Study of Grains and Boundaries of Molybdenum Diselenide and

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Study of grains and boundaries of molybdenum diselenide and tungsten diselenide using liquid crystal Muhammad Arslan Shehzad, Sajjad Hussain, Junsu Lee, Jongwan Jung, Naesung Lee, Gunn Kim, and Yongho Seo Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b04491 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017

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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* 1

2

Graphene Research Institute, Sejong University, Seoul 143-747, Republic of Korea Faculty of Nanotechnology & Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea 3 Department of Physics & Astronomy, Sejong University, Seoul 143-747, Republic of Korea

Abstract: Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs) i.e. MoSe2 and WSe2. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe2 and WSe2 on silicon, using optical birefringence of 2D layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From numerical calculation, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with 3-fold symmetric alignment could be utilized for display applications.

Keywords: MoSe2, WSe2, 2D material, CVD growth, Liquid crystal alignment, grain, grain boundaries *Corresponding author: [email protected] and [email protected]

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1. Introduction: Liquid crystal (LC) molecules have received great attention after the discovery of tunable alignment behavior on polymer surface by Mauguin in 1911. Major display related technologies are established on the re-orientation mechanism of liquid crystal molecules in response to the electric field. This change in the order of arrangement of LC molecules can cause changes in the optical properties.1 Most of the liquid crystal-based applications use nematic phase because of their flexibility in arrangement. Surface anchoring property of nematic phase is an important parameter in order to alter the controllability of devices.2, 3 Alignment of LC can be achieved via different routes i.e. microgrooves at the surface of the alignment film,4 crystalline polymer surface where the LC is oriented preferred crystalline structure,5 and polycrystalline areas with a preferred sequence of orientation.6 This prerequisite of this phenomenon of alignment via surface planes of numerous substrates is a configuration which must have preferential orientation in order to achieve tailored alignment patterns. The principal advantage for nematic LC-based displays was low power consumption. From the past few decades, a lot of work was done to develop multistable symmetric displays. This symmetry was generated by rubbing in different directions to adjust the photonic band gap.7 Pattering in a square form was used to obtain bistable four-fold symmetric patterns,8,

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similar triangles and hexagons were made in order to get tristable symmetry.10 Previously we suggested an idea to use 2D hexagonal planes i.e. graphene or hexagonal boron nitride in order to obtain hexastable displays apart from rubbing schemes.11 Previously, scanning tunneling microscopy was used to observe the alignment behavior of LC molecules on the surface of transition metal dichalcogenides (TMDs).12, 13 More recently, direct


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observation of grains and boundaries of MoS2 were obtained using LC.14 Although we have the basic knowledge of alignment, it is of significance to comprehend the mechanism by which such liquid crystal alignment methods work. It is well known that LC director exhibits a correlation with the graphene and boron nitride (BN) edges.15,16 LC technique gives us the versatility in order to determine the grains of TMDs without any damage to the grown materials.17, 18 In this study, we employed a type of LC molecule (5CB) to directly visualize grains and boundaries of CVD grown MoSe2 and WSe2. Due to van der Waals growth and assembly of TMD materials with band gap, they attracted significant attention of researchers recently. A lot of efforts have been devoted to grow MoSe2 and WSe2, with versatility in shape and thickness.19-24 In-depth alignment behavior was studied, and a numerical simulation observed that the LC molecules preferred to align with 3-fold rotational symmetry, instead of 6-fold symmetry observed previously in the case of graphene.11 It was observed that a preferential alignment direction was determined by an anisotropic grain shape. This work will not only help the efforts focused on visualizing the grain boundaries of TMD materials but also help to understand how grains can be controlled for large scale synthesis of these materials.

2. Result and Discussion: 2.1. Growth of MoSe2 and WSe2: Growth of molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) was performed using the same process as we discussed before for molybdenum disulphide (MoS2), and it was observed that grain boundaries could influence electrical properties.25, 26 Here, the synthesis was done via sputtering and CVD-based selenization mechanism (See Methods). First, Mo (or W) seed layer was deposited on silicon oxide using sputtering (Figure 1a, b). Selenization was done


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to diffuse selenium in an oxide film during the CVD process (Figure 1c). Optical images depict a different color after each step to the interference. Green color was observed after sputtering which forms oxide layer of transition metal. Later in the selenization process, selenium was diffused into the oxide layer, and yellowish color was obtained.27 A detailed synthesis process is discussed in the method section and supporting information. Atomic force microscopy (AFM) topography was applied on the edge of the grown WSe2 to estimate the thickness, which was estimated as 1.5 ± 0.1 nm (Figure S1a, c). Raman spectra of CVD grown WSe2 showed good crystallinity of few-layer structure (Figure S1b). Similarly, AFM scan of grown MoSe2 also showed smooth morphology of the film (Figure S2a). Raman spectra revealed out of plane and in-plane characteristic modes with difference of ~18 cm-1 which could be a characteristic correspondence to multilayer of grown film. From the AFM image on the edge, the thickness of 1.6 ± 0.1nm was estimated (Figure S2c, d). 2.2. Liquid Crystal Alignment Process: To directly observe the grains and boundaries of grown films, LC was coated on the 2D surface. It was heated above isotropic phase (60oC) and subsequently cooled down to ambient temperature to be nematic phase. Interestingly, we observed WSe2 grains and boundaries which were not visible without liquid crystal, as shown in Figure 1(c). After cooling below isotropic temperature, LC molecules were aligned along the different direction of grains which was observed via a polarized microscope, as shown in Figure 1(d). Schematics (Figure 1(e)) explains that the different grains indicate different colors. LC was coated, and unidirectional rubbed layer was placed on it with spacing of 2 µm using spacers. A high quality of grains contrast was obtained due to the unidirectional alignment of one end of LC compared to that without rubbed layer (Figure S3).


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Figure 1: Growth of MoSe2 & WSe2 and alignment of LC: Optical images depict different colors after each process. (a-b) Optical microscope images were taken before and after sputtering on bare silicon in order to deposit oxide layer of molybdenum. (c) After oxide layer deposition, selenization was done to diffuse selenium in oxide film. (d) Liquid crystal was then spin coated on the WSe2 sample. (e) Schematics show growth of layered materials which include different grains with grain boundaries. LC molecules align along the crystal orientations of grains, and can be observed via polarized light.

2.3. LC alignment behavior on TMDs: As discussed in the introduction, previously it was considered that LC alignment is purely dependent on the patterning of microgrooves of the underlying surface. Later it was observed that LC could align itself depending on the compatibility of alignment layer.5 Optical microscope image of LC on MoSe2 with no polarized light shows a smooth film, as shown in Figure 2(a). A polarized optical microscope (POM) image of the same area (Figure 2(b)) with cross-polarizer clearly depicts grains and boundaries, implying that alignment of LC molecules was determined by the crystalline orientation of the grain. Different grains were marked, and the sample was rotated in clockwise direction as shown in Figure 2(c). Grain 2 pointed by arrows exhibited bright color at 17o and was completely turned dark at 46o. A whole rotation-cycle was completed, and transmittance of each grain was recorded as a function of the angle. Transmittance versus rotation-angle was plotted and fitted to a sinusoidal function with a 90o period to obtain the phase difference for the grain, as shown in Figure 2(d). Similarly, more grains were selected and phases were calculated (See Figure S4). A histogram indicating phase versus number of grains shown in Fig. 2(e) exhibits three peaks with 15o spacing in the range from 0 to 90o, which was confirmed in the other 2D materials. Previously it was observed that LC tend to align with six preferential angles with 15o spacing in the range from 0 to 90o on graphene and hexagonal boron nitride.11 The hexagonal lattice crystals have the 60o symmetry orientation preferred by


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LC molecules, and preferential angles with 15o spacing was explained by the tilted tail of the LC molecule.28 However, the significant difference for MoSe2 is that all peaks in the histogram are concentrated within 30o (from -5o to 25o), and there is forbidden band with 60o interval.

Figure 2: Liquid crystal alignment on molybdenum diselenide (MoSe2): (a) Optical microscope image of MoSe2 with no polarized light show smooth film with liquid crystal on it. (b) POM image with cross-polarizer clearly depicts domains and boundaries. (c) The sample was rotated in clockwise, and different domains were marked on POM images. (d) Transmittance vs. rotation-angle was plotted and was fitted to calculate the phase difference of a grain. (e) Phases for 18 grains were calculated, and its histogram was plotted indicating three preferential orientations.

In order to confirm this result, the same experiments were performed on a likewise grown WSe2 film. Basically the same results were obtained, as shown in Figs. 3. More than 20 grains were selected, and the phase was calculated (Figure S5). It was observed that in both the cases LCs


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Figure 3: Liquid crystal alignment on tungsten diselenide (WSe2): WSe2 was CVD grown (as explained) and liquid crystal was then spin coated on the same sample. (a) Optical microscope image of WSe2 with no polarized light show uniform film. (b) POM image illustrates grains and boundaries. (c) The sample was rotated in the clockwise direction, and the contrast of each domain were traced; grain 1 with arrows shows bright color at 0o and completely turns dark at 40o. A rotation-cycle was completed and transmittance of each grain was calculated. (d) Transmittance vs. rotational angle was plotted and fitted to a sinusoidal function. More grains were selected and phase was calculated. (e) Phase vs. number of grains also showed three isolated peaks on WSe2 surface. the 60o forbidden region. If the grains on the sample have random crystal orientation, the phases also should be random, and the data points in the histogram should be scattered widely. Judging from the existence of the particular peaks, the crystal orientation of the grains is correlated together. Therefore, we claim that the grains have a parallel crystal orientation, at least in the local area. As the silicon oxide substrate, is not a single crystal, the parallel orientation can be


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caused by the unidirectional continuous flow of the source gases in the CVD process. The existence of a forbidden region of the histogram data can be attributed to the anisotropic shape of the grains. As the grains have an elongated shape, collective ordering of LC molecules has preferential orientations to be more parallel to the elongated axis.

Figure 4: Schematics of alignment and HRTEM analysis of MoSe2 & WSe2 films: (a) LC molecules may align on three zigzag and three armchair edges which give freedom to align along 6 different angles with 6-fold rotational symmetry. Three-fold symmetry can be found if LC molecules tend to align on one preferential edge state i.e. zigzag or armchair. TEM analysis of grown films was performed on MoSe2 (b) and WSe2 (c), respectively. The images show high crystallinity of the grown films with continuous lattice fringes with a lattice spacing of 2.80Å.


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For a hexagonal symmetric substrate, LC molecules can align on three zigzag and three armchair directions which collectively give freedom to align along 30o of symmetry, as shown in Figure 4(a). Three-fold symmetry can be applicable if LC molecules tend to align on one preferential direction i.e. zigzag or armchair. Previously we have shown that the alignment of LC of graphene and hexagonal boron nitride was based on the same anchoring force for zigzag and armchair directions, and bending of the molecule.28 As compared with graphene, the lower alignment symmetry for these TMD materials can be attributed to the larger thickness of a layer with 3 atomic z-positions in a unit cell. It can be observed that boundary lines are evident between different grains of MoSe2 and WSe2, as shown in Figure 1(d) and 2(b). The orientation of LC at the boundary lines is not random, but has a local optical axis parallel to the boundary.29 The interaction energy between LC molecules can ଵ

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be defined as ܷ௫ = ଶ ‫[ܭ‬డ௥ ೔ ]ଶ , where K is an elastic constant and డ௥ ೔ is the spatial gradient of the ೕ

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director n(r).29 To minimize Ux, LC molecules prefer to choose the direction most parallel to the boundary among possible orientations.29 High resolution transmission electron microscopy (HRTEM) was employed to identify atomic structures of the grown films. For TEM analysis, grown films were transferred to a copper grid with a carbon mesh (Quantifoil, Micro Tools, Jena, Germany) having a diameter of 1 µm, as shown in Figure S7. Poly(methyl methacrylate) (PMMA) was coated on the films grown on silicon. Potassium hydroxide (KOH) was used to etch the oxide layer on silicon. Once etching was done, both MoSe2 and WSe2 films were transferred to the grid. TEM images of films of MoSe2 (b) and WSe2 (c) are shown in Figure 4. High resolution images show high crystalinity of the films with continuous lattice fringes, and the lattice spacing calculated from the inverse fast Fourier transform (FFT) images was 2.8 Å, which is similar to the values reported by other


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researchers.30 The regular honeycomb patterns of atoms are found in xy plane, implying that it was grown in the horizontal direction. The FFT image, shown in the insets of Figure 4b-c corresponding to the diffraction spots of MoSe2 and WSe2 layers, exhibits a hexagonal pattern, confirming a single orientated crystal structure of the sample. Occasionally, different crystal orientations of grains were found in TEM (Figure S8), but most areas showed parallel orientation of grains. On the other hand, vertical growth can also be found depending on the growth conditions.31 It was observed that a thick oxide layer sputtered leads to vertical growth of MoSe2 (Figure S10). 2.4. Density Functional Theory Calculations:


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Figure 5: DFT calculation results for the optimized geometries of 5CB molecule on (a) graphene, (b) MoSe2, and (c) WSe2. 5CB molecule in upper (lower) panels are in armchair (zigzag) direction, where d is the distance between 5CB molecule and the substrate, and Ead is the adsorption energy of liquid crystal molecules.

To deeply understand alignment behavior of LC molecules on different hexagonal surfaces, density functional theory (DFT) was employed to determine the energetically stable adsorption configurations of LC molecules. Adsorption energy, Ead, is defined as Ead = E5CB + Esub - E5CB32 sub,

where E5CB and Esub are the total energies for the isolated 5CB molecule and the isolated 2D

substrate, respectively, while E5CB-sub is the total energy for the 2D substrate with the adsorbate 5CB molecule.33 Before considering for 5CB adsorption on TMDs, we considered different types of stacking for 5CB adsorption on graphene i.e. AB, AA, and AA' as discussed in Figure S11. It was observed that the most preferable stacking configuration was AB (AB-like) stacking throughout the substrates. Due to different atomic configuration of dichalcogenides compared to that of 2D hexagonal planes, DFT calculation results was extended to two different coverages in AB alignment for the case of MoSe2 and WSe2: for “AB-Mo stacking”, a molybdenum (or tungsten) atom is located at the center of a phenyl ring of 5CB. For “AB-Se stacking”, a selenium atom is located at the center of the ring. It was observed that selenium centered alignment was more stable, compared to the molybdenum centered one (Figure S12). To further study the alignment of LC molecules in different directions of 2D materials i.e. zigzag (ZZ) and armchair (AC) direction, AB stacking with selenium centered alignment was considered. Assuming that LC molecule was aligned on graphene, MoSe2 and WSe2 in ZZ and AC directions, the adsorption energy was calculated at different angles. 5CB molecules in upper panels are in armchair direction and lower three are in zigzag direction with 30 degrees of


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rotation, respectively in Figure 5, where d is the distance between 5CB molecule and the substrate layer. From the comparison between graphene and TMDs, it was observed that adsorption energy for the case of graphene on the zigzag and armchair direction was almost the same (1.81 and 1.79 eV/molecule, respectively) confirming that the probability of alignment of LC molecules on graphene is equal for the zigzag and armchair directions. On the other hand, this gap was quite enough in the case of MoSe2 and WSe2 i.e. ~0.12 eV/molecule. This confirms that the armchair direction has higher probability of alignment compared with the zigzag direction of MoSe2 and WSe2. This low degeneracy of rotational symmetry can be an advantage for discrete color implementation for the display application.

3. Conclusion: In this study, liquid crystal (5CB) was aligned to directly visualize grains and grain boundaries of CVD grown MoSe2 and WSe2. In-depth orientation behavior was discussed, and interestingly, we found that the grains have parallel crystalline orientations in local area. From the DFT calculation, a higher adsorption energy of armchair direction causes LC molecules to align on it, compared to that of zigzag direction which leads to 3-fold symmetric alignment. This work will not only help the efforts focused on understanding the relationship between growth condition and grain structure of TMD materials, but also help to be utilized for display application.


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Methods: Synthesis of MoSe2 /WSe2 films using two-step technique: 2D molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) were synthesized using sputtering and selenization processes. This two-step process includes oxide layer deposition using sputtering chamber under high vacuum of 1 × 10-6 Torr. Mo/Wo were then deposited on SiO2/Si substrates at room temperature using molybdenum oxide or tungsten oxide target (99.99 % pure) in argon gas. A 25 W power was supplied to the sputter target using an RF power generator. In order to increase the crystallinity, samples were then annealed before exposing them to selenium environment. Selenium powder (99.99% purity) of 0.3 g was placed upstream in the CVD chamber and evaporated at 120 °C with Ar (100 sccm) as the carrier gas, and growth was done at 650 °C for 10 s, where pressure of CVD chamber was kept at 2 x10-2 mTorr.

Fabrication of LC-TMDs aligned Cell: In order to check the alignment of liquid crystal on 2D MoSe2 and WSe2, commercially available nematic liquid crystal (5CB-Sigma Aldrich) was used. 0.5 µl LC was spin coated (3000 rpm) for 40 s on sputtered and CVD grown films to obtain ~2µm thick layer. It was heated to be isotropic phase, up to 60o (clearing point) for 15 minutes followed by cooling in ambient. Liquid crystal was directly aligned along the surface of grown hexagonal films, and grains and boundaries could be observed under a microscope with a cross polarizer. On the other hand, polyvinyl alcohol (PVA) was spin coated on glass slide at 3000 rpm in order to obtain ~2 µm thick layer. This PVA film was then unidirectionally rubbed and was placed over the LC coated sample with ~5 µm cell gap. Cell gap was controlled using thin tape or micro beads between two sides.


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TEM Analysis: In order to do TEM analysis, grown films were transferred to a TEM grid (Quantifoil) with a hole 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 and PMMA was removed using acetone. (Figure S7)

Characterization: LC alignment, grain boundaries, and grain orientation were observed using a polarized optical microscope (Olympus POM - BX-51). Axis of polarizer was adjusted as indicated by arrows in the captured images. Raman spectroscopy with excitation wavelength of 514 nm was measured by using a commercial equipment (micro-Raman, Renishaw), and power was kept below 1.0 mW to avoid laser-induced heating. The laser spot size of Raman spectroscopy was 1 ± 0.2 µm. AFM imaging was done in tapping mode, in ambient condition (n-Tracer, NanoFocus).

Computational simulation We utilized a plane-wave basis set and the projector augmented wave (PAW) method34, implemented using the Vienna ab initio simulation package (VASP)35. The kinetic cutoff energy was 400 eV, and GGA/PBE36 and optB86b-vdW method37, 38 were used for exchange-correlation energy functional and van der Waals correction, respectively.

ACKNOWLEDGMENTS This research was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government’s Ministry of Trade, Industry & Energy (No. 20154030200630). This work was also supported by the industrial research innovation program (10051701), funded by the Ministry of


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Trade, Industry and Energy (MOTIE). GK acknowledges a financial support of the Priority Research Center Program (Grant No. 2010-0020207) of the Korean Government.

Supporting Information Available To support our claims in main text, additional experimental details including POM, TEM analysis, density function theory (DFT) comparison with different alignment angles are included in supporting information. This material can be accessible at http://pubs.acs.org. Note: The authors declare no competing financial interest. REFERENCES: 1. Yang, D.-K.; Wu, S.-T., Fundamentals of liquid crystal devices. 2nd ed.; John Wiley & Sons Inc.: Chichester, West Sussex, United Kingdom, 2014. 2. Collings, P. J.; Patel, J. S., Handbook of liquid crystal research. Oxford University Press: New York ; Oxford, 1997; p xv, 600 p. 3. Stohr, J.; Samant, M. G.; Luning, J.; Callegari, A. C.; Chaudhari, P.; Doyle, J. P.; Lacey, J. A.; Lien, S. A.; Purushothaman, S.; Speidell, J. L. Science 2001, 292, (5525), 2299-2302. 4. Berreman, D. W. Phys Rev Lett 1972, 28, (26), 1683-1686. 5. Geary, J. M.; Goodby, J. W.; Kmetz, A. R.; Patel, J. S. J Appl Phys 1987, 62, (10), 4100-4108. 6. Toney, M. F.; Russell, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J. M.; Kumar, S. K. Nature 1995, 374, (6524), 709-711. 7. Choi, S. S.; Morris, S. M.; Coles, H. J.; Huck, W. T. S. Appl Phys Lett 2007, 91, (23), 231110 1-3. 8. Kim, J. H.; Yoneya, M.; Yamamoto, J.; Yokoyama, H. Applied Physics Letters 2001, 78, (20), 30553057. 9. Niitsuma, J.; Yoneya, M.; Yokoyama, H. J Appl Phys 2012, 111, (10), 103507 1-6. 10. Kim, J. H.; Yoneya, M.; Yokoyama, H. Nature 2002, 420, (6912), 159-162. 11. Shehzad, M. A.; Tien, D. H.; Iqbal, M. W.; Eom, J.; Park, J. H.; Hwang, C.; Seo, Y. Sci Rep-Uk 2015, 5, 13331 1-8. 12. Hara, M.; Iwakabe, Y.; Tochigi, K.; Sasabe, H.; Garito, A. F.; Yamada, A. Nature 1990, 344, (6263), 228-230. 13. Iwakabe, Y.; Hara, M.; Kondo, K.; Tochigi, K.; Mukoh, A.; Yamada, A.; Garito, A. F.; Sasabe, H. Jpn J Appl Phys 1 1991, 30, (10), 2542-2546. 14. Kim, D. W.; Ok, J. M.; Jung, W. B.; Kim, J. S.; Kim, S. J.; Choi, H. O.; Kim, Y. H.; Jung, H. T. Nano Lett 2015, 15, (1), 229-234. 15. Yu, J. S.; Ha, D. H.; Kim, J. H. Nanotechnology 2012, 23, (39), 395704 1-5. 16. Park, J. H.; Park, J. C.; Yun, S. J.; Kim, H.; Luong, D. H.; Kim, S. M.; Choi, S. H.; Yang, W.; Kong, J.; Kim, K. K.; Lee, Y. H. Acs Nano 2014, 8, (8), 8520-8528. 17. Kim, D. W.; Kim, Y. H.; Jeong, H. S.; Jung, H. T. Nat Nanotechnol 2012, 7, (1), 29-34. 18. Son, J. H.; Baeck, S. J.; Park, M. H.; Lee, J. B.; Yang, C. W.; Song, J. K.; Zin, W. C.; Ahn, J. H. Nat Commun 2014, 5, 3484 1-7.


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19. Shaw, J. C.; Zhou, H. L.; Chen, Y.; Weiss, N. O.; Liu, Y.; Huang, Y.; Duan, X. F. Nano Res 2014, 7, (4), 511-517. 20. Wang, X. L.; Gong, Y. J.; Shi, G.; Chow, W. L.; Keyshar, K.; Ye, G. L.; Vajtai, R.; Lou, J.; Liu, Z.; Ringe, E.; Tay, B. K.; Ajayan, P. M. Acs Nano 2014, 8, (5), 5125-5131. 21. Chang, Y. H.; Zhang, W.; Zhu, Y.; Han, Y.; Pu, J.; Chang, J. K.; Hsu, W. T.; Huang, J. K.; Hsu, C. L.; Chiu, M. H.; Takenobu, T.; Li, H.; Wu, C. I.; Chang, W. H.; Wee, A. T. S.; Li, L. J. Acs Nano 2014, 8, (8), 8582-8590. 22. Liu, B. L.; Fathi, M.; Chen, L.; Abbas, A.; Ma, Y. Q.; Zhou, C. W. Acs Nano 2015, 9, (6), 6119-6127. 23. Gong, Y. J.; Lei, S. D.; Ye, G. L.; Li, B.; He, Y. M.; Keyshar, K.; Zhang, X.; Wang, Q. Z.; Lou, J.; Liu, Z.; Vajtai, R.; Zhou, W.; Ajayan, P. M. Nano Lett 2015, 15, (9), 6135-6141. 24. Eichfeld, S. M.; Hossain, L.; Lin, Y. C.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kim, M. J.; Mayer, T. S.; Redwing, J. M.; Robinson, J. A. Acs Nano 2015, 9, (2), 2080-2087. 25. Shehzad, M. A.; Hussain, S.; Khan, M. F.; Eom, J.; Jung, J.; Seo, Y. Nano Res 2015, 9, (2), 380-391. 26. Hussain, S.; Shehzad, M. A.; Vikraman, D.; Khan, M. F.; Singh, J.; Choi, D.-C.; Seo, Y.; Eom, J.; Lee, W.-G.; Jung, J. Nanoscale 2016, 8, (7), 4340-4347. 27. Jeon, J.; Jang, S. K.; Jeon, S. M.; Yoo, G.; Jang, Y. H.; Park, J. H.; Lee, S. Nanoscale 2015, 7, (5), 1688-1695. 28. Arslan Shehzad, M.; Hoang Tien, D.; Waqas Iqbal, M.; Eom, J.; Park, J. H.; Hwang, C.; Seo, Y. Sci Rep 2015, 5, 13331 1-8. 29. Lee, B. W.; Clark, N. A. Science 2001, 291, (5513), 2576-2580. 30. Duan, X. D.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Li, H. L.; Wu, X. P.; Tang, Y.; Zhang, Q. L.; Pan, A. L.; Jiang, J. H.; Yu, R. Q.; Huang, Y.; Duan, X. F. Nat Nanotechnol 2014, 9, (12), 1024-1030. 31. Kong, D. S.; Wang, H. T.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Nano Lett 2013, 13, (3), 1341-1347. 32. Lee, J.; Min, K. A.; Hong, S.; Kim, G. Chem Phys Lett 2015, 618, 57-62. 33. Barbero, G.; Evangelista, L. R. Liquid Crystals Reviews 2014, 2, (1), 72-72. 34. Kresse, G.; Joubert, D. Phys Rev B 1999, 59, (3), 1758-1775. 35. Kresse, G.; Furthmuller, J. Phys Rev B 1996, 54, (16), 11169-11186. 36. Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys Rev Lett 1996, 77, (18), 3865-3868. 37. Klimes, J.; Bowler, D. R.; Michaelides, A. J Phys-Condens Mat 2010, 22, (7), 022201 38. Klimes, J.; Bowler, D. R.; Michaelides, A. Phys Rev B 2011, 83, (19), 195131


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Study of Grains and Boundaries of Molybdenum Diselenide and

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