model.For the LC, the median YME values calculated from single stiffness maps averaged 17.2 (±2.7) kPa in normal eyes and 10.1 (±1.4) kPa in PEX eyes, indicating a significant PEX-related decrease in stiffness by over 40% (P < 0.01). The corresponding YME values for the ppSC, which revealed a 9-fold higher tissue stiffness than in the LC, averaged 158.3 (±59.8) kPa for control and 85.8 (±16.9) kPa for PEX samples.AFM was proven suitable for determining the stiffness of ONH tissues, encouraging further large-scale analyses. The marked decrease in stiffness, implying an increased deformability of the ONH in PEX eyes, may reflect an inherent tissue weakness rendering these eyes more vulnerable to glaucomatous damage.
EVALuna2:21095

Evaluation of techniques for estimating the power spectral density of RR-intervals under paced respiration conditions - Schaffer, Thorsten; Hensel, Bernhard; Weigand, Christian; Schüttler, Jürgen; Jeleazcov, Christian (Journal of Clinical Monitoring and Computing, 2014)
Heart rate variability (HRV) analysis is increasingly used in anaesthesia and intensive care monitoring of spontaneous breathing and mechanical ventilated patients. In the frequency domain, different estimation methods of the power spectral density (PSD) of RR-intervals lead to different results. Therefore, we investigated the PSD estimates of fast Fourier transform (FFT), autoregressive modeling (AR) and Lomb-Scargle periodogram (LSP) for 25 young healthy subjects subjected to metronomic breathing. The optimum method for determination of HRV spectral parameters under paced respiration was identified by evaluating the relative error (RE) and the root mean square relative error (RMSRE) for each breathing frequency (BF) and spectral estimation method. Additionally, the sympathovagal balance was investigated by calculating the low frequency/high frequency (LF/HF) ratio. Above 7 breaths per minute, all methods showed a significant increase in LF/HF ratio with increasing BF. On average, the RMSRE of FFT was lower than for LSP and AR. Therefore, under paced respiration conditions, estimating RR-interval PSD using FFT is recommend.
EVALuna2:16636

Evidence for crossed Andreev reflection in superconductor-ferromagnet hybrid structures - Beckmann, Detlef; Weber, Heiko B.; von Löhneysen, Hilbert (Physical Review Letters, 2004)
We have measured the nonlocal resistance of aluminum-iron spin-valve structures fabricated by e-beam lithography and shadow evaporation. The sample geometry consists of an aluminum bar with two or more ferromagnetic wires forming point contacts to the aluminum at varying distances from each other. In the normal state of aluminum, we observe a spin-valve signal which allows us to control the relative orientation of the magnetizations of the ferromagnetic contacts. In the superconducting state, at low temperatures and excitation voltages well below the gap, we observe a spin-dependent nonlocal resistance which decays on a smaller length scale than the normal-state spin-valve signal. The sign, magnitude, and decay length of this signal are consistent with predictions made for crossed Andreev reflection.

Evidence of Tailoring the Interfacial Chemical Composition in Normal Structure Hybrid Organohalide Perovskites by a Self-Assembled Monolayer - Will, Johannes; Hou, Yi; Scheiner, Simon; Pinkert, Ute; Hermes, Ilka M.; Weber, Stefan A.L.; Hirsch, Andreas; Halik, Marcus; Brabec, Christoph; Unruh, Tobias (ACS Applied Materials and Interfaces, 2018)
Current-voltage hysteresis is a major issue for normal architecture organo-halide perovskite solar cells. In this manuscript we reveal a several Å thick Methylammonium Iodide (MAI) rich interface between the perovskite and the metal oxide. Surface functionalization via selfassembled monolayers (SAMs) allowed us to control the composition of the interface monolayer from Pb poor to Pb rich, which in parallel suppresses hysteresis in perovskite solar cells. The bulk of the perovskite films is not affected by the interface engineering and remains highly crystalline in surface normal direction over the whole film thickness. The subnm structural modifications of the buried interface were revealed by x-ray reflectivity (XRR), which is most sensitive to monitor changes in the mass density of only several Å thin interfacial layers as a function of substrate functionalization. From Kelvin probe force microscopy (KPFM) on a solar cell cross section study, we further demonstrate local variations of the potential on different electron transporting layers (ETLs) within a solar cell. Based on these findings we present a unifying model explaining hysteresis in perovskite solar cells, giving for the first time insight into one crucial aspect of hysteresis and paving the way for new strategies in the fields of perovskite based opto-electronic devices.
Keywords: perovskite solar cell; hysteresis; buried interface; x-ray reflectivity; Kelvin probe force microscopy

Evolution of quantum criticality in CeNi9-xCuxGe4 - Peyker, Ludwig; Gold, Christian; Scheidt, Ernst-Wilhelm; Scherer, Wolfgang; Donath, J. G.; Gegenwart, Philipp; Mayr, Franz; Unruh, Tobias; Eyert, Volker; Bauer, Ernst; Michor, Herwig (Journal of Physics: Condensed Matter, 2009)
Crystal structure, specific heat, thermal expansion, magnetic susceptibility and electrical resistivity studies of the heavy fermion system CeNi9-xCuxGe4 (0 <= x <= 1) reveal a continuous tuning of the ground state by Ni/Cu substitution from an effectively fourfold-degenerate non-magnetic Kondo ground state of CeNi9Ge4 (with pronounced non-Fermi-liquid features) towards a magnetically ordered, effectively twofold-degenerate ground state in CeNi8CuGe4 with T-N = 175 +/- 5 mK. Quantum critical behavior, C/T proportional to chi proportional to - ln T, is observed for x congruent to 0.4. Hitherto, CeNi9-xCuxGe4 represents the first system where a substitution-driven quantum phase transition is connected not only with changes of the relative strength of the Kondo effect and RKKY interaction, but also with a reduction of the effective crystal field ground state degeneracy.

Evolution of the Ligand Shell Around Small ZnO Nanoparticles During the Exchange of Acetate by Catechol: A Small Angle Scattering Study - Schindler, Torben; Lin, Wei; Schmutzler, Tilo; Lindner, Peter; Peukert, Wolfgang; Segets, Doris; Unruh, Tobias (ChemNanoMat, 2018)
The core‐shell structure of colloids surrounded by ligands is of great importance for their later application as it can significantly alter the chemical and physical properties of the nanoparticles (NPs). A combination of small angle X‐ray and neutron scattering (SAXS/SANS) in the native solution with additional ex situ measurements (titration‐UV) was applied to study the NP/ligand interface of ZnO NPs after functionalization with catechol derivatives. Based on SAXS data, it was found that within the multimodal particle size distribution the fraction of agglomerates is shifted to smaller sizes and nearly disappeared upon the binding of ethyl 3,4‐dihydroxybenzoate (CAT) molecules. This is ascribed to improved stabilization at the primary particle level by CAT molecules. By combining the neutron scattering contrast with the input of bound CAT molecules from a previously developed titration‐UV method, the heterogeneous composition of the ligand shell became accessible for the first tim
Keywords: quantum dots; Raman spectroscopy; core-shell; nanoparticles; small angle scattering

Exact and fast calculation of the X-ray pair distribution function - Neder, Reinhard; Proffen, Thomas (Journal of Applied Crystallography, 2020)
A fast and exact algorithm to calculate the powder pair distribution function (PDF) for the case of periodic structures is presented. The new algorithm calculates the PDF by a detour via reciprocal space. The calculated normalized total powder diffraction pattern is transferred into the PDF via the sine Fourier transform. The calculation of the PDF via the powder pattern avoids the conventional simplification of X-ray and electron atomic form factors. It is thus exact for these types of radiation, as is the conventional calculation for the case of neutron diffraction. The new algorithm further improves the calculation speed. Additional advantages are the improved detection of errors in the primary data, the handling of preferred orientation, the ease of treatment of magnetic scattering and a large improvement to accommodate more complex instrumental resolution functions.
CRIS-Team WoS Importer:2020-06-19

Exchange splitting of image states on Fe/Cu(100) and Co/Cu(100) - Fauster, Thomas; et al. (Physical Review B, 1996)
The image states on epitaxial films of iron and cobalt on Cu(100) have been investigated. For iron the energies can be related to the variation of the film morphology with coverage due to the extreme surface sensitivity of the image states. With the help of spin-selective excitations of electrons into the image states, the exchange splitting of the first image state on 7 ML of Fe on Cu(100) could be determined to 65 +/- 15 meV. The exchange splitting for a 6-ML Co film is 55 +/- 10 meV. These results are in agreement with a ferromagnetic order of the surfaces and can be obtained without magnetization of the sample.

Excitation-energy dependence of optically induced ESRin a-Si:H - Ristein, Jürgen; et al. (Physical Review B, 1989)
UnivIS-Import:2015-03-05:Pub.1989.nat.dphy.IAP.exphys.excita_9

Excitation spectroscopy of photoluminescence in a-Si:H - Ristein, Jürgen; et al. (Solar Cells, 1989)
The photoluminescence (PL) excitation spectrum (intensity of PL as a function of the wavelength of the exciting light) has been measured over the range 1.0 - 1.25 eV for PL at 0.79 eV using a free electron laser (FEL) as the excitation source. At all wavelengths, the PL intensity varies with incident laser power, I, as I0.9 independent of wavelength. The resulting PL excitation spectrum has two distinguishing features, an exponential rise at lower energies followed by a leveling-off above about 1.15 eV. Although the precise functional form of the data at low energies is not uniquely determined, the data are consistent with an exponential rise whose slope is the same as that measured in optical absorption at higher energies. © 1989.
UnivIS-Import:2015-03-05:Pub.1989.nat.dphy.IAP.exphys.excita

Exciton binding energies in carbon nanotubes from two-photon photoluminescence - Maultzsch, Janina; et al. (Physical Review B, 2005)
Excitonic effects in the linear and nonlinear optical properties of single-walled carbon nanotubes are manifested by photoluminescence excitation experiments and ab initio calculations. One- and two-photon spectra showed a series of exciton states; their energy splitting is the fingerprint of excitonic interactions in carbon nanotubes. By ab initio calculations we determine the energies, wave functions, and symmetries of the excitonic states. Combining experiment and theory we find binding energies of 0.3-0.4 eV for nanotubes with diameters between 6.8 and 9.0 angstrom.

Excitonic absorption spectra of metallic single-walled carbon nanotubes - Malic, Ermin; Maultzsch, Janina; Reich, Stephanie; Knorr, Andreas (Physical Review B, 2010)
We present microscopic calculations of the absorption spectra of metallic single-walled carbon nanotubes. We address the controversial question of the excitonic binding energies in metallic nanotubes as well as the excitonic character of higher transitions. In spite of the strong screening, we observe binding energies in the range of 100 meV for metallic nanotubes with diameters in the range of 1-2.2 nm. Characteristic features of the absorption spectra, such as peak splitting and an asymmetric peak shape, are observed. The splitting is due to the trigonal warping effect. The peak shoulder at high energies is a result of an overlap of the excitonic excitation with the free-particle Van Hove singularity. Furthermore, we find higher transitions to be also significantly influenced by excitons. Our approach is based on density matrix, which allows the investigation of the chirality and diameter dependence of the excitonic binding energy for the first four optical transitions for a variety of different metallic and semiconducting nanotubes.

Excitonic Rayleigh scattering spectra of metallic single-walled carbon nanotubes - Malic, Ermin; Maultzsch, Janina; Reich, Stephanie; Knorr, Andreas (Physical Review B, 2010)
We have performed microscopic calculations of the Rayleigh scattering cross section for arbitrary metallic single-walled carbon nanotubes. The focus of our investigations lies on excitonic effects and their influence on the characteristic features in a Rayleigh scattering spectrum. Our approach is based on density-matrix theory including tight-binding energies, the carrier-light coupling as well as the carrier-carrier interaction. Due to the refractive-index contribution to the scattering cross section, we observe characteristic features in Rayleigh spectra, such as a strong deviation from the Lorentz peak shape and the larger oscillator strength of the lower-lying transition M(ii)(-) in the double-peaked structure, independently of the chiral angle and the diameter of the investigated nanotubes. We observe excitonic binding energies in the range of 60-80 meV for metallic nanotubes with diameters of 1.5-2.5 nm. The overlap of the excitonic transition with the close-by continuum has a significant influence on the peak shape and a minor influence on the peak intensity ratios. The presented results are in good agreement with recent experimental data.

Excitons and Trions in One-Photon- and Two-Photon-Excited MoS2: A Study in Dispersions - Wibmer, Leonie; Lages, Sebastian; Unruh, Tobias; Guldi, Dirk Michael (Advanced Materials, 2018)
Herein, various dispersions of MoS2 obtained by means of liquid phase exfoliation are spectroscopically, (spectro-) electrochemically, and microscopically characterized. At the core of these studies are transient absorption assays. Importantly, small-angle X-ray scattering measurements are employed to corroborate the exfoliated character of the MoS2 flakes in dispersion, on the one hand, and to correlate the results with TEM, AFM, and Raman characterization in the solid state, on the other. It is, then, demonstrated that transient absorption spectroscopy responds sensitively not only to changes in the sample preparation but also to instrumental and environmental parameters. It is documented that the spectroscopic features and their underlying lifetimes are tuneable on the femto-, pico-, and nanosecond scales by changing, for example, the centrifugation speed, the pump fluence, or the temperature. In other words, transient absorption spectroscopy provides an in situ method to quantitatively characterize liquid dispersions of MoS2 without facing the problems of reaggregated samples due to their drying for microscopic assays. The most far reaching results stem from resonantly and nonresonantly changing the pump fluence to characterize either single- or multiple-excited-state species such as excitons, trions, and bi-/multiexcitons and to follow their formation and deactivation pattern.
Keywords: 2D materials; MoS2; pump-probe spectroscopy; small-angle X-ray scattering (SAXS)

Excitons in carbon nanotubes - Maultzsch, Janina; et al. (physica status solidi (b), 2006)
We present two-photon excitation luminescence experiments on carbon nanotubes which show the excitonic origin of the optical excitations. The two-photon allowed exciton state, which has even parity under rotation about the U-axis, is roughly 300 meV above the one-photon active, odd-parity state. This indicates exciton binding energies on the order of 400 meV for nanotubes with diameters around 8 A. Ab-initio calculations of the exciton wavefunctions and energies are in good agreement with our experimental results, confirming the predictions on the symmetry of the exciton states. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Excitons in twisted van der Waals bilayers: Internal structure and ultrafast dynamics - Mooshammer, Fabian; Merkl, Philipp; Ovesen, Simon; Brem, Samuel; Girnghuber, Anna; Lin, Kai-Qiang; Liebich, Marlene; Yong, Chaw-Keong; Gillen, Roland; Maultzsch, Janina; Lupton, John M.; Malic, Ermin; et al. (2020 45TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER, AND TERAHERTZ WAVES (IRMMW-THZ), 2020)
CRIS-Team WoS Importer:2021-07-30

Experimental and theoretical Raman analysis of functionalized diamantane - Meinke, Reinhard; Richter, Robert; Moeller, Thomas; Tkachenko, Boryslav A.; Schreiner, Peter R.; Thomsen, Christian; Maultzsch, Janina (Journal of Physics B-Atomic Molecular and Optical Physics, 2013)
Diamantane functionalized with amino, hydroxyl and thiol groups has been analyzed by Raman spectroscopy. The peaks in the spectra were assigned by supporting density functional theory computations. Two fingerprint regions for a fast determination of functionalization type and site were identified. With their help we can distinguish two different sites of the functional group. Functionalization in the 1-position leads to Raman activation and splitting of degenerate vibrational modes and hence to a more complex Raman spectrum and strong shifts of several modes. Small shifts of the same modes, on the other hand, indicate functionalization in the 4-position. We observe an anomaly in the mass dependence of the functional group, which we assign to the influence of the electronegativity on the vibrational frequency. Additionally, we can distinguish between the functional groups by the characteristic bands of the stretch vibration.

Experimental considerations in electron beam transport on a nanophotonic chip using alternating phase focusing - Shiloh, Roy; Chlouba, Tomas; Hommelhoff, Peter (Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics, 2022)
Not long after the laser was invented, it has been marked as a candidate source of strong, high-frequency electromagnetic radiation for the acceleration of particles. Indeed, while today's complex particle accelerator facilities are an astonishing culmination of decades of work contributed by generations of physicists, engineers, and a host of scientists, new trends and acceleration technologies have recently been proposed and demonstrated. One of these technologies involves the miniaturization of particle accelerators, which is achieved by replacing the radiofrequency (RF) electromagnetic fields accelerating the particles with fields in the optical frequency range using lasers. This entails using nanophotonics structures to provide the required field distribution. Recently, individual elements toward the nanophotonics counterpart of RF accelerators have been demonstrated. Similarly, active electron transport through such a structure has been shown, which was based on the concept of alternating phase focusing. In this contribution, we discuss and augment on the recently demonstrated principle of alternating phase focusing using optical frequencies and provide new insights from relevant simulations and experiments. In particular, we show how to identify possible imprecisions and parasitic effects from time-delay scans and discuss how the transmission of electrons through the nanometric structure depends on the temporal overlap between electron and laser pulses. We also show how the incidence angle of the electron beam can affect the measured transmission of electrons through the structure.
CRIS-Team WoS Importer:2021-12-31

Experimental Determination of the Phonon-Eigenvectors of Silicon Carbide by Raman Spectroscopy - Hundhausen, Martin; Ley, Lothar; et al. (Materials Science Forum, 2002)
The phonon frequencies of silicon carbide (SiC) depend on the atomic masses of the constituent elements Si and C. In general, the frequency decreases, when the mass of one kind of atom is increased by isotopic substitution. Quantitatively, the shift of a phonon line in the Raman spectrum due to such a substitution depends on the phonon eigenvector of the isotopically modified constituent atom. We measure the frequency shifts for Raman active modes in SiC polytypes, when the 12C-isotope is replaced by 13C and calculate the absolute value of the eigenvector of the carbon sublattice from these results. With these results a validation and improvement of the force constants used in a linear-chain model to calculate phonon dispersion curves is possible. © 2002 Trans Tech Publications.
UnivIS-Import:2015-04-14:Pub.2002.nat.dphy.IAP.LAP.experi_5
Keywords: Phonon dispersion curves; Phonon eigenvectors; Raman spectroscopy

Experimental evidence for Phonon folding in compositional amorphous Superlattices - Hundhausen, Martin; Ley, Lothar; et al. (Journal of Non-Crystalline Solids, 1985)
We give experimental evidence for the folding of LA-phonons in a-Si:H/a-SiNx:H compositional superlattices with periods ranging from 60 to 400 Å. We observe two peaks between 5 and 70 cm-1 in the Raman spectra of these samples which we identify with transitions into the second and third branch of the folded longitudinal acoustic phonon dispersion curve. The dispersion of these branches was determined by varying the superlattice period and the laser frequency. The result is well described by a continuum theory that takes the differences in density and sound velocity of a-Si:H and a-SiNx:H into account. © 1985.
UnivIS-Import:2015-03-05:Pub.1985.nat.dphy.IAP.exphys.experi

Experimental Evidence for Quantum Interference and Vibrationally Induced Decoherence in Single-Molecule Junctions - Ballmann, Stefan; Härtle, Rainer; Brana Coto, Pedro; Elbing, Mark; Mayor, Marcel; Bryce, Martin; Thoss, Michael; Weber, Heiko B. (Physical Review Letters, 2012)
We analyze quantum interference and decoherence effects in single-molecule junctions both experimentally and theoretically by means of the mechanically controlled break junction technique and density-functional theory. We consider the case where interference is provided by overlapping quasidegenerate states. Decoherence mechanisms arising from electronic-vibrational coupling strongly affect the electrical current flowing through a single-molecule contact and can be controlled by temperature variation. Our findings underline the universal relevance of vibrations for understanding charge transport through molecular junctions. © 2012 American Physical Society.
UnivIS-Import:2015-04-14:Pub.2012.nat.dphy.IAP.LAP.experi

Experimental studies of the electronic structure of graphene - Bostwick, Aaron; Mcchesney, Jessica; Ohta, Taisuke; Rotenberg, Eli; Seyller, Thomas; Horn, Karsten (Progress in Surface Science, 2009)
Graphene, the single layer of hexagonally coordinated carbon, is a two-dimensional material with many unusual properties; its physical realization a few years ago has caused a storm of activities in the solid state physics and materials science communities. The intriguing ``massless Dirac Fermion'' character of its charge carriers renders graphene a unique study object in condensed matter physics, and we discuss how surface-related techniques such as photoemission, STM and LEED play a prominent role in these investigations. We report on experimental studies of the growth and electronic structure of epitaxial single and few layer graphene on silicon carbide. The unusual band structure of single layer graphene and its evolution as layers are added towards bulk graphite is studied. In the special case of the bilayer, the opening of a gap by inducing an asymmetry through the influence of doping is examined. Finally, the influence of many body processes on the spectral function is discussed on the basis of high resolution photoemission data. The discussion of these aspects gives a comprehensive overview of the electronic structure of graphene as examined by experiment.
EAM Import::2019-03-14

Extended van Hove Singularity and Superconducting Instability in Doped Graphene - Mcchesney, J. L.; Bostwick, Aaron; Ohta, Taisuke; Seyller, Thomas; Horn, Karsten; Gonzalez, Jose; Rotenberg, Eli (Physical Review Letters, 2010)
We have investigated the effects of doping on a single layer of graphene using angle-resolved photoemission spectroscopy. We show that many-body interactions severely warp the Fermi surface, leading to an extended van Hove singularity (EVHS) at the graphene M point. The ground state properties of graphene with such an EVHS are calculated, analyzing the competition between a magnetic instability and the tendency towards superconductivity. We find that the latter plays the dominant role as it is enhanced by the strong modulation of the interaction along the Fermi line, leading to an energy scale for the onset of the pairing instability as large as 1 meV when the Fermi energy is sufficiently close to the EVHS.
EAM Import::2019-03-14

Extension of the LOPLS-AA Force Field for Alcohols, Esters, and Monoolein Bilayers and its Validation by Neutron Scattering Experiments. - Pluhackova, Kristyna; Morhenn, Humphrey; Lautner, Lisa; Lohstroh, Wiebke; Nemkovski, Kirill; Unruh, Tobias; Böckmann, Rainer (Journal of Physical Chemistry B, 2015)
The recently presented LOPLS-AA all-atom force field for long hydrocarbon chains, based on the OPLS-AA force field, was extended to alcohols, esters, and glyceryl monooleate (GMO) lipids as a model lipid. Dihedral angles were fitted against high level ab initio calculations, and ester charges were increased to improve their hydration properties. Additionally, the ester Lennard-Jones parameters were readjusted to reproduce experimental liquid bulk properties, densities, and heats of vaporization. This extension enabled the setup of LOPLS-AA parameters for GMO molecules. The properties of the lipid force field were tested for the liquid-crystalline phase of a GMO bilayer. The obtained area per lipid for GMO is in good agreement with experiment. Additionally, the lipid dynamics on the subpicosecond to the nanosecond time scale is in excellent agreement with results from time-of-flight (TOF) quasielastic neutron scattering (QENS) experiments on a multilamellar monoolein system, enabling here for the first time the critical evaluation of the short-time dynamics obtained from a molecular dynamics simulation of a membrane system.

Extreme Events through Prevailing Backscattering and Their Suppression by a Focusing Nonlinearity - Schultheiß, Vincent; Wimmer, Martin; Malzer, Stefan; Peschel, Ulf (Physical Review X, 2018)
We investigate linear and nonlinear light propagation in weakly perturbed AlGaAs slab waveguides whose fabrication induced local defects give rise to random multiple scattering. The statistical evaluation of the intensity distribution at the rear facet reveals an abundance of high-amplitude events in the linear regime, contradicting the predictions of the central limit theorem by far. In this article we propose weak successive backscattering to be a generator of high-amplitude events, while a focusing nonlinearity of the Kerr type counterintuitively diminishes them, thus effectively randomizing the intensity distribution. We support our experimental findings with numerical simulations based on the stray matrix approach introduced by Edrei et al. [Phys. Rev. Lett. 62, 2120 (1989)]. This novel proposed mechanism for the formation of extreme events in the context of linear wave dynamics represents the first encounter of the phenomenon in the limit of weak scattering.
CRIS-Team WoS Importer:2019-02-20

Extreme localization of electrons in space and time - Hommelhoff, Peter; Kealhofer, Catherine; Aghajani-Talesh, Anoush; Sortais, Yvan R. P.; Foreman, Seth M.; Kasevich, Mark A. (Ultramicroscopy, 2009)
Electron emission from sharp metal tips can take place on sub-femtosecond time scales if the emission is driven by few cycle femtosecond laser pulses. Here we outline the experimental prerequisites in detail, discuss emission regimes and relate them to recent experiments in the gas phase (attosecond physics). We present a process that leads to single atom tip emitters that are stable under laser illumination and conclude with a discussion of how to achieve short electron pulses at a target. © 2008 Elsevier B.V. All rights reserved.
UnivIS-Import:2015-04-14:Pub.2009.nat.dphy.IAP.exphys.extrem
Keywords: Femtosecond laser; Field emission; Field ion microscope; Laser-matter interaction; Tunnel ionization

Ex vivo expanded NK Cells from Melanoma Patients receive their Migration in a 3D Model and lyse autologous Melanoma Cells - Rossner, S.; Czerwinski, Tina; Mark, Christoph; Gross, S.; Karg, Margarete; Bosch, J.; Schuler-Thurner, Beatrice; Fabry, Ben; Schuler, Gerold; Bosch-Voskens, C. (2018)
EVALuna2:34775

Fabrication and characterization of Ag- and Ga-doped mesoporous glass-coated scaffolds based on natural marine sponges with improved mechanical properties - Ciraldo, Francesca; Arango-Ospina, Marcela; Goldmann, Wolfgang H.; Beltran, Ana M.; Detsch, Rainer; Gruenewald, Alina; Roether, Judith; Boccaccini, Aldo R. (Journal of Biomedical Materials Research Part A, 2020)
Natural marine sponges were used as sacrificial template for the fabrication of bioactive glass-based scaffolds. After sintering at 1050°C, the resulting samples were additionally coated with a silicate solution containing biologically active ions (Ag and Ga), well-known for their antibacterial properties. The produced scaffolds were characterized by superior mechanical properties (maximum compressive strength of 4 MPa) and total porosity of 80% in comparison to standard scaffolds made by using PU foam templates. Direct cell culture tests performed on the uncoated and coated samples showed positive results in terms of adhesion, proliferation, and differentiation of MC3T3-E1 cells. Moreover, vascular endothelial growth factor (VEGF) secretion from cells in contact with scaffold dissolution products was measured after 7 and 10 days of incubation, showing promising angiogenic results for bone tissue engineering applications. The antibacterial potential of the produced samples was assessed by performing agar diffusion tests against both Gram-positive and Gram-negative bacteria.
CRIS-Team Scopus Importer:2020-12-04
Keywords: antibacterial; bioactive glass; bone tissue engineering; ion delivery; scaffolds; VEGF

Fabrication and characterization of copper(II)-chitosan complexes as antibiotic-free antibacterial biomaterial - Gritsch, Lukas; Lovell, Christopher; Goldmann, Wolfgang; Boccaccini, Aldo R. (Carbohydrate Polymers, 2018)
<span style="color: rgb(80, 80, 80); font-family: Arial, Helvetica, "Lucida Sans Unicode", "Microsoft Sans Serif", "Segoe UI Symbol", STIXGeneral, "Cambria Math", "Arial Unicode MS", sans-serif; font-size: medium;">We produced and characterized copper(II)-chitosan complexes fabricated via in-situ precipitation as antibiotic-free antibacterial biomaterials. Copper was bound to chitosan from a dilute acetic acid solution of chitosan and copper(II) chloride exploiting the ability of the polysaccharide to chelate metal ions. The influence of copper(II) ions on the morphology, structure and hydrophobicity of the complexes was evaluated using scanning electron microscopy, energy-dispersive X-ray spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy and static contact-angle measurements. To assess the biological response to the materials, cell viability and antibacterial assays were performed using mouse embryonic fibroblasts and both Gram-positive and −negative bacteria. Combined analysis of cell and bacterial studies identified a threshold concentration at which the material shows outstanding antibacterial properties without significantly affecting fibroblast viability. This key outcome sets copper(II)- chitosan as a promising biomaterial and encourages further investigation on similar systems toward the development of new antibiotic-free antibacterial technologies.
Keywords: ChitosanCopperChelationTherapeutic ionsAntimicrobial materialsAntibiotic-free

Fabrication and characterization of glassy carbon membranes - Koval, Yury; Geworski, Andreas; Gieb, Klaus; Lazareva, Irina; Müller, Paul (Journal of Vacuum Science & Technology B, 2014)
In this work, the authors focus on a method to fabricate arbitrary shaped free standing membranes with a thickness less than 20nm, produced from different polymers with the help of low-energy ion irradiation. The authors analyze the thickness of the membranes and its dependence on the details of the irradiation process. In order to tune the properties of the suspended membranes, an additional ion irradiation step has been used. This step is applied to already suspended membranes and leads to several effects, such as heating, shape transformation, etc. These effects were analyzed for irradiation with Ar and He ions. The authors have found that He irradiation has a significant advantage over Ar irradiation providing strained, smooth, and homogeneous membranes. In order to measure the electrical properties of the suspended membranes, the authors invented a new method to contact the membranes. These low resistance contacts can be achieved as the authors describe in detail. The membranes electrical properties after He ion irradiation at different temperatures are presented. Finally, the authors analyze Raman spectra, and thermal and electrical conductivity of the highly conducting membranes. The authors conclude that after high temperature He ion irradiation the membranes consist of material similar in properties to the glassy carbon obtained by pyrolysis. However, this method does not require high temperature pyrolysis step, which makes integration with on-chip electronics more feasible. © 2014 American Vacuum Society.

Fabrication and structural characterization of diamond-coated tungsten tips - Tafel, Alexander; Wu, Mingjian; Spiecker, Erdmann; Hommelhoff, Peter; Ristein, Jürgen (Diamond and Related Materials, 2019)
Coating metal nanotips with a negative electron affinity material like hydrogen-terminated diamond bears promise for a high brightness photocathode. We report a recipe on the fabrication of diamond-coated tungsten tips. A tungsten wire is etched electrochemically to a nanometer sharp tip, dip-seeded in diamond suspension and subsequently overgrown with a diamond film by plasma-enhanced chemical vapor deposition. With dip-seeding only, the seeding density declines towards the tip apex due to seed migration during solvent evaporation. The migration of seeds can be counteracted by nitrogen gas flow towards the apex, which makes coating of the apex with nanometer-thin diamond possible. At moderate gas flow, diamond grows homogeneously at shaft and apex whereas at high flow diamond grows in the apex region only. With this technique, we achieve a thickness of a few tens of nanometers of diamond coating within less than 1 μm away from the apex. Conventional transmission electron microscopy (TEM), electron diffraction and electron energy loss spectroscopy confirm that the coating is composed of dense nanocrystalline diamond with a typical grain size of 20 nm. High resolution TEM reveals graphitic paths between the diamond grains.
CRIS-Team Scopus Importer:2019-07-02
Keywords: Conformal coating; EELS; HRTEM; Nanocrystalline diamond; Nanoemitter

Fabrication of genuine single-quantum-dot light-emitting diodes - Schmidt, Ralf; Scholz, Uwe; Vitzthum, Markus; Fix, Richard; Metzner, Claus; Kailuweit, Peter; Reuter, Dirk; Wieck, Andreas; Hübner, M.C.; Stufler, Stefan; Zrenner, Arthur; Malzer, Stefan; et al. (Applied Physics Letters, 2006)
We present a simple approach for the fabrication of genuine single quantum-dot light-emitting diodes. A submicron wide bottom contact stripe is formed by focused ion beam implantation doping into a GaAs buffer layer. Successive overgrowth with a thin intrinsic layer incorporating self-assembled InAs quantum dots, followed by a top contact layer of complementary doping type and standard photolithographic processing, allows for electrical cross sections in the sub- μ m range. In devices with sufficiently low dot densities, only one single dot is expected to be electrically addressed. Both the observed current versus voltage characteristics and the evolution of the electroluminescence spectra as a function of applied voltage clearly demonstrate that this goal has been achieved. © 2006 American Institute of Physics.

Fabrication of layered nanostructures by successive electron beam induced deposition with two precursors: Protective capping of metallic iron structures - Schirmer, Michael; Walz, Marie-Madeleine; Papp, Christian; Kronast, F.; Gray, A.X.; Balke, B.; Cramm, S.; Fadley, C.S.; Steinrück, Hans-Peter; Marbach, Hubertus (Nanotechnology, 2011)
We report on the stepwise generation of layered nanostructures via electron beam induced deposition (EBID) using organometallic precursor molecules in ultra-high vacuum (UHV). In a first step a metallic iron line structure was produced using iron pentacarbonyl; in a second step this nanostructure was then locally capped with a 2-3nm thin titanium oxide-containing film fabricated from titanium tetraisopropoxide. The chemical composition of the deposited layers was analyzed by spatially resolved Auger electron spectroscopy. With spatially resolved x-ray absorption spectroscopy at the Fe L edge, it was demonstrated that the thin capping layer prevents the iron structure from oxidation upon exposure to air. © 2011 IOP Publishing Ltd.

Facile one‑pot synthesis of water‑soluble fcc FePt3 alloy nanostructures - Kizaloğlu Akbulut, Melek; Harreiß, Christina; Löffler, Mario; Mayrhofer, Karl J.J.; Schöbitz, Michael; Bachmann, Julien; Spiecker, Erdmann; Hock, Rainer; Kryschi, Carola (SN Applied Sciences, 2020)
Proccessible FePt3 alloy nanoparticles with sizes smaller than 50 nm open the avenue to novel magnetic sensor, catalytic and biomedical applications. Our research objective was to establish a highly scalable synthesis technique for production of single-crystalline FePt3 alloy nanoparticles. We have elaborated a one-pot thermal decomposition technique for the synthesis of superparamagnetic FePt3 nanoparticles (FePt3 NPs) with mean sizes of 10 nm. Subsequent tiron coating provided water solubility of the FePt3 NPs and further processibility as bidental ligands enable binding to catalyst surfaces, smart substrates or biosensors. The chemical composition, structure, morphology, magnetic, optical and crystallographic properties of the FePt3 NPs were examined using high resolution transmission electron microscopy, high angle annular dark field-scanning transmission electron microscopy, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy mapping, Fourier transform infrared-attenuated total reflection, X-ray powder diffraction, X-ray photoelectron spectroscopy, vibrating sample magnetometry and UV–Vis absorption spectroscopy.<br
Keywords: Iron-platinum alloy; FePt3 nanoparticles; XPS; XRD analysis; Thermal decomposition route

FAIRmat Guide to Writing Data Management Plans A Practical Guide for the Condensed-Matter Physics and Materials-Science Communities - Mansour, Ahmed E.; Rotheray, Lucia; Helbig, Kerstin; Botti, Silvana; Weber, Heiko B.; Aeschlimann, Martin; Draxl, Claudia (1st Conference on Research Data Infrastructure (CoRDI), 2023)
Research data management is becoming an increasingly important topic due to the growing amounts of types, formats, and sizes of data produced by scientific research. In addition, a growing demand to make data accessible and comprehensible requires standardizing, managing, and planning the data life-cycle. For this reason, many funding agencies now require a data management plan (DMP) as part of submitted research proposals. While some of them and other scientific bodies offer DMP templates, there is no one-size-fits-all solution, due to the heterogeneity of data generated by different scientific disciplines. Here, we present as an example FAIRmat’s effort in enhancing data literacy on the topic of DMP aiming to guide physicists and materials scientists to writing DMPs that comply with the requirements of the German Research Foundation (Deutsche Forschungsgemeinschaft, DF

FAIR Research Data With NOMAD FAIRmat's Distributed, Schema-based Research-data Infrastructure to Harmonize RDM in Materials Science - Scheidgen, Markus; Brückner, Sebastian; Brockhauser, Sandor; Ghiringhelli, Luca M.; Dietrich, Felix; Mansour, Ahmed E.; Marquez, Jose A.; Albrecht, Martin; Weber, Heiko B.; Botti, Silvana; Aeschlimann, Martin; Draxl, Claudia (1st Conference on Research Data Infrastructure (CoRDI), 2023)
Scientific research is becoming increasingly data centric, which requires more effort to manage, share, and publish data. NOMAD is a web-based platform that provides research data management (RDM) for materials-science data. In addition to core RDM functions like uploading and sharing files, NOMAD automatically extracts structured data from supported file formats, normalizes, and converts data from these formats. NOMAD provides an extendable framework for managing not just files, but structured machine-actionable harmonized and inter-operable data. This is the basis for a faceted search with domain-specific filters, a comprehensive API, structured data entry via customizable ELNs, integrated data-analysis and machine-learning tools. NOMAD is run as a free public service and can additionally be operated by research institutes. Connecting NOMAD installations through the public services will allow a federated data infrastructure to share data between research institutes and further harmonize RDM within a large research domain such as materials scienc

Fast phenotyping of monocytes mechanics under the cytokine storm - Eroles, Mar; Gerum, Richard; Fabry, Ben; Rico, Felix (EUROPEAN BIOPHYSICS JOURNAL WITH BIOPHYSICS LETTERS, 2021)
CRIS-Team WoS Importer:2021-08-13

Fast temporal fluctuations in single-molecule junctions - Ochs, Rolf; Secker, Daniel; Elbing, Mark; Mayor, Marcel; Weber, Heiko B. (Faraday Discussions, 2006)
The noise within the electrical current through single-molecule junctions is studied at cryogenic temperature. The organic sample molecules were contacted with the mechanically controlled break-junction technique. The noise spectra refer to a system where only few Lorentzian fluctuators occur in the conductance. The frequency dependence shows qualitative variations from sample to sample.

Femtosecond Laser-Induced Electron Emission from Nanodiamond-Coated Tungsten Needle Tips - Tafel, Alexander; Meier, Stefan; Ristein, Jürgen; Hommelhoff, Peter (Physical Review Letters, 2019)
We present femtosecond laser-induced electron emission from nanodiamond-coated tungsten tips. Based on the shortness of the femtosecond laser pulses, electrons can be photoexcited for wavelengths from the infrared (1932 nm) to the ultraviolet (235 nm) because multiphoton excitation becomes efficient over the entire spectral range. Depending on the laser wavelength, we find different dominant emission channels identified by the number of photons needed to emit electrons. Based on the band alignment between tungsten and nanodiamond, the relevant emission channels can be identified as specific transitions in diamond and its graphitic boundaries. It is the combination of the character of initial and final states (i.e., bulk or surface-near, direct or indirect excitation in the diamond band structure), the number of photons providing the excitation energy, and the peak intensity of the laser pulses that determines the dominant excitation channel for photoemission. A specific feature of the hydrogen-terminated nanodiamond coating is its negative electron affinity that significantly lowers the work function and enables efficient emission from the conduction band minimum into vacuum without an energy barrier. Emission is stable for bunch charges of up to 400 electrons per laser pulse. We infer a normalized emittance of <0.20 nm rad and a normalized peak brightness of >1.2×1012 A m−2 sr−1. The properties of these tips are encouraging for their use as laser-triggered electron sources in applications such as ultrafast electron microscopy as well as diffraction and novel photonics-based laser accelerators.<br
Keywords: diamond-coated nanotip; ultrafast electron sources; emittance; brightness; electron microscopy

Femtosecond laser meets field emission tip - a sensor for the carrier envelope phase? - Hommelhoff, Peter; Kealhofer, Catherine; Kasevich, Mark A. (Proceedings of the 2006 IEEE International Frequency Control Symposium and Expositions, 2006)
UnivIS-Import:2015-04-16:Pub.2006.nat.dphy.IAP.exphys.femtos

Femtosecond two-photon photoemission studies of image-potential states - Fauster, Thomas; Reuss, Christian; Shumay, Igor; Weinelt, Martin (Chemical Physics, 2000)
High-resolution two-photon photoemission studies with femtosecond time resolution permit the accurate determination of decay and dephasing processes for image-potential states. The influence of adsorbates on the respective inelastic and quasielastic scattering processes is investigated for Cu on Cu(100) and Cu(111). The results are discussed in relation to previous work for CO on Cu(100). (C) 2000 Elsevier Science B.V. All rights reserved.

Fermi level on hydrogen terminated diamond surfaces - Ristein, Jürgen; Ley, Lothar; et al. (Applied Physics Letters, 2003)
The characterization of hydrogen terminated patterns contacted with gold and aluminum on (100) diamond surface using atomic force microscopy was discussed. Taking into account the negative electron affinity of -1.3 eV and a band gap of 5.5 eV the Fermi energy was found to be 0.7 eV deep in the valence band.
UnivIS-Import:2015-03-09:Pub.2003.nat.dphy.IAP.exphys.fermil

Few-electron correlations after ultrafast photoemission from nanometric needle tips - Meier, Stefan; Heimerl, Jonas; Hommelhoff, Peter (Nature Physics, 2023)
Free electrons are essential in such diverse applications as electron microscopes, accelerators and photoemission spectroscopy. However, the space charge effects of many electrons are often a problem and, when confined to extremely small space–time dimensions, even two electrons can interact strongly. Here we demonstrate that the resulting Coulomb repulsion can be highly advantageous, as it leads to strong electron–electron correlations. We show that femtosecond laser-emitted electrons from nanometric needle tips are highly anticorrelated in terms of energy because of dynamic Coulomb repulsion, with a visibility of 56%. We extract a mean energy splitting of 3.3 eV and a correlation decay time of 82 fs. The energy-filtered electrons display a sub-Poissonian number distribution with a second-order correlation function as small as g (2) = 0.34, implying that shot-noise-reduced pulsed electron beams can be realized by simple energy filtering. We also reach the strong-field regime of laser-driven electron emission and gain insights into how the electron correlations of the different electron classes (direct or rescattered) are influenced by the strong laser fields.
Created from Fastlane, Scopus look-up

Few-Layer Antimonene by Liquid-Phase Exfoliation - Gibaja, Carlos; Rodriguez-San-Miguel, David; Ares, Pablo; Gomez-Herrero, Julio; Varela, Maria; Gillen, Roland; Maultzsch, Janina; Hauke, Frank; Hirsch, Andreas; Abellan Saez, Gonzalo; Zamora, Felix (Angewandte Chemie International Edition, 2016)
We report on a fast and simple method to produce highly stable isopropanol/water (4:1) suspensions of few-layer antimonene by liquid-phase exfoliation of antimony crystals in a process that is assisted by sonication but does not require the addition of any surfactant. This straightforward method generates dispersions of few-layer antimonene suitable for on-surface isolation. Analysis by atomic force microscopy, scanning transmission electron microscopy, and electron energy loss spectroscopy confirmed the formation of high-quality few-layer antimonene nanosheets with large lateral dimensions. These nanolayers are extremely stable under ambient conditions. Their Raman signals are strongly thickness-dependent, which was rationalized by means of density functional theory calculations.
Keywords: Antimonene; Atomic force microscopy; Liquid-phase exfoliation; Raman spectroscopy; Two-dimensional materials

Fiber alignment in 3D collagen networks as a biophysical marker for cell contractility - Böhringer, David; Bauer, Andreas; Moravec, Ivana; Bischof, Lars; Kah, Delf-Thorge Eric; Mark, Christoph; Grundy, Thomas J.; Görlach, Ekkehard; O'Neill, Geraldine M.; Budday, Silvia; Strissel, Pamela; Strick, Reiner; et al. (Matrix Biology, 2023)
Cells cultured in 3D fibrous biopolymer matrices exert traction forces on their environment that induce deformations and remodeling of the fiber network. By measuring these deformations, the traction forces can be reconstructed if the mechanical properties of the matrix and the force-free matrix configuration are known. These requirements limit the applicability of traction force reconstruction in practice. In this study, we test whether force-induced matrix remodeling can instead be used as a proxy for cellular traction forces. We measure the traction forces of hepatic stellate cells and different glioblastoma cell lines and quantify matrix remodeling by measuring the fiber orientation and fiber density around these cells. In agreement with simulated fiber networks, we demonstrate that changes in local fiber orientation and density are directly related to cell forces. By resolving Rho-kinase (ROCK) inhibitor-induced changes of traction forces, fiber alignment, and fiber density in hepatic stellate cells, we show that the method is suitable for drug screening assays. We conclude that differences in local fiber orientation and density, which are easily measurable, can be used as a qualitative proxy for changes in traction forces. The method is available as an open-source Python package with a graphical user interface.
CRIS-Team Scopus Importer:2023-12-22
Keywords: Biopolymer Networks; Fiber Orientation; Mechanobiology; Traction Force Microscopy

Fiber chirped pulse amplifier at 2.08 μm emitting 383-fs pulses at 10 nJ and 7 MHz - Hoogland, Heinar; Wittek, Steffen; Hänsel, Wolfgang; Stark, Sebastian; Holzwarth, Ronald (Optics Letters, 2014)
An all-polarization maintaining (PM) fiber chirped pulse amplifier system at 2.08 μm based on thulium:holmium codoped gain fibers is reported. An inhouse built oscillator emits pulses at a repetition rate of 7 MHz with a spectral full width at half-maximum (FWHM) bandwidth of 23.5 nm at 2.8 mW average output power. The pulses are temporally stretched and subsequently amplified in a double-stage amplifier setup. The stretched pulses are compressed to 383 fs by use of a Martinez-style setup at an output pulse energy of 10.2 nJ. By neglecting temporal stretching, high peak powers in a single amplifier stage led to Raman soliton formation at 2.3 μm.

Fiber optic based system for polarization sensitive spectroscopy of semiconductor quantum structures - Arora, Ashish; Karmakar, Biswajit; Sharma, Sayan; Schardt, Michael; Malzer, Stefan; Bansal, Bhavtosh; Döhler, Gottfried; Arora, Brij Mohan (Review of Scientific Instruments, 2010)
We describe an optical fiber based setup for performing polarization resolved magneto-optical spectroscopy measurements under low temperatures (∼4 K) and high magnetic fields (∼8 T). The measurements are performed in a windowless helium Dewar. Circularly polarized light is produced inside the Dewar by inserting the polarizing elements between the fiber end and the sample. Photoconductivity spectra of a GaAs/AlGaAs multiquantum-well sample have been measured over the photon energy range of 1.5-1.7 eV in left and right circularly polarized light under crossed magnetic and electric fields. It is shown that reversing the direction of magnetic field produces the same spectral changes as caused by changing the direction of circular polarization with the optical components. © 2010 American Institute of Physics.

Fibre-based pressure-controlled sources for quantum optics - Joly, Nicolas; Hammer, Jakob; Chekhova, Maria (2021 Conference on Lasers and Electro-Optics, CLEO 2021 - Proceedings, 2021)
We present progress towards the generation of photon triplet states and realisation of a tunable source of entangled photons. In both cases, control of the phase-matching is achieved through gas-pressure tuning.
CRIS-Team Scopus Importer:2021-12-24

Fibre-optic photon-number squeezing in the normal group-velocity dispersion regime - Sizmann, Andreas; Fauster, Thomas; Leuchs, Gerd; et al. (Journal of Modern Optics, 1998)
The nonlinear optical Kerr effect, acting on optical pulses in fibres, creates spectral sidebands and noise correlations between these sidebands. The reduction of photon-number fluctuations of these pulses below the shot-noise limit by spectral filtering is well established in the anomalous dispersion regime which allows for soliton formation. Here it is demonstrated that a significant quantum-noise reduction with spectral filtering can also be reached for pulses in the normal dispersion regime. The filter function was optimized and the power dependence of the noise reduction was investigated. The best squeezing result is (1.2 ± 0.2) dB (corresponding to (2.6 ± 0.7) dB inferred for 100% detection efficiency).
UnivIS-Import:2015-03-05:Pub.1998.nat.dphy.optik.1optik.fibreo

Fibrous protein-based hydrogels for cell encapsulation - Silva, Raquel; Fabry, Ben; Boccaccini, Aldo R. (Biomaterials, 2014)
Tissue scaffolds play a vital role in tissue engineering by providing a native tissue-mimicking environment for cells, with the aim to promote cell proliferation, proper cell differentiation, and regeneration. To better mimic the microenvironment of native tissues, novel techniques and materials have emerged in recent years. Among them, hydrogels formed from self-assembled biopolymer networks are particularly interesting. This paper reviews the fabrication and use of fibrous protein-based hydrogels, with an emphasis on silk, keratin elastin and resilin proteins. Hydrogels formed by these proteins show close structural, chemical and mechanical similarities with the extracellular matrix, typically good biological compatibility, and they can trigger specific cellular responses. In addition, these hydrogels can be degraded in the body by proteolytic enzymes. For these reasons, fibrous protein hydrogels are one of the most versatile materials for tissue engineering. © 2014 Elsevier Ltd.
Keywords: Biomaterials; Cell-encapsulation; Fibrous-proteins; Hydrogels

Field-effect transistor made of individual V2O5 nanofibers - Kim, Gyutae; Muster, Jörg; Krstic, Vojislav; Park, YungWoo; Park, Jin-gyu; Roth, Siegmar; Burghard, Marko (Applied Physics Letters, 2000)
A field-effect transistor (FET) with a channel length of ∼100 nm was constructed from a small number of individual V2O5 fibers of the cross section 1.5 nm×10 nm. At low temperature, the conductance increases as the gate voltage is changed from negative to positive values, characteristic of a FET with n-type enhancement mode. The carrier mobility, estimated from the low-field regime, is found to increase from 7.7×10−5 cm2/V s at T=131 K to 9.6×10−3 cm2/V s at T=192 K with an activation energy of Ea=0.18 eV. The nonohmic current/voltage dependence at high electric fields was analyzed in the frame of small polaron hopping conduction, yielding a nearest-neighbor hopping distance of ∼4 nm.
Keywords: Nanofiber; Hopping transport; Field effect transistors; Polarons; Electronic transport

Field effect transistors on graphitized polymer surfaces - Koval, Yury; Lazareva, Irina; Müller, Paul; Müller, Klaus; Henkel, Karsten; Friedrich, Dieter (physica status solidi (b), 2011)
Conducting properties of several polymers after low-energy ion irradiation were investigated. The enhancement of conductance induced by the ions is discussed in terms of graphitization. Field effect transistors (FET) on the graphitized polymer surfaces were studied. We found that the field effect mobility depends on the conductivity of the graphitized surfaces according to the power relation σ μ. Such behavior is typical for disordered organic semiconductors with a variable range hopping mechanism of conductance. In contrast, the graphitized surfaces with a semimetallic type of conductance demonstrate a high carrier mobility 1cm/Vs, which is almost independent on the conductance. These semimetallic materials consist of 2 and 3nm graphene patches and are characterized by a significantly smaller disorder level. However, for a feasible FET application the carrier concentration in the graphitized surfaces must be reduced. Possibilities to improve the performance of the FETs are discussed. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Keywords: Conductivity; Field effect transistor; Graphitization; Ion irradiation; Mobility; Polymers

Field emission measurements with micrometre resolution on carbon nanostructures - Stammler, Martin; Ristein, Jürgen; Ley, Lothar; et al. (Diamond and Related Materials, 1999)
The field emission properties of carbon nanostructures grown in a microwave plasma chemical vapour deposition system under bias conditions were investigated. Different carbon nanostructures, namely nanocrystalline diamond films (sample A) and nanotubes (sample B), were grown under conditions similar to those employed during the bias pretreatment for diamond growth. Morphological and structural properties of the carbon nanostructures were investigated by scanning electron microscopy, Raman and photoelectron spectroscopy. The field emission properties of the samples were measured by means of a field emission scanning microscope using resolutions of 3 and 40 μm, respectively. The local emission follows the Fowler-Nordheim law up to 1 mA/mm2 (3 mA/mm2) for sample A (sample B). Higher currents lead for sample A to an improved emission behaviour, while on sample B the application of higher currents can be detrimental in some cases. The average onset field strength (measured at a current of 0.5 nA) was as low as 55 V/μm (sample A) and 9 V/μm (sample B), respectively, compared with 200 V/μm and more for CVD diamond films.
UnivIS-Import:2015-03-05:Pub.1999.nat.dphy.IAP.exphys.fielde
Keywords: Field emission; Nanocrystalline diamond; Nanotubes; Scanning electron microscopy

Field Emission Tip as a Nanometer Source of Free Electron Femtosecond Pulses - Hommelhoff, Peter; Sortais, Yvan R. P.; Aghajani-Talesh, Anoush; Kasevich, Mark A. (Physical Review Letters, 2006)
We report a source of free electron pulses based on a field emission tip irradiated by a low-power femtosecond laser. The electron pulses are shorter than 70 fs and originate from a tip with an emission area diameter down to 2 nm. Depending on the operating regime we observe either photofield emission or optical field emission with up to 200 electrons per pulse at a repetition rate of 1 GHz. This pulsed electron emitter, triggered by a femtosecond oscillator, could serve as an efficient source for time-resolved electron interferometry, for time-resolved nanometric imaging and for synchrotrons. © 2006 The American Physical Society.
UnivIS-Import:2015-04-14:Pub.2006.nat.dphy.IAP.exphys.fielde

Field enhanced conductivity in a-Si:H thin film transistors - Hundhausen, Martin; Ley, Lothar; et al. (Journal of Non-Crystalline Solids, 1993)
We have used amorphous silicon thin film transistors (TFT) to study the excitation and the distribution of electrons in the band tail states in the presence of strong electric fields at low temperature. The dark conductivity shows the same features as field enhanced conductivity (FEC) in doped a-Si:H, albeit at much smaller fields. The magnitude of the IR-photoconductivity measured between 120 and 260 meV is independent of gate or source-drain voltage as long as the source-drain current is kept constant. We consider a model in which electrons are excited by the field from the Fermi energy (EF) to an energy higher up in the band tail. The IR-photocurrent spectra reflect then the carrier distribution that results from the thermalization of electrons back to EF. © 1993.
UnivIS-Import:2015-03-05:Pub.1993.nat.dphy.IAP.exphys.fielde

Field localization and rescattering in tip-based photoemission - Yalunin, Sergey V.; Herink, Georg; Solli, Daniel R.; Krüger, Michael; Hommelhoff, Peter; Diehn, Manuel; Munk, Axel; Ropers, Claus (Annalen Der Physik, 2013)
UnivIS-Import:2015-04-14:Pub.2013.nat.dphy.IAP.exphys.fieldl

Filamin A is essential for active cell stiffening but not passive stiffening under external force - Kaszar, Karen E.; Nakamura, Fumihiko; Hu, Shaohua; Kollmannsberger, Philip; Fabry, Ben; Stossel, Thomas Peter; Wang, Ning; Weitz, David A. (Journal of Biophysics, 2009)

Film Thickness of Pb Islands on the Si(111) Surface - Spaeth, Th.; Popp, Matthias Albert; Hoffmann-Vogel, R. (Physical Review Letters, 2020)
We analyze topographic scanning force microscopy images together with Kelvin probe images obtained on Pb islands and on the wetting layer on Si(111) for variable annealing times. Within the wetting layer we observe negatively charged Si-rich areas. We show evidence that these Si-rich areas result from islands that have disappeared by coarsening. We argue that the islands are located on Si-rich areas inside the wetting layer such that the Pb/Si interface of the islands is in line with the top of the wetting layer rather than with its interface to the substrate. We propose that the Pb island heights are one atomic layer smaller than previously believed. For the quantum size effect bilayer oscillations of the work function observed in this system, we conclude that for film thicknesses below 9 atomic layers large values of the work function correspond to even numbers of monolayers instead of odd ones. The atomically precise island height is important to understand ultrafast "explosive" island growth in this system.
CRIS-Team WoS Importer:2020-01-24

Fingerprints of a size-dependent crossover in the dimensionality of electronic conduction in Au-seeded Ge nanowires - Kolesnik-Gray, Maria; Collins, Gillian; Holmes, Justin D.; Krstic, Vojislav (Beilstein Journal of Nanotechnology, 2016)
We studied the electrical transport properties of Au-seeded germanium nanowires with radii ranging from 11 to 80 nm at ambient conditions. We found a non-trivial dependence of the electrical conductivity, mobility and carrier density on the radius size. In particular, two regimes were identified for large (lightly doped) and small (stronger doped) nanowires in which the charge-carrier drift is dominated by electron-phonon and ionized-impurity scattering, respectively. This goes in hand with the finding that the electrostatic properties for radii below ca. 37 nm have quasi one-dimensional character as reflected by the extracted screening lengths.
Keywords: electrical transport; germanium nanowires; quasi-1D confinement; screening length; VLS growth

FINITE-TEMPERATURE FERROMAGNETISM OF NICKEL - Fauster, Thomas; et al. (Physical Review B, 1989)

FIR response of intrinsic Josephson Junctions - Rother, Stephan; Koval, Yury; Müller, Paul; Kasai, Y.; Nakajima, K.; Darula, M. (IEEE Transactions on Applied Superconductivity, 2001)
We present experiments on the far infrared response of intrinsic Josephson junctions in Bi 2Sr 2CaCu 2O 8+x (BSCCO). We detected first order Shapiro steps on resistive branches in a frequency range between 584 GHz and 2.5 THz. Due to the very low power coupled into the junctions the suppression of the critical current was only weak. With increasing number of junctions in the resistive state a higher amplitude of the Shapiro steps was measured. As the frequencies are well above the plasma frequency of our junctions, the steps are expected to be stable at any power level. To verify this, step heights were studied at various power levels as a function of frequency. The impedance matching of the system improved considerably when several junctions have switched into the resistive state.
UnivIS-Import:2015-03-09:Pub.2001.nat.dphy.IAP.pephys.firres
Keywords: Far infrared; Impedance match; Intrinsic Josephson junctions; Shapiro steps

First and second optical transitions in single-walled carbon nanotubes: a resonant Raman study - Telg, Hagen; Maultzsch, Janina; Reich, Stephanie; Thomsen, Christian (physica status solidi (b), 2007)
Resonant Raman spectroscopy was performed to study electron-phonon coupling in single-walled carbon nanotubes separated in solution. By varying the excitation energy from 1.26 eV to 1.93 eV we obtained radial breathing mode resonance profiles of the first and second optical transitions E, and E2, of the (9, 1) and (8,3) tubes. We observe up to 16 times stronger Raman intensities for the E, transitions which can mostly be attributed to a two times broader linewidth of the E-22 transition. Comparison of the matrix element ratio M-11/M-22 to theoretical predictions on the electron-phonon coupling show a deviation of a factor 1.7 which might-be associated with the change of the exciton-photon matrix element. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

First- and second-order Raman spectroscopy of monoclinic β- Ga2 O3 - Janzen, Benjamin M.; Gillen, Roland; Galazka, Zbigniew; Maultzsch, Janina; Wagner, Markus R. (Physical Review Materials, 2022)
We employ a combined experimental-theoretical study of the first- and second-order Raman modes of monoclinic β-Ga2O3. Gallium oxide has attracted considerable interest due to its deep-UV band gap paired with a high critical field strength, offering promising applications in power electronics. A crucial prerequisite for the future development of Ga2O3-based devices is a detailed understanding of the lattice vibrations, i.e., phonons, as they govern important material properties such as elasticity, thermal conductivity, temperature-dependence of the band gap, or free-carrier transport. Polarized micro-Raman spectroscopy measurements on the (010) and (2¯01) planes of β-Ga2O3 single crystals enable the determination of the phonon frequencies of all 15 first-order and more than 40 second-order Raman modes. The experimental results are correlated with calculations of the mode frequencies, the phonon dispersion relation, and the phonon density of states using density functional perturbation theory. By applying a group-theoretical analysis, we are able to distinguish between overtones and combinational modes and identify the high-symmetry points in the Brillouin zone that contribute to the observed second-order modes. Based on this information, we demonstrate the simultaneous determination of Raman, IR, and acoustic phonons in β-Ga2O3 by second-order Raman spectroscopy.
CRIS-Team Scopus Importer:2022-06-17

First results with the neutron backscattering and TOF spectrometer option BATS on IN16B - Appel, Markus; Frick, Bernhard; Magerl, Andreas (Physica B-Condensed Matter, 2019)
We report on the status of the ongoing upgrade of the neutron backscattering spectrometer IN16B with the new BATS option, which enables operation of the instrument in inverted Time-of-Flight mode. First commissioning results are available, confirming the expectations and demonstrating its capabilities. Using Si 111 analysers, BATS allows to record neutron spectra with variable energy resolution down to 1.5 mu eV with momentum transfers up to 1.9 angstrom(-1), reaching energy transfers beyond - 600 mu eV and achieving a signal to noise ratio above 10(4).
CRIS-Team WoS Importer:2019-04-30

Fission of super-heavy nuclei explored with skyrme forces - Schindzielorz, Nils; Erler, J.; Klüpfel, P.; Reinhard, Paul-Gerhard; Hager, Georg (International Journal of Modern Physics E-Nuclear Physics, 2009)
We present a large scale survey of life-times for spontaneous fission in the regime of super-heavy elements (SHE), i.e. nuclei with Z=104-122. This is done on the basis of the Skyrme-Hartree-Fock model. The axially symmetric fission path is computed using a quadrupole constraint. Self-consistent cranking is used for the collective masses and associated quantum corrections. The actual tunneling probability is estimated by the WKB approximation. Three typical Skyrme forces are used to explore the sensitivity of the results. Benchmarks in the regime Z=104-108 show an acceptable agreement. The general systematics reflects nicely the islands of shell stabilization and the crossover from α-decay to fission for the decay chains from the region of Z/N=118/176. © 2009 World Scientific Publishing Company.
CRIS-Team Scopus Importer:2023-07-24

Flexible sample cell for real-time GISAXS, GIWAXS and XRR: design and construction - Berlinghof, Marvin; Bär, Christian; Haas, D.; Bertram, F.; Langner, Stefan; Osvet, Andres; Chumakov, A.; Will, Johannes; Schindler, Torben; Zech, Tobias; Brabec, Christoph; Unruh, Tobias (Journal of Synchrotron Radiation, 2018)
Since the properties of functional materials are highly dependent on their specific structure, and since the structural changes, for example during crystallization, induced by coating and annealing processes are significant, the study of structure and its formation is of interest for fundamental and applied science. However, structure analysis is often limited to ex situ determination of final states due to the lack of specialized sample cells that enable real-time investigations. The lack of such cells is mainly due to their fairly complex design and geometrical restrictions defined by the beamline setups. To overcome this obstacle, an advanced sample cell has been designed and constructed; it combines automated doctor blading, solvent vapor annealing and sample hydration with real-time grazing-incidence wide- and small-angle scattering (GIWAXS/GISAXS) and X-ray reflectivity (XRR). The sample cell has limited spatial requirements and is therefore widely usable at beamlines and laboratory-scale instruments. The cell is fully automatized and remains portable, including the necessary electronics. In addition, the cell can be used by interested scientists in cooperation with the Institute for Crystallography and Structural Physics and is expandable with regard to optical secondary probes. Exemplary research studies are presented, in the form of coating of P3HT:PC61PM thin films, solvent vapor annealing of DRCN5T:PC71BM thin films, and hydration of supported phospholipid multilayers, to demonstrate the capabilities of the in situ cell.
Keywords: in situ; GIWAXS; GISAXS; XRR; thin film

Flow and hydrodynamic shear stress inside a printing needle during biofabrication - Müller, Sebastian J.; Mirzahossein, Elham; Iftekhar, Emil N.; Bächer, Christian; Schrüfer, Stefan; Schubert, Dirk W.; Fabry, Ben; Gekle, Stephan (PLoS ONE, 2020)
We present a simple but accurate algorithm to calculate the flow and shear rate profile of shear thinning fluids, as typically used in biofabrication applications, with an arbitrary viscosity-shear rate relationship in a cylindrical nozzle. By interpolating the viscosity with a set of power-law functions, we obtain a mathematically exact piecewise solution to the incompressible Navier-Stokes equation. The algorithm is validated with known solutions for a simplified Carreau-Yasuda fluid, full numerical simulations for a realistic chitosan hydrogel as well as experimental velocity profiles of alginate and chitosan solutions in a microfluidic channel. We implement the algorithm in an easy-to-use Python tool, included as Supplementary Material, to calculate the velocity and shear rate profile during the printing process, depending on the shear thinning behavior of the bioink and printing parameters such as pressure and nozzle size. We confirm that the shear stress varies in an exactly linear fashion, starting from zero at the nozzle center to the maximum shear stress at the wall, independent of the shear thinning properties of the bioink. Finally, we demonstrate how our method can be inverted to obtain rheological bioink parameters in-situ directly before or even during printing from experimentally measured flow rate versus pressure data.
CRIS-Team Scopus Importer:2020-08-07

Fluctuations and jump dynamics in atomic friction experiments - Maier, Sabine; Sang, Yi; Filleter, Tobin; Grant, Martin; Bennewitz, Roland; Meyer, Ernst; et al. (Physical Review B, 2005)
Atomic stick-slip processes have been studied in detail by means of friction force microscopy with high spatial and temporal resolution. The influence of the tip-sample contact on the thermal fluctuations of the force sensor and on the dynamics of the stick-slip process are characterized. Results are compared with simulations based on an extended Tomlinson model including thermal fluctuations. A correlation between the duration of the atomic slip event and the atomic structure of the contact is establishe

Fluctuations of cytoskeleton-bound microbeads - the effect of bead-receptor binding dynamics - Metzner, Claus; Raupach, Carina; Mierke, Claudia; Fabry, Ben (Journal of Physics: Condensed Matter, 2010)

Flux-enhanced monochromator by ultrasound excitation of annealed Czochralski-grown silicon crystals - Hock, Rainer; et al. (Applied Physics A-Materials Science & Processing, 2003)
The neutron flux from monochromator crystals can be increased by ultrasound excitation or by strain fields. Rocking curves of both a perfect float-zone silicon crystal and an annealed Czochralski silicon crystal with oxygen precipitates were measured at various levels of ultrasound excitation on a cold-neutron backscattering spectrometer. We find that the effects of the dynamic strain field from the ultrasound and the static strain field from the defects are not additive. Rocking curves were also taken at different ultrasound frequencies near resonance of the crystal/ultrasound-transducer system with a time resolution of 1 min. Pronounced effects of crystal heating are observed, which render the conditions for maximum neutron reflectivity delicate.

Focal Adhesion Kinase Stabilizes the Cytoskeleton - Fabry, Ben; Klemm, Anna; Kienle, Sandra; Schäffer, Tilman; Goldmann, Wolfgang (Journal of Biophysics, 2011)

Force microscopy on insulators: Imaging of organic molecules - Maier, Sabine; et al. (Journal of Physics: Conference Series, 2005)
So far, most of the high resolution scanning probe microscopy studies of organic molecules were restricted to metallic substrates. Insulating substrates are mandatory when the molecules need to be electrically decoupled in a electronic circuit. In such a case, atomic force microscopy is required. In this paper we will discuss our recent studies on different organic molecules deposited on KBr surfaces in ultra-high vacuum, and then imaged by AFM at room temperature. The distance between tip and surface was controlled either by the frequency-shift of the cantilever resonance or by the excitation signal required to keep the oscillation amplitude constant. Advantages and drawbacks of both techniques are discussed. The high mobility of the molecules, due to their weak interaction with the substrate, hinders the formation of regular self assembled structures. To overcome this problem we created artificial structures on the surface by annealing and by electron irradiation, which made possible the growth of the molecules onto step edges and their confinement into rectangular pits. © 2005 IOP Publishing Ltd.
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Formation and Decomposition of CO2 Intercalated Graphene Oxide - Eigler, Siegfried; Dotzer, Christoph; Hirsch, Andreas; Enzelberger-Heim, Michael; Müller, Paul (Chemistry of Materials, 2012)
The formation, stability, and decomposition of CO intercalated graphene oxide was analyzed by FTIR, TGAMS, TGA-IR, AFM, and SEM for the first time. We found that the formation starts at 50 °C and develops up to 120 °C. The formation process can be best observed by FTIR spectroscopy, and the product is stable at ambient conditions. At higher temperatures, the decomposition of CO intercalated graphene oxide occurs due to the release of water, CO , and CO that can be monitored by TGA-MS and TGA-IR analysis. AFM and SEM images can visualize the formation of blisters in GO films that become instable at 210 °C. We further prepared graphene oxide with a low water-content and found that the formation of CO was significantly suppressed and CO became the major species responsible for the weight loss. In addition we prepared OH treated graphene oxide to elucidate the formation process of CO and found C O O by TGA-MS analysis that proves the crucial role of water during CO formation. From these experiments we propose that hydrate species are key-intermediates for the formation of CO . Hence, it seems likely that rearrangement reactions that can proceed via hydrate intermediates, known from organic chemistry, are probably responsible for the formation of carboxylic acids at the edges of graphene oxide sheets after sonication of graphite oxide. Further, our investigations prove that graphene oxide is less stable than shown by TGA measurements. This has a high impact on the electronic properties of reduced graphene oxide, especially for all those using it for electronic applications. © 2012 American Chemical Society.

Formation of Cu2SnSe3 from stacked elemental layers investigated by combined in situ X-ray diffraction and differential scanning calorimetry techniques - Wibowo, Rachmat Adhi; Möckel, Stefan; Yoo, Hye Sun; Hölzing, Astrid; Hock, Rainer; Wellmann, Peter (Journal of Alloys and Compounds, 2014)
Stacked elemental layers of Mo/Cu/Sn and Mo/Cu/Sn/Se were employed as samples for investigating the formation reaction of Cu-Sn intermetallic compounds as well as CuSnSe phases by in situ technique of X-ray diffraction and differential scanning calorimetry. The use of a combined in situ technique allows a real-time observation on solid-state reactions as well as any crystalline phase changes during annealing towards the crystallization of CuSnSe. It is found that Cu and Sn form intermetallic compounds of CuSn, CuSn and CuSn as the annealing temperature rises from 30 to 550 C. The reaction of Se with Cu to form a CuSe phase dominates the binary phase formation at a low annealing temperature. The annealing of a stacked Mo/Cu/Sn/Se layer suggests that only CuSn intermetallic compound directly acts as a reactant for the Cu-selenide phase formation. A SnSe phase mostly forms from a liquid-state reaction of Sn and Se above the Sn melting point. The in situ investigation also reveals a complete set of Cu-selenide peritectic decompositions of CuSe → CuSe → CuSe at 360 and 412 C. The formation of CuSnSe phase starts at 450 C as a product from a reaction between Cu Se and SnSe in a presence of liquid Se. Comparisons on the initial formation temperatures of all involved phases and on the formation pathways between CuSnSe and CuSnS are discussed as well. © 2013 Elsevier B.V. All rights reserved.
Keywords: Intermetallics; Thermoelectric materials; Solid state reactions; Phase transitions

Formation of CuInSe2 by the annealing of stacked elemental layers - analysis by in situ high-energy powder diffraction - Hock, Rainer; et al. (Thin Solid Films, 2003)
Phases and phase transitions in three binary systems, Cu-Se, In-Se, Cu-In and in the ternary and quaternary systems Cu-In-Se (Ga) were investigated by in situ high energy powder diffraction in a temperature range from 25 to 550 degreesC. Results for the binary systems are compared to the known equilibrium phase diagrams of Cu-In, Cu-Se and In-Se. Above 225 degreesC Cu-In and Cu-Se follow the equilibrium phase diagrams. For In-Se significant deviations from the equilibrium diagram are observed. The results on binary systems yielded the basis for the qualitative phase analysis of the phase sequences observed in the CuInSe2. precursors during thermal anneal. On ternary and quaternary systems Cu-In-Se-(Ga) the reaction path to the formation of CuInSe2 could be deter-mined in real time. Deviations of phase sequences from the equilibrium phase diagrams are attributed to the digression from the chemical rather than the thermal equilibrium. CuInSe2 finally crystallises from the direct precursors Cu2Se (Cu2-xSe, respectively) and InSe within a melt rich in selenium. The influence of gallium and sodium on the phase sequences and the resulting formation of CuInSe2, are discussed. Both, Na and Ga promote the crystallisation of Cu2Se, the direct precursor phase for CuInSe2. A comparison of the crystallographic structures of Cu2Se and InSe shows that epitaxial growth of InSe (0001) on Cu2Se (111) lattice planes is feasible. Based upon the experimental and crystallographic analysis a qualitative model for CuInSe, crystallisation from the precursors in the melt is developed. (C) 2003 Elsevier Science B.V All rights reserved.
Keywords: solar cell; CuInSe2; in situ powder diffraction; phase analysis

Formation of highly ordered molecular porous 2D networks from cyano-functionalized porphyrins on Cu(111) - Steinrück, Hans-Peter; Adhikari, Rajan; Siglreithmaier, Gretel; Gurrath, Martin; Meusel, Manuel; Kuliga, Jan; Lepper, Michael; Hölzel, Helen; Jux, Norbert; Meyer, Bernd; Marbach, Hubertus (Chemistry - A European Journal, 2020)
We investigated the adsorption of three related cyano-functionalized tetraphenyl porphyrin derivatives on Cu(111) by scanning tunneling microscopy (STM) in ultra-high vacuum (UHV) with the goal to identify the role of the cyano group and the central Cu atom for the intermolecular and supramolecular arrangement. The porphyrin derivatives studied were Cu-TCNPP, Cu-cisDCNPP, and 2H-cisDCNPP, that is, Cu-5,10,15,20-tetrakis-(p-cyano)-phenyl-porphyrin, Cu-meso-cis-di(p-cyano)-phenyl-porphyrin and 2H-meso-cis-di(p-cyano)-phenyl-porphyrin, respectively. Starting from different structures obtained after deposition at room temperature, all three molecules form the same long-range ordered hexagonal honeycomb-type structure with triangular pores and three molecules per unit cell. For the metal-free 2H-cisDCNPP, this occurs only after self-metalation upon heating. The structure-forming elements are pores with a distance of 3.1 nm, formed by triangles of porphyrins fused together by cyano-Cu-cyano interactions with Cu adatoms. This finding leads us to suggest that two cyano-phenyl groups in the "cis" position is the minimum prerequisite to form a highly ordered 2D porous molecular pattern. The experimental findings are supported by detailed density functional theory calculations to analyze the driving forces that lead to the formation of the porous hexagonal honeycomb-type structure.
CRIS-Team Scopus Importer:2020-07-03
Keywords: porphyrins coordinated networks cyano functionalization scanning tunneling microscopy (STM) density-functional theory (DFT) calculations

Formation of Highly Ordered VO2 Nanotubular/Nanoporous Layers and Their Supercooling Effect in Phase Transitions - Yang, Yang; Lee, Kiyoung; Zobel, Mirijam; Mackovic, Mirza; Unruh, Tobias; Spiecker, Erdmann; Schmuki, Patrik (Advanced Materials, 2012)
The fabrication of self-organized VO2 nanotubular/nanoporous layers is demonstrated by self-organizing anodization, followed by a suitable heat treatment. These VO2 layers show a reversible metal to insulator transition (MIT) at 70 and 44 °C, when heating and cooling, respectively.
Keywords: vanadium dioxide; nanoporous layers; anodization; supercooling; metal-insulator transition

Formation of liquid crystalline phases in aqueous suspensions of platelet-like tripalmitin nanoparticles - Schmiele, Martin; Gehrer, Simone; Westermann, Martin; Steiniger, Frank; Unruh, Tobias (Journal of Chemical Physics, 2014)
Suspensions of platelet-like shaped tripalmitin nanocrystals stabilized by the pure lecithin DLPC and the lecithin blend S100, respectively, have been studied by small-angle x-ray scattering (SAXS) and optical observation of their birefringence at different tripalmitin (PPP) concentrations phi(PPP). It could be demonstrated that the platelets of these potential drug delivery systems start to form a liquid crystalline phase already at pharmaceutically relevant concentrations phi(PPP) of less than 10 wt. %. The details of this liquid crystalline phase are described here for the first time. As in a previous study [A. Illing et al., Pharm. Res. 21, 592 (2004)] some platelets are found to self-assemble into lamellar stacks above a critical tripalmitin concentration phi(st)(PPP) of 4 wt. %. In this study another critical concentration phi(lc)(PPP) approximate to 7 wt. % for DLPC and phi(lc)(PPP) approximate to 9 wt. % for S100 stabilized dispersions, respectively, has been observed phi(lc)(PPP) describes the transition from a phase of randomly oriented stacked lamellae and remaining non-assembled individual platelets to a phase in which the stacks and non-assembled platelets exhibit an overall preferred orientation. A careful analysis of the experimental data indicates that for concentrations above phi(lc)(PPP) the stacked lamellae start to coalesce to rather small liquid crystalline domains of nematically ordered stacks. These liquid crystalline domains can be individually very differently oriented but possess an overall preferred orientation over macroscopic length scales which becomes successively more expressed when further increasing phi(PPP). The lower critical concentration for the formation of liquid crystalline domains of the DLPC-stabilized suspension compared to phi(lc)(PPP) of the S100-stabilized suspension can be explained by a larger aspect ratio of the corresponding tripalmitin platelets. A geometrical model based on the excluded volumes of individual platelets and stacked lamellae has been developed and successfully applied to reproduce the critical volume fractions for both, the onset of stack formation and the appearance of the liquid crystalline phase. (C) 2014 AIP Publishing LLC.

Formation of molecular wires on nanostructured KBr - Maier, Sabine; et al. (Journal of Physics: Conference Series, 2007)
The formation of molecular wires on a nanostructured KBr(001) substrate is observed by non- contact atomic force microscopy. Locally polarized cyano porphyrin molecules evaporated on this sample assemble in a linear fashion along straight pit edges forming one dimensional structures. © 2007 IOP Publishing Ltd.
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Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses - Zhao, Quanzhong; Malzer, Stefan; Wang, Lijun (Optics Letters, 2007)
The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is investigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of the ripple structure turned left or right by 45° with respect to the incident plane when using right and left circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential applications in large-area structuring of metals. © 2007 Optical Society of America.

Formation of the interface between c-Si(111) and diamond-like carbon studied with photoelectron spectroscopy - Ristein, Jürgen; Ley, Lothar; et al. (Applied Surface Science, 1998)
The hydrogen-terminated Si(111) surface was used as a substrate to grow amorphous hydrogenated carbon (a-C:H) films by means of layer-by-layer plasma deposition. Electronic and structural properties of the forming interface were determined by in situ core level and valence band photoelectron spectroscopy. From the core level spectra the gradual formation of an initially carbon-rich and eventually stoichiometric silicon carbide interface layer is inferred with a final thickness of 18 Å. The silicon core level data indicate that the substrate is hydrogen-passivated owing to the plasma exposure. On the final heterostructure, the valence band edge of the diamond-like carbon is found to be 0.4 eV below that of the interfacial a-SiC:H and also below that of the silicon, with the offset amounting to 0.7 ± 0.1 eV. © 1998 Elsevier Science B.V.
UnivIS-Import:2015-03-05:Pub.1998.nat.dphy.IAP.exphys.format
Keywords: Band bending; Band offset; Photoelectron spectroscopy; Silicon-carbon interface

Formation reactions of chalcopyrite compounds and the role of sodium doping - Hock, Rainer; et al. (ELSEVIER SCIENCE SA, 2007)
The selenisation of the thin film precursors indium, Cu-In and Cu-In-Ga has been studied by in-situ X-ray diffraction. The influence of sodium (loping on the kinetics of the solid-state reactions observed agrees with the predictions derived from a phenomenological model presented earlier. In this model a layer of sodium polyselenide is assumed to form at the surface of the precursor, or around each crystal grain. This layer might be responsible for impeded ion exchange in certain solid-state reactions. (C) 2006 Elsevier B.V. All rights reserved.
Keywords: chalcogenides; thin films; crystal growth; X-ray diffraction

Fractional derivatives embody essential features of cell rheological behavior - Fabry, Ben; et al. (Annals of Biomedical Engineering, 2003)

Fractional Quantum Conductance Plateaus in Mosaic-Like Conductors and Their Similarities to the Fractional Quantum Hall Effect - Kißlinger, Ferdinand; Rienmüller, Dennis; Ott, Christian; Kampert, Erik; Weber, Heiko B. (Annalen Der Physik, 2019)
A simple route to generate magnetotransport data is reported that results in fractional quantum Hall plateaus in the conductance without invoking strongly correlated physics. Ingredients to the generating model are conducting tiles with integer quantum Hall effect and metallic linkers, further Kirchhoff rules. When connecting few identical tiles in a mosaic, fractional steps occur in the conductance values. Richer spectra representing several fractions occur when the tiles are parametrically varied. Parts of the simulation data are supported with purposefully designed graphene mosaics in high magnetic fields. The findings emphasize that the occurrence of fractional conductance values, in particular in two‐terminal measurements, does not necessarily indicate interaction‐driven physics. The importance of an independent determination of charge densities is underscored and similarities with and differences to the fractional quantum Hall effect are critically discusse

Free electron laser stochastic spectroscopy revealing silicon bond softening dynamics - De Angelis, Dario; Principi, Emiliano; Bencivenga, Filippo; Fausti, Daniele; Foglia, Laura; Klein, Yishay; Manfredda, Michele; Mincigrucci, Riccardo; Montanaro, Angela; Pedersoli, Emanuele; Pelli Cresi, Jacopo Stefano; Perosa, Giovanni; et al. (Physical Review B, 2023)
Time-resolved x-ray emission/absorption spectroscopy (Tr-XES/XAS) is an informative experimental tool sensitive to electronic dynamics in materials, widely exploited in diverse research fields. Typically, Tr-XES/XAS requires x-ray pulses with both a narrow bandwidth and subpicosecond pulse duration, a combination that in principle finds its optimum with Fourier transform-limited pulses. In this work, we explore an alternative experimental approach, capable of simultaneously retrieving information about unoccupied (XAS) and occupied (XES) states from the stochastic fluctuations of broadband extreme ultraviolet pulses of a free electron laser. We used this method, in combination with singular-value decomposition and Tikhonov regularization procedures, to determine the XAS/XES response from a crystalline silicon sample at the L2,3 edge, with an energy resolution of a few tens of meV. Finally, we combined this spectroscopic method with a pump-probe approach to measure structural and electronic dynamics of a silicon membrane. Tr-XAS/XES data obtained after photoexcitation with an optical laser pulse at 390 nm allowed us to observe perturbations of the band structure, which are compatible with the formation of the predicted precursor state of a nonthermal solid-liquid phase transition associated with a bond softening phenomenon.
CRIS-Team Scopus Importer:2023-08-04

Free-film small-angle neutron scattering: a novel container-free in situ sample environment with minimized H/D exchange - Krauss, Sebastian W.; Schweins, Ralf; Magerl, Andreas; Zobel, Mirijam (Journal of Applied Crystallography, 2019)
The development of a container-free sample environment which is particularly well suited for in situ reaction studies of liquid samples by small-angle neutron scattering and related techniques is reported. Hydrogen exchange with the humidity from air is reduced by an encapsulating setup in a bag filled with an inert gas such as He. The effectiveness of this measure is quantitatively accessed by infrared absorption and gravimetry, and further correlated with neutron scattering.
CRIS-Team WoS Importer:2019-04-16

Friction and Dissipation in Epitaxial Graphene Films - Filleter, Tobin; Mcchesney, J. L.; Bostwick, Aaron; Rotenberg, Eli; Emtsev, K. V.; Seyller, Thomas; Horn, K.; Bennewitz, Roland (Physical Review Letters, 2009)
We have studied friction and dissipation in single and bilayer graphene films grown epitaxially on SiC. The friction on SiC is greatly reduced by a single layer of graphene and reduced by another factor of 2 on bilayer graphene. The friction contrast between single and bilayer graphene arises from a dramatic difference in electron-phonon coupling, which we discovered by means of angle-resolved photoemission spectroscopy. Bilayer graphene as a lubricant outperforms even graphite due to reduced adhesion.
EAM Import::2019-03-14

From arrays of THz antennas to large-area emitters - Malzer, Stefan; Döhler, Gottfried; Garcia-Munoz, Enrique; Preu, Sascha; Bauerschmidt, Sebastian; Montero-De-Paz, J.; Ugarte-Munoz, E.; Rivera-Lavado, Alejandro; Segovia-Vargas, Daniel; Gonzalez-Posadas, V. (IEEE Transactions on Terahertz Science and Technology, 2013)
Arrays of coherently driven photomixers with antenna (antenna emitter arrays, AEAs) have been evaluated as a possibility to overcome the power limitations of individual conventional photomixers with antenna ('antenna emitters', AEs) for the generation of continuous-wave (CW) THz radiation. In this paper, 'large area emitters' (LAEs) are proposed as an alternative approach, and compared with AEAs. In this antenna-free new scheme of photomixing, the THz radiation originates directly from the acceleration of photo-induced charge carriers generated within a large semiconductor area. The quasi-continuous distribution of emitting elements corresponds to a high-density array and results in favorable radiation profiles without side lobes. Moreover, the achievable THz power is expected to outnumber even large AEAs. Last not least, the technological challenge of fabricating LAEs appears to be significantly less demanding. © 2011-2012 IEEE.

From evaporation-induced self-assembly to shear-induced alignment - Srikantharajah, Rubitha; Schindler, Torben; Landwehr, Inga; Romeis, Stefan; Unruh, Tobias; Peukert, Wolfgang (Nanoscale, 2016)
The functionality of compact nanostructured thin films depends critically on the degree of order and hence on the underlying ordering mechanisms during film formation. For dip coating of rigid nanorods the counteracting mechanisms, evaporation-induced self-assembly (EISA) and shear-induced alignment (SIA) have recently been identified as competing ordering mechanisms. Here, we show how to achieve highly ordered and homogeneous thin films by controlling EISA and SIA in dip coating. Therefore we identify the influences of the process parameters including temperature, initial volume fraction and nanorod aspect ratio on evaporation-induced convective flow and externally applied shear forces and evaluate the resulting films. The impact of evaporation and shear can be distinguished by analysing film thickness, surface order and bulk order by careful in situ SAXS, Raman and SEM-based image analysis. For the first time we derive processing guidelines for the controlled application of EISA and SIA towards highly ordered thin nematic films.

From isolated diamondoids to a van-der-Waals crystal: A theoretical and experimental analysis of a trishomocubane and a diamantane dimer in the gas and solid phase - Tyborski, Christoph; Meinke, Reinhard; Gillen, Roland; Bischoff, Tobias; Knecht, Andre; Richter, Robert; Merli, Andrea; Fokin, Andrey A.; Koso, Tetyana V.; Rodionov, Vladimir N.; Schreiner, Peter R.; Moeller, Thomas; et al. (Journal of Chemical Physics, 2017)
The electronic properties of sp(2)/sp(3) diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.5 eV-1 eV. The pi -> pi(*) transition is responsible for large absorption in both gas and crystalline phases. It further causes a large increase in the Raman intensity of the C=C stretch vibration when excited resonantly. By resonance Raman spectroscopy we were able to determine the C=C bond length of the trishomocubane dimer to exhibit 1.33 angstrom in the ground and 1.41 angstrom in the excited stat

From Powder to Solution: Hydration Dependence of Human Hemoglobin Dynamics Correlated to Body Temperature - Stadler, Andreas M.; Digel, Ilya; Embs, Jan Peter; Unruh, Tobias; Tehei, Moeava; Zaccai, Giuseppe; Büldt, Georg; Artmann, Gerhard (Biophysical Journal, 2009)
A transition in hemoglobin (Hb), involving partial unfolding and aggregation, has been shown previously by various biophysical methods. The correlation between the transition temperature and body temperature for Hb from different species, suggested that it might be significant for biological function. To focus on such biologically relevant human Hb dynamics, we studied the protein internal picosecond motions as a response to hydration, by elastic and quasielastic neutron scattering. Rates of fast diffusive motions were found to be significantly enhanced with increasing hydration from fully hydrated powder to concentrated Hb solution. In concentrated protein solution, the data showed that amino acid side chains can explore larger volumes above body temperature than expected from normal temperature dependence. The body temperature transition in protein dynamics was absent in fully hydrated powder, indicating that picosecond protein dynamics responsible for the transition is activated only at a sufficient level of hydration. A collateral result from the study is that fully hydrated protein powder samples do not accurately describe all aspects of protein picosecond dynamics that might be necessary for biological function.

From strong-field physics in and at nanoscale matter to photonics-based laser accelerators - Kozak, Martin; Higuchi, Takuya; McNeur, Joshua; Shiloh, Roy; Heide, Christian; Paschen, Timo; Yousefi, Peyman; Sturm, Constanze; Li, Ang; Illmer, Johannes; Meier, Stefan; Schönenberger, Norbert; et al. (205, 2019)
New ways of controlling quasi-free and free electrons by means of phase-controlled ultrashort laser pulses are demonstrated: from strong-field physics in the conducting 2-d material graphene and at the surface of nanostructures, to laser acceleration of free electrons with a nanophotonic structure, and the demonstration of the longitudinal Kapitza-Dirac effec

From Thermogelling Hydrogels toward Functional Bioinks: Controlled Modification and Cytocompatible Crosslinking - Hahn, Lukas; Beudert, Matthias; Gutmann, Marcus; Kessler, Larissa; Stahlhut, Philipp; Fischer, Lena; Karakaya, Emine; Lorson, Thomas; Thievessen, Ingo; Detsch, Rainer; Luehmann, Tessa; Luxenhofer, Robert (Macromolecular Bioscience, 2021)
Hydrogels are key components in bioink formulations to ensure printability and stability in biofabrication. In this study, a well-known Diels-Alder two-step post-polymerization modification approach is introduced into thermogelling diblock copolymers, comprising poly(2-methyl-2-oxazoline) and thermoresponsive poly(2-n-propyl-2-oxazine). The diblock copolymers are partially hydrolyzed and subsequently modified by acid/amine coupling with furan and maleimide moieties. While the thermogelling and shear-thinning properties allow excellent printability, trigger-less cell-friendly Diels-Alder click-chemistry yields long-term shape-fidelity. The introduced platform enables easy incorporation of cell-binding moieties (RGD-peptide) for cellular interaction. The hydrogel is functionalized with RGD-peptides using thiol-maleimide chemistry and cell proliferation as well as morphology of fibroblasts seeded on top of the hydrogels confirm the cell adhesion facilitated by the peptides. Finally, bioink formulations are tested for biocompatibility by incorporating fibroblasts homogenously inside the polymer solution pre-printing. After the printing and crosslinking process good cytocompatibility is confirmed. The established bioink system combines a two-step approach by physical precursor gelation followed by an additional chemical stabilization, offering a broad versatility for further biomechanical adaptation or bioresponsive peptide modification.
CRIS-Team Scopus Importer:2021-08-06
Keywords: biofabrication; bioprinting; chemical crosslinking; hydrogels

Früh zur Datenkompetenz - Krieger, Michael; Weber, Heiko B.; van Eldik, Christopher (Physik Journal, 2022)

FUCCI Reporter Gene-Based Cell Cycle Analysis - Fischer, Lena; Thievessen, Ingo (Humana Press Inc., 2023)
One of the most commonly assessed parameters in cellular analyses is the proliferative activity of a cell population. The fluorescence ubiquitin cell cycle indicator (FUCCI)-based system allows live and in vivo observation of cell cycle progression. Based on the mutually exclusive activity of two fluorescently labeled proteins cdt1 and geminin during the G0/1 and S/G2/M phases of the cell cycle, individual cells can be assigned to their respective cell cycle phase by fluorescence imaging of the nucleus. Here, we describe the generation of NIH/3T3 cells containing the FUCCI reporter system by lentiviral transduction and their use in 3D culture assays. The protocol can be adapted to other cell lines.
CRIS-Team Scopus Importer:2023-05-26
Keywords: Cdt1; Cell cycle analysis; Cell proliferation; FUCCI; Geminin; Lentivirus

Fuctionalization of single-walled carbon nanotubes with organo-lithium compounds: A combined XPS, STM, and AFM study - Graupner, Ralf; Abraham, Jürgen; Vencelová, Andrea; Lauffer, Peter; Ley, Lothar; Hirsch, Andreas (Electronic Properties of Novel Nanostructures, 2005)
A covalent sidewall functionalization of Single-Walled Carbon Nanotubes (SWCNTs) has been achieved by a reaction with butyllithium in benzene. The nucleophilic attack of the lithium organyl leads to a transfer of nagative charge to the SWCNTs which is balanced by the positive charge of the Li-ions. In our investigation we were able to verify the negative charge on the SWCNTs by a concomitant shift of the binding energy of the C 1s core levels of the SWCNTs in X-ray induced photoelectron spectroscopy (XPS). Moreover, scanning tunneling microscopy (STM) reveals a decoration of the tubewalls by features which exhibit the expected threefold symmetry of the butyl-groups. The charging of the SWCNTs in solution could also be verified by applying a homogeneous electric field using Au coated glass electrodes. The electrodes were chraracterized by XPS and atomic force microscopy (AFM). These measurements reveal the appearance of Li on one electrode, and the occurance of SWCNTs on the counter electrode. © 2005 American Institute of Physics.

Full-Spectrum InP-Based Quantum Dots with Near-Unity Photoluminescence Quantum Efficiency - Van Avermaet, Hannes; Schiettecatte, Pieter; Hinz, Sandra; Giordano, Luca; Ferrari, Fabio; Nayral, Celine; Delpech, Fabien; Maultzsch, Janina; Lange, Holger; Hens, Zeger (ACS nano, 2022)
Photoluminescent color conversion by quantum dots (QDs) makes possible the formation of spectrum-ondemand light sources by combining blue LEDs with the light generated by a specific blend of QDs. Such applications, however, require a near-unity photoluminescence quantum efficiency since self-absorption magnifies disproportionally the impact of photon losses on the overall conversion efficiency. Here, we present a synthesis protocol for forming InP-based QDs with +90% quantum efficiency across the full visible spectrum from blue/cyan to red. The central features of our approach are as follows: (1) the formation of InP core QDs through one-batch-one-size reactions based on aminophosphine as the phosphorus precursor, (2) the introduction of a core/ shell/shell InP/Zn(Se,S)/ZnS structure, and (3) the use of specific interfacial treatments, most notably the saturation of the ZnSe surface with zinc acetate prior to ZnS shell growth. Moreover, we adapted the composition of the Zn(Se,S) inner shell to attain the intended emission color while minimizing line broadening induced by the InP/ZnS lattice mismatch. The protocol is established by analysis of the QD composition and structure using multiple techniques, including solid-state nuclear magnetic resonance spectroscopy and Raman spectroscopy, and verified for reproducibility by having different researchers execute the same protocol. The realization of full-spectrum, +90% quantum efficiency will strongly facilitate research into light-matter interaction in general and luminescent color conversion in particular through InP-based QDs.
CRIS-Team WoS Importer:2022-07-08

Functional behavior of chitosan/gelatin/silica-gentamicin coatings by electrophoretic deposition on surgical grade stainless steel - Aydemir, Tuba; Liverani, Liliana; Pastore, Juan I.; Ceré, Silvia M.; Goldmann, Wolfgang; Boccaccini, Aldo R.; Ballarre, Josefina (Materials Science and Engineering C, 2020)
Metals are used in several orthopedic applications as fixation elements for the stabilization of fractures or as prostheses. One of the most common orthopedic biomaterials in many developing countries is surgical grade stainless steel (SS). However, its use as permanent implant in orthopedic surgery is conditioned due to its limited corrosion resistance in physiological media, lack of osseointegration, and absence of antibacterial effect. The aim of this work is to generate a degradable coating with antibacterial properties for stainless steel to be used in implants/medical devices. The coating is composed of a biopolymer/silica-gentamicin nanoparticles composite obtained by electrophoretic deposition (EPD) on surgical grade stainless steel plates. The coating surface was characterized by microscopic examination, and in vitro performance was evaluated after immersion in phosphate-buffered saline (PBS) solution, simulated body fluid (SBF), and cell culture medium, to analyze coating degradation, antibiotics release, cell attachment (ST-2 stromal cells), and antibacterial (Escherichia coli and Staphylococcus aureus) properties. EPD coatings were uniform and covered homogeneously the surface of the SS substrate. Also the distribution of silica-gentamicin nanoparticles was homogeneous on the coated area. The degradation of the chitosan-gelatin coatings was evident after 7 days of immersion. The gentamicin release led to excellent antibacterial behavior at 24 h, meanwhile the cell proliferation (at 7 days culture) was not inhibited. The results show that the coating system exhibits promising behavior which could contribute to prevent hospital infections at early implantation times.
CRIS-Team Scopus Importer:2020-06-02
Keywords: Chitosan; Electrophoretic deposition; Gelatin; Prevention of infections; Silica-gentamicin nanoparticles; Stainless steel implants

Functional genomics meta-analysis to identify gene set enrichment networks in cardiac hypertrophy - Angeloni, Miriam; Thievessen, Ingo; Engel, Felix; Magni, Paolo; Ferrazzi, Fulvia (Biological Chemistry, 2021)
In order to take advantage of the continuously increasing number of transcriptome studies, it is important to develop strategies that integrate multiple expression datasets addressing the same biological question to allow a robust analysis. Here, we propose a meta-analysis framework that integrates enriched pathways identified through the Gene Set Enrichment Analysis (GSEA) approach and calculates for each meta-pathway an empirical p-value. Validation of our approach on benchmark datasets showed comparable or even better performance than existing methods and an increase in robustness with increasing number of integrated datasets. We then applied the meta-analysis framework to 15 functional genomics datasets of physiological and pathological cardiac hypertrophy. Within these datasets we grouped expression sets measured at time points that represent the same hallmarks of heart tissue remodeling ('aggregated time points') and performed meta-analysis on the expression sets assigned to each aggregated time point. To facilitate biological interpretation, results were visualized as gene set enrichment networks. Here, our meta-analysis framework identified well-known biological mechanisms associated with pathological cardiac hypertrophy (e.g., cardiomyocyte apoptosis, cardiac contractile dysfunction, and alteration in energy metabolism). In addition, results highlighted novel, potentially cardioprotective mechanisms in physiological cardiac hypertrophy involving the down-regulation of immune cell response, which are worth further investigation.
CRIS-Team Scopus Importer:2021-06-11
Keywords: cardiac hypertrophy; pathological hypertrophy; pathway network; physiological hypertrophy; transcriptome meta-analysis

Functionalization of Silver Nanowires Surface using Ag-C Bonds in a Sequential Reductive Method - Bashouti, Muhammad Y.; Resch, Sebastian; Ristein, Jürgen; Mackovic, Mirza; Spiecker, Erdmann; Waldvogel, Siegfried. R.; Christiansen, Silke. H. (ACS Applied Materials and Interfaces, 2015)
Silver nanowires (Ag-NW) assembled in interdigitated webs have shown an applicative potential as transparent and conducting electrodes. However, upon integration in practical device designs, the presence of silver oxide, which instantaneously forms on the Ag-NW surfaces in ambient conditions, is unwanted. Here, we report on the functionalization of Ag-NWs with 4-nitrophenyl moieties through A-C bonds using a versatile two step reduction process, i.e., ascorbate reduction combined electrografting. We show that 40% of the Ag atop sites were terminated and provide high surface stability toward oxidation for more than 2 months while keeping the same intrinsic conductivity as in bulk silver.
Keywords: silver nanowires; transparent electrodes; Ag-C bonds; electrografting; XPS; surface functionalization

Functionalization of single-walled carbon nanotubes by aromatic molecules studied by scanning tunneling microscopy - Lauffer, Peter; Jung, Adrian; Graupner, Ralf; Hirsch, Andreas; Ley, Lothar (physica status solidi (b), 2006)
A functionalization of single-walled carbon nanotubes (SWCNTs) without the necessity to introduce defects to the atomic structure of the graphitic lattice can be achieved via π-stacking interaction of the SWCNTs sidewalls with aromatic molecules. In our work we study the functionalization of SWCNTs with two different aromatic molecules by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS Functionalization was performed in solution and the SWCNTs so obtained were transfered to a sputter-annealed Au(111) substrate using dry transfer techniques in vacuum to avoid surface contamination. Differential conductance imaging of the atomically resolved functionalized SWCNT reveals a substructure which can be attributed to the molecular orbitals of aminoanthracene. These findings are compared to the STM images obtained from the bare molecule prepared by thermal evaporation on a clean Au(111) surface which are interpreted with the help of quantum chemical calculations of the molecular orbital structure. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

Fundamental Insights into the Degradation and Stabilization of Thin Layer Black Phosphorus - Abellan, Gonzalo; Wild, Stefan; Lloret, Vicent; Scheuschner, Nils; Gillen, Roland; Mundloch, Udo; Maultzsch, Janina; Varela, Maria; Hauke, Frank; Hirsch, Andreas (Journal of the American Chemical Society, 2017)
Herein, we have developed a systematic study on the oxidation and passivation of mechanically exfoliated black phosphorus (BP). We analyzed the strong anisotropic behavior of BP by scanning Raman microscopy providing an accurate method for monitoring the oxidation of BP-via statistical Raman spectroscopy. Furthermore, different factors influencing the environmental instability of the BP, i.e., thickness, lateral dimensions or visible light illumination, have been investigated in detail. Finally, we discovered that the degradation of few-layer BP flakes of <10 nm can be suppressed for months by using ionic liquids, paving the way for the development of BP-based technologie

Ga and Ce ion-doped phosphate glass fibres with antibacterial properties and their composite for wound healing applications - Lapa, Agata; Cresswell, Mark; Campbell, Ian; Jackson, Phil; Goldmann, Wolfgang; Detsch, Rainer; Parsons, Andrew; Ahmed, Ifty; Boccaccini, Aldo R. (Journal of Materials Chemistry B, 2019)
Novel gallium/cerium-doped phosphate glass fibres (PGF) were successfully manufactured by the melt-quenching and melt-spinning process. The amorphous character of the materials produced was confirmed using X-ray powder diffraction (XRD), and the elemental composition was investigated with X-ray fluorescence confirming the presence of 2 mol% of Ga2O3 or CeO2. Fourier Transform Infrared Spectroscopy (FTIR) confirmed the presence of Q1 and Q2 structural phosphate species. Mechanical properties of the PGFs revealed tensile strength values of 428 ± 94 MPa and 379 ± 80 MPa, with elastic modulus values of 45 ± 4 GPa and 54 ± 9 GPa for Ce-PGF (diameter 25 μm) and Ga-PGF (diameter 18 μm), respectively. The influence of both dopants on the glass degradation properties was evaluated by tests in deionised water, which revealed a decreased dissolution rate for gallium-doped PGF in comparison to cerium-doped PGF. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) measurements were used to explore ion release in cell culture medium, while ICP-mass spectrometry (ICP-MS) was used to measure ion release in deionised water. These techniques showed controlled release of therapeutic and antibacterial ions from the PGF. Antibacterial properties of Ce-PGF and Ga-PGF, based on turbidity measurements, were confirmed against Gram-positive bacteria. Moreover, Ce-doped phosphate glass fibres did not disturb the proliferation of human epidermal keratinocyte (HaCaT) cells or the mobility of mice embryonic fibroblasts (MEF). Applying an in vitro scratch assay showed full wound closure after 24 h of indirect incubation with Ga-PGF. Due to their superior processability as compared with Ga-PGFs, a fully degradable mesh based on Ce-PGF was designed and found to achieve high water uptake (up to 800%), suggesting its suitability for wound healing applications.
CRIS-Team Scopus Importer:2019-11-22

Gain enhancement by dielectric horns in the Terahertz band - Malzer, Stefan; Andres-Garcia, Belen; Garcia-Munoz, Enrique; Segovia-Vargas, Daniel; Bauerschmidt, Sebastian; Preu, Sascha; Döhler, Gottfried; Wang, Lijun (IEEE Transactions on Antennas and Propagation, 2011)
A new geometry for the design of antennas in the Terahertz band is presented. The structure is based on a horn antenna etched in the substrate and fed with a planar printed antenna used for generation of terahertz radiation, designed for the 200 GHz to 3 THz range. For the proposed antenna, the energy distribution through the substrate is reduced towards an increase in the gain of the system, at least, 8 dB in a 1:10 bandwidth. The structure has been measured showing the expected behavior in the low band. © 2006 IEEE.

Gallium- and Cerium-Doped Phosphate Glasses with Antibacterial Properties for Medical Applications - Lapa, Agata; Cresswell, Mark; Campbell, Ian; Jackson, Phil; Goldmann, Wolfgang H.; Detsch, Rainer; Boccaccini, Aldo R. (Advanced Engineering Materials, 2020)
Novel cerium- (Ce) and gallium (Ga)-doped phosphate glasses (PGs) are successfully obtained by the peroxidation method. The new glasses are characterized using Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and pH measurements. A strong correlation between glass properties and their composition is found. The incorporation of Ga/Ce in the glass structure is confirmed by peak shifts in Raman and FTIR spectra. Degradation tests conducted in water confirm the soluble character of the PGs and reveal the influence of Ga and Ce on the degradation rate. Ga-doped glasses are found to be less soluble than Ce-doped ones (43%, 33%, 21%, and 16% of dry mass remained after 7 weeks for 7Ga, 5Ga, 5Ce, and 7Ce, respectively). Biological evaluation using ST-2 cells shows adequate cell response with a cell viability of 80% measured using the indirect contact method depending on the composition. The cell viability decreases with an increase in Ga content and increases with Ce content. The antibacterial character of Ga/Ce-doped PGs is confirmed by turbidity measurements against Escherichia coli and Staphylococcus carnosus. The novel Ce/Ga-doped PGs exhibiting antibacterial properties and biocompatibility reported here are interesting for tissue engineering and wound healing applications.
CRIS-Team Scopus Importer:2020-03-17
Keywords: antibacterial properties; cerium; gallium; ion release; medical applications; phosphate glasses

Gallium Oxide for High-Power Optical Applications - Deng, Huiyang; Leedle, Kenneth J.; Miao, Yu; Black, Dylan S.; Urbanek, Karel E.; McNeur, Joshua; Kozak, Martin; Ceballos, Andrew; Hommelhoff, Peter; Solgaard, Olav; Byer, Robert L.; Harris, James S. (Advanced Optical Materials, 2020)
Gallium oxide (Ga2O3) is an emerging wide-bandgap transparent conductive oxide (TCO) with potential applications for high-power optical systems. Herein, Ga2O3 fabricated nanostructures are described, which demonstrate high-power laser induced damage threshold (LIDT). Furthermore, the demonstration of an electron accelerator based on Ga2O3 gratings is reported. These unique Ga2O3 nanostructures provide acceleration gradients exceeding those possible with conventional RF accelerators due to the high breakdown threshold of Ga2O3. In addition, the laser damage threshold and acceleration performance of a Ga2O3-based dielectric laser accelerator (DLA) are compared with those of a DLA based on sapphire, a material known for its high breakdown strength. Finally, the potential of Ga2O3 thin-film coatings as field reduction layers for Si nanostructures is shown; they potentially improve the effective LIDT and performance of Si-based DLAs and other high-power optical structures. These results could provide a foundation for new high-power optical applications with Ga2O3.
CRIS-Team WoS Importer:2020-01-31

Gallium-rich Pd-Ga phases as supported liquid metal catalysts - Taccardi, Nicola; Grabau, Mathias; Debuschewitz, Jonas; Distaso, Monica; Brandl, Marco; Hock, Rainer; Maier, Florian; Papp, Christian; Erhard, Jannis; Neiß, Christian; Peukert, Wolfgang; Görling, Andreas; et al. (Nature chemistry, 2017)
A strategy to develop improved catalysts is to create systems that merge the advantages of heterogeneous and molecular catalysis. One such system involves supported liquid-phase catalysts, which feature a molecularly defined, catalytically active liquid film/droplet layer adsorbed on a porous solid support. In the past decade, this concept has also been extended to supported ionic liquid-phase catalysts. Here we develop this idea further and describe supported catalytically active liquid metal solutions (SCALMS). We report a liquid mixture of gallium and palladium deposited on porous glass that forms an active catalyst for alkane dehydrogenation that is resistant to coke formation and is thus highly stable. X-ray diffraction and X-ray photoelectron spectroscopy, supported by theoretical calculations, confirm the liquid state of the catalytic phase under the reaction conditions. Unlike traditional heterogeneous catalysts, the supported liquid metal reported here is highly dynamic and catalysis does not proceed at the surface of the metal nanoparticles, but presumably at homogeneously distributed metal atoms at the surface of a liquid metallic phas

G(-) and G(+) in the Raman spectrum of isolated nanotube: a study on resonance conditions and lineshape - Telg, Hagen; Fouquet, M.; Maultzsch, Janina; Wu, Y.; Chandra, B.; Hone, James; Heinz, T. F.; Thomsen, Christian (physica status solidi (b), 2008)
We analyze the high-energy Raman modes, G(+) and G(-), in a pair of one metallic and one semiconducting nanotube grown across a 100 m wide slit. By combining Raman resonance profiles of the radial breathing mode and the high-energy modes, we assign the broad G(-) peak to a metallic and the G(+) peak to a semiconducting nanotube. Considering theoretical predictions we show that both peaks, G(-) and G(+), originate from the LO phonon. The G(-) peak is the longitudinal mode of the metallic tube; it is broadened and downshifted due to strong electron-phonon coupling in the metallic nanotube. The GO peak is due to the longitudinal mode in the semiconducting tube. An asymmetric lineshape of the G(-) peak agrees with observations of the asymmetry to be an intrinsic feature of metallic nanotubes. (C) 2008 WILEY-VCH Veriag GmbH & Co. KGaA, Weinheim

Gap-size dependence of optical near fields in a variable nanoscale two-tip junction - Heimerl, Jonas; Higuchi, Takuya; Ammon, Maximilian Michael; Schneider, M. Alexander; Hommelhoff, Peter (Physical Review B, 2020)
When nanometric metal structures are illuminated with light, large optical near fields arise. These fields are even further enhanced when two nanometer-sized structures are brought in nanometric distance. Then the individual near fields of the structures couple. Measuring the coupled optical near fields directly is hard. Here we demonstrate an scanning tunneling microscope-based setup which allows us to probe the coupling of optical near fields from two nanometer-sharp metal needles facing each other. By measuring the emitted photocurrent as a function of the gap distance, we retrieve the peak fields at the tip apices. The comparison to optical near-field simulations shows that large field enhancement of 15.9 is present at the tip apex. This is an enhancement 4-times larger compared to the individual nanotip. The demonstrated sensitivity of the coupling on the gap size is of large interest for future applications in ultrafast near-field microscopy.
CRIS-Team WoS Importer:2020-03-20

Gaseous nitriding of Co-10 at% and -15 at% Cr alloys at 400 °C and 450 °C - Akhlaghi, Maryam; Hock, Rainer; Dallmann, Johannes; Krapf, Anna; Merle, Benoit; Hofsäss, Hans; Körner, Carolin; Leineweber, Andreas (Journal of Alloys and Compounds, 2022)
Co-10 at% and -15 at% Cr with the stable hexagonal close packed (hcp) crystal structure at room temperature have been subjected to gaseous nitriding treatment using nitriding temperatures of 400 °C and 450 °C far below the austenite start temperature. Using X-ray and electron backscatter diffraction (EBSD), structural changes as a result of nitrogen incorporation into the initial hcp lattice can be divided into the following steps: (i) expansion of the lattice due to N uptake yielding hcp (hcpexp) at nitriding temperature of 400 °C, (ii) gradual transformation of hcpexp into an expanded face-centered cubic phase (fccexp) by increasing time at 400 °C/ or temperature to 450 °C (mixture of two phases), (iii) development of stable nitrided layer containing fccexp using moderate nitriding times up to 3 h at 450 °C and (iv) decomposition of fccexp layer into CrN nitrides and less expanded austenite and the reverse transformation from fcc to hcp at near-surface regions at 450 °C after prolonged nitriding time of 24 h. Due to occurrence of step (iv), the square-root treatment-time dependence of the nitrided layer thickness breaks down. Nitrogen-depth profiles obtained for Co-15 at% Cr by glow-discharge optical emission spectroscopy (GDOES) along with the Non-Rutherford MeV proton backscattering spectrometry reveal a nitrogen supersaturation of around 20 at% at surface-adjacent areas in hcpexp. A relatively large unit volume expansion has been recognized for hcpexp with a change of c/a ratio of around 0.6%. Hardness-depth profiles obtained for nitrided Co-10 at% and -15 at% Cr alloys using nanoindentation evidence an increase in hardness inside the nitrided layer, compared to the untreated core.
Keywords: Nitride materials; Precipitation; Phase transitions; Rutherford backscattering; scanning electron microscopy; SEM; X-ray diffraction

Gated Epitaxial Graphene Devices - Waldmann, Daniel; Jobst, Johannes; Speck, Florian; Seyller, Thomas; Krieger, Michael; Weber, Heiko B. (Materials Science Forum, 2012)
A bottom gate scheme is presented to tune the charge density of epitaxial graphene via a gate voltage while leaving the surface open for further manipulation or investigation. Depending on the doping concentration of the buried gate layer, the temperature and illumination, the bottom gate structure can be operated in two regimes with distinct capacitances. A model is proposed, which quantitatively describes the gate operation. The model is verified by a control experiment with an illuminated gate structure using UV light. Using UV illumination the Schottky capacitor (SC) regime, which provides improved gate efficiency, can be used even at low temperatures. © (2012) Trans Tech Publications.
UnivIS-Import:2015-04-14:Pub.2012.nat.dphy.IAP.LAP.gatede
Keywords: Bottom gate; Buried gate; Epitaxial graphene

Gateless patterning of epitaxial graphene by local intercalation - Sorger, Christian; Hertel, Stefan; Jobst, Johannes; Steiner, Christian; Meil, Katharina; Ullmann, Konrad; Albert, A.; Wang, Yabin; Krieger, Michael; Ristein, Jürgen; Maier, Sabine; Weber, Heiko B. (Nanotechnology, 2015)
We present a technique to pattern the charge density of a large-area epitaxial graphene sheet locally without using metallic gates. Instead, local intercalation of the graphene-substrate interface can selectively be established in the vicinity of graphene edges or prede fined voids. It provides changes of the work function of several hundred meV, corresponding to a conversion from n-type to p-type charge carriers. This assignment is supported by photoelectron spectroscopy, scanning tunneling microscopy, scanning electron microscopy and Hall effect measurements. The technique introduces materials contrast to a graphene sheet in a variety of geometries and thus allows for novel experiments and novel functionalities.
UnivIS-Import:2015-04-14:Pub.2015.nat.dphy.IAP.LAP.gatele
Keywords: Graphene; Intercalation; Patterning

Gating effect in the I-V characteristics of iodine doped polyacetylene nanofibers - Park, Jin-gyu; Kim, Gyutae; Krstic, Vojislav; Lee, Sang-hun; Kim, Byunghoon; Burghard, Marko; Park, YungWoo; et al. (Synthetic Metals, 2001)
Keywords: Polyacetylene; FET; Nanofiber; Electrical transport

Generating few-cycle pulses for nanoscale photoemission easily with an erbium-doped fiber laser - Thomas, Sebastian; Holzwarth, Ronald; Hommelhoff, Peter (Optics Express, 2012)
We demonstrate a simple setup capable of generating four-cycle pulses at a center wavelength of 1700nm for nanoscale photoemission. Pulses from an amplified erbium-doped fiber laser are spectrally broadened by propagation through a highly non-linear fiber. Subsequently, we exploit dispersion in two different types of glass to compress the pulses. The pulse length is estimated by measuring an interferometric autocorrelation trace and comparing it to a numerical simulation. We demonstrate highly non-linear photoemission of electrons from a nanometric tungsten tip in a hitherto unexplored pulse parameter range. © 2012 Optical Society of America.
UnivIS-Import:2015-04-14:Pub.2012.nat.dphy.IAP.exphys.genera

Generation and Characterization of Attosecond Microbunched Electron Pulse Trains via Dielectric Laser Acceleration - Schönenberger, Norbert; Mittelbach, Anna; Yousefi, Peyman; McNeur, Joshua; Niedermayer, Uwe; Hommelhoff, Peter (Physical Review Letters, 2019)
Dielectric laser acceleration is a versatile scheme to accelerate and control electrons with the help of femtosecond laser pulses in nanophotonic structures. We demonstrate here the generation of a train of electron pulses with individual pulse durations as short as 270±80 attoseconds(FWHM), measured in an indirect fashion, based on two subsequent dielectric laser interaction regions connected by a free-space electron drift section, all on a single photonic chip. In the first interaction region (the modulator), an energy modulation is imprinted on the electron pulse. During free propagation, this energy modulation evolves into a charge density modulation, which we probe in the second interaction region (the analyzer). These results will lead to new ways of probing ultrafast dynamics in matter and are essential for future laser-based particle accelerators on a photonic chip.

Generation of 1.5 cycle pulses at 780 nm at oscillator repetition rates with stable carrier-envelope phase - Dienstbier, Philip; Tani, Francesco; Higuchi, Takuya; Travers, John C.; Russell, Philip St. J.; Hommelhoff, Peter (Optics Express, 2019)
We demonstrate a spectral broadening and compression setup for carrier-envelopephase (CEP) stable sub-10-fs Ti:sapphire oscillator pulses resulting in 3.9 fs pulses spectrallycentered at 780 nm. Pulses from the oscillator with 2 nJ energy are launched into a 1 mm longall-normal dispersive solid-core photonic crystal fiber and spectrally broadened to more than oneoctave. Subsequent pulse compression is achieved with a phase-only 4f pulse shaper. Secondharmonic frequency resolved optical gating with a ptychographic reconstruction algorithm isused to obtain the spectral phase, which is fed back as a phase mask to the shaper display forpulse compression. The compressed pulses are CEP stable with a long term standard deviationof 0.23 rad for the CEP noise and 0.32 rad for the integrated rms phase jitter. The high totalthroughput of 15% results in a remaining pulse energy of about 300 pJ at 80 MHz repetition rate.With these parameters and the ability to tailor the spectral phase, the system is well suited forwaveform sensitive photoemission experiments with needle tips or nanostructures and can beeasily adapted to other sub-10 fs ultra-broadband Ti:sapphire oscillator
Keywords: Ultra-short laser pulses; pulse shaping; spectral broadening; carrier-envelope stability; laser source development

Geometrically asymmetric optical cavity for strong atom-photon coupling - Kawasaki, Akio; Braverman, Boris; Pedrozo-Penafiel, Edwin; Shu, Chi; Colombo, Simone; Li, Zeyang; Ozel, Ozge; Chen, Wenlan; Salvi, Leonardo; Heinz, André; Levonian, David; Akamatsu, Daisuke; et al. (Physical Review A, 2019)
Optical cavities are widely used to enhance the interaction between atoms and light. Typical designs using a geometrically symmetric structure in the near-concentric regime face a tradeoff between mechanical stability and high single-atom cooperativity. To overcome this limitation, we design and implement a geometrically asymmetric standing-wave cavity. This structure, with mirrors of very different radii of curvature, allows strong atom-light coupling while exhibiting good stability against misalignment. We observe effective cooperativities ranging from ηeff=10 to ηeff=0.2 by shifting the location of the atoms in the cavity mode. By loading Yb171 atoms directly from a mirror magneto-optical trap into a one-dimensional optical lattice along the cavity mode, we produce atomic ensembles with collective cooperativities up to Nη=2×104. This system opens a way to preparing spin squeezing for an optical lattice clock and to accessing a range of nonclassical collective states.
CRIS-Team Scopus Importer:2019-05-02

Geometric Constraints Enable Self-Supervised Sinogram Inpainting in Sparse-View Tomography - Wagner, Fabian; Thies, Mareike; Maul, Noah; Pfaff, Laura; Aust, Oliver; Pechmann, Sabrina; Syben, Christopher; Maier, Andreas (17th International Meeting on Fully Three-Dimensional Image Reconstruction in Radiology and Nuclear Medicine (Fully3D), 2023)
The diagnostic quality of computed tomography (CT) scans is usually restricted by the induced patient dose, scan speed, and image quality. Sparse-angle tomographic scans reduce radiation exposure and accelerate data acquisition, but suffer from image artifacts and noise. Existing image processing algorithms can restore CT reconstruction quality but often require large training data sets or can not be used for truncated objects. This work presents a self-supervised projection inpainting method that allows optimizing missing projective views via gradient-based optimization. By reconstructing independent stacks of projection data, a self-supervised loss is calculated in the CT image domain and used to directly optimize projection image intensities to match the missing tomographic views constrained by the projection geometry. Our experiments on real X-ray microscope (XRM) tomographic mouse tibia bone scans show that our method improves reconstructions by 3.1-7.4%/7.7-17.6% in terms of PSNR/SSIM with respect to the interpolation baseline. Our approach is applicable as a flexible self-supervised projection inpainting tool for tomographic applications.<br

Geometry of the (2x1) reconstruction of diamond(111) - Heinz, Klaus; Maier, Florian; Ristein, Jürgen; Ley, Lothar; et al. (Journal of Physics: Condensed Matter, 2002)
UnivIS-Import:2015-04-14:Pub.2002.nat.dchph.lpc2._geome

Germanium Incorporation during PVT Bulk Growth of Silicon Carbide - Hens, Philip; Künecke, Ulrike; Konias, Katja; Hock, Rainer; Wellmann, Peter (Materials Science Forum, 2009)
Silicon carbide as a material for electronic devices still has Substantial problems concerning its structural quality and defects. It has been shown that dopants call have a big influence on structural properties like poly-type stability and dislocation statistics [1]. We will discuss the effect of an isoelectronic dopant in silicon carbide. Germanium, being a member of the 4(th) group in the periodic table of elements like silicon and carbon, will not influence the electrical properties of the material such as e.g. aluminum. In Our experiments we reached concentrations of up to 1*10(20) cm(-3). We have observed all impact on the polytype stability during sublimation growth with in-situ germanium incorporation. We investigated all influence on the dislocation statistics during growth and, hence, varying germanium concentration. We found only a slight decrease in mobility during Hall measurements but no severe changes in electrical properties of the materia
Keywords: Bulk Growth; Dislocation; EDX; Germanium; Hall; Lattice Parameter

Germanium nanowire synthesis from fluorothiolate-capped gold nanoparticles in supercritical carbon dioxide - Krstic, Vojislav; Collins, Gillian; Kolesnik, Maria; Krstic, Vojislav; Holmes, Justin D. (Chemistry of Materials, 2010)
Ge nanowires seeded from Au nanoparticles capped with fluorothiolate ligands were synthesized in supercritical carbon dioxide (sc-CO2) by the thermal decomposition of diphenylgermane (DPG) at a temperature of 380 °C and a pressure of 25.7 MPa. Both perfluorinated and semifluorinated capped Au nanoparticles acted as effective catalysts for growing Ge nanowires, with mean diameters of 11 nm (σ = 2.8) and 14 nm (σ = 3.5), respectively. The mean diameter of the Ge nanowires grown from the fluorous-capped Au nanoparticles were considerably smaller than those synthesized from dodecanethiol-capped nanoparticles in sc-toluene, under the same reaction conditions, i.e., 28 nm and σ = 10.3. Differences in the ligand conformations on the surface of the Au nanoparticles and phase separation of the fluorocarbon/CO2 and hydrocarbon/toluene systems gave rise to greater steric stabilization of the fluorous-capped Au nanoparticles in CO2, resulting in small diameter nanowires with a relatively narrow size distribution. Electrical analysis of the nanowires showed them to be p-type (hole) semiconductors.<br

Giant Faraday rotation in single- and multilayer graphene - Crassee, Iris; Levallois, Julien; Walter, Andrew L.; Ostler, Markus; Bostwick, Aaron; Rotenberg, Eli; Seyller, Thomas; Van Der Marel, Dirk; Kuzmenko, Alexey B. (Nature Physics, 2010)
The rotation of the polarization of light after passing a medium in a magnetic field, discovered by Faraday, is an optical analogue of the Hall effect, which combines sensitivity to the carrier type with access to a broad energy range. Up to now the thinnest structures showing the Faraday rotation were several-nanometre-thick two-dimensional electron gases. As the rotation angle is proportional to the distance travelled by the light, an intriguing issue is the scale of this effect in two-dimensional atomic crystals or films—the ultimately thin objects in condensed matter physics. Here we demonstrate that a single atomic layer of carbon—graphene—turns the polarization by several degrees in modest magnetic fields. Such a strong rotation is due to the resonances originating from the cyclotron effect in the classical regime and the inter-Landau-level transitions in the quantum regime. Combined with the possibility of ambipolar doping, this opens pathways to use graphene in fast tunable ultrathin infrared magneto-optical devices.
EAM Import::2019-03-13

Giant Rashba Splitting in CH3NH3PbBr3 Organic-Inorganic Perovskite - Niesner, Daniel; Wilhelm, Max; Levchuk, Ievgen; Osvet, Andres; Shrestha, Shreetu; Batentschuk, Miroslaw; Brabec, Christoph; Fauster, Thomas (Physical Review Letters, 2016)
As they combine decent mobilities with extremely long carrier lifetimes, organic-inorganic perovskites open a whole new field in optoelectronics. Measurements of their underlying electronic structure, however, are still lacking. Using angle-resolved photoelectron spectroscopy, we measure the valence band dispersion of single-crystal CH3NH3PbBr3. The dispersion of the highest energy band is extracted applying a modified leading edge method, which accounts for the particular density of states of organic-inorganic perovskites. The surface Brillouin zone is consistent with bulk-terminated surfaces both in the low-temperature orthorhombic and the high-temperature cubic phase. In the low-temperature phase, we find a ring-shaped valence band maximum with a radius of 0.043 Å-1, centered around a 0.16 eV deep local minimum in the dispersion of the valence band at the high-symmetry point. Intense circular dichroism is observed. This dispersion is the result of strong spin-orbit coupling. Spin-orbit coupling is also present in the room-temperature phase. The coupling strength is one of the largest ones reported so far.

Graphene-based electro-absorption modulator integrated in a passive polymer waveguide platform - Kleinert, Moritz; Herziger, Felix; Reinke, Philipp; Zawadzki, Crispin; de Felipe, Davide; Brinker, Walter; Bach, Heinz-Gunter; Keil, Norbert; Maultzsch, Janina; Schell, Martin (Optical Materials Express, 2016)
A graphene-based electro-absorption modulator has been integrated into a passive polymer waveguide platform for the first time. The opto-electronic properties of the structure are investigated with numerical simulations and measurements of a fabricated device. The graphene layers transferred to the polymer substrate were analyzed by means of Raman spectroscopy and the results indicate a high crystalline quality of the two-dimensional material. The voltage-dependent transmission through a 25 mu m long device has been measured in the telecommunications-relevant wavelength range between 1500 nm and 1600 nm yielding an extinction ratio of 0.056 dB/mu m. (C) 2016 Optical Society of America

Graphene grown on Ge(001) from atomic source - Lippert, Gunther; Dabrowski, J; Schroeder, Thomas; Schubert, Markus Andreas; Yamamoto, Yuji; Herziger, Felix; Maultzsch, Janina; Baringhaus, Jens; Tegenkamp, Christoph; Asensio, M.-C.; Avila, J; Lupina, G (Carbon, 2014)
Among the many anticipated applications of graphene, some - such as transistors for Si microelectronics - would greatly benefit from the possibility to deposit graphene directly on a semiconductor grown on a Si wafer. We report that Ge(001) layers on Si(001) wafers can be uniformly covered with graphene at temperatures between 800 degrees C and the melting temperature of Ge. The graphene is closed, with sheet resistivity strongly decreasing with growth temperature, weakly decreasing with the amount of deposited C, and reaching down to 2 k Omega/square. Density functional theory calculations indicate that the major physical processes affecting the growth are (1) substitution of surface Ge by C, (2) interaction between C clusters and Ge monomers, and (3) formation of bonds between graphene edge and Ge(001), and that the processes 1 and 2 are surpassed by CH2 surface diffusion when carbon is delivered from CH4. The results of this study indicate that graphene can be produced directly at the active region of the transistor in a process compatible with the Si technology (C) 2014 Elsevier Ltd. All rights reserved.

Graphene growth by metal etching on Ru(0001) - Starodub, Elena; Maier, Sabine; Stass, Ingeborg; Bartelt, Norm C.; Feibelman, Peter J.; Salmeron, Miquel; McCarty, Kevin (Physical Review B, 2009)
Low-energy electron microscopy reveals a mode of graphene growth on Ru(0001) in which Ru atoms are etched from a step edge and injected under a growing graphene sheet. Based on density-functional calculations, we propose a model wherein injected Ru atoms form metastable islands under the graphene. Scanning tunneling microscopy reveals that dislocation networks exist near step edges, consistent with some of the injected atoms being incorporated into the topmost Ru layer, thereby increasing its density.
Keywords: density functional theory; edge dislocations; electron microscopy; etching; graphene; island structure; ruthenium; scanning tunnelling microscopy

Graphene layers on silicon carbide studied by Raman spectroscopy - Röhrl, Jonas; Hundhausen, Martin; Emtsev, Konstantin; Seyller, Thomas; Ley, Lothar (Materials Science Forum, 2008)
UnivIS-Import:2015-03-09:Pub.2008.nat.dphy.IAP.exphys.graphe

Graphene-metal interface: Two-terminal resistance of low-mobility graphene in high magnetic fields - Krstic, Vojislav; Obergfell, Dirk; Hansel, Stefan; Rikken, Geert L.J.A.; Blokland, Janneke H.; Ferreira, Mauro S.; Roth, Siegmar (Nano Letters, 2008)
The two-terminal magnetotransport of a single graphene layer was investigated up to a field of 55 T. The dependence of the electron transmission probability at the organo−metallic interface between the graphene and the metal electrodes was studied as a function of filling factor and electron density. A resistance-plateau spanning several tens of tesla width was observed. We argue that this plateau originates from an augmented sublattice spin-splitting due to the high surface-impurity concentration of the graphene layer. At electron densities close to the Dirac point, fingerprints of a thermally activated energy gap were observed.

Graphene nanoribbons with mixed cove-cape-zigzag edge structure - Shinde, Prashant P.; Liu, Jia; Dienel, Thomas; Gröning, Oliver; Dumslaff, Tim; Mühlinghaus, Markus; Narita, Akimitsu; Müllen, Klaus; Pignedoli, Carlo A.; Fasel, Roman; Ruffieux, Pascal; Passerone, Daniele (Carbon, 2021)
A recently developed bottom-up synthesis strategy enables the fabrication of graphene nanoribbons with well-defined width and non-trivial edge structures from dedicated molecular precursors. Here we discuss the synthesis and properties of zigzag nanoribbons (ZGNRs) modified with periodic cove-cape-cove units along their edges. Contrary to pristine ZGNRs, which show antiferromagnetic correlation of their edge states, the edge-modified ZGNRs exhibit a finite single particle band gap without localized edge states. We report the on-surface synthesis of such edge-modified ZGNRs and discuss tunneling conductance dI/dV spectra and dI/dV spatial maps that reveal a noticeable localization of electronic states at the cape units and the opening of a band gap without presence of edge states of magnetic origin. A thorough ab initio investigation of the electronic structure identifies the conditions under which antiferromagnetically coupled, edge-localized states reappear in the electronic structure. Further modifications of the ribbon structure are proposed that lead to an enhancement of such features, which could find application in nanoelectronics and spintronics.
CRIS-Team Scopus Importer:2021-01-22
Keywords: Bottom-up Synthesis; Clar's structure; Density functional theory; Edge modification; Electronic structure; Graphene Nanoribbons

Graphene Ohmic Contacts to n-type Silicon Carbide (0001) - Hertel, Stefan; Finkler, Andreas; Krieger, Michael; Weber, Heiko B. (Materials Science Forum, 2015)
Epitaxial graphene on silicon carbide (SiC) can easily be grown by thermal decomposition. A welldefined epitaxial interface between graphene and substrate is formed, especially when the silicon face of hexagonal polytypes is employed. It is found that as-grown monolayer graphene with interfacial buffer layer provides perfectly ohmic contacts to n-type SiC - even to low-doped epitaxial layers without contact implantation. Contact resistances to highly doped samples are competitive with conventional annealed nickel (Ni) contacts; a direct comparison of Ni and graphene contacts on 4H-SiC resulted in an one order of magnitude reduction of the contact resistance in the case of graphene contacts. On highly doped 6H-SiC, a specific contact resistance as low as ρC = 5.9·10-6 Ωcm2 was found. This further improvement compared to 4H-SiC is assigned to better matching of work functions at the Schottky-like interface.
UnivIS-Import:2017-11-07:Pub.2015.nat.dphy.IAP.LAP.graphe
Keywords: Epitaxial graphene; Ohmic contact; Transfer length method

Graphene on Si(111)7x7 - Ochedowski, O.; Begall, G.; Scheuschner, N.; El Kharrazi, M.; Maultzsch, Janina; Schleberger, M. (Nanotechnology, 2012)
We demonstrate that it is possible to mechanically exfoliate graphene under ultrahigh vacuum conditions on the atomically well defined surface of single crystalline silicon. The flakes are several hundred nanometers in lateral size and their optical contrast is very faint, in agreement with calculated data. Single-layer graphene is investigated by Raman mapping. The graphene and 2D peaks are shifted and narrowed compared to undoped graphene. With spatially resolved Kelvin probe measurements we show that this is due to p-type doping with hole densities of n(h) similar or equal to 6 x 10(12) cm(-2). The in vacuo preparation technique presented here should open up new possibilities to influence the properties of graphene by introducing adsorbates in a controlled way.

Graphene oxide: Efficiency of reducing agents - Eigler, Siegfried; Grimm, Stefan; Enzelberger-Heim, Michael; Müller, Paul; Hirsch, Andreas (Chemical Communications, 2013)
The efficiency of reducing agents for the reduction of graphene oxide (GO) could be probed by scanning Raman spectroscopy. A film of graphene flakes derived from GO was probed to be graphene like. We also focus on the surface quality of reduced GO (rGO). © 2013 The Royal Society of Chemistry.

Graphene-Templated Growth of Pd Nanoclusters - Gotterbarm, Karin; Steiner, Christian; Bronnbauer, Carina; Bauer, Udo; Steinrück, Hans-Peter; Maier, Sabine; Papp, Christian (Journal of Physical Chemistry C, 2014)
Graphene grown on Rh(111) was used as a template for the growth of Pd nanoclusters. Using high-resolution synchrotron radiation-based X-ray photoelectron spectroscopy, we studied the deposition of Pd on corrugated graphene in situ. From the XP spectra, we deduce a cluster-by-cluster growth mode. The formation of clusters with 3 nm diameter was confirmed by low-temperature scanning tunneling microscopy measurements. The investigation of the thermal stability of the Pd particles showed three characteristic temperature regimes: Up to 550 K restructuring of the particles takes place, between 550 and 750 K the clusters coalesce into larger agglomerates, and finally between 750 and 900 K Pd intercalates between the graphene layer and the Rh surface.

Graphitization of polymer surfaces by low-energy ion irradiation - Lazareva, Irina; Koval, Yury; Alam, Sahabul; Strömsdörfer, Stephan; Müller, Paul (Applied Physics Letters, 2007)
The surface of several polymers was graphitized by low-energy ion irradiation. Their conducting properties were studied as a function of the energy of the ions and the irradiation temperature. It was found that at rather modest ion energies (∼1000 eV) and irradiation temperatures (<400 °C), polymer surfaces transform to a graphitized state. The graphitized layer consists of overlapping graphite islands with a diameter of 1-3 nm and exhibits a semimetallic conductivity. Gradually reducing the energy of the ions and the irradiation temperature, the authors observed a transition from semimetallic to variable range hopping conductivity. © 2007 American Institute of Physics.

Graphitization of polymer surfaces by scanning ion irradiation - Koval, Yury (Applied Physics Letters, 2014)
Graphitization of polymer surfaces was performed by low-energy Ar and He ion irradiation. A method of scanning irradiation was implemented. It was found that by scanning ion irradiation, a significantly higher electrical conductivity in the graphitized layers can be achieved in comparison with a conventional broad-beam irradiation. The enhancement of the conductance becomes more pronounced for narrower and better collimated ion beams. In order to analyze these results in more detail, the temperature dependence of conductance of the irradiated samples was investigated. The results of measurements are discussed in terms of weak localization corrections to conductance in disordered metals. The observed effects can be explained by enlargement of graphitic patches, which was achieved with the scanning ion irradiation method.

Growth and electrical transport of germanium nanowires - Gu, G.; Burghard, Marko; Kim, Gyutae; Duesberg, Georg S.; Chiu, P.; Krstic, Vojislav; Roth, Siegmar; Han, W. (Journal of Applied Physics, 2001)
Single crystalline germanium nanowires have been synthesized from gold nanoparticles based on a vapor–liquid–solid growth mechanism. Germanium powder was evaporated at 950 °C, and deposited onto gold nanoparticles at 500 °C using argon as a carrier gas. The diameter of the germanium nanowires ranged from 20 to 180 nm when gold thin films were utilized as the substrate, while the nanowires grown from 10 nm Au particles showed a narrower diameter distribution centered at 28 nm. The growth direction of germanium nanowires is along the [111] direction, determined by high resolution transmission electron microscopy. Transport measurements on individual Ge nanowires indicate that the wires are heavily doped during growth and that transport data can be explained by the thermal fluctuation tunneling conduction model.
Keywords: Transmission electron microscopy; Chemical vapor deposition; Chemical elements; Transition metals; Thin films; Nanoparticles; Thermal fluctuations

Growth and electronic structure of boron-doped graphene - Gebhardt, Julian; Koch, Roland; Zhao, Wei; Höfert, Oliver; Gotterbarm, Karin; Mammadov, Samir; Papp, Christian; Görling, Andreas; Steinrück, Hans-Peter; Seyller, Thomas (Physical Review B, 2013)
The doping of graphene to tune its electronic properties is essential for its further use in carbon-based electronics. Adapting strategies from classical silicon-based semiconductor technology, we use the incorporation of heteroatoms in the 2D graphene network as a straightforward way to achieve this goal. Here, we report on the synthesis of boron-doped graphene on Ni(111) in a chemical vapor deposition process of triethylborane on the one hand and by segregation of boron from the bulk of the substrate crystal on the other hand. The chemical environment of boron was determined by x-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy was used to analyze the impact on the band structure. Doping with boron leads to a shift of the graphene bands to lower binding energies. The shift depends on the doping concentration and for a doping level of 0.3 ML a shift of up to 1.2 eV is observed. The experimental results are in agreement with density-functional calculations. Furthermore, our calculations suggest that doping with boron leads to graphene preferentially adsorbed in the top-fcc geometry, since the boron atoms in the graphene lattice are then adsorbed at substrate fcc-hollow sites. The smaller distance of boron atoms incorporated into graphene compared to graphene carbon atoms leads to a bending of the doped graphene sheet in the vicinity of the boron atoms. By comparing calculations of doped and undoped graphene on Ni(111), as well as the respective freestanding cases, we are able to distinguish between the effects that doping and adsorption have on the band structure of graphene. Both doping and bonding to the surface result in opposing shifts on the graphene bands. © 2013 American Physical Society.

Growth and electronic structure of nitrogen-doped graphene on Ni(111) - Koch, Roland; Weser, M.; Zhao, Wei; Vines, Francesc; Gotterbarm, Karin; Kozlov, Sergey M.; Höfert, Oliver; Ostler, Markus; Papp, Christian; Gebhardt, Julian; Steinrück, Hans-Peter; Görling, Andreas; et al. (Physical Review B, 2012)
We report on experimental and theoretical investigations of nitrogen-doped graphene. The incorporation of nitrogen was achieved during chemical-vapor deposition on Ni(111) using pyridine as a precursor. The obtained graphene layers were investigated using photoelectron spectroscopy. By studying C 1s and N 1s core levels, we show that the nitrogen content is influenced by the growth temperature and determine the atomic arrangement of the nitrogen atoms. Valence-band photoelectron spectra show that the incorporation of nitrogen leads to a broadening of the photoemission lines and a shift of the π band. Density functional calculations for two possible geometric arrangements, the substitution of carbon atoms by nitrogen and vacancies in the graphene sheet with pyridinic nitrogen at the edges, reveal that the two arrangements have opposite effects on the band structure. For the present experimental approach, vacancies with pyridinic nitrogen are dominant. In the latter case the vacancies generated by the nitrogen doping, not the nitrogen itself, have the main effect on the band structure. By intercalating gold between the doped graphene layer and the Ni(111) substrate electronic decoupling is achieved. After intercalation the doping remains. © 2012 American Physical Society.

Growth and nucleation regimes in boron doped silicon by dynamical x-ray diffraction - Will, Johannes; Gröschel, Alexander; Bergmann, Christoph; Weißer, Matthias; Magerl, Andreas (Applied Physics Letters, 2014)
The oxygen precipitation of highly (17.5 m Omega cm) and moderately (4.5 Omega cm) boron (B) doped silicon (Si) crystals at 780 degrees C is investigated by following in-situ the evolution of diffraction Pendellosung oscillations. All samples show an initial diffusion-driven growth process which may change over into Ostwald ripening. For the highly doped sample and involving a nucleation step at 450 degrees C for 30 h, the precipitate density rho is enhanced by a factor of 8 as compared to the moderately doped sample. The influence of a high B concentration on rho is dramatically higher for the samples directly heated to 780 degrees C, where an enhancement factor of 80 is found. Considering Ostwald ripening as a second growth regime reveals consistent ripening rates and surface energies sigma with those found at 900 degrees C in a previous publication. (c) 2014 AIP Publishing LLC.

GROWTH AND STRUCTURE OF AG ON PD(111) STUDIED BY PHOTOELECTRON FORWARD SCATTERING USING A 2-DIMENSIONAL DISPLAY-TYPE ANALYZER - Fauster, Thomas; et al. (Physical Review B, 1993)
For an investigation of the growth and the structure of thin Ag films on Pd(111), the angular distribution of core-level photoelectrons emitted at high kinetic energy (> 500 eV) was measured with a two-dimensional display-type electron spectrometer. At room temperature, Ag grows epitaxially on Pd(111) in the layer-by-layer mode. The Ag films have fcc crystal structure and grow with a stacking fault at the interface relative to the Pd(111) substrate. Low-energy electron-diffraction-intensity measurements show that the first Ag layer is pseudomorphic with the Pd(111) substrate and the stacking fault occurs between the first and the second Ag layers.

Growth and Structure of the First Layers of Ice on Ru(0001) and Pt(111) - Maier, Sabine; Lechner, B.A.J; Somorjai, G.A.; Salmeron, Miquel (Journal of the American Chemical Society, 2016)
Scanning tunneling microscopy was used to probe the structure and growth of the first few layers of water on Ru(0001) and Pt(111) at the molecular level. The surface bound first layer is composed of a mixture of water molecules forming hexagonal structures, both in registry and out-of registry with the substrate atoms. The hexagons are connected by pentagonal and heptagonal units. At temperatures below 140 K, this layer structure gives rise to the growth of metastable amorphous structures in the second and higher layers. We found that in the transition from amorphous to crystalline ice the structure of the original bottom layer changes to one in perfect local registry with the hexagonal surfaces of Ru(0001) and Pt(111). We further discovered structural defects in the form of extended one-dimensional lines of five- and eight-membered rings that are domain boundaries and stacking faults in the growing ice layers, which lead to the formation of metastable cubic ice.

GROWTH AND STRUCTURE OF THIN AU FILMS ON AG(111) - Fauster, Thomas; et al. (Physical Review B, 1994)
The growth and the structure of thin Au films on Ag(111) were studied by measuring the angular distribution of core-level photoelectrons emitted at high kinetic energy using a two-dimensional display-type electron spectrometer. Additional information was obtained by angle-integrated photoelectron spectroscopy and by thermal desorption spectrometry of NO. We find, even at room temperature, intermixing induced by surface diffusion. This conclusion is corroborated by the study of Au films deposited on (1-3)-monolayer-high Ag films grown on Pd(111).

GROWTH AND STRUCTURE OF THIN CO FILMS ON CU(111) STUDIED BY FULL-SOLID-ANGLE X-RAY PHOTOELECTRON DISTRIBUTIONS - Fauster, Thomas; et al. (Physical Review B, 1993)
Photoelectron spectroscopy at kinetic energies of almost-equal-to 570 eV has been performed on a Cu(111) surface with Co coverages of up to 50 monolayers. The angle-integrated data indicate that tall islands develop with Cu atoms in the top layers. The angular distributions of the photoelectrons were measured using a display-type analyzer with an opening angle of 88-degrees. The Co emission patterns always show some threefold symmetry corresponding to the fcc structure of the clean Cu(111) surface. Most of the Co atoms are in the hcp structure and this fraction increases with coverage.

Growth of 3C-SiC Bulk Material by the Modified Lely-Method - Krieger, Michael; Hundhausen, Martin; Ley, Lothar; et al. (Materials Science Forum, 2004)
The growth of cubic 3C-SiC single crystals by the modified Lely method is reported. Free-standing 200μm thick 3C-SiC epilayers were employed as seed crystals. The source material consisted of stoichiometric SiC; in addition, a separate Si source was deposited in the furnace at a temperature of about 1500°C. The temperature of the seed crystal was kept in the range of (1850-1950)°C. The growth rate is equal to 0.05mm/h and a nitrogen donor concentration of (2-6) ×1018 cm-3 is determined in the grown 3C-SiC.
UnivIS-Import:2015-04-14:Pub.2004.nat.dphy.IAP.LAP.growth_2
Keywords: 3C-SiC; Bulk growth; Modified Lely method

GROWTH OF AG, AN, AND CO ON CU(111) STUDIED BY HIGH-RESOLUTION SPECTROSCOPY OF IMAGE STATES - Fauster, Thomas; et al. (Surface Science, 1995)
Image states are located a few angstroms in front of metal surfaces and constitute a highly sensitive probe of surface properties. With high-resolution two-photon photoemission the image states were measured during the growth of ultrathin epitaxial films of Ag, Au, and Co on Cu(111) at room temperature. For Ag on Cu(111) the image state of the substrate is observed up to the coverage of one monolayer (ML) as expected for a layer-by-layer growth. The observation of an unshifted Ag image state in the submonolayer regime indicates Ag islands of more than 100 Angstrom diameter. For Au on Cu(111) the image state of the substrate is observed for coverages up to 1.7 ML. The simultaneous observation of image states from 1 and 2 ML high islands as well as from the clean copper surface is a clear proof for the growth of three-dimensional islands. For Co on Cu(111) a similar growth mode is found. Annealing experiments reveal a segregation of 1 ML of Cu onto the Co films at temperatures between 100 and 200 degrees C depending on the Co coverage.
Keywords: ANGLE RESOLVED PHOTOEMISSION; COBALT; COPPER; EPITAXY; GOLD; LOW INDEX SINGLE CRYSTAL SURFACES; SILVER; SINGLE CRYSTAL EPITAXY; SURFACE ELECTRONIC PHENOMENA; VISIBLE AND ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY; WORK FUNCTION MEASUREMENTS

Growth of cubic SiC single crystals by the physical vapor transport technique - Semmelroth, Kurt; Krieger, Michael; Pensl, Gerhard; Nagasawa, H.; Püsche, Roland; Hundhausen, Martin; Ley, Lothar; Nerding, Melanie; Strunk, Horst Paul (Journal of Crystal Growth, 2007)
Suitable process parameters for the growth of cubic 3C-SiC single crystals via the seeded physical vapor transport (PVT) technique, also known as the modified Lely method, have been determined. Free-standing, 200 μm thick 3C-SiC epilayers with (0 0 1)- or (0 0 1̄)-face grown on undulant Si (0 0 1) as well as 3C-SiC platelets with [1 1 1]- or [1̄ 1̄ 1̄]-orientation grown by thermal decomposition of methyl trichlorosilane in hydrogen were employed as seed crystals. The source material consisted of stoichiometric SiC; in addition, a separate Si source was deposited in the furnace at a temperature of about 1500 °C. The temperature of the seed crystals was kept at about 1900 °C. Stable growth of 3C-SiC bulk material of high crystalline quality was reached on 3C-SiC seed crystals with (0 0 1)-face providing a low density of planar defects and at near-thermal-equilibrium conditions resulting in a reduction of internal stress and as a consequence in avoiding the generation of new extended crystal defects. The growth rate achieved under these conditions was approximately 0.05 mm/h. The nitrogen donor concentration in the grown 3C-SiC crystals was determined to be equal to (2-6)×1018 cm-3. © 2007 Elsevier B.V. All rights reserved.
UnivIS-Import:2015-04-14:Pub.2007.nat.dphy.IAP.LAP.growth
Keywords: A2. Bulk growth; A2. Modified Lely method; A2. Physical vapor transport; B1. 3C-SiC

Growth of diamond on silicon during the bias pretreatment in chemically vapour deposition of polycristalline diamond films - Ristein, Jürgen; Hundhausen, Martin; Ley, Lothar; et al. (Journal of Applied Physics, 1996)
UnivIS-Import:2015-03-05:Pub.1996.nat.dphy.IAP.exphys.growth

Growth of high-quality homoepitaxial diamond films by HF-CVD - Stammler, Markus; Eisenbeiß, Henrick; Ristein, Jürgen; Neubauer, Jürgen; Göbbels, Matthias; Ley, Lothar; Ristein, Jürgen; Ley, Lothar (Diamond and Related Materials, 2002)
Defined growth of high-quality crystalline diamond films is necessary for manufacturing of electronic devices. Several groups have already worked on this topic and the main challenges remaining are the suppression of crystalline defects, the reduction of impurities and higher growth rates. High-quality homoepitaxial diamond films were produced in a hot filament chemical vapor deposition (HF-CVD) system with and without addition of nitrogen to the gas phase. Substrates were (100)-oriented synthetic Ib diamonds (Sumitomo) with a maximum misorientation angle of 0.3° as determined by X-ray diffraction. By adjusting the α parameter independently to a value close to two using polycrystalline diamond films on silicon growth rates of up to 500 nm/h could be achieved while simultaneously keeping the density of unepitaxial crystallites below 10 cm-2. The surface roughness (RMS) was below 1 nm in a 1 × 1-μm2 area. Impurities were studied by photoluminescence and cathodoluminescence spectroscopy. For the films deposited without intentional nitrogen addition, nitrogen-related impurities could only be detected by low temperature photoluminescence indicating a very low concentration. At 1.68 eV we observed the signature of the Si-vacancy complex probably caused by evaporation of Si from the tungsten filament. © 2002 Elsevier Science B.V. All rights reserved.
UnivIS-Import:2015-03-09:Pub.2002.nat.dgeo.IGM.LM.growth
Keywords: Diamond growth and characterization; HF-CVD; Homoepitaxy; Luminescene

Growth of Monolayer WS2Single Crystals with Atmospheric Pressure CVD: Role of Temperature - Xie, Yong; Wang, Guanfei; Wang, Zhan; Nan, Tang; Wang, Haolin; Wang, Yabin; Zhan, Yongjie; Jie, Wanqi; Ma, Xiaohua (MRS Advances, 2019)
It has been demonstrated that the introduction of NaCl can significantly improve the quality of monolayer WS 2 at the growth temperatures ranging from 700°C to 850°C by atmospheric pressure chemical vapor deposition (APCVD) without the assistant of hydrogen. Here, the influence of NaCl on the nucleation and growth of WS 2 has been thoroughly investigated. The morphology and quality of WS 2 grown with different temperatures are discussed by optical microscope, Raman and Photoluminescence (PL) spectra. It was found that amount of NaCl can efficiently influence the morphology and quality of WS 2 crystals. PL intensity of WS 2 crystal increases around three times from the center region to the edge of an individual domain, which may be attributed to the appearance of small triangle hollows formed during the growth at the edge of single crystal WS 2 .
CRIS-Team Scopus Importer:2019-05-09
Keywords: 2D materials; chemical vapor deposition (CVD) (deposition); optical properties; scanning electron microscopy (SEM)

Growth of polycrystalline phosphourous-doped CVD diamond layers - Lazea, A.; Mortet, Vincent; D'Haen, J.; Geithner, Peter; Ristein, Jürgen; D'Olieslaeger, M.; Haenen, Ken (Chemical Physics Letters, 2008)
UnivIS-Import:2015-03-09:Pub.2008.nat.dphy.IAP.exphys.growth

GROWTH OF ULTRATHIN IRON FILMS ON CU(001) - AN ION-SCATTERING SPECTROSCOPY STUDY - Fauster, Thomas; et al. (Surface Science, 1993)
Low-energy ion scattering has been applied to study the pseudomorphic growth of ultrathin fcc iron films on a Cu(001) single-crystal surface. At all coverages - even as low as 0.1 monolayer (ML) - iron is found to about equal amounts in both the surface and the first subsurface layer, since pat of the iron atoms are incorporated into the original copper surface. This is particular evident for the 0.1 ML film, which does not exhibit any iron defects, such as steps or adatoms. Instead copper surface defects become visible upon iron deposition. Near 2 ML the substrate is covered for the most part by a relatively smooth bilayer indicating the coalescence of the iron islands. Up to a film thickness of around 6 ML the surface defect density of the iron layers decreases with increasing coverage and raises again at larger coverages. Above approximately 10 ML the pseudomorphic fcc growth breaks down and domains of bcc iron with (110) orientation are formed.

GROWTH OF ULTRATHIN SI FILMS ON FCC CO(100) SURFACES - Fauster, Thomas; et al. (Surface Science, 1995)
Ultrathin Si layers were deposited at room temperature on fee Co(100) films grown epitaxially on Cu(100). The surfaces were studied by low-energy electron diffraction (LEED), Auger electron, and photoelectron spectroscopy using synchrotron radiation. The growth mode was studied by Auger uptake curves. From the Si2p core-level shifts the existence of a unique adsorption site is inferred at low coverages. Photoelectron forward-scattering experiments revealed no pronounced binding directions, whereas LEED patterns were observed at all coverages. This is compatible with the growth of an ordered layer of Si on top of the Co(100) substrate at low coverages followed by the growth of a disordered Si layer with holes.
Keywords: AUGER ELECTRON SPECTROSCOPY; CHEMISORPTION; COBALT; GROWTH; SILICON; SOFT X-RAY PHOTOELECTRON SPECTROSCOPY

Guest host interaction and low energy host structure dynamics in tin clathrates - Christensen, Sebastian; Bjerg, Lasse; Kaltzoglou, Andreas; Juranyi, Fanni; Fässler, Thomas; Unruh, Tobias; Christensen, Mogens (Journal of Applied Physics, 2013)
The two binary clathrates with vacancies (square) Rb8Sn44 square(2) and Cs8Sn44 square(2) have been examined using powder inelastic neutron scattering (INS). Rattling energies of Rb and Cs are found to be similar by both experiment and calculations, h omega(Cs) =h omega(Rb)vertical bar(Exp) = 0.98(1) and h omega(Cs) =h omega(Rb)vertical bar(Calc) = 1.0, despite the significant mass difference: m(Cs)/m(Rb) = 1.6, which shows that guest-host interaction is non-negligible for the studied system. For Rb8Sn44 square(2), a low energy phonon mode is observed at approximate to 3.5 meV, below the phonon mode which in the literature is attributed to the guest atom. The 3.5 meV mode is interpreted to have significant spectral weight of Sn host atoms based on temperature dependence and comparison with published theoretical phonon calculations. The record of low thermal conductivity of the tin clathrates can be attributed to the host structure dynamics rather than the guest atom rattling. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793081]

Head/tail interaction of vinculin influences cell mechanical behavior - Diez, Gerold; Auernheimer, Vera; Fabry, Ben; Goldmann, Wolfgang (Biochemical and Biophysical Research Communications, 2011)

Heterojunction based hybrid silicon nanowire solar cell: surface termination, photoelectron and photoemission spectroscopy study - Bashouti, Muhammad Y.; Pietsch, Matthias; Broenstrup, Gerald; Sivakov, Vladimir; Ristein, Jürgen; Christiansen, Silke (Progress in Photovoltaics: Research and Applications, 2013)
UnivIS-Import:2015-03-09:Pub.2013.nat.dphy.IAP.exphys.hetero

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering - Dolle, Christian; Oestreicher, Victor; Ruiz, Alberto M.; Kohring, Malte; Garnes-Portolés, Francisco; Wu, Mingjian; Sánchez-Santolino, Gabriel; Seijas-Da Silva, Alvaro; Alcaraz, Marta; Eggeler, Yolita M.; Spiecker, Erdmann; Canet-Ferrer, Josep; et al. (Journal of the American Chemical Society, 2023)
High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of “electronic grade” structural quality. Intriguingly, the material presents a sulfur-alkyl-functionalized reconstructed surface that prevents it from oxidation and leads to a tuned electronic structure that results from the altered arrangement of the surface. The metallic behavior of the hybrid is supported by ab initio predictions and room temperature transport measurements of individual nanoflakes. Our findings indicate how surface reconstructions in two-dimensional (2D) systems can promote unexpected properties that can pave the way to new functionalities and devices. Moreover, this scalable synthetic process opens new avenues for applications in plasmonics or electronic (and spintronic) device fabrication. Beyond electronics, this 2D hybrid material may be of interest in organic catalysis, biomedicine, or energy storage and conversion.<br

Hierarchical Assembly and Sensing Activity of Patterned Graphene-Hamilton Receptor Nanostructures - Bao, Lipiao; Zhao, Baolin; Ali, Muhammad; Assebban, M'hamed; Yang, Bowen; Kohring, Malte; Ryndyk, Dmitry A.; Heine, Thomas; Weber, Heiko B.; Halik, Marcus; Hauke, Frank; Hirsch, Andreas (Advanced Materials Interfaces, 2022)

Hierarchical on-surface synthesis and electronic structure of carbonyl-functionalized one- and two-dimensional covalent nanoarchitectures - Steiner, Christian; Gebhardt, Julian; Ammon, Maximilian Michael; Yang, Zechao; Heidenreich, Alexander; Hammer, Natalie; Görling, Andreas; Kivala, Milan; Maier, Sabine (Nature Communications, 2017)
The fabrication of nanostructures in a bottom-up approach from specific molecular precursors offers the opportunity to create tailored materials for applications in nanoelectronics. However, the formation of defect-free two-dimensional (2D) covalent networks remains a challenge, which makes it difficult to unveil their electronic structure. Here we report on the hierarchical on-surface synthesis of nearly defect-free 2D covalent architectures with carbonyl-functionalized pores on Au(111), which is investigated by low-temperature scanning tunnelling microscopy in combination with density functional theory calculations. The carbonyl-bridged triphenylamine precursors form six-membered macrocycles and one-dimensional (1D) chains as intermediates in an Ullmann-type coupling reaction that are subsequently interlinked to 2D networks. The electronic band gap is narrowed when going from the monomer to 1D and 2D surface-confined π-conjugated organic polymers comprising the same building block. The significant drop of the electronic gap from the monomer to the polymer confirms an efficient conjugation along the triphenylamine units within the nanostructures.

High-energy phonon branches of an individual metallic carbon nanotube - Maultzsch, Janina; et al. (Physical Review Letters, 2003)
We present excitation-energy dependent Raman measurements between 2.05 and 2.41 eV on the same individual carbon nanotube. We find a change in the Raman frequencies of both the D mode (63 cm(-1)/eV) and the high-energy modes. The observed frequencies of the modes at approximate to1600 cm(-1) as a function of laser-energy map the phonon dispersion relation of a metallic tube near the Gamma point of the Brillouin zone. Our results prove the entire first-order Raman spectrum in single-wall carbon nanotubes to originate from double-resonant scattering. Moreover, we confirm experimentally the phonon softening in metallic tubes by a Peierls-like mechanism.

High field transport in the inversion layer of amorphous silicon thin film transistors - Hundhausen, Martin; Ley, Lothar; et al. (Journal of Non-Crystalline Solids, 1996)
The temperature and field dependence of the channel conductivity of a short-channel amorphous silicon thin film transistor were measured. The data are analyzed in the framework of the effective temperature model in which it is assumed that the transport properties depend only on an effective temperature that is calculated from the electric field and temperature. It is found that this model has to be refined in order to take into account the energy dependence of the localization length of the conduction band tail states and the field dependence of the dimensionless factor entering the formula for the effective temperature.
UnivIS-Import:2015-03-05:Pub.1996.nat.dphy.IAP.exphys.highfi

High-Force Magnetic Tweezers with Force Feedback for Biological Applications - Kollmannsberger, Philip; Fabry, Ben (Review of Scientific Instruments, 2007)

High-Force Magnetic Tweezers with Hysteresis-Free Force Feedback - Kah, Delf-Thorge Eric; Dürrbeck, Christopher; Schneider, Werner; Fabry, Ben; Gerum, Richard (Biophysical Journal, 2020)
Magnetic tweezers based on a solenoid with an iron alloy core are widely used to apply large forces (∼100 nN) onto micron-sized (∼5 μm) superparamagnetic particles for mechanical manipulation or microrheological measurements at the cellular and molecular level. The precision of magnetic tweezers, however, is limited by the magnetic hysteresis of the core material, especially for time-varying force protocols. Here, we eliminate magnetic hysteresis by a feedback control of the magnetic induction, which we measure with a Hall sensor mounted to the distal end of the solenoid core. We find that the generated force depends on the induction according to a power-law relationship and on the bead-tip distance according to a stretched exponential relationship. Combined, they describe with only three parameters the induction-force-distance relationship, enabling accurate force calibration and force feedback. We apply our method to measure the force dependence of the viscoelastic and plastic properties of fibroblasts using a protocol with stepwise increasing and decreasing forces. We group the measured cells in a soft and a stiff cohort and find that softer cells show an increasing stiffness but decreasing plasticity with higher forces, indicating a pronounced stress stiffening of the cytoskeleton. By contrast, stiffer cells show no stress stiffening but an increasing plasticity with higher forces. These findings indicate profound differences between soft and stiff cells regarding their protection mechanisms against external mechanical stress. In summary, our method increases the precision, simplifies the handling, and extends the applicability of magnetic tweezers.
CRIS-Team Scopus Importer:2020-06-23

High-frequency dynamics of vitreous GeSe2 - Orsingher, Laura; Baldi, Giacomo; Fontana, Aldo; Bove, Livia E.; Unruh, Tobias; Orecchini, A.; Petrillo, Caterina; Violini, N.; Sacchetti, Francesco (Physical Review B, 2010)
The vibrational dynamics of vitreous germanium diselenide (v-GeSe2), an amorphous glass belonging to the family of continuous random network forming glasses, has been investigated by means of inelastic neutron scattering. We employed three different neutron spectrometers, each of them providing different energy resolutions and spanning different portions of the energy-wave-vector ((h) over bar omega,Q) plane, which partially overlap so that complementary information can be obtained. Two well-defined peaks are resolved in the dynamic structure factor in a broad Q region, below and above the position of the first sharp diffraction peak in the static structure factor. The low-lying excitation is nondispersive and located around the boson peak frequency. The high-frequency mode presents a pseudoperiodic behavior and it can be associated to the high-frequency counterpart of the longitudinal acoustic (LA) mode. In the low-Q region this mode shows a positive dispersion of its apparent sound velocity and an attenuation which follows a Q(2) law. The combined use of a set of appropriate instruments allows an accurate determination of the nonergodicity parameter f(Q). The LA dispersion curve of v-GeSe2 is similar to that measured in other glasses where a second excitation appears in the spectra. On the contrary other glass forming materials, where this second mode is not detected, present a sinusoidal-like dispersion curve with a deeper minimum in the second pseudo-Brillouin zone.

Highly Coherent Electron Beam from a Laser-Triggered Tungsten Needle Tip. - Ehberger, Dominik; Hammer, Jakob; Eisele, Max; Krüger, Michael; Noe, Jonathan; Högele, Alexander; Hommelhoff, Peter (Physical Review Letters, 2015)
We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission triggered by a near-ultraviolet laser with a photon energy of 3.1 eV and compare it to dc-field emission. A carbon nanotube is brought into close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes, we deduce an upper limit of the effective source radius both in laser-triggered and dc-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80±0.05) nm in laser-triggered and (0.55±0.02) nm in dc-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discusse

Highly Efficient and Reversible Covalent Patterning of Graphene: 2D-Management of Chemical Information - Wei, Tao; Kohring, Malte; Chen, Muqing; Yang, Shangfeng; Weber, Heiko B.; Hauke, Frank; Hirsch, Andreas (Angewandte Chemie International Edition, 2020)
Patterned graphene-functionalization with a tunable degree of functionalization can tailor the properties of graphene. Here, we present a new reductive functionalization approach combined with lithography rendering patterned graphene-functionalization easily accessible. Two types of covalent patterning of graphene were prepared and their structures were unambiguously characterized by statistical Raman spectroscopy together with scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS). The reversible defunctionalization processes, as revealed by temperature-dependent Raman spectroscopy, enable the possibility to accurately modulate the degree of functionalization by annealing. This allows for the management of chemical information through complete write/store/erase cycles. Based on our strategy, controllable and efficient patterning graphene-functionalization is no longer a challenge and facilitates the development of graphene-based devices.
CRIS-Team WoS Importer:2020-02-11

Highly Localized Optical Field Enhancement at Neon Ion Sputtered Tungsten Nanotips - Paschen, Timo; Brückner, Leon; Wu, Mingjian; Spiecker, Erdmann; Hommelhoff, Peter (Nano Letters, 2023)
We present laser-driven rescattering of electrons at a nanometric protrusion (nanotip), which is fabricated with an in situ neon ion sputtering technique applied to a tungsten needle tip. Electron energy spectra obtained before and after the sputtering show rescattering features, such as a plateau and high-energy cutoff. Extracting the optical near-field enhancement in both cases, we observe a strong increase of more than 2-fold for the nanotip. Accompanying finite-difference time-domain (FDTD) simulations show a good match with the experimentally extracted near-field strengths. Additionally, high electric field localization for the nanotip is found. The combination of transmission electron microscope imaging of such nanotips and the determination of the near-field enhancement by electron rescattering represent a full characterization of the electric near-field of these intriguing electron emitters. Ultimately, nanotips as small as single nanometers can be produced, which is of utmost interest for electron diffraction experiments and low-emittance electron sources.<br

Highly Ordered Monolayers of μm-Sized Polystyrene Spheres Studied by Grazing-Incidence Small-Angle X-ray Scattering, Simulations, and Geometrical Calculations - Maiti, Santanu; Senavirathna, L. N.; Minguez Bacho, Ignacio; Menath, Johannes; Gruber, W.; Vogel, Nicolas; Bachmann, Julien; Unruh, Tobias (Langmuir, 2024)
Unraveling the two-dimensional (2D) structural ordering of colloidal particles assembled at a flat surface is essential for understanding and optimizing their physical properties. So far, grazing-incidence small-angle X-ray scattering (GISAXS) has been widely used to determine crystallographic information on 2D self-assembled structures of nanosize objects. However, solving the structure of 2D lattices consisting of micrometer (μm)-sized objects still remains a challenge using scattering methods. Here, a model 2D SCALMS (supported catalytically active liquid metal solution) template is fabricated from μm-sized polystyrene (PS) spheres that form a monolayer on top of the flat solid support. GISAXS patterns of the sample were collected for rotation angles around its surface normal in steps of 3°. For every rotation angle, different Bragg-type interference maxima along the out-of-plane (qz) direction were observed. On the basis of simulations of GISXAS patterns of single domains of ordered particle arrangements using the distorted wave Born approximation (DWBA) and validation against a simple geometrical scattering model, the interference maxima could nicely be interpreted to originate from a monolayer of the μm-sized spherical particles which are arranged in domains of hexagonal 2D paracrystalline order. This novel GISAXS evaluation technique serves as a proof of principle for determining the μm-size periodicity of 2D crystalline domains and demonstrates its potential to spatially resolve the relative orientations of such domains with respect to a reference direction.
CRIS-Team Scopus Importer:2024-02-09

Highly Stable Lasing from Solution-Epitaxially Grown Formamidinium-Lead-Bromide Micro-Resonators - Afify, Hany A.; Sytnyk, Mykhailo; Rehm, Viktor; Barabash, Anastasiia; Mashkov, Oleksandr; Osvet, Andres; Volobuev, Valentine V.; Korczak, Jędrzej; Szczerbakow, Andrzej; Story, Tomasz; Götz, Klaus; Unruh, Tobias; et al. (Advanced Optical Materials, 2022)
High-quality epitaxial growth of oriented microcrystallites on a semiconductor substrate is demonstrated here for formamidinium lead bromide perovskite, by drop casting of precursor solutions in air. The microcrystallites exhibit green photoluminescence at room temperature, as well as lasing with low thresholds. Lasing is observed even though the substrate is fully opaque at the lasing wavelengths, and even though it has a higher refractive index as the perovskite active material. Moreover, the lasing is stable for more than 109 excitation pulses, which is more than what is previously achieved for devices kept in the air. Such highly stable lasing under pulsed excitation represents an important step towards continuous mode operation or even electrical excitation in future perovskite-based devices.
CRIS-Team Scopus Importer:2022-04-14
Keywords: halide perovskites; lasing; microresonators; solution epitaxy

High-order above-threshold photoemission from nanotips controlled with two-color laser fields - Seiffert, Lennart; Paschen, Timo; Hommelhoff, Peter; Fennel, Thomas (Journal of the Physics B: Atomic and Molecular Physics, 2018)
We investigate the process of phase-controlled high-order above-threshold photoemission from metallic nanotips under bichromatic laser fields. Experimental photoelectron spectra resulting from two-color excitation with a moderately intense near-infrared fundamental field (1560 nm) and its weak second harmonic show a strong sensitivity on the relative phase and clear indications for a plateau-like structure that is attributed to elastic backscattering. To explore the relevant control mechanisms, characteristic features, and particular signatures from the near-field inhomogeneity, we performed systematic quantum simulations employing a one-dimensional nanotip model. Besides rich phase-dependent structures in the simulated above-threshold ionization photoelectron spectra we find ponderomotive shifts as well as substantial modifications of the rescattering cutoff as function of the decay length of the near-field. To explore the quantum or classical nature of the observed features and to discriminate the two-color effects stemming from electron propagation and from the ionization rate we compare the quantum results to classical trajectory simulations. We show that signatures from direct electrons as well as the modulations in the plateau region mainly stem from control of the ionization probability, while the modulation in the cutoff region can only be explained by the impact of the two-color field on the electron trajectory. Despite the complexity of the phase-dependent features that render two-color strong-field photoemission from nanotips intriguing for sub-cycle strong-field control, our findings support that the recollision features in the cutoff region provide a robust and reliable method to calibrate the relative two-color phas
Keywords: nanotips; coherent control; two-color; photoemission

High-order harmonic generation driven by metal nanotip photoemission: theory and simulations - Ciappina, M. F.; Perez-Hernandez, J. A.; Shaaran, T.; Lewenstein, Maciej; Krüger, Michael; Hommelhoff, Peter (Physical Review A, 2014)
We present theoretical predictions of high-order-harmonic generation resulting from the interaction of short femtosecond laser pulses with metal nanotips. It is demonstrated that high-energy electrons can be generated using nanotips as sources; furthermore, the recollision mechanism is proven to be intrinsically linked to this high-energy tail. If recollision exists, it should be possible to convert the energy gained by the electron in the continuum in a high-energy photon. Consequently, the emission of harmonic radiation appears to be viable. We employ a quantum mechanical time-dependent approach to model the electron dipole moment including both the laser experimental conditions and the bulk matter properties. The use of metal tips should provide an alternative way of generating coherent XUV light with a femtosecond laser field. © 2014 American Physical Society.
UnivIS-Import:2015-04-14:Pub.2014.nat.dphy.IAP.exphys.highor

High-performance direct conversion X-ray detectors based on sintered hybrid lead triiodide perovskite wafers - Shrestha, Shreetu; Fischer, Rene; Matt, Gebhard; Feldner, Patrick; Michel, Thilo; Osvet, Andres; Levchuk, Ievgen; Merle, Benoit; Golkar, Saeedeh; Chen, Haiwei; Tedde, Sandro F.; Schmidt, Oliver; et al. (Nature Photonics, 2017)
Lead halide perovskite semiconductors are in general known to have an inherently high X-ray absorption cross-section and a significantly higher carrier mobility than any other low-temperature solution-processed semiconductor. So far, the processing of several-hundred-micrometres-thick high-quality crystalline perovskite films over a large area has been unresolved for efficient X-ray detection. In this Article, we present a mechanical sintering process to fabricate polycrystalline methyl ammonium lead triiodide perovskite (MAPbI(3)) wafers with millimetre thickness and well-defined crystallinity. Benchmarking of the MAPbI(3) wafers against state-of-the-art CdTe detectors reveals competitive conversion efficiencies of 2,527 mu C Gy(air)(-1) cm(-2) under 70 kV(p) X-ray exposure. The high ambipolar mobility-lifetime product of 2 x 10(-4) cm(2) V-1 is suggested to be responsible for this exceptionally high sensitivity. Our findings inform a new generation of highly efficient and low-cost X-ray detectors based on perovskite wafer

High Performance X-ray to Current Converters Fabricated Via Sintering or Melting of a Metal-halide Perovskite - Matt, Gebhard; Levchuk, Ievgen; Knüttel, Judith; Shrestha, Shreetu; Dallmann, Johannes; Hock, Rainer; Heiß, Wolfgang; Brabec, Christoph (nanoGe Fall Meeting 2019, 2019)
The search for an ideal X-ray sensitive photo-conductor is an ongoing task since the most semiconductors do not absorb high energy radiation effectively. However, the latter is an intrinsic property of the lead-halide perovskites semiconductors due to high radiation attenuation by the lead and halide ions combined with there good charge transport properties. In this presentation we will review our latest efforts utilizing organic-inorganic (MAPbI3) as well as a pure inorganic metal-halide perovkite (CsPbBr3) for X-ray to current converters. We demonstrate a sintering as well as a melting process which leads to the formation of several hundred µm thick and crystalline samples. Most notable for these samples are the good charge transport properties and unusual high mobility- lifetime products ( 1E-3 cm^2/V). The achieved X-ray to current conversation rate up to 2500 µC/Gycm2 is on par to the current state-of-the-art Cd(Zn)Te detector technology and our findings inform on a low-cost scaleable technology for the next generation of high energy detector technology.

High power terahertz photomixer arrays - Andres-Garcia, Belen; Garcia-Munoz, Enrique; Segovia-Vargas, Daniel; Bauerschmidt, Sebastian; Döhler, Gottfried; Preu, Sascha; Malzer, Stefan; Lu, Hong; Gossard, Art (6th European Conference on Antennas and Propagation, EuCAP 2012, 2012)
Two solutions for high power terahertz radiation are discussed in this paper. The first solution is based on classical array theory, presenting an array of 55 elements, where we achieve upon combining the beams an increased directivity of 18 dB in comparison with the 8 dB of an isolated antenna. Measurements for the different elements of the array are presented. The second solution is the use of large area emitters, where the larger area allows a higher illumination power having a higher radiated power. © 2012 IEEE.

High-resolution scanning tunneling microscopy imaging of mesoscopic graphene sheets on an insulating surface - Stolyarova, Elena; Rim, K.T.; Ryu, Sunmin; Maultzsch, Janina; Kim, Philip; Brus, Louis E.; Heinz, Tony F.; Hybertsen, M; Flynn, G. W. (Proceedings of the National Academy of Sciences of the United States of America, 2007)
We present scanning tunneling microscopy (STM) images of singlelayer graphene crystals examined under ultrahigh vacuum conditions. The samples, with lateral dimensions on the micrometer scale, were prepared on a silicon dioxide surface by direct exfoliation of crystalline graphite. The single-layer films were identified by using Raman spectroscopy. Topographic images of single-layer samples display the honeycomb structure expected for the full hexagonal symmetry of an isolated graphene monolayer. The absence of observable defects in the STM images is indicative of the high quality of these films. Crystals composed of a few layers of graphene also were examined. They exhibited dramatically different STM topography, displaying the reduced threefold symmetry characteristic of the surface of bulk graphit
Keywords: two-dimensional; graphite; nanoscience

High Resolution Scanning Tunneling Microscopy of a 1D Coordination Polymer with Imidazole-Based N,N,O Ligands on HOPG - Fischer, Nina V.; Mitra, Utpal; Warnick, Karl-Georg; Dremov, Viatcheslav; Stocker, Michael; Wölfle, Thorsten; Hieringer, Wolfgang; Heinemann, Frank Wilhelm; Burzlaff, Nicolai; Görling, Andreas; Müller, Paul (Chemistry - A European Journal, 2014)
Novel k(3)-N,N,O ligands tend to form 1D coordination polymer strands. Deposition of 1D structures on highly oriented pyrolytic graphite (HOPG) was achieved from diluted solutions and polymer strands have been studied on HOPG by AFM/STM. Single strands were mapped by STM and their electronic properties were subsequently characterized by current imaging tunneling spectroscopy (CITS). Periodic density functional calculations simulating a polymer strand deposited on a HOPG surface are in agreement with the zig-zag structure indicated by experimental findings. Both the observed periodicity and the Zn-Zn distances can be reproduced in the simulations. Van der Waals interactions were found to play a major role for the geometry of the isolated polymer strand, for the adsorption geometry on HOPG, as well as for the adsorption energy.
Keywords: atomic force microscopy; coordination polymers; ligand design; scanning tunneling microscopy; zinc

High resolution surface sensitive C1s core level spectra of clean and hydrogen terminated diamond (100) and (111) surfaces - Maier, Florian; Ristein, Jürgen; Ley, Lothar; et al. (Physical Review B - Condensed Matter and Materials Physics, 1998)
UnivIS-Import:2015-03-05:Pub.1998.nat.dphy.IAP.exphys.highre

High resolution time-of-flight MR-angiography at 7 T exploiting VERSE saturation, compressed sensing and segmentation - Meixner, Christian; Liebig, Patrick; Speier, Peter; Forman, Christoph; Hensel, Bernhard; Schmidt, Manuel; Saake, Marc; Uder, Michael; Dörfler, Arnd; Heidemann, Robin M.; Schmitter, Sebastian; Nagel, Armin Michael (Magnetic Resonance Imaging, 2019)
Background: 3D Time-of-Flight (TOF) MR-angiography (MRA) substantially benefits from ultra-high magnetic field strengths (≥7 T) due to increased Signal-to-Noise ratio and improved contrast. However, high-resolution TOF-MRA usually requires long acquisition times. In addition, specific absorption rate constraints limit the choice of optimal pulse sequence parameters, especially if venous saturation is employed. Purpose: To implement and evaluate an arterial TOF-MRA for accelerated high-resolution angiography at ultra-high magnetic field strength. Field strengths/sequence: 7 T modified gradient-echo TOF sequence including venous saturation using Variable-Rate Selective Excitation (VERSE), Compressed Sensing (CS) and sparse application of saturation pulses, called segmentation, were included for acceleration. Assessment: To analyze the acceleration techniques all volunteers were examined with the same protocols. CS with different sampling patterns and regularization factors as well as segmentation were applied for acceleration. For comparison, conventional acceleration techniques were applied (GRAPPA PAT 3 and Partial Fourier (6/8 in slice/phase encoding)). Images were co-registered and 40 mm thick transversal maximum intensity projections were created to calculate the relative number of vessels. To analyze the visibility of small vessels, the lenticulostriate arteries (LSA) were examined. This was done via multiscale vessel enhancement filtering in a VOI and quantification via Fiji ImageJ as well as qualitatively evaluation by two radiologists. Additionally, the venous/arterial vessel-to-background ratios (vVBR/aVBR) were calculated for chosen protocols. Results: For the acceleration of a high resolution TOF-MRA (0.31 mm isotropic), under-sampling of 9.6 showed aliasing artifacts, whereas 7.2 showed no aliasing. The regularization factor R had a strong impact on the image quality according to smoothing (R = 0.01 to R = 0.005) and noise (R = 0.0005 to R = 0.00005). With the alternating sampling patterns it was shown that the k-space center should not be under-sampled too much. Additionally segmentation could be verified to be feasible for stronger acceleration with sufficient venous suppression. Conclusion: The combination of several independent techniques (VERSE, CS with acceleration factor 7.2, R = 0.001, Poisson disc radius of 80%, 3 segments) enables the application of high-resolution (0.31 mm isotropic) TOF-MRA with venous saturation at 7 T in clinical time settings (TA ≈ 5 min) and within the SAR limits.
CRIS-Team Scopus Importer:2019-08-30
Keywords: 7 Tesla; Compressed sensing; MR-angiography; Time-of-flight; Ultra-high field; VERSE

High Sensitivity photoelectron yield spectroscopy with computer-calculated electron optics - Ristein, Jürgen; Ley, Lothar; et al. (Review of Scientific Instruments, 1993)
An improved spectrometer design for photoelectron yield spectroscopy is described. Basis of the new spectrometer is a computer-designed electron optics that images the emission spot into the electron multiplier and thereby improves the sensitivity for electrons emitted from the sample. The collection efficiency was experimentally determined to 98.3%. A background of stray electrons which limits the ultimate sensitivity of the method could be substantially reduced by minimizing multiple beam reflections and by coating critical surfaces with a high work function material such as platinum. As a result, photoyield spectra of Au with a dynamical range of ten orders of magnitude are routinely obtained.
UnivIS-Import:2015-03-05:Pub.1993.nat.dphy.IAP.exphys.highse

High spatial coherence in multiphoton-photoemitted electron beams - Meier, Stefan; Higuchi, Takuya; Nutz, Manuel; Högele, Alexander; Hommelhoff, Peter (Applied Physics Letters, 2018)
Nanometer-sharp metallic tips are known to be excellent electron emitters. They are used in highest-resolution electron microscopes in cold field emission mode to generate the most coherent electron beam in continuous-wave operation. For time-resolved operation, sharp metal needle tips have recently been triggered with femtosecond laser pulses. We show here that electrons emitted with near-infrared femtosecond laser pulses at laser oscillator repetition rates show the same spatial coherence properties as electrons in cold field emission mode in cw operation. From electron interference fringes, obtained with the help of a carbon nanotube biprism beam splitter, we deduce a virtual source size of less than (0.65 ± 0.06) nm for both operation modes, a factor of ten smaller than the geometrical source size. These results bear promise for ultrafast electron diffraction, ultrafast electron microscopy, and other techniques relying on highly coherent and ultrafast electron beam
Keywords: ultrafast electron sources; metallic needle tips; spatial coherence; femtosecond pulses; source size estimation

High visibility in two-color above-threshold photoemission from tungsten nanotips in a coherent control scheme - Paschen, Timo; Förster, Michael; Krüger, Michael; Lemell, Christoph; Wachter, Georg; Libisch, Florian; Madlener, Thomas; Burgdörfer, Joachim; Hommelhoff, Peter (Journal of Modern Optics, 2017)
In this article, we present coherent control of above-threshold photoemission from a tungsten nanotip achieving nearly perfect modulation. Depending on the pulse delay between fundamental ((Formula presented.)) and second harmonic ((Formula presented.)) pulses of a femtosecond fiber laser at the nanotip, electron emission is significantly enhanced or depressed during temporal overlap. Electron emission is studied as a function of pulse delay, optical near-field intensities, DC bias field and final photoelectron energy. Under optimized conditions modulation amplitudes of the electron emission of 97.5% are achieved. Experimental observations are discussed in the framework of quantum-pathway interference supported by local density of states simulation
Keywords: density functional theory; electron emitter; Nanotip; two-color laser field

Hindered Water Motions in Hardened Cement Pastes Investigated over Broad Time and Length Scales - Bordallo, Heloisa Nunes; Aldridge, Laurence P.; Fouquet, Peter; Pardo, Luis Carlos; Unruh, Tobias; Wuttke, Joachim; Yokaichiya, Fabiano (ACS Applied Materials and Interfaces, 2009)
We investigated the dynamics of confined water indifferent hydrated cement pastes With minimized contributions of capillary water. It was found that the water motions are extremely reduced compared to those of bulk water The onset of water mobility, which was modified by the local environment, was investigated with elastic temperature scans using the high-resolution neutron backscattering instrument SPHERES Using a Cauchy-Lorenz distribution. the quasi-elastic signal observed in the spectra obtained by the backscattering spectrometer was analyzed, leading to the identification of rotational motions with relaxation times of 0 3 ns Additionally, neutron spin echo (NSE) spectroscopy was used to measure the water diffusion over the local network of pores The motions observed in the NSE time scale were characterized by diffusion constants ranging from 0 6 to 1 1 x 10(-9) m(2) s(-1) most likely related to water molecules removed from the interface In summary, our results indicate that the local diffusion observed in the gel pores of hardened cement pastes is on the order of that found in deeply supercooled water Finally, the importance of the magnetic properties of cement pastes were discussed in relation to the observation of a quasi-elastic signal on the dried sample spectra measured using the time-of-flight spectrometer
Keywords: cement paste; pore structure; neutron scattering; water dynamics; water diffusion; magnetic relaxation

Host guest chemistry and supramolecular doping in triphenylamine-based covalent frameworks on Au(111) - Steiner, Christian; Fromm, Lukas; Gebhardt, Julian; LIU, YI; Heidenreich, Alexander; Hammer, Natalie; Görling, Andreas; Kivala, Milan; Maier, Sabine (Nanoscale, 2021)
The post-synthetic modification of covalent organic frameworks (COFs) via host-guest chemistry is an important method to tailor their electronic properties for applications. Due to the limited structural control in the assembly of two-dimensional surface-supported COFs, supramolecular networks are traditionally used at present for host-guest experiments on surfaces, which lack structural and thermal stability, however. Here, we present a combined scanning tunneling microscopy and density functional theory study to understand the host-guest interaction in triphenylamine-based covalently-linked macrocycles and networks on Au(111). These triphenylamine-based structures feature carbonyl and hydrogen functionalized pores that create preferred adsorption sites for trimesic acid (TMA) and halogen atoms. The binding of the TMA through optimized hydrogen-bond interactions is corroborated by selective adsorption positions within the pores. Band structure calculations reveal that the strong intermolecular charge transfer through the TMA bonding reduces the band gap in the triphenylamine COFs, demonstrating the concept of supramolecular doping by host-guest interactions in surface-supported COFs. Halogen atoms selectively adsorb between two carbonyl groups at Au hollow sites. The mainly dispersive interaction of the halogens with the triphenylamine COF leads to a small downshift of the bands. Most of the halogens change their adsorption position selectively upon annealing near the desorption temperature. In conclusion, we demonstrate evidence for supramolecular doping via post-synthetic modification and to track chemical reactions in confined space. This journal is
CRIS-Team Scopus Importer:2021-06-18

Hot-Volumes as Uniform and Reproducible SERS-Detection Enhancers in Weakly-Coupled Metallic Nanohelices - Krstic, Vojislav; Caridad, Jose M.; Winters, Sinead; Mccloskey, David; Duesberg, Georg S.; Donegan, John F.; Krstic, Vojislav (Scientific Reports, 2017)
Reproducible and enhanced optical detection of molecules in low concentrations demands simultaneously intense and homogeneous electric fields acting as robust signal amplifiers. To generate such sophisticated optical near-fields, different plasmonic nanostructures were investigated in recent years. These, however, exhibit either high enhancement factor (EF) or spatial homogeneity but not both. Small interparticle gaps or sharp nanostructures show enormous EFs but no near-field homogeneity. Meanwhile, approaches using rounded and separated monomers create uniform near-fields with moderate EFs. Here, guided by numerical simulations, we show how arrays of weakly-coupled Ag nanohelices achieve both homogeneous and strong near-field enhancements, reaching even the limit forreproducible detection of individual molecules. The unique near-field distribution of a single nanohelix consists of broad hot-spots, merging with those from neighbouring nanohelices in specific array configurations and generating a wide and uniform detection zone (“hot-volume”). We experimentally assessed these nanostructures via surface-enhanced Raman spectroscopy, obtaining a corresponding EF of 107 and a relative standard deviation <10%. These values demonstrate arrays of nanohelices as state-of-the-art substrates for reproducible optical detection as well as compelling nanostructures for related fields such as near-field imaging.<br

Hot volumes for highly enhanced and reproducible SERS detection in weakly-coupled metallic nanohelices - Caridad, Jose M.; Winters, Sinead; McCloskey, David; Duesberg, Georg S.; Donegan, John F.; Krstic, Vojislav (Scientific Reports, 2017)
Reproducible and enhanced optical detection of molecules in low concentrations demands simultaneously intense and homogeneous electric fields acting as robust signal amplifiers. To generate such sophisticated optical near-fields, different plasmonic nanostructures were investigated in recent years. These, however, exhibit either high enhancement factor (EF) or spatial homogeneity but not both. Small interparticle gaps or sharp nanostructures show enormous EFs but no near-field homogeneity. Meanwhile, approaches using rounded and separated monomers create uniform near-fields with moderate EFs. Here, guided by numerical simulations, we show how arrays of weakly-coupled Ag nanohelices achieve both homogeneous and strong near-field enhancements, reaching even the limit forreproducible detection of individual molecules. The unique near-field distribution of a single nanohelix consists of broad hot-spots, merging with those from neighbouring nanohelices in specific array configurations and generating a wide and uniform detection zone (“hot-volume”). We experimentally assessed these nanostructures via surface-enhanced Raman spectroscopy, obtaining a corresponding EF of 107 and a relative standard deviation <10%. These values demonstrate arrays of nanohelices as state-of-the-art substrates for reproducible optical detection as well as compelling nanostructures for related fields such as near-field imaging.
Keywords: Electronics; photonics and device physics; Materials for optics; Nanoscale materials; Optical materials and structures

How Does Water Wet a Surface? - Maier, Sabine; Salmeron, Miquel (Accounts of Chemical Research, 2015)
In this Account, a selection of recent STM results on the water adsorption and dissociation on close-packed metal surfaces is reviewed, with a particular focus on Ru(0001). The Ru(0001) surface is one where water adsorbs intact in a metastable state at low temperatures and where partially dissociated layers are formed at temperatures above similar to 150 K. First, we will describe the structure of intact water clusters starting with the monomer up to the monolayer. We show that icelike wetting layers do not occur on close-packed metal surfaces but instead hydrogen bonded layers in the form of a mixture of pentagonal, hexagonal, and heptagonal molecular rings are observed. Second, we will discuss the dissociation mechanism of water on Ru(0001). We demonstrate that water adsorption changes from dissociative to molecular as a function of the oxygen preadsorbed on Ru. Finally, we briefly review recent STM experiments on bulk ice (I-h and I-c) and water adsorption on insulating thin films. We conclude with an outlook illustrating the manipulation capabilities of STM in respect to probe the proton and hydrogen dynamics in water clusters.

Human laryngeal mucus from the vocal folds: Rheological characterization by particle tracking microrheology and oscillatory shear rheology - Peters, Gregor; Wendler, Olaf; Böhringer, David; Gostian, Antoniu Oreste; Müller, Susanna; Canziani, Herbert; Hesse, Nicolas; Semmler, Marion; Berry, David A.; Kniesburges, Stefan; Peukert, Wolfgang; Döllinger, Michael (Applied Sciences, 2021)
Mucus consistency affects voice physiology and is connected to voice disorders. Neverthe-less, the rheological characteristics of human laryngeal mucus from the vocal folds remain unknown. Knowledge about mucus viscoelasticity enables fabrication of artificial mucus with natural properties, more realistic ex-vivo experiments and promotes a better understanding and improved treatment of dysphonia with regard to mucus consistency. We studied human laryngeal mucus samples from the vocal folds with two complementary approaches: 19 samples were successfully applied to particle tracking microrheology (PTM) and five additional samples to oscillatory shear rheology (OSR). Mucus was collected by experienced laryngologists from patients together with demographic data. The analysis of the viscoelasticity revealed diversity among the investigated mucus samples according to their rigidity (absolute G’ and G”). Moreover some samples revealed throughout solid-like character (G’ > G”), whereas some underwent a change from solid-like to liquid-like (G’ < G”). This led to a subdivision into three groups. We assume that the reason for the differences is a variation in the hydration level of the mucus, which affects the mucin concentration and network formation factors of the mucin mesh. The demographic data could not be correlated to the differences, except for the smoking behavior. Mucus of predominant liquid-like character was associated with current smokers.
CRIS-Team Scopus Importer:2021-04-16
Keywords: Human laryngeal mucus; Oscillatory shear rheology; Particle tracking microrheology; Viscoelasticity; Vocal folds