Transition-metal dioxide (TMDO)Transition-Metal Dioxide (TMDO) and transition-metal dichalcogenide (TMDC)Transition-Metal Dichalcogenide (TMDC) monolayers with MX2 stoichiometry consist of a central sublayer of transition-metal atoms, M = Cr, Mo, and W, sandwiched between two sublayers of chalcogenide atoms, X = O, S, Se, and Te. These compounds present an ideal platform for exploring 2D chemistry and physics owing to their various combinations of elements and many existing polytypes. A characteristic feature controlling the physicochemical properties of this group of compounds is the half-filled d band (d2 group) of the transition metals. An early comprehensive study of monolayers with MX2 stoichiometry presented the structural, chemical, and physical data of 88 compounds based on first-principles calculations. The monolayers consist of three sublayers bonded by varying covalent and ionic forces, and the few-layer assemblies usually are bound to each other by much weaker van der Waals attraction. The fabrication of monolayers and few-layer sheets is possible by micromechanical cleavage or liquid-phase exfoliationLiquid-phase exfoliation if layered bulk crystals exist. Chemical vapor deposition (CVD)Chemical Vapor Deposition (CVD) and metal organic chemical vapor deposition (MOCVD) allow the bottom-up synthesisSynthesis of any stable compound. The controlled growth of high-quality single monolayers is possible by molecular-beam epitaxyEpitaxy (MBE), which uses ultrahigh vacuum (UHV) and therefore allows in situ detection of the growth process, e.g., by reflection high-energy electron diffraction (RHEED) and thus the analysis of intrinsic properties. IndentationIndentation measurements, providing Young’s moduli by atomic force microscopy (AFM)Atomic Force Microscopy (AFM), reveal the strongly varying stiffnessStiffness and ultimate strengthUltimate strength of monolayers and few-layer systems. For example, Young’s modulusYoung’s modulus of MoS2 is 176 N/m, and the maximum breaking strain is 0.10, to be compared with 340 N/m and 0.25 for grapheneGraphene, respectively. Some of the best studied monolayers (MoS2, WS2, MoSe2, WSe2, and MoTe2) possess a direct band gap and qualify for applications in electronics and optoelectronicsOptoelectronics. Monolayer structures, which have no inversion centers, qualify for potential applications in the new field of valleytronics. For heavy atoms with d orbitals, strong spin–orbit coupling (SOC)Spin-orbit coupling (SOC) removes spin degeneracy in the valence and conduction bands. The resulting energy splitting between the spin-up and spin-down states opens access to novel devices in spintronicsSpintronics.

Group–VIB Transition-Metal Dioxide and Dichalcogenide Monolayers

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发表日期：

2025年

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Peter Hess

摘要

Transition-metal dioxide (TMDO)Transition-Metal Dioxide (TMDO) and transition-metal dichalcogenide (TMDC)Transition-Metal Dichalcogenide (TMDC) monolayers with MX2 stoichiometry consist of a central sublayer of transition-metal atoms, M = Cr, Mo, and W, sandwiched between two sublayers of chalcogenide atoms, X = O, S, Se, and Te. These compounds present an ideal platform for exploring 2D chemistry and physics owing to their various combinations of elements and many existing polytypes. A characteristic feature controlling the physicochemical properties of this group of compounds is the half-filled d band (d2 group) of the transition metals. An early comprehensive study of monolayers with MX2 stoichiometry presented the structural, chemical, and physical data of 88 compounds based on first-principles calculations. The monolayers consist of three sublayers bonded by varying covalent and ionic forces, and the few-layer assemblies usually are bound to each other by much weaker van der Waals attraction. The fabrication of monolayers and few-layer sheets is possible by micromechanical cleavage or liquid-phase exfoliationLiquid-phase exfoliation if layered bulk crystals exist. Chemical vapor deposition (CVD)Chemical Vapor Deposition (CVD) and metal organic chemical vapor deposition (MOCVD) allow the bottom-up synthesisSynthesis of any stable compound. The controlled growth of high-quality single monolayers is possible by molecular-beam epitaxyEpitaxy (MBE), which uses ultrahigh vacuum (UHV) and therefore allows in situ detection of the growth process, e.g., by reflection high-energy electron diffraction (RHEED) and thus the analysis of intrinsic properties. IndentationIndentation measurements, providing Young’s moduli by atomic force microscopy (AFM)Atomic Force Microscopy (AFM), reveal the strongly varying stiffnessStiffness and ultimate strengthUltimate strength of monolayers and few-layer systems. For example, Young’s modulusYoung’s modulus of MoS2 is 176 N/m, and the maximum breaking strain is 0.10, to be compared with 340 N/m and 0.25 for grapheneGraphene, respectively. Some of the best studied monolayers (MoS2, WS2, MoSe2, WSe2, and MoTe2) possess a direct band gap and qualify for applications in electronics and optoelectronicsOptoelectronics. Monolayer structures, which have no inversion centers, qualify for potential applications in the new field of valleytronics. For heavy atoms with d orbitals, strong spin–orbit coupling (SOC)Spin-orbit coupling (SOC) removes spin degeneracy in the valence and conduction bands. The resulting energy splitting between the spin-up and spin-down states opens access to novel devices in spintronicsSpintronics.