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Supporting Information Study of Grains and Boundaries of Molybdenum Supporting Information Study of grains and boundaries of molybdenum diselenide and tungsten diselenide using liquid crystal Muhammad Arslan Shehzad1,2, Sajjad Hussain1,2, Junsu Lee3, Jongwan Jung1,2, Naesung Lee2, Gunn Kim1,3*, Yongho Seo1,2* 1Graphene Research Institute, Sejong University, Seoul 143-747, Republic of Korea 2Faculty of Nanotechnology & Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea 3Department of Physics & Astronomy, Sejong University, Seoul 143-747, Republic of Korea Figure S1: Surface topography and Raman analysis of WSe2 2 (a) 0.1×0.1µm AFM scan of grown WSe2 in contact mode, in ambient revealed smooth morphology of grown film. (b) Raman spectrum of CVD grown WSe2 shows sharp E1g mode at -1 -1 1 -1 ~250 cm while A1g mode at 258 cm . Difference between E1g and A1g mode was ~ 8 cm which could be the characteristic correspondence to relaxation of multiple layers. This also confirms the few-layer structure of grown films. (c-d) AFM topography was taken on the edge to estimate the thickness of the grown film which was valued as 1.5 ± 0.1 nm. Figure S2: Surface topography and Raman analysis of MoSe2 (a) AFM scan of grown MoSe2 also showed smooth morphology of grown film. (b) Raman spectra -1 of CVD grown MoSe2 showed sharp out of plane phonon (OC) mode at 240 cm while in plane vibration mode (IC) at 258 cm-1 and in-plane relative motion of transition metal and chalcogen atom (IMC) mode at 288 cm-1.2 Difference between OC and IC modes was ~18 cm-1 which could be the characteristic correspondence to relaxation due to more than single layer. (c-d) To further confirm the thickness of film, AFM was performed on the edge which was estimated as 1.6 ± 0.1 nm. Figure S3: Effect of rubbed cover glass on LC aligned MoSe2 (a-c) & WSe2 (d-f) (a) and (d) show POM images of liquid crystal aligned grains without rubbed layer. Afterwards unidirectional rubbed layer aligned on the surface and different polarized directions were observed playing with the analyzer rotation (b, e). It was observed that covered glass with unidirectional rubbing could increase the quality of the grains contrast which might be due to the unidirectional alignment of one end of the liquid crystal molecules which was further confirmed with different analyzer direction (c, f). Figure S4: Analysis of each grain with liquid crystal alignment on MoSe2 (a) POM image was marked with different domains with independent numbers. 18 domains were selected and transmittance was plotted against rotational angle (b). Fitting was done to calculate phase of each domain and to find single crystal area. It was observed the domain No; 11,14,15,16 and 17 have the same phases of ~25o similarly 2, 4, 5, 12 13 and 14 have the same phase of ~85o with some exceptions. Figure S5: Analysis of each grain with liquid crystal alignment on WSe2 (a) POM image marked with different domains with independent numbers. More than 20 domains were marked as we did previously and transmittance was plotted against rotational angle (b). Fitting was done to calculate phase of each domain and to find single crystal area. It was observed the domain No; 1,2,5,8, 13, 18 and 20 have the same phases of ~75o similarly 3, 4, 6, 7, 10, 11, 12, 14, 16, 17 and 19 have the same phase of ~15o Spacing between these two phases was observed as 60o which confirmed the 120o of symmetry in the alignment of LC molecules Figure S6: Effect of analyzer rotation (a) CVD grown MoSe2 was aligned and domains were observed under polarized microscope. (a-f) A specific domain was marked and observed under different analyzer angles. It was observed that the dark domain has opposite alignment considering the adjacent domains. (c) Once analyzer is being rotated from 0 to almost 70o, the color of dark domain becomes bright and that of adjacent domains becomes dark. This process was repeatable once rotation was from 90 to 180o. From this analysis it was confirmed that these two domains have opposite alignment. Figure S7: Transfer of CVD grown films on copper grid (a) For TEM analysis, the grown films were transferred to a copper grid (Quantifoil) having diameter of 1µm. PMMA was coated on the grown films on silicon. KOH was further used to etch oxide layer of silicon. Once etching was done, both MoSe2 & WSe2 films were transferred to the grid as shown in (b) and (c) respectively. Figure S8: HRTEM of MoSe2 (a) and WSe2 (b) HRTEM was employed to on different areas of grown films. (a) and (b) Multi-orientation of grown structure was observed which could be assigned to the variation in the CVD growth parameters. Spreading of FFT three dark spots in the inset could be assigned to Moiré pattern by superposition of stacked trilayers with some tilted angle.3,4 Figure S9: Effect of selenium annealing on liquid crystal alignment LC alignment was used to determine the quality of grown films (a-c) MoSe2 and (d-f) WSe2. After sputtering selenium annealing was done in CVD chamber for 130, 150 and 180 sec. (a, d) Hint of LC alignment was observed in 130sec of growth we may corresponds to initial nucleation. Similarly, partial alignment can be observed in the case of 150sec of growth (b, e). (c, f) It was observed that LC alignment could be used in order to optimize the growth process as 180sec of growth was effective considering the visualization of grains and boundaries in both MoSe2 and WSe2. Figure S10: TEM analysis on vertical growth of MoSe2 It was observed that thick oxide layer sputtered leads to give vertical grown MoSe2. TEM analysis shown vertical grown MoSe2 structures with FFT image showed single orientation of grown structures. Figure S11: Geometry optimization for most favorable stacking configuration of biphenyl of 5CB on graphene Considering the ideal environment having biphenyl of 5CB and symmetric graphene three different stacking configurations were obtained: AB, AA, and AA' stackings. It was observed that AB stacking was the most favorable with the highest adsorption energy of 1.127 eV/molecule. On the other hand, AA and AA' stacking configurations have adsorption energies of 1.045 eV/molecule and 1.114 eV/molecule, respectively. This result was further used to study alignment behavior of LC on TMD materials. Figure S12: Atomic structures and adsorption energies of 5CB molecule on MoSe2 In terms of adsorption of 5CB molecule on MoSe2 (WSe2) substrate, we considered 8 different alignment states: (a) AA stacking, (b, c) AA' (1) ((2)) stacking that left (right) one phenyl group of the 5CB molecule adsorbed on the MoSe2 with the AA' stacking, (d, e) AB-Mo (1) ((2)) stacking that left (right) one phenyl group of the 5CB molecule adsorbed on the MoSe2 with the AB stacking and the Mo atom is at the center of the phenyl hexagon, (f, g) AB-Se (1) ((2)) stacking that left (right) one phenyl group of the 5CB molecule adsorbed on the MoSe2 with the AB stacking and the Se atom is at the center of the phenyl hexagon, and (h) Tail on Se stacking that the tail group of the 5CB molecule aligns with the top Se atoms. Corresponding adsorption energies are 1.608, 1.702, 1.636, 1.692, 1.571, 1.660, 1.712, and 1.622 eV/molecule in order from (a) to (h), respectively. Taking account of all configurations, AB-Se (2) stacking was considered as the most stable state and was used for further analysis. Figure S13: Rotation of biphenyl group from armchair to zigzag edge of graphene Biphenyl aligned on the armchair edge direction (AED) was rotated considering a fixed rotation axis indicated by an arrow, and adsorption energy was calculated after each 3o of intervals. It was observed that adsorption energy linearly decrease moving from the AED to the zigzag edge direction (ZED) of hexagonal surface. Calculated Ead are 1.127, 1.109 and 1.091 eV/molecule for 0o, 15o and 30o respectively. This result was obtained at the fixed rotation axis, without considering the translational shift of the molecule to more stable location. Figure S14: Optical microscopic images of (a) tungsten oxide and (b) tungsten diselenide. (c- e) Alignment of LC on sputtered and annealed tungsten oxide film. (f-h) LC alignment after selenization. As shown in Figure S14 (a), tungsten oxide was sputtered on silicon at room temperature and annealed in a CVD chamber without selenization. (b) Similarly another sample was prepared with same condition and selenization was done via CVD. (c-e) To compare both samples, POM measurement with LC was done, and it was found that grains are appeared even in as-sputtered samples before selenization. (f-h) After selenization those grains became mature and clear alignment was observed. This confirms the argument that premature crystallinity of metal oxide film could leads to anisotropic behavior of TMD film after selenium vaporization. Figure S15: TEM imaging to observe large scale grains and boundaries of WSe2. (a) Bright field image, (b) diffraction pattern, and (c-e) dark field images at different aperture locations. (f) From different contrasts, grains with different orientations were estimated. In order to observe grains and boundaries of our grown films, we applied a diffraction-filter in dark field TEM imaging. In Figure S15 (a), bright field image of transferred film showed almost smooth surface with no hint of grain boundaries. (b) Diffraction pattern showed multi-orientation of grains. Placing an aperture in the diffraction plane revealed corresponding dark field images.
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