Abstract text (incl. figure legends and references)

The structure and morphology of an adsorbed atomic or molecular species depend critically on the interaction with the substrate that provides the energy landscape that governs the reaction dynamics and, in turn, influences the final product.^1-4 Surprisingly we found in an earlier scanning transmission electron microscopy study that the morphology and size of the forming cesium iodide (CsI) clusters drastically changes when adsorbed on single layer graphene (SLG) or bi-layer graphene (BLG) surfaces.⁴ The formation of 2D-CsI crystals is observed explicitly on BLG and 3D-molecular clusters consisting of 4, 6, 7, and 8 atoms are only formed on SLG. As a possible hypothesis, the observed discrepancy in the adsorption behavior of the CsI clusters on SLG and BLG was discussed in the light of the presence of additional conjugated pi-electrons in the BLG, stimulating the growth of 2D crystals.4 However a thorough experimental study comparing the adsorption characteristic of CsI clusters in dependence on the numbers of graphene layers as well as on the TEM imaging parameters has not been performed so far.

Here, we investigate the structure, chemistry, and stability of 2D-CsI crystals for varying number of graphene layers (1-5) using the sub-angstrom low-voltage electron microscope (SALVE) operated at electron accelerating voltages from 40 to 80 kV. We used commercially available CVD-grown single and bi-layer graphene on a copper foil (Graphenea S.A.). In the first step, copper foil is etched in an 8% ammonium persulfate solution. Then the SLG and BLG are transferred to the platinum coated (thickness ~ 10 nm) perforated silicon nitride TEM grid. Subsequently, by directly stacking another SLG or BLG onto the already transferred SLG/BLG onto a TEM grid, a combination of two, three, four, and five stacked layers of graphene sheets are prepared. In order to minimize the hydrocarbon contamination, the TEM grid was annealed at 300^o C in the air for 30 minutes, where the platinum layer, deposited on the TEM grid, catalyzes the oxidation of the hydrocarbon contaminations. Finally, CsI cluster ions (MX)_nM⁺ (where M and X represent the cation and anion, respectively) are deposited using the electrospray ion-beam deposition (ES-IBD) technique⁵ on free-standing graphene TEM grids under high vacuum conditions. In addition, we employ ab-initio calculations to gain comprehensive insight regarding the stability of 2D-CsI crystals to the underlying graphene sheets.

In this work, we combine two unique techniques: low-voltage spherical and chromatic aberration-corrected transmission electron microscopy and highly pure, chemically selective deposition of mass-filtered molecular ion beams to further unravel the mystery of number of layer-dependent growth of 2D-CsI crystals. Furthermore, we anticipate that our atomically resolved number of graphene-layer-dependent and electron accelerating voltage-dependent analysis not only sheds light into the adsorption and growth behavior of CsI clusters but also helps to understand crystal growth of other alkali-iodide species on graphene.

References:
1. Hardcastle, T. P., et al. Physical Review B 87.19 (2013): 195430.
2. Al Balushi, Z. Y., et al. Nature materials 15.11 (2016): 1166-1171.
3. Shi, G., et al. Nature Chemistry 10.7 (2018): 776-779.
4. Vats, N., et al. ACS nano 14.4 (2020): 4626-4635.
5. Rauschenbach, S., et al. small 2.4 (2006): 540-547.