二维过渡金属碲化物单层生长机理与本征缺陷的研究

范德华二维（2D）材料具有较弱的层间作用力和较强的层内共价键，当单个分子层从其块状母体剥离出来时往往会表现出奇特的物理性质，吸引着众多科研工作者对其结构与性质开展广泛的研究。2D过渡金属碲化物（TMTs）由于原子级的厚度、丰富的晶体结构和多种有趣的物质形态成为铁电存储器，低功耗器件，光电转换器件等应用的重要候选材料。然而，Ⅵ 族TMTs在低维极限下是一类空气敏感的2D材料，这种特性严重制约着其本征晶体结构和物理性质的研究，以及未来在工业界的大规模应用。

化学气相沉积法（CVD）在生长大面积、高质量、原子级厚度2D材料方面具有独特的优势，已经成功实现了石墨烯、二硫化钼（MoS₂）等单层2D材料的晶圆级单晶样品生长。由于空气敏感特性，CVD生长的碲化物样品都要经历空气暴露，导致样品的本征结构和物性被破坏。更重要的是，空气敏感特性严重制约着单层碲化物的晶圆级单晶和碲化物异质结的生长。此外，目前CVD生长的TMTs样品均为多晶薄膜，其内部含有多种缺陷结构，比如点缺陷、边缘重构和晶界。这些缺陷结构阻碍着晶体内部电子的传输，严重制约着TMTs材料不同物质形态的出现，例如超导态，绝缘态等。

原子分辨扫描透射电子显微镜（STEM）的高角度环形暗场像（HAADF）能够获得晶格点位中与原子序数相关的衬度信息。在2D极限下，单个TMTs分子层中仅有少数几个原子层厚度，在已知元素组成的条件下能够通过STEM-HAADF图像识别出每一种元素在晶格中的原子占位，为揭示相应的物理性质提供了精确的原子级结构信息，是分析上述大范围CVD生长碲化物薄膜中缺陷结构、相组成与化学配比的强力工具。虽然单层TMTs是过渡金属硫族化合物（TMDCs）中的一员，但是它却拥有着与众不同的各向异性晶体结构。尽管关于TMTs的物性研究不断取得新的发现，然而仍然由于空气敏感特性，关于其在单层极限下的原子级结构表征停滞不前。

针对以上问题，本文以空气敏感的单层MoTe₂和WTe₂为主体材料，以构建其原子级结构与物性之间的关联为研究目标，首先搭建了惰性氛围保护的手套箱互连系统，验证了该系统对空气敏感单层TMTs的有效保护。然后，以生长晶圆级单晶碲化物为目标，通过盐辅助的方法实现了厘米级多晶连续单层碲化物薄膜的生长，并自主搭建多通道气流控制系统，探索了晶圆级单晶碲化物薄膜生长的可行性。通过原子级的STEM表征分析了空气敏感碲化物的面内点缺陷结构，边缘结构，晶界拼接结构，结合密度泛函理论（DFT）计算分析了每种结构的热力学稳定性的差异。在此基础上，进一步实现了基于空气敏感碲化物的新型异质结生长：成功获得垂直堆垛的MoTe₂异相双层异质结和MoTe₂-WTe₂面内拼接单层异质结，通过原子级的STEM表征确认了MoTe₂双层异质结的层间堆垛结构以及面内MoTe₂-WTe₂异质结的原子级拼接界面。MoTe₂双层异质结的生长为进一步构建大尺寸垂直异质结的器件奠定了基础。面内MoTe₂-WTe₂异质结的构建为进一步研究界面拼接处的超导性与拓扑边缘态的耦合提供了物质基础。

Van der Waals two-dimensional (2D) materials are characterized by weak interlayer forces and strong intralayer covalent bonds. Separating a single molecule layer from its bulk matrix often reveals unique physical properties, which have attracted considerable attention from researchers investigating its structure and properties. Of particular interest are 2D transition metal tellurides (TMTs), which are promising materials for ferroelectric storage, low-power devices, and photoelectric conversion due to their atomic-level thickness, rich crystal structures, and various interesting physical states. Nonetheless, the air-sensitivity of group VI TMTs in their monolayer limit significantly restricts the investigation of their intrinsic crystal structure and physical properties, as well as their potential for large-scale industrial applications.

Chemical vapor deposition (CVD) offers unique advantages in the growth of large-area, high-quality, monolayer 2D materials. Specifically, it has enabled the wafer-scale single crystal growth of graphene, molybdenum disulfide (MoS₂), and other monolayer 2D materials. However, TMTs grown by the CVD method are currently subject to air exposure, which leads to the destruction of their intrinsic structure and physical properties due to their air-sensitive nature. Moreover, the air sensitivity significantly constrains the growth of wafer-scale single crystal TMTs and their heterojunctions. Furthermore, the TMT samples currently produced by the CVD method are polycrystalline films with various defect structures inside, such as point defects, edge reconstructions, and grain boundaries. These defect structures impede the transport of electrons inside the crystal and severely constrain the emergence of different physical states of TMTs, such as the superconducting state and the insulating state.

High-angle annular dark-field (HAADF) imaging with scanning transmission electron microscopy (STEM) at the atomic level enables the acquisition of lattice-site-specific information related to atomic number. In the 2D limit, a single TMTs molecular layer is only a few atomic layers thick. When the elemental composition is known, each element’s atomic occupancy in the lattice can be identified using STEM-HAADF imaging. This technique provides precise atomic-level structural information that is essential for uncovering the corresponding physical properties. Despite being a member of transition metal dichalcogenides (TMDCs), monolayer TMTs has a distinctive anisotropic crystal structure. Although new discoveries about the physical properties of TMTs are constantly being reported, the characterization of its atomic-level structure under the air-sensitive nature is still lacking.

In this study, air-sensitive monolayer MoTe₂ and WTe₂ were selected as the primary materials, and the goal was to establish a correlation between their atomic-scale structure and physical properties. Firstly, an interconnection system between glove boxes, protected by an inert gas atmosphere (GIS), was constructed, and its effectiveness in protecting air-sensitive monolayer TMTs was verified. Next, centimeter-scale continuous polycrystalline monolayer telluride films were grown using a salt-assisted method. To achieve the growth of wafer-scale single crystal tellurides, a multi-channel airflow control system was autonomously constructed to explore the feasibility of such growth. The in-plane point defect structure, edge structure, and grain boundary structure of air-sensitive tellurides were characterized using atomic-level STEM analysis. In addition, the corresponding thermodynamic stability differences of each structure were analyzed using density functional theory (DFT) calculations. Based on these findings, the growth of heterostructures based on air-sensitive tellurides was achieved, including the vertical stacking of MoTe₂ bilayer structures and the in-plane splicing heterojunction of MoTe₂-WTe₂. The atomic-scale STEM characterization confirmed the stacking structure of the MoTe₂ bilayer heterostructure, as well as the atomic-scale splicing interface of the MoTe₂-WTe₂.

[1] GEIM A K, GRIGORIEVA I V. Van der Waals heterostructures[J]. Nature, 2013, 499: 419–425.
[2] DUONG D L, YUN S J, LEE Y H. Van der waals layered materials: opportunities and challenges[J]. ACS Nano, 2017, 11: 11803–11830.
[3] NOVOSELOV K S, MISHCHENKO A, CARVALHO A, et al. 2D materials and van der Waals heterostructures[J]. Science, 2016, 353: 6298.
[4] DEAN C, YOUNG A F, WANG L, et al. Graphene based heterostructures[J]. Solid State Communications, 2012, 152: 1275–1282.
[5] HAIGH S J, GHOLINIA A, JALIL R, et al. Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices[J]. Nature Materials, 2012, 11: 764–767.
[6] GORBACHEV R V., GEIM A K, KATSNELSON M I, et al. Strong Coulomb drag and broken symmetry in double-layer graphene[J]. Nature Physics, 2012, 8: 896–901.
[7] PONOMARENKO L A, YANG R, MOHIUDDIN T M, et al. Effect of a high-κ environment on charge carrier mobility in graphene[J]. Physical Review Letters, 2009, 102: 100–103.
[8] ZONG X, HU H, OUYANG G, et al. Black phosphorus-based van der Waals heterostructures for mid-infrared light-emission applications[J]. Light: Science and Applications, 2020, 9: 114.
[9] LIU X, HERSAM M C. Borophene-graphene heterostructures[J]. 2019, 5: 6444.
[10] CHEN Y, SUN M. Two-dimensional WS2/MoS2heterostructures: Properties and applications[J]. Nanoscale, 2021, 13: 5594–5619.
[11] FU L, SUN Y, WU N, et al. Direct growth of MoS2/h-BN heterostructures via a sulfide-resistant alloy[J]. ACS Nano, 2016, 10: 2063–2070.
[12] LI D, XIONG W, JIANG L, et al. Multimodal nonlinear optical imaging of MoS2 and mos2-based van der waals heterostructures[J]. ACS Nano, 2016, 10: 3766–3775.
[13] FLÖRY N, JAIN A, BHARADWAJ P, et al. A WSe2/MoSe2 heterostructure photovoltaic device[J]. Applied Physics Letters, 2015, 107: 2–6.
[14] NOVOSELOV K S, GEIM A K, MOROZOV S V, et al. Electric field effect in atomically thin carbon films[J]. Science, 2004, 306: 666-669.
[15] NOVOSELOV K S, JIANG Z, ZHANG Y, et al. Room-temperature quantum hall in graphene[J]. Science, 2007, 315: 1379.
[16] CHANGGU LEE, XIAODING WEI, JEFFREY W. KYSAR, et al. Measurement of the elastic properties and intrinsic strength of monolayer graphene[J]. Science, 2008, 321: 382–385.
[17] DIAS A C, QU F, AZEVEDO D L, et al. Band structure of monolayer transition-metal dichalcogenides and topological properties of their nanoribbons: Next-nearest-neighbor hopping[J]. Physical Review B, 2018, 98: 075202.
[18] MAK K F, LEE C, HONE J, et al. Atomically thin MoS2: A new direct-gap semiconductor[J]. Physical Review Letters, 2010, 105: 2–5.
[19] ZHAO W, RIBEIRO R M, TOH M, et al. Origin of indirect optical transitions in few-layer MoS2, WS2, and WSe2[J]. Nano Letters, 2013, 13: 5627–5634.
[20] SPLENDIANI A, SUN L, ZHANG Y, et al. Emerging photoluminescence in monolayer MoS2[J]. Nano Letters, 2010, 10: 1271-1275.
[21] WANG Q H, KALANTAR-ZADEH K, KIS A, et al. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides[J]. Nature Nanotechnology, 2012, 7: 699–712.
[22] SCHREPPLER S, SPETHMANN N, BRAHMS N, et al. The valley Hall effect in MoS2 transistors[J]. Science , 2014, 344: 1486–1489.
[23] CHHOWALLA M, SHIN H S, EDA G, et al. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets[J]. Nature Chemistry, 2013, 5: 263–275.
[24] TANG S, ZHANG C, WONG Di, et al. Quantum spin Hall state in monolayer 1T’-WTe2[J]. Nature Physics, 2017, 13: 683–687.
[25] FEI Z, ZHAO W, PALOMAKI T A, et al. Ferroelectric switching of a two-dimensional metal[J]. Nature, 2018, 560: 336–339.
[26] ZHANG X, LU Q, LIU W, et al. Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films[J]. Nature Communications, 2021, 12: 2492.
[27] SAJADI E, PALOMAKI T, FEI Z, et al. Gate-induced superconductivity in a monolayer topological insulator[J]. Science, 2018, 362: 922–925.
[28] HŸTCH M J, SNOECK E, KILAAS R. Quantitative measurement of displacement and strain fields from HREM micrographs[J]. Ultramicroscopy, 1998, 74: 131–146.
[29] KEUM D H, CHO S, KIM J H, et al. Bandgap opening in few-layered monoclinic MoTe2[J]. Nature Physics, 2015, 11: 482–486.
[30] WANG L, GUTIÉRREZ-LEZAMA I, BARRETEAU C, et al. Tuning magnetotransport in a compensated semimetal at the atomic scale[J]. Nature Communications, 2015, 6: 1–7.
[31] CHANG T R, XU S Y, CHANG G, et al. Prediction of an arc-tunable Weyl Fermion metallic state in MoxW1-xTe2[J]. Nature Communications, 2016, 7: 1–9.
[32] XU X, ZHANG Z, DONG J, et al. Ultrafast epitaxial growth of metre-sized single-crystal graphene on industrial Cu foil[J]. Science Bulletin, 2017, 62: 1074–1080.
[33] CHEN T-A, CHUU C-P, TSENG C-C, et al. Wafer-scale single-crystal hexagonal boron nitride monolayers on Cu (111)[J]. Nature, 2020, 579: 219–223.
[34] LI T, GUO W, MA L, et al. Epitaxial growth of wafer-scale molybdenum disulfide semiconductor single crystals on sapphire[J]. Nature Nanotechnology, 2021, 16: 1201-1207.
[35] LI N, WANG Q, SHEN C, et al. Large-scale flexible and transparent electronics based on monolayer molybdenum disulfide field-effect transistors[J]. Nature Electronics, 2020, 3: 711–717.
[36] PACE S, MARTINI L, CONVERTINO D, et al. Synthesis of large-scale monolayer 1T′-MoTe2 and its stabilization via scalable h-BN encapsulation[J]. ACS Nano, 2021, 15: 4213–4225.
[37] KRIVANEK O L, CHISHOLM M F, NICOLOSI V, et al. Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy[J]. Nature, 2010, 464: 571–574.
[38] MOON J Y, KIM M, KIM S Il, et al. Layer-engineered large-area exfoliation of graphene[J]. Science Advances, 2020, 6: 1–8.
[39] RASOOL H I, OPHUS C, KLUG W S, et al. Measurement of the intrinsic strength of crystalline and polycrystalline graphene[J]. Nature Communications, 2013, 4: 1–7.
[40] GMITRA M, KONSCHUH S, ERTLER C, et al. Band-structure topologies of graphene: Spin-orbit coupling effects from first principles[J]. Physical Review B - Condensed Matter and Materials Physics, 2009, 80: 1–5.
[41] ZHANG Y, TAN Y W, STORMER H L, et al. Experimental observation of the quantum Hall effect and Berry’s phase in graphene[J]. Nature, 2005, 438: 201–204.
[42] CHANG K, MEI Z, WANG T, et al. MoS2/graphene cocatalyst for efficient photocatalytic H2 evolution under visible light irradiation[J]. ACS Nano, 2014, 8: 7078–7087.
[43] DAVID L, BHANDAVAT R, SINGH G. MoS2/graphene composite paper for sodium-ion battery electrodes[J]. ACS Nano, 2014, 8: 1759–1770.
[44] LIU L, FENG P, SHEN X. Structural and electronic properties of h-BN[J]. Physical Review B - Condensed Matter and Materials Physics, 2003, 68: 1–8.
[45] ACUN A, ZHANG L, BAMPOULIS P, et al. Germanene: The germanium analogue of graphene[J]. Journal of Physics Condensed Matter, 2015, 27: 443002.
[46] ZHU F F, CHEN W J, XU Y, et al. Epitaxial growth of two-dimensional stanene[J]. Nature Materials, 2015, 14: 1020–1025.
[47] MOLLE A, GOLDBERGER J, HOUSSA M, et al. Buckled two-dimensional Xene sheets[J]. Nature Materials, 2017, 16: 163–169.
[48] KIM J, BAIK S S, RYU S H, et al. Observation of tunable band gap and anisotropic Dirac semimetal state in black phosphorus[J]. Science, 2015, 349: 723–726.
[49] LUO W, ZEMLYANOV D Y, MILLIGAN C A, et al. Surface chemistry of black phosphorus under a controlled oxidative environment[J]. Nanotechnology, 2016, 27: 434002.
[50] RADISAVLJEVIC B, RADENOVIC A, BRIVIO J, et al. Single-layer MoS2 transistors[J]. Nature Nanotechnology, 2011, 6: 147–150.
[51] QIAN X, LIU J, FU L, et al. Quantum spin hall effect in two-dimensional transition metal dichalcogenides[J]. Science, 2014, 346: 1344–1347.
[52] SAJADI E, PALOMAKI T, FEI Z, et al. Gate-induced superconductivity in a monolayer topological insulator[J]. Science, 2018, 362: 922–925.
[53] YANG Q, WU M, LI J. Origin of two-dimensional vertical ferroelectricity in WTe2 bilayer and multilayer[J]. Journal of Physical Chemistry Letters, 2018, 9: 7160–7164.
[54] FEI Z, PALOMAKI T, WU S, et al. Edge conduction in monolayer WTe2[J]. Nature Physics, 2017, 13: 677–682.
[55] YUAN S, LUO X, CHAN H L, et al. Room-temperature ferroelectricity in MoTe2 down to the atomic monolayer limit[J]. Nature Communications, 2019, 10: 2–7.
[56] QI Y, NAUMOV P G, ALI M N, et al. Superconductivity in Weyl semimetal candidate MoTe2[J]. Nature Communications, 2016, 7: 1–7.
[57] WANG W, KIM S, LIU M, et al. Evidence for an edge supercurrent in the Weyl superconductor MoTe2[J]. Science, 2020, 368: 534–537.
[58] WANG Y, XIAO J, ZHU H, et al. Structural phase transition in monolayer MoTe2 driven by electrostatic doping[J]. Nature, 2017, 550: 487–491.
[59] CHO S, KIM S, KIM J H, et al. Phase patterning for ohmic homojunction contact in MoTe2[J]. Science, 2015, 349: 625–628.
[60] HUANG B, CLARK G, NAVARRO-MORATALLA E, et al. Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit[J]. Nature, 2017, 546: 270–273.
[61] GONG C, LI L, LI Z, et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals[J]. Nature, 2017, 546: 265–269.
[62] Koma A. Van der Waals epitaxy for highly lattice-mismatched systems[J]. Journal of Crystal Growth, 1999, 201: 236-241.
[63] DUAN X, WANG C, SHAW J C, et al. Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions[J]. Nature Nanotechnology, 2014, 9: 1024–1030.
[64] ZHANG Y, LV Q, WANG H, et al. Simultaneous electrical and thermal rectification in a monolayer lateral heterojunction[J]. Science, 2022, 378: 169–175.
[65] ZHAO W, FEI Z, SONG T, et al. Magnetic proximity and nonreciprocal current switching in a monolayer WTe2 helical edge[J]. Nature Materials, 2020, 19: 503–507.
[66] CAO Y, FATEMI V, DEMIR A, et al. Correlated insulator behaviour at half-filling inmagic-angle graphene superlattices[J]. Nature, 2018, 556: 80–84.
[67] CAO Y, FATEMI V, FANG S, et al. Unconventional superconductivity in magic-angle graphene superlattices[J]. Nature, 2018, 556: 43–50.
[68] PARK J M, CAO Y, WATANABE K, et al. Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene[J]. Nature, 2021, 590: 249–255.
[69] NAYLOR C H, PARKIN W M, GAO Z, et al. Large-area synthesis of high-quality monolayer 1T’-WTe2 flakes[J]. 2D Materials, 2017, 4: 021008.
[70] HUANG Y, PAN Y H, YANG R, et al. Universal mechanical exfoliation of large-area 2D crystals[J]. Nature Communications, 2020, 11: 2453.
[71] JIA Y, WANG P, CHIU C L, et al. Evidence for a monolayer excitonic insulator[J]. Nature Physics, 2022, 18(1): 87–93.
[72] HE Q, LI P, WU Z, et al. Molecular beam epitaxy scalable growth of wafer-scale continuous semiconducting monolayer MoTe2 on inert amorphous dielectrics[J]. Advanced Materials, 2019, 31: 1–11.
[73] CHEN J, WANG G, TANG Y, et al. Quantum effects and phase tuning in epitaxial hexagonal and monoclinic MoTe2 monolayers[J]. ACS Nano, 2017, 11: 3282–3288.
[74] EMPANTE T A, ZHOU Y, KLEE V, et al. Chemical vapor deposition growth of few-layer MoTe2 in the 2H, 1T′, and 1T phases: Tunable properties of MoTe2 films[J]. ACS Nano, 2017, 11: 900–905.
[75] LI J, CHENG S, LIU Z, et al. Centimeter-scale, large-area, few-layer 1T′-WTe2 films by chemical vapor deposition and its long-term dtability in ambient condition[J]. Journal of Physical Chemistry C, 2018, 122: 7005–7012.
[76] ZHOU J, LIU F, LIN J, et al. Large-area and high-quality 2D transition Metal Telluride[J]. Advanced Materials, 2017, 29: 1603471.
[77] PARK J C, YUN S J, KIM H, et al. Phase-Engineered Synthesis of Centimeter-Scale 1T′- and 2H-Molybdenum Ditelluride Thin Films[J]. ACS Nano, 2015, 9: 6548–6554.
[78] HYNEK D J, SINGHANIA R M, XU S, et al. Cm2-scale synthesis of MoTe2 thin films with large grains and layer control[J]. ACS Nano, 2021, 15: 410–418.
[79] SUNG J H, HEO H, SI S, et al. Coplanar semiconductor–metal circuitry defined on few-layer MoTe2 via polymorphic heteroepitaxy[J]. Nature Nanotechnology, 2017, 12: 1064–1070.
[80] XU X, PAN Y, LIU S, et al. Seeded 2D epitaxy of large-area single-crystal films of the van der Waals semiconductor 2H MoTe2[J]. Science, 2021, 372: 195–200.
[81] KÖNIG M, MOLENKAMP L W, QI X, et al. Quantum spin Hall insulator state in HgTe Quantum Wells[J]. Science, 2007, 318: 766–770.
[82] BERNEVIG B A, HUGHES T L., Zhang S. Quantum spin Hall effect and topological phase transition in HgTe quantum wells[J]. Science, 2006, 314: 1757–1761.
[83] KNEZ I, DU R R, SULLIVAN G. Evidence for helical edge modes in inverted InAs/GaSb quantum wells[J]. Physical Review Letters, 2011, 107: 1–5.
[84] FATEMI V, WU S, CAO Y, et al. Electrically tunable low-density superconductivity in a monolayer topological insulator[J]. Science, 2018, 362: 926–929.
[85] SCOTT J F. Applications of modern ferroelectrics[J]. Science, 2007, 315: 954–959.
[86] DAWBER M K, Rabe M, Scott J F. Physics of thin-film ferroelectric oxides[J]. Reviews of Modern Physics, 2005, 77: 1083-1130.
[87] GAO W, ZHU Y, WANG Y, et al. A review of flexible perovskite oxide ferroelectric films and their application[J]. Journal of Materiomics, 2020, 6: 1–16.
[88] SAKAI H, IKEURA K, BAHRAMY M S, et al. Critical enhancement of thermopower in a chemically tuned polar semimetal MoTe2[J]. Science Advances, 2016, 2: 1–8.
[89] KIM T H, PUGGIONI D, YUAN Y, et al. Polar metals by geometric design[J]. Nature, 2016, 533: 68–72.
[90] BENEDEK N A, BIROL T. "Ferroelectric" metals reexamined: Fundamental mechanisms and design considerations for new materials[J]. Journal of Materials Chemistry C, 2016, 4: 4000–4015.
[91] SHI Y, GUO Y, WANG X, et al. A ferroelectric-like structural transition in a metal[J]. Nature Materials, 2013, 12: 1024–1027.
[92] ANDERSON P W, BLOUNT E I. Symmetry considerations on martensitic transformations: "ferroelectric" metals?[J]. Physical Review Letters, 1965, 14(13): 532.
[93] FATEMI V, GIBSON Q D, WATANABE K, et al. Magnetoresistance and quantum oscillations of an electrostatically tuned semimetal-to-metal transition in ultrathin WTe2[J]. Physical Review B, 2017, 95: 1–5.
[94] RAJAPITAMAHUNI A, HOFFMAN J, AHN C H, et al. Examining graphene field effect sensors for ferroelectric thin film studies[J]. Nano Letters, 2013, 13: 4374–4379.
[95] JINDAL A, SAHA A, LI Z, et al. Coupled ferroelectricity and superconductivity in bilayer Td-MoTe2[J]. Nature, 2023, 613: 48–52.
[96] LÜPKE F, WATERS D, DE LA BARRERA S C, et al. Proximity-induced superconducting gap in the quantum spin Hall edge state of monolayer WTe2[J]. Nature Physics, 2020, 16: 526–530.
[97] WILLIAMS B, CARTER C B. Transmission Electron Microscopes[M]. Boston, MA: Springer US, 1996: 3-17.
[98] LIN J, ZHOU J, ZULUAGA S, et al. Anisotropic ordering in 1T′ molybdenum and tungsten ditelluride layers alloyed with sulfur and selenium[J]. ACS Nano, 2018, 12: 894–901.
[99] DENG Y, LI P, ZHU C, et al. Controlled synthesis of MoxW1-xTe2 atomic layers with emergent quantum states[J]. ACS Nano, 2021, 15: 11526–11534.
[100] ZHOU W, ZOU X, NAJMAEI S, et al. Intrinsic structural defects in monolayer molybdenum disulfide[J]. Nano Letters, 2013, 13: 2615–2622.
[101] VAN DER ZANDE A M, HUANG P Y, CHENET D A, et al. Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide[J]. Nature Materials, 2013, 12: 554–561.
[102] CUI J, LI P, ZHOU J, et al. Transport evidence of asymmetric spin–orbit coupling in few-layer superconducting 1Td -MoTe2[J]. Nature Communications, 2019, 10: 1–8.
[103] RHODES D A, JINDAL A, YUAN N F Q, et al. Enhanced superconductivity in monolayer Td-MoTe2[J]. Nano Letters, 2021, 21: 2505–2511.
[104] NIU K, WENG M, LI S, et al. Direct visualization of large‐scale intrinsic atomic lattice structure and its collective anisotropy in air‐sensitive monolayer 1T’‐ WTe2[J]. Advanced Science, 2021, 8: 2101563.
[105] ZHOU J, LIN J, HUANG X, et al. A library of atomically thin metal chalcogenides[J]. Nature, 2018, 556: 355–359.
[106] LI S, LIN Y C, LIU X Y, et al. Wafer-scale and deterministic patterned growth of monolayer MoS2 via vapor-liquid-solid method[J]. Nanoscale, 2019, 11: 16122–16129.
[107] LI S. Salt-assisted chemical vapor deposition of two-dimensional transition metal dichalcogenides[J]. iScience, 2021, 24: 103229.
[108] WANG J, XU X, CHENG T, et al. Dual-coupling-guided epitaxial growth of wafer-scale single-crystal WS2 monolayer on vicinal a-plane sapphire[J]. Nature Nanotechnology, 2022, 17: 33-38.
[109] LIU L, LI T, MA L, et al. Uniform nucleation and epitaxy of bilayer molybdenum disulfide on sapphire[J]. Nature Nanotechnology, 2022, 605: 69-75.
[110] WANG Q, LI N, TANG J, et al. Wafer-scale highly oriented monolayer MoS2 with large domain sizes[J]. Nano Letters, 2020, 20: 7193–7199.
[111] CHO S, KANG S H, YU H S, et al. Te vacancy-driven superconductivity in orthorhombic molybdenum ditelluride[J]. 2D Materials, 2017, 4: 021030.
[112] CHEN W H, KAWAKAMI N, LIN J J, et al. Noncentrosymmetric characteristics of defects on WTe2[J]. Physical Review B, 2022, 106: 1–6.
[113] MUECHLER L, HU W, LIN L, et al. Influence of point defects on the electronic and topological properties of monolayer WTe2[J]. Physical Review B, 2020, 102: 1–6.
[114] HUANG C, NARAYAN A, ZHANG E, et al. Inducing strong superconductivity in WTe2 by a proximity effect[J]. ACS Nano, 2018, 12: 7185–7196.
[115] DEVARAKONDA A, CHECKELSKY J G. Monolayers have the edge[J]. Nature Physics, 2017, 13: 630–631.
[116] SHI Y, KAHN J, NIU B, et al. Imaging quantum spin Hall edges in monolayer WTe2[J]. Science Advances, 2019, 5: 1–7.
[117] PENG L, YUAN Y, LI G, et al. Observation of topological states residing at step edges of WTe2[J]. Nature Communications, 2017, 8: 1–7.
[118] KONONOV A, KONONOV A, ABULIZI G, et al. One-dimensional edge transport in few-layer WTe2[J]. Nano Letters, 2020, 20: 4228–4233.
[119] OK S, MUECHLER L, DI SANTE D, et al. Custodial glide symmetry of quantum spin Hall edge modes in monolayer WTe2[J]. Physical Review B, 2019, 99: 1–5.
[120] WANG G Y, XIE W, XU D, et al. Formation mechanism of twin domain boundary in 2D materials: The case for WTe2[J]. Nano Research, 2019, 12: 569–573.
[121] KIM H W, KANG S H, KIM H J, et al. Symmetry dictated grain boundary state in a two-dimensional topological insulator[J]. Nano Letters, 2020, 20: 5837–5843.
[122] XU K, PAN Y, YE S, et al. Shear anisotropy-driven crystallographic orientation imaging in flexible hexagonal two-dimensional atomic crystals[J]. Applied Physics Letters, 2019, 115: 063101.
[123] LIU D, HONG J, WANG X, et al. Diverse atomically sharp interfaces and linear dichroism of 1T’ ReS2-ReSe2 lateral p–n heterojunctions[J]. Advanced Functional Materials, 2018, 28: 1804696.
[124] CHEN X, LEI B, ZHU Y, et al. Diverse spin-polarized in-gap states at grain boundaries of rhenium dichalcogenides induced by unsaturated Re-Re bonding[J]. ACS Materials Letters, 2021, 3: 1513–1520.
[125] LI S, LIN Y C, ZHAO W, et al. Vapour-liquid-solid growth of monolayer MoS2 nanoribbons[J]. Nature Materials, 2018, 17: 535–542.
[126] YANG S, XU X, XU W, et al. Large-scale vertical 1T′/2H MoTe2 nanosheet-based heterostructures for low contact resistance transistors[J]. ACS Applied Nano Materials, 2020, 3: 10411–10417.
[127] ZHANG X, JIN Z, WANG L, et al. Low contact barrier in 2H/1T′ MoTe2 in-plane heterostructure synthesized by chemical vapor deposition[J]. ACS Applied Materials and Interfaces, 2019, 11: 12777–12785.
[128] XU X, CHEN S, LIU S, et al. Millimeter-scale single-crystalline semiconducting MoTe2 via solid-to-solid phase transformation[J]. Journal of the American Chemical Society, 2019, 141: 2128–2134.
[129] HOANG A T, SHINDE S M, KATIYAR A K, et al. Orientation-dependent optical characterization of atomically thin transition metal ditellurides[J]. Nanoscale, 2018, 10: 21978–21984.
[130] LIU L, WU J, WU L, et al. Phase-selective synthesis of 1T′ MoS2 monolayers and heterophase bilayers[J]. Nature Materials, 2018, 17: 1108–1114.
[131] HULMAN M, SOJKOVÁ M, VÉGSÖ K, et al. Polarized Raman reveals alignment of few-layer MoS2 films[J]. Journal of Physical Chemistry C, 2019, 123: 29468–29475.
[132] ZHANG S, MAO N, ZHANG N, et al. Anomalous polarized raman scattering and large circular intensity differential in layered triclinic ReS2[J]. ACS Nano, 2017, 11: 10366–10372.
[133] CHEN S Y, NAYLOR C H, GOLDSTEIN T, et al. Intrinsic phonon bands in high-quality monolayer T’ molybdenum ditelluride[J]. ACS Nano, 2017, 11: 814–820.
[134] BARONI S, GIRONCOLI S D, CORSO A D, et al. Phonons and related crystal properties from density-functional perturbation theory[J]. 2001, 73: 515-562.
[135] Kresse G, Furthmuller J. Bayesian optimization for calibrating and selecting hybrid-density functional models[J]. Journal of Physical Chemistry A, 2020, 124: 4053–4061.
[136] OHFUCHI M, SEKINE A. Quantum spin hall states in 2D monolayer WTe2/MoTe2 lateral heterojunctions for topological quantum computation[J]. ACS Applied Nano Materials, 2023, 6: 2020–2026.