Supporting Information Identifying and Visualizing the Edge Terminations of Single-Layer MoSe2 Island Epitaxially Grown on Au(111) Jianchen Lu, De-Liang Bao, Kai Qian, Shuai Zhang, Hui Chen, Xiao Lin*, Shi-Xuan Du*, Hong-Jun Gao Institute of Physics & University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China To whom correspondence should be addressed. E-mail: xlin@ucas.ac.cn, sxdu@iphy.ac.cn S1
Figure S1. (a) STM image showing that the atom-like small protrusions and single-layer MoSe2 islands coexist on the Au surface. The white arrow indicates the small protrusions. (b) STM image of Au(111) surface with only selenium atoms deposition. (c) The line profile from the blue dashed line in (a), indicating the height of a MoSe2 island is about 2.30 Å. Scanning parameters: (a) Vs = 0.4 V, It = 0.5 na; (b) Vs = 1.0 V, It = 0.74 na. As seen from Figure S1a, the clear atomic like protrusions reside on the Au(111) surface with forms of atomic chains and islands. Figure S1b shows the STM image of a control experiment, in which we only deposited selenium atoms onto Au(111). It clearly shows that Se atoms on Au surface are atomic chains and atomically resolved triangular islands. The distance between two atoms is about 5.0 Å ± 0.05 Å. The features are the same as the protrusions in Figure S1a, which means the atom-like protrusions are Se clusters and the most possible situation is that Se atoms chemisorb onto Au surfaces along the 112 direction of Au. S2
Figure S2. (a) Large-scale STM image of MoSe2 islands on Au(111) after several growth cycles, showing the high-coverage of MoSe2 islands (~0.7 ML). (b) Large-scale, atomic-resolution STM image, simultaneously showing MoSe2 lattice and moiré pattern. (c) Fast Fourier Transform (FFT) of (b). The outmost six points represent the MoSe2 lattice and the inner six points correspond to the moiré superstructure. The blue and black arrows indicate that the MoSe2 lattice and the moiré superstructure have the same orientations. Scanning parameters: (a) Vs = 2 V, It = 0.05 na; (b) Vs = 0.1 V, It = 2.0 na. S3
Figure S3. Normalized formation energies of MoSe2 ribbons with different edges. The chemical potential of Se is chosen in the range of 4.44 ev < < 3.36 ev, corresponding to Mo-rich and Se-rich cases. Relative stability of ribbons with different edges were tested by comparing the normalized formation energy. We adopted a widely used formula to calculate the formation energy as the function of the chemical potential of Se [ref 32]: Ef = Erbn NMoEMoSe2 NSeμSe; Ef * = Ef/2 where is the total energy of the ribbons, is the energy per MoSe2 unit in a perfect monolayer, is the number of Mo atoms in the ribbons, is the number of extra Se atoms in the ribbon with respect to the stoichiometry in a perfect MoSe2 lattice and is the chemical potential of Se. Note that the formation energy is normalized over 2, the number of edges in ribbon models, to compare the stability for different MoSe2 ribbons. Similar as ref 32, for Se-rich condition, the chemical potential of Se,, was chosen as the energy of a Se atom in Se8 S4
molecules. For Se-poor condition, is chosen as =1 2, which reflects plenty of Mo in a bulk form. Figure S3 shows atomic structures and the formation energies of several ribbons with different edges. It is clear that ribbon-v has the smallest formation energy except at extreme Se-rich condition. Considering the epitaxial process and experimentally no observable Se islands, we can exclude the extreme Se-rich condition. In together with the excellent agreement between the simulated and observed STM images, we can conclude that ribbon-v is the experimentally observed configuration. S5
Figure S4. Checking electronic structure of the middle of the ribbon. The grey curve represents the PDOS of the atoms in the middle of the ribbon (grey shadow in MoSe2 structural model), while the yellow shadow represents the DOS of a perfect MoSe2 monolayer. Obviously, the grey curve matches the yellow shadow very well, indicating the atoms in the middle of the ribbon are in the intrinsic electronic states of MoSe2. i.e., the width of the ribbon we used is enough to isolate the edge states. S6
Figure S5. (a) MoSe2 island on Au(111) substrate. (b) Line profile along the black dashed line in (a). Scanning parameters: (a) Vs = 1.4 V, It = 0.1 na. Figure S5 shows the line profile along the black dashed line in the MoSe2 island. It clearly demonstrates that Se edge (shorter edge) is higher than Mo edge (longer edge) at this bias voltage ( 1.4 V). This feature fits well with the simulated STM image at bias voltage of 1.5 V in Figure 4b, where the Se edge is brighter than the Mo edge. S7
Figure S6. Simulated STM images of all considered configurations. Structure I, II, III, and IV are the configurations mentioned in the main text. And the right images are the corresponding simulated STM images. In order to explain the identification of the edge configuration, and makes it much easier to understand. We did the simulated STM images for each edge configuration (Figure S6). Obviously, the STM simulations of configurations I, II, III, and IV differ from the STM images (Figure 4). Comparing the atomically resolved STM image with the simulated image for configuration V (Figure 4), it s clear that the bright brim features are unique to the proposed edges. S8