Electronic structure of monolayer 1T′−MoTe2 grown by molecular beam epitaxy

Shujie Tang, Chaofan Zhang, Chunjing Jia, Hyejin Ryu, Choongyu Hwang, Makoto Hashimoto, Donghui Lu,Zhi Liu, Thomas P. Devereaux, Zhi-Xun Shen, and Sung-Kwan Mo

Citation: APL Materials 6, 026601 (2018);View online: https://doi.org/10.1063/1.5004700View Table of Contents: http://aip.scitation.org/toc/apm/6/2Published by the American Institute of Physics

APL MATERIALS 6, 026601 (2018)

Electronic structure of monolayer 1T′-MoTe2 grownby molecular beam epitaxy

Shujie Tang,1,2,3,4,5 Chaofan Zhang,1,2,6,7 Chunjing Jia,1,2 Hyejin Ryu,3,8

Choongyu Hwang,9 Makoto Hashimoto,10 Donghui Lu,10 Zhi Liu,4,5

Thomas P. Devereaux,1,2 Zhi-Xun Shen,1,2,a and Sung-Kwan Mo3,a1Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory,2575 Sand Hill Road, Menlo Park, California 94025, USA2Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics,Stanford University, Stanford, California 94305, USA3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley,California 94720, USA4State Key Laboratory of Functional Materials for Informatics, Shanghai Instituteof Microsystem and Information Technology, Chinese Academy of Sciences,Shanghai 200050, China5School of Physical Science and Technology, Shanghai Tech University,Shanghai 200031, China6College of Optoelectronic Science and Engineering, National University of DefenseTechnology, Changsha 410073, China7Interdisciplinary Center of Quantum Information, National University of Defense Technology,Changsha 410073, China8Max Planck-POSTECH/Hsinchu Center for Complex Phase Materials, Max PlankPOSTECH/Korea Research Initiative (MPK), Gyeongbuk 37673, South Korea9Department of Physics, Pusan National University, Busan 46241, South Korea10Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory,2575 Sand Hill Road, Menlo Park, California 94025, USA

(Received 14 September 2017; accepted 23 October 2017; published online 14 November 2017)

Monolayer transition metal dichalcogenides (TMDCs) in the 1T′ structural phase havedrawn a great deal of attention due to the prediction of quantum spin Hall insulatorstates. The band inversion and the concomitant changes in the band topology inducedby the structural distortion from 1T to 1T′ phases are well established. However,the bandgap opening due to the strong spin-orbit coupling (SOC) is only verified for1T′-WTe2 recently and still debated for other TMDCs. Here we report a successfulgrowth of high-quality monolayer 1T′-MoTe2 on a bilayer graphene substrate throughmolecular beam epitaxy. Using in situ angle-resolved photoemission spectroscopy(ARPES), we have investigated the low-energy electronic structure and Fermi surfacetopology. The SOC-induced breaking of the band degeneracy points between thevalence and conduction bands is clearly observed by ARPES. However, the strength ofSOC is found to be insufficient to open a bandgap, which makes monolayer 1T′-MoTe2

on bilayer graphene a semimetal. © 2017 Author(s). All article content, except whereotherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/1.5004700

Two-dimensional (2D) transition metal dichalcogenide (TMDC) is a versatile material platform,in which electrical, optical, and topological properties can be controlled through thickness, strain,field, and other perturbations.1–4 The most well-studied example is the 2H-MX2 (M = Mo, W;X = S, Se) semiconductor that makes a transition from indirect bandgap to direct bandgap onlyin the monolayer,4,5 with well-pronounced spin-splitting6,7 and valley degrees of freedom.8,9 Otherstructural phases stemming from different stacking orders between transitional metal and chalcogenlayers deliver distinct physical properties even with the same constituent atoms.

aElectronic addresses: zxshen@stanford.edu and skmo@lbl.gov

2166-532X/2018/6(2)/026601/7 6, 026601-1 © Author(s) 2017

026601-2 Tang et al. APL Mater. 6, 026601 (2018)

1T′-MX2 (M = Mo, W; X = S, Se, Te) has gained particular interest regarding its topologi-cal properties. While its three-dimensional bulk form has been explored in terms of type-II Weylsemimetal,10–13 the monolayer is predicted to host a quantum spin Hall (QSH) insulator.3 A QSHinsulator, or a two-dimensional (2D) topological insulator, is a topologically nontrivial quantumstate, which is hallmarked by the helical edge state protected by time reversal symmetry and the bulk(as opposed to the edge) bandgap opening due to strong spin-orbit coupling (SOC).14–16 The resultingtransport properties exhibit quantized Hall conductance even in the absence of magnetic field, andspin-polarized edge current is expected to be useful for the spintronic applications. The QSH phasein 1T′ TMDC has been recently realized only in 1T′-WTe2.17,18 It is of great interest whether it ispossible to realize a QSH state in other 1T′ TMDCs and whether fundamental parameters such asbulk bandgap would be different from those of 1T′-WTe2.

Many efforts have been devoted to achieve a QSH state in monolayer 1T′-MoTe2.3,19–21 Fromthe electronic structure point of view, two critical elements of achieving a QSH state in group VITMDCs are the band inversion caused by the structural distortion from the 1T phase to 1T′ phaseand the 2D bulk bandgap induced by strong SOC.18 The former has been well established by theprevious theoretical calculations.3,19,20 However, as shown in Table I, the calculated bandgap sizevaries significantly depending on calculation methods, even to a degree that it is not clear whether1T′-MoTe2 is a semiconductor or a semimetal. Optical absorption and transport measurements reporta 60 meV bulk gap in few-layer mechanically exfoliated (ME) 1T′-MoTe2,20 while CVD-grownmonolayer 1T′-MoTe2 shows a metallic transport behavior.21 Whether the SOC is strong enoughto open a bulk gap is still not very clear. A characterization tool that can directly visualize theband structure and the size of the bandgap, such as angle-resolved photoemission spectroscopy(ARPES),22,23 would provide a clearer insight on this aspect.

In this letter, we report a successful growth of monolayer 1T′-MoTe2 on a bilayer graphene(BLG) substrate using molecular beam epitaxy (MBE). The electronic structure of epitaxial1T′-MoTe2 has been investigated by in situ ARPES. We found that the SOC indeed breaks theband degeneracy points and separates the valence and conduction bands. However, the strength ofSOC is not enough to open a 2D bulk bandgap, which makes monolayer 1T′-MoTe2 on BLG asemimetal.

Both thin film growth and ARPES characterization of 1T′-MoTe2 were performed at the Beamline10.0.1, Advanced Light Source, Lawrence Berkeley National Laboratory. 1T′-MoTe2 was grownby MBE with a base pressure of ∼2 × 10�10 Torr. Mo (99.99% purity) and Te (99.999% purity)were evaporated by an e-beam evaporator and a Knudsen cell, respectively, with the flux ratio of∼1:20. The substrate was BLG prepared by vacuum graphitization of 6H–SiC(0001).24 The substratetemperature was set at 280 ◦C for growth. The growth rate was 15 min per layer. Following the growth,a 20-min post-annealing process at the same temperature was performed. Additional ARPES mea-surements were performed at the Beamline 5-4, Stanford Synchrotron Radiation Lightsource (SSRL),SLAC National Accelerator Laboratory. The band structure calculation is performed by using the gen-eralized gradient approximation method with Perdew-Burke-Ernzerhof (PBE) exchange-correlationfunctional as implemented in the VASP.25 The lattice constants used are a = 6.38 Å and b = 3.45 Åfor 1T′-MoTe2.

TABLE I. Gap sizes derived from different calculation methods and experimental measurements.

Calculation or Sample fabricationexperiment method method and thickness Gap size (eV) Bulk conductivity References

PBE . . . −0.262 Metallic 3

G0W0 . . . −0.300 Metallic 3

PBE + HSE06 . . . 0.03 Semiconductor 20

Transport CVD monolayer . . . Metallic 21

Optical absorption ME few layer 0.06 Semiconductor 20

ARPES MBE monolayer . . . Metallic This work

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FIG. 1. MBE growth of 1T′-MoTe2. (a) Crystal structure of 1T′-MoTe2, side view (upper panel) and top view (lower panel).(b) Brillouin zone of 1T′-MoTe2. (c) RHEED patterns of graphene substrate (top) and after monolayer 1T′-MoTe2 film growth(bottom). (d) Core level spectrum of the 1T′-MoTe2 thin film; the inset is the zoom-in of the boxed part.

The crystal structure of 1T′-MoTe2 is illustrated in Fig. 1(a). It can be seen as distorted 1T-MoTe2,for which Mo atoms are in octahedral coordination with Te atoms. With the distortion, Mo atomsshift off the center of Te octahedra along X direction forming zigzag metal chains along the Y direc-tion. The Te atoms also shift accordingly, making two types of Te with inequivalent coordinations.Figure 1(b) shows the Brillion zone of 1T′-MoTe2. The reflection high-energy electron diffraction(RHEED) patterns before and after the thin film growth are shown in Fig. 1(c). The sharp diffractionstripes after growth indicate the high crystalline quality of the 1T′-MoTe2 thin film. The thermal sta-bility of the sample was checked by annealing the sample at 350 ◦C under Te atmosphere. No obviouschange in the RHEED pattern was observed, indicating that growth temperature 280 ◦C is far belowthe decomposing temperature, thus retaining high crystallinity of the sample.26 Using the lattice con-stant of BLG ∼2.46 Å as a reference, one can get the lattice parameter of 1T′-MoTe2 on BLG ∼6.3 Å,which is consistent with reported values.3 Figure 1(d) is the angle-integrated core level spectrum,clearly showing the characteristic Mo 4p peak and Te 4d peaks. Figure 2 is the low-energy electron

FIG. 2. LEED pattern of the 1T′-MoTe2 film. (a) LEED pattern (94 eV) of the film. Blue solid circles are the first-orderreflection from graphene. The blue dotted circles indicate the reflections from 1T′-MoTe2. (b) Superposition of the Brillionzones of 1T′-MoTe2 (blue, green, and red rectangles) and BLG (red hexagon).

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diffraction (LEED) pattern taken with an electron kinetic energy of 94 eV to reveal the surface sym-metry and lattice structure. The blue solid circles indicate the first-order diffraction from a BLGsubstrate. The six spots surrounding each of them come from 6

√3R30◦ superlattice diffractions

between graphene and SiC.24 Due to the symmetry mismatch between three-fold rotational sym-metric BLG and two-fold symmetric 1T′-MoTe2, there are three energetically equivalent rotationalalignments between BLG and 1T′-MoTe2.27 This results in three sets of reciprocal lattices superposedin both LEED and ARPES data. The LEED pattern from different domains of 1T′-MoTe2 is indicatedby the dotted lines with different colors in Fig. 2(a). Using the in-plane lattice constant of graphenea = 2.46 Å, one can get the lattice constants of 1T′-MoTe2, a = 6.3 Å and b = 3.4 Å, consistent withthe previously reported values3 as well as with that from our RHEED measurement. Figure 2(b)shows the superposition of the Brillion zones from three equivalent 1T′-MoTe2 and graphenelattices.

Figure 3 show the overall electronic structure and Fermi surface (FS) topology from ARPESmeasurements on monolayer 1T′-MoTe2/BLG. Figure 3(a) is the FS intensity map, which shows six-fold symmetry due to the superposition of three 1T′-MoTe2 domains with 120◦ rotation with respectto each other. One may extract a single-domain FS from the ARPES data as shown in Fig. 3(b). Thereare one hole pocket located at the zone center, two electron pockets along the ΓY direction, and twomore electron pockets located at the zone boundaries. The topology of the FS plays a key role inunderstanding the transport properties of the semimetal in which the hole and electron carriers coexist.Perfect electron-hole compensation is proposed to be responsible for the non-saturating magneto-resistance in three-dimensional bulk 1T′-WTe2.28 However, too many bands crossing the Fermi energy(EF) in a confined momentum space have challenged ARPES measurements and interpretations.29,30

The simplified FS in monolayer 1T′-MoTe2 makes the evaluation easier and reliable. We have obtainedthe concentration ratio between the n and p type carriers ∼6:10. Considering that the volume of theelectron (hole) pocket increase (decrease) very quickly above EF, one may easily achieve the balanceof n and p carries with slightly doping on the film to realize the electron-hole compensation conditionin the monolayer.1,31

Figure 3(c) is the overall band structure along the ΓY direction measured by in situ ARPES,superimposed with the theoretical calculation made with the PBE method. Due to the superimposedrotational domains, the signal from theΓP direction [Fig. 1(b)] overlaps with that from theΓY directionwithin a single detection plane. However, the low-energy bands from ΓP are contained within theenvelop of ΓY bands in both theory and experiment. The domain size of the film is estimated to betens of nanometers,1,18 which call for further studies with characterization tools with high spatialresolution such as STM and nano-ARPES to disentangle the superposition of signals from domainswith different orientations. Overall, the experimental band structure and FS topology from ARPESagree well with the theoretical calculation.

FIG. 3. Electronic structure of monolayer 1T′-MoTe2. (a) Fermi surface intensity map from ARPES measurement of mono-layer 1T′-MoTe2. The intensity is integrated within a ±10 meV window around the Fermi energy. The red, blue, and greenrectangles indicate three Brillion zones from three rotational domains, 120◦ with respect to each other. (b) Fermi surfacetopology extracted from ARPES data for a single domain. The red and blue pockets represent the hole and electron pockets,respectively. (c) The second derivative of the ARPES spectrum along ΓY superposed with PBE calculation. The red and bluelines are calculated bands along the ΓY and ΓP directions, respectively.

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Now we focus on the low-energy electronic structure right near EF. According to theoreticalcalculations,3,19,20 the transition from 1T to 1T′ inverts the band order leading to a band degeneracypoint between the valence and conduction bands at ∆ point [Fig. 4(a), left]. The SOC then lifts thedegeneracy at the cross point [Fig. 4(a), right] to make the system to be an insulator with a bulkbandgap or a semimetal, depending on the strength of the coupling. The low-energy band dispersionalong both ΓX and ΓY directions [Figs. 4(b) and 4(c)] and corresponding momentum distributioncurves (MDCs) and energy distribution curves (EDCs) along the ΓY direction [Figs. 4(d) and 4(e)]clearly indicate well-separated valence and conduction bands confirming the scenario mentionedabove. However, the band structure measured by ARPES clearly exhibits that the hole band at Γcrosses EF, while the conduction band minimum locates well below EF, leading to a FS with bothelectron and hole pockets. This shows that monolayer 1T′-MoTe2 on BLG is a semimetal with amoderate strength of SOC.

Considering the weaker SOC in Mo compared to W, in general, one would expect that theSOC-induced energy bandgap in 1T′-MoTe2 is smaller than that in 1T′-WTe2. In addition, strainand electron-electron interaction are believed to play important roles in the low-energy electronicstructure of 1T′-MoTe2.3,32 As shown in the calculations,3,32 the size of the gap is very sensitiveto the lattice constant. However, a large strain, ∼4%–6% of the lattice constant, is needed to liftthe crossover of the conduction and valence bands and to open a bulk gap. It has been known thatvan der Waals epitaxy adopted in our growth of 1T′-MoTe2 on graphene minimizes the strain effectcaused by lattice mismatch.33 A strong electron-electron correlation, which is largely screened inthe bulk 1T′-MoTe2, could also enlarge the separation between the conduction band and the valenceband.32 As calculated in Ref. 32, even with a very strong on-site Coulomb repulsion∼5 eV, the bandgapremains closed for 1T′-MoTe2. In our 1T′-MoTe2/BLG thin films, the screening from conductinggraphene substrate lowers the strength of on-site Coulomb interactions, which would act against thebulk gap opening due to the electron-electron interaction.

In summary, high-quality monolayer 1T′-MoTe2 has been grown on the BLG substrate by MBE.In situ LEED and ARPES measurements are performed to characterize the crystal and electronicstructure. It has been shown that three equivalent rotational domains of 1T′-MoTe2 coexist andcontribute to our spectra. The overall electronic structure obtained from ARPES exhibits excellentagreement with theoretical calculations, including the predicted degeneracy lifting induced by SOC.However, the splitting due to the SOC is not large enough to open a 2D bulk gap. The valence bandmaximum locates above the conduction band minimum resulting in a FS with both electron and hole

FIG. 4. Detailed view of the low-energy electronic structure of monolayer 1T′-MoTe2. (a) The band evolution of 1T′-TMDCsunder strong and weak SOC. (b) and (c) are detailed ARPES spectra along the ΓX and ΓY directions, respectively. (d) and (e)are MDCs and EDCs along the ΓY direction corresponding to the boxed area in (c).

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pockets. Our measurements confirm that the epitaxially grown monolayer 1T′-MoTe2 on BLG is asemimetal.

The ARPES and thin film growth works at the Stanford Institute for Materials and Energy Sci-ences and Stanford University are supported by the Office of Basic Energy Sciences, Division ofMaterials Science and AFOSR Grant No. FA9550-14-1-0277 through the University of Washingtonthat supports the ALS activity by the Stanford team. Research performed at ALS (thin film growthand ARPES) is supported by the Office of Basic Energy Sciences, U.S. DOE under Contract No.DE-AC02-05CH11231. SSRL is supported by the Office of Basic Energy Sciences, U.S. DOE underContract No. DE-AC02-76SF00515. Z.L. is supported by the National Natural Science Foundationof China (No. 11227902). S.T. acknowledges the support by CPSF-CAS Joint Foundation for Excel-lent Postdoctoral Fellows. H.R. and C.H. acknowledge the support from the NRF, Korea throughMax Planck Korea/POSTECH Research Initiative (No. 2011-0031558) and Basic Science ResearchProgram (No. 2015R1C1A1A01053065). A portion of the computational work was performed usingthe resources of the National Energy Research Scientific Computing Center (NERSC) supported bythe U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-05CH11231.

1 Y. Zhang, T.-R. Chang, B. Zhou, Y.-T. Cui, H. Yan, Z. Liu, F. Schmitt, J. Lee, R. Moore, Y. Chen, H. Lin, H.-T. Jeng,S.-K. Mo, Z. Hussain, A. Bansil, and Z.-X. Shen, “Direct observation of the transition from indirect to direct bandgap inatomically thin epitaxial MoSe2,” Nat. Nanotechnol. 9(2), 111–115 (2014).

2 S. B. Desai, G. Seol, J. S. Kang, H. Fang, C. Battaglia, R. Kapadia, J. W. Ager, J. Guo, and A. Javey, “Strain-inducedindirect to direct bandgap transition in multilayer WSe2,” Nano Lett. 14(8), 4592–4597 (2014).

3 X. Qian, J. Liu, L. Fu, and J. Li, “Quantum spin Hall effect in two-dimensional transition metal dichalcogenides,” Science346(6215), 1344–1347 (2014).

4 K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, “Atomically thin MoS2: A new direct-gap semiconductor,” Phys. Rev.Lett. 105(13), 136805 (2010).

5 A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C.-Y. Chim, G. Galli, and F. Wang, “Emerging photoluminescence inmonolayer MoS2,” Nano Lett. 10(4), 1271–1275 (2010).

6 J. M. Riley, F. Mazzola, M. Dendzik, M. Michiardi, T. Takayama, L. Bawden, C. Granerod, M. Leandersson,T. Balasubramanian, M. Hoesch, T. K. Kim, H. Takagi, W. Meevasana, P. Hofmann, M. S. Bahramy, J. W. Wells, andP. D. C. King, “Direct observation of spin-polarized bulk bands in an inversion-symmetric semiconductor,” Nat. Phys.10(11), 835–839 (2014).

7 S.-K. Mo, C. Hwang, Y. Zhang, M. Fanciulli, S. Muff, J. H. Dil, Z.-X. Shen, and Z. Hussain, “Spin-resolved photoemissionstudy of epitaxially grown MoSe2 and WSe2 thin films,” J. Phys.: Condens. Matter 28(45), 454001 (2016).

8 K. F. Mak, K. He, J. Shan, and T. F. Heinz, “Control of valley polarization in monolayer MoS2 by optical helicity,” Nat.Nanotechnol. 7(8), 494–498 (2012).

9 K. F. Mak, K. L. McGill, J. Park, and P. L. McEuen, “The valley Hall effect in MoS2 transistors,” Science 344(6191),1489–1492 (2014).

10 K. Deng, G. Wan, P. Deng, K. Zhang, S. Ding, E. Wang, M. Yan, H. Huang, H. Zhang, Z. Xu, J. Denlinger, A. Fedorov,H. Yang, W. Duan, H. Yao, Y. Wu, S. Fan, H. Zhang, X. Chen, and S. Zhou, “Experimental observation of topological Fermiarcs in type-II Weyl semimetal MoTe2,” Nat. Phys. 12(12), 1105–1110 (2016).

11 A. Tamai, Q. S. Wu, I. Cucchi, F. Y. Bruno, S. Ricco, T. K. Kim, M. Hoesch, C. Barreteau, E. Giannini, C. Besnard,A. A. Soluyanov, and F. Baumberger, “Fermi arcs and their topological character in the candidate type-II Weyl semimetalMoTe2,” Phys. Rev. X 6(3), 031021 (2016).

12 Y. Sun, S.-C. Wu, M. N. Ali, C. Felser, and B. Yan, “Prediction of Weyl semimetal in orthorhombic MoTe2,” Phys. Rev. B92(16), 161107 (2015).

13 J. Jiang, Z. K. Liu, Y. Sun, H. F. Yang, C. R. Rajamathi, Y. P. Qi, L. X. Yang, C. Chen, H. Peng, C. C. Hwang, S. Z. Sun,S. K. Mo, I. Vobornik, J. Fujii, S. S. P. Parkin, C. Felser, B. H. Yan, and Y. L. Chen, “Signature of type-II Weyl semimetalphase in MoTe2,” Nat. Commun. 8, 13973 (2017).

14 S. Murakami, “Quantum spin Hall effect and enhanced magnetic response by spin-orbit coupling,” Phys. Rev. Lett. 97(23),236805 (2006).

15 B. A. Bernevig, T. L. Hughes, and S.-C. Zhang, “Quantum spin Hall effect and topological phase transition in HgTe quantumwells,” Science 314(5806), 1757–1761 (2006).

16 C. L. Kane and E. J. Mele, “Quantum spin Hall effect in graphene,” Phys. Rev. Lett. 95(22), 226801 (2005).17 Z. Fei, T. Palomaki, S. Wu, W. Zhao, X. Cai, B. Sun, P. Nguyen, J. Finney, X. Xu, and D. H. Cobden, “Edge conduction in

monolayer WTe2,” Nat. Phys. 13(7), 677–682 (2017).18 S. Tang, C. Zhang, D. Wong, Z. Pedramrazi, H.-Z. Tsai, C. Jia, B. Moritz, M. Claassen, H. Ryu, S. Kahn, J. Jiang,

H. Yan, M. Hashimoto, D. Lu, R. G. Moore, C.-C. Hwang, C. Hwang, Z. Hussain, Y. Chen, M. M. Ugeda, Z. Liu, X. Xie,T. P. Devereaux, M. F. Crommie, S.-K. Mo, and Z.-X. Shen, “Quantum spin Hall state in monolayer 1T′-WTe2,” Nat. Phys.13(7), 683–687 (2017).

19 D.-H. Choe, H.-J. Sung, and K. J. Chang, “Understanding topological phase transition in monolayer transition metaldichalcogenides,” Phys. Rev. B 93(12), 125109 (2016).

20 D. H. Keum, S. Cho, J. H. Kim, D.-H. Choe, H.-J. Sung, M. Kan, H. Kang, J.-Y. Hwang, S. W. Kim, H. Yang, K. J. Chang,and Y. H. Lee, “Bandgap opening in few-layered monoclinic MoTe2,” Nat. Phys. 11(6), 482–486 (2015).

026601-7 Tang et al. APL Mater. 6, 026601 (2018)

21 C. H. Naylor, W. M. Parkin, J. Ping, Z. Gao, Y. R. Zhou, Y. Kim, F. Streller, R. W. Carpick, A. M. Rappe, M. Drndic,J. M. Kikkawa, and A. T. C. Johnson, “Monolayer single-crystal 1T′-MoTe2 grown by chemical vapor deposition exhibitsweak antilocalization effect,” Nano Lett. 16(7), 4297–4304 (2016).

22 A. Damascelli, Z. Hussain, and Z.-X. Shen, “Angle-resolved photoemission studies of the cuprate superconductors,” Rev.Mod. Phys. 75(2), 473–541 (2003).

23 S.-K. Mo, “Angle-resolved photoemission spectroscopy for the study of two-dimensional materials,” Nano Convergence4(1), 6 (2017).

24 W. Qingyan, Z. Wenhao, W. Lili, H. Ke, M. Xucun, and X. Qikun, “Large-scale uniform bilayer graphene prepared byvacuum graphitization of 6H-SiC(0001) substrates,” J. Phys.: Condens. Matter 25(9), 095002 (2013).

25 G. Kresse and J. Furthmuller, “Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basisset,” Phys. Rev. B 54(16), 11169–11186 (1996).

26 B. K. Choi, M. Kim, K.-H. Jung, J. Kim, K.-S. Yu, and Y. J. Chang, “Temperature dependence of band gap in MoSe2 grownby molecular beam epitaxy,” Nanoscale Res. Lett. 12(1), 492 (2017).

27 E. Wang, H. Ding, A. V. Fedorov, W. Yao, Z. Li, Y.-F. Lv, K. Zhao, L.-G. Zhang, Z. Xu, J. Schneeloch, R. Zhong, S.-H. Ji,L. Wang, K. He, X. Ma, G. Gu, H. Yao, Q.-K. Xue, X. Chen, and S. Zhou, “Fully gapped topological surface states inBi2Se3 films induced by a d-wave high-temperature superconductor,” Nat. Phys. 9(10), 621–625 (2013).

28 M. N. Ali, J. Xiong, S. Flynn, J. Tao, Q. D. Gibson, L. M. Schoop, T. Liang, N. Haldolaarachchige, M. Hirschberger,N. P. Ong, and R. J. Cava, “Large, non-saturating magnetoresistance in WTe2,” Nature 514(7521), 205–208 (2014).

29 S. Thirupathaiah, R. Jha, B. Pal, J. S. Matias, P. K. Das, P. K. Sivakumar, I. Vobornik, N. C. Plumb, M. Shi, R. A. Ribeiro,and D. D. Sarma, “MoTe2: An uncompensated semimetal with extremely large magnetoresistance,” Phys. Rev. B 95(24),241105 (2017).

30 W. Chen-Lu, Z. Yan, H. Jian-Wei, L. Guo-Dong, L. Ai-Ji, Z. Yu-Xiao, S. Bing, L. Jing, H. Cheng, D. Ying, L. De-Fa,H. Yong, H. Shao-Long, Z. Lin, Y. Li, H. Jin, W. Jiang, M. Zhi-Qiang, S. You-Guo, J. Xiao-Wen, Z. Feng-Feng, Z. Shen-Jin,Y. Feng, W. Zhi-Min, P. Qin-Jun, X. Zu-Yan, C. Chuang-Tian, and Z. Xing-Jiang, “Evidence of electron-hole imbalance inWTe2 from high-resolution angle-resolved photoemission spectroscopy,” Chin. Phys. Lett. 34(9), 097305 (2017).

31 M. Kang, B. Kim, S. H. Ryu, S. W. Jung, J. Kim, L. Moreschini, C. Jozwiak, E. Rotenberg, A. Bostwick, and K. S. Kim,“Universal mechanism of band-gap engineering in transition-metal dichalcogenides,” Nano Lett. 17(3), 1610–1615 (2017).

32 H.-J. Kim, S.-H. Kang, I. Hamada, and Y.-W. Son, “Origins of the structural phase transitions in MoTe2 and WTe2,” Phys.Rev. B 95(18), 180101 (2017).

33 K. S. Novoselov, A. Mishchenko, A. Carvalho, and A. H. Castro Neto, “2D materials and van der Waals heterostructures,”Science 353(6298), aac9439 (2016).