Graphene‐supported microwell liquid cell for in situ studies in TEM and SEM

Liquid cell transmission electron microscopy (LCTEM) is used for in situ investigations of dynamic nanoparticulate processes in aqueous and nonaqueous solutions. In contrast to complementary techniques like conventional (S)TEM, cryo‐TEM or SAXS, LCTEM ensures real‐time high‐resolution imaging and is used in various research fields like biology, electrochemistry and materials science [1]. For our approach we make use of a modified microwell liquid cell layout combining the benefits of the microwell liquid cell design used by Dukes et al. [2] and the graphene liquid cell shown by Yuk et al. [3] for in situ (S)TEM and SEM experiments. In this approach, the liquid specimen is confined between amorphous silicon nitride microwells and multilayer graphene (cf. fig. 1). This enables improved imaging conditions compared to other cell designs because the cell profits from the advantages of (i) a robust encasement combined with an ultrathin and electrically conducting low‐Z material and (ii) a constant liquid film‐thickness defined by the nitride wells. Furthermore, in this design window bulging is largely suppressed due to small microwell diameters on the order of 5 µm. The liquid cells are processed under clean room conditions via conventional semiconductor technology as well as bulk micromachining. In a first step a silicon nitride layer is deposited onto an oxidized thin silicon wafer via LPCVD followed by structuring the front and back‐side by photolithography and reactive ion etching. For the simplified device layout no mask alignment is necessary which minimizes the failure probability and improves the yield of the fabrication process. Furthermore, the number of process steps could be considerably reduced compared to the fabrication process of conventional static liquid cells. Bulk micromachining of the silicon wafer is done by an anisotropic potassium hydroxide wet‐etching process. The filling and vacuum tight sealing of the cell is conducted in one step by transferring the graphene directly onto a droplet of the specimen solution. When the liquid dries the graphene adheres to the silicon nitride and encloses small amounts of the fluid within the microwells. The compatibility to conventional specimen holders makes the liquid cells feasible for the use with various kinds of TEMs and SEMs.

As first application and test of this optimized liquid cell design electron beam induced growth and degradation phenomena of Au‐nanoparticles in HAuCl₄‐solution have been studied. Figure 2 shows snapshots of an in situ TEM investigation monitoring the dissolution of a Au‐nanoplatelet at a liquid‐gas interface and the subsequent growth of smaller particles above (or below) the gas bubble. This redistribution of material is facilitated by the high mobility of gold‐atoms in solution as well as by reactive species generated by electron beam irradiation of the aqueous solution. The experiment was carried out in TEM bright‐field mode using a Philips CM‐30 (S)TEM operated at 300 kV. In order to demonstrate that the cell can be equally used inside an SEM figure 3 shows snapshots of an in situ study of electron beam induced dendrite‐growth of Au‐nanostructures in a 1 mM HAuCl₄‐solution. Here, imaging was carried out in STEM mode using a FEI Helios Nanolab 660 operated at 29 keV primary electron energy.