Substrate-Selective Morphology of Cesium Iodide Clusters on Graphene

Formation and characterization of low-dimensional nanostructures is crucial for controlling the properties of two-dimensional (2D) materials such as graphene. Here, we study the structure of low-dimensional adsorbates of cesium iodide (CsI) on free-standing graphene using aberration-corrected transmission electron microscopy at atomic resolution. CsI is deposited onto graphene as charged clusters by electrospray ion-beam deposition. The interaction with the electron beam forms two-dimensional CsI crystals only on bilayer graphene, while CsI clusters consisting of 4, 6, 7, and 8 ions are exclusively observed on single-layer graphene. Chemical characterization by electron energy-loss spectroscopy imaging and precise structural measurements evidence the possible influence of charge transfer on the structure formation of the CsI clusters and layers, leading to different distances of the Cs and I to the graphene.

Time-series HRTEM image of crystalline 3D-CsI nanoparticle with magnified inset of crystal planes. (d-g) boxed average intensity profile from the inset of (a,b and c). The error bar in the measurement is ± 0.02 nm.

S2 Formation of 2D-Cesium-Iodide crystal on bi-layer graphene (BLG)
A large area of a graphene sample with CsI clusters was illuminated with a parallel electron beam for 20 minutes in STEM. A HAADF image was acquired from the interface region between SLG and BLG ( Figure S2). Two-dimensional cesium-iodide crystals formed selectively on BLG whereas on SLG only single atoms or clusters of atoms without any trace of 2D crystal formation were observed. Inset is the FFT of the corresponding HRTEM image. In the FFT, bright spots corresponding to graphene are marked by dotted white circles. Six spots correspond to one set of a graphene lattice and another six spots to another set of a graphene lattice, which comprises to a total of 12 spots (all marked by white dotted circle) that represent the underlying bi-layer graphene with a misorientation of 11 degrees between them. (d) The 3D-CsI nanoparticle has completely decomposed into an amorphous structure and some atoms from the decomposed nanoparticle have rearranged into 2D crystals (enclosed by red-dashed boxes).

S2.3 Distinguishing single-layer graphene (SLG) from bi-layer graphene (BLG)
In order to distinguish SLG from BLG and vice versa, we use the FFT from the HRTEM image.
The FFT of SLG shows six inner spots and six outer spots, respectively. Each set of six spots forms a hexagon. The six inner spots originate from the zig-zag pattern and the outer 6 spots to the armchair pattern of the graphene lattice. Now when we have two sheets of graphene with a misorientation between them, there exist 12 spots in the inner and 12 spots in the outer ring (see figure S2.3b). In figure S2.3b, spots corresponding to one layer of graphene are marked by dotted green circles and spots from the second layer are marked by white dotted circles. Hence, in the inner ring, there exist 12 spots (6 green and 6 white) and the angle between neighbouring white and green circles tells us the misorientation angle between the two graphene sheets.

S3 Counting of atoms in 2D-Cesium-Iodide crystals on BLG
Total numbers of atoms present in 2D crystal were counted on two occasions. First, when the 2D crystals were first observed and secondly just before their disappearance from the TEM field of view. The size of the 2D crystal was observed to shrink before disappearance (see Figure S3). The total number of atoms counted in Figure S3a

S3.2 Qualitative representation of expansion of 2D-CsI crystal on graphene w.r.t. 3D-CsI
Single layer slices were constructed (via VESTA crystal software) from two lattice constants namely 1) using 3D-CsI (bulk structure) and 2) Using lattice constant from the experimentally found 2D crystal in this work. These slices were stacked over each other for comparing slight elongation that 2D crystal undergoes over is 3D counterpart when observed from [100] viewing direction.  Table S1. Dimension-dependent bond length comparison of Cesium-Iodide in experiment.
Cs-I bond length 0.337 nm 0.34 nm 0.395 nm

S4 Mapping positions of atoms in CsI atomic clusters on single-layer graphene lattice
Few HRTEM images acquired in time series (5 in this case) were summed to produce an averaged HRTEM image. In the next step, Gaussian-blur filter of radius sigma of 1 nm was applied to the averaged image for better signal-to-noise ratio. In the final filtered image, carbon atoms in the graphene lattice are clearly visible and are marked using red dots and green dots (see Figure S4. a-d). Then we model the system by overlaying atomic clusters on to the graphene lattice.