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The Evolution of Two-Dimensional Mo <sub/> 1- <i>x</i> </sub> W <sub/><i>x</i> </sub> S <sub/>2</sub> Alloy-Based Vertical Heterostructures with Various Composition Ranges <i>via</i> Manipulating the Mo/W Precursors
摘要： Two-dimensional (2D) layered transition metal dichalcogenides (TMDCs), for example, MoS2 and WS2, are gaining widespread attention at present due to their distinct physical properties. Developing the deeper potential of 2D TMDCs demands fine spatial modulation of chemical compositions and electrical performance to produce desired heterostructures. In this study, we report a one-step chemical vapor deposition (CVD) synthesis of compositionally tunable WS2-Mo1-xWxS2 vertical heterostructures by adjusting the stacking sequence of Mo/W films as the precursors. Detailed Raman and photoluminescence analyses confirm that as-grown samples present clear structural and optical modulations. The evolution of these heterostructures with different composition ranges is discussed on the basis of the tunable vapor pressure. The present study provides an alternative strategy to facilitate the development of 2D semiconductor heterostructures, which is an essential step towards realizing functional electronics and optoelectronics.
关键词： Heterostructures,Optical properties,Two-dimensional materials,Chemical vapor deposition,Transition metal dichalcogenides
更新于2025-09-10 09:29:36
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A Scanning Tunneling Microscopy Study of Monolayer and Bilayer Transition-Metal Dichalcogenides Grown by Molecular-Beam Epitaxy
摘要： This review presents an account of some recent scanning tunneling microscopy and spectroscopy (STM/S) studies of monolayer and bilayer transition-metal dichalcogenide (TMD) films grown by molecular-beam epitaxy (MBE). In addition to some intrinsic properties revealed by STM/S, defects such as inversion domain boundaries and point defects, their properties and induced effects, are presented. More specifically, the quantum confinement and moiré potential effects, charge state transition, quasi-particle interference and structural phase transition as revealed by STM/S are described.
关键词： transition-metal dichalcogenides,quantum confinement,structural phase transition,molecular-beam epitaxy,moiré potential,quasi-particle interference,scanning tunneling microscopy,charge state transition
更新于2025-09-09 09:28:46
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A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers
摘要： The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by the recombination of electrons in the conduction band with holes in the spin-orbit split valence bands, and there are two distinct emission features referred to as the A-peak (ground state exciton) and B-peak (higher spin-orbit split state). The intensity ratio of these two features varies widely, and several contradictory interpretations have been reported. In this work, we analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and identify the underlying cause of observed variations in emission profile. We determine that PL variations arise from differences in the non-radiative recombination associated with defect densities. Therefore, the relative intensities of the A- and B-emission features can be used to qualitatively assess the non-radiative recombination and a low B/A ratio is indicative of low defect density and high sample quality. We also performed polarization-resolved PL measurements. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the B-exciton to the A-exciton of the valence band. Our work clarifies disparities reported in the literature relating to the emission profile and provides a straightforward means to assess sample quality.
关键词： transition metal dichalcogenides,valley polarization,A-exciton,sample quality,B-exciton,monolayers,photoluminescence
更新于2025-09-09 09:28:46
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Nonlinear optical susceptibility of atomically thin WX2 crystals
摘要： We have studied tungsten diselenide (WSe2) and tungsten disulfide (WS2) monolayer materials in second harmonic generation spectroscopy and microscopy experiments. Ultra-broadband continuum pulses served as the fundamental beam while its second harmonic spectrum in the visible and ultraviolet (UV) range was detected and analyzed with a better than 0.3 nm spectral resolution (< 2 meV). We provide dispersion data and absolute values for χ(2) for the materials within a photon energy range of 2.3–3.2 eV. Fine spectral features that were detected within the dispersion data for the optical nonlinearities indicate the impact of near bandgap exciton transitions. The fundamental bandgap of 2.35 eV and exciton binding energy of 0.38 eV were determined from the measurements for WS2 monolayers while the corresponding values in WSe2 monolayers were 2.22 eV and 0.71 eV. Ranges for the absolute values of the sheet nonlinearity for WS2 and WSe2 are shown to be 0.58–1.65 × 10?18 m2/V and 0.21–0.92 × 10?18 m2/V, correspondingly.
关键词： Nonlinear spectroscopy and microscopy,Transition metal dichalcogenides,Two-dimensional semiconductors,Second harmonic generation,Optical nonlinearity in semiconductors,Monolayer crystals,Optics at interfaces
更新于2025-09-09 09:28:46
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Two-Dimensional Electronics and Optoelectronics: Present and Future
摘要： Since the successful isolation of graphene a little over a decade ago, a wide variety of two-dimensional (2D) layered materials have been studied. They cover a broad spectrum of electronic properties, including metals, semimetals, semiconductors, and insulators. Many of these 2D materials have demonstrated promising potential for electronic and optoelectronic applications.
关键词： transition metal dichalcogenides,black phosphorus,optoelectronics,electronics,two-dimensional materials,graphene
更新于2025-09-09 09:28:46
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Electrically and optically tunable responses in graphene/ transition metal dichalcogenide heterostructures
摘要： Heterostructures involving layered two dimensional (2D) transitional metal dichalcogenides (TMDCs) are not only fundamentally interesting to explore emerging properties at atomically thin limit, but also technically important to achieve novel optoelectronic devices. However, achieving tunable optoelectronic properties and clarifying interlayer processes (charge transfer, energy transfer) in 2D heterostructures remain to be part of the key challenges so far. Here, by fabricating heterostructures of graphene and monolayer TMDCs (n-type MoS2 and p-type WSe2), we demonstrate both electrically and optically tunable responses of the heterostructures, revealing the critical interface processes between graphene and TMDCs. In MoS2/graphene heterostructure, electron transfer from MoS2 to graphene is observed and gate-tunable interface relaxation induces the electrically controlled photoluminescence (PL). While in WSe2/graphene heterostructure, electron transfer from graphene to WSe2 is observed and the PL is tuned by carrier density, which can be controlled by the gate voltage. The interlayer process can also be modulated by laser intensity, which enables photo-induced doping on graphene and optically tunable electrical characteristics of graphene. Combining the tunable Fermi level of graphene and strong light-matter interaction of monolayer TMDCs, our demonstrations are important to the design of multifunctional and efficient optoelectronic devices with TMDCs/graphene heterostructures.
关键词： photo-induced doping,heterostructure,charge transfer,optoelectronic,electrically tunable,transition metal dichalcogenides
更新于2025-09-09 09:28:46
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Band structure and giant Stark effect in two-dimensional transition-metal dichalcogenides
摘要： We present a comprehensive study of the electronic structures of 192 configurations of 39 stable, layered, transition-metal dichalcogenides using density-functional theory. We show detailed investigations of their monolayer, bilayer, and trilayer structures’ valence-band maxima, conduction-band minima, and band gap responses to transverse electric fields. We also report the critical fields where semiconductor-to-metal phase transitions occur. Our results show that band gap engineering by applying electric fields can be an effective strategy to modulate the electronic properties of transition-metal dichalcogenides for next-generation device applications.
关键词： two-dimensional materials,band structure,giant-Stark effect,density functional theory,electric field,transition metal dichalcogenides
更新于2025-09-09 09:28:46
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Radiative lifetime of localized excitons in transition-metal dichalcogenides
摘要： Disorder derived from defects or local strain in monolayer transition-metal dichalcogenides (TMDs) can lead to a dramatic change in the physical behavior of the interband excitations, producing inhomogeneous spectral broadening and localization leading to radiative lifetime increase. In this study, we have modeled the surface disorder of a monolayer TMD sample through a randomized potential in the layer plane. We show that this model, applied to a monolayer of WSe2, allows us to simulate the spectra of localized exciton states as well as their radiative lifetime. In this context, we give an in depth study of the in?uence of the disorder potential parameters on the optical properties of these defects through energies, density of states, oscillator strengths, photoluminescence (PL) spectroscopy, and radiative lifetime at low temperature (4 K). We demonstrate that localized excitons have a longer emission time than free excitons, in the range of tens of picoseconds or more, the radiative decay time depending strongly on the disorder parameter and dielectric environment. Finally, in order to prove the validity of our model, we compare it to available experimental results of the literature.
关键词： disorder potential,localized excitons,radiative lifetime,transition-metal dichalcogenides,photoluminescence spectroscopy
更新于2025-09-09 09:28:46
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Stable 1T Tungsten Disulfide Monolayer and Its Junctions: Growth and Atomic Structures
摘要： Transition metal dichalcogenides in the 1T phase have been a subject of increasing interest partly due to their fascinating physical properties and partly to their potential applications in the next generation of electronic devices, including supercapacitors, electrocatalytic hydrogen evolution, and phase-transition memories. The primary method for obtaining 1T WS2 or MoS2 has been using ion intercalation in combination with solution-based exfoliation. The resulting flakes are small in size and tend to aggregate upon deposition, forming an intercalant-TMD complex with small 1T and 1T’ patches embedded in the 2H matrix. Existing growth methods have, however, produced WS2 or MoS2 solely in the 2H phase. Here we have refined the growth approach to obtain monolayer 1T WS2 up to 80 μm in size based on chemical vapor deposition. With the aid of synergistic catalysts (iron oxide and sodium chloride), 1T WS2 can nucleate in the infant stage of the growth, forming special butterfly-like single crystals with the 1T phase in one wing and the 2H phase in the other. Distinctive types of phase boundaries are discovered at the 1T/2H interface. The 1T structure thus grown is thermodynamically stable over time and even persists at a high temperature above 800 ?C, allowing for a stepwise edge epitaxy of lateral 1T heterostructures. Atomic images show that the 1T WS2/MoS2 heterojunction features a coherent and defectless interface with a sharp atomic transition. The stable 1T phase represents a missing piece of the puzzle in the research of atomic thin van der Waals crystals, and our growth approach provides an accessible way of filling this gap.
关键词： 1T WS2,phase transition,chemical vapor deposition,transition metal dichalcogenides,heterojunctions
更新于2025-09-04 15:30:14
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Electric-field induced structural transition in vertical MoTe2- and Mo1–xWxTe2-based resistive memories
摘要： Transition metal dichalcogenides have attracted attention as potential building blocks for various electronic applications due to their atomically thin nature and polymorphism. Here, we report an electric-field-induced structural transition from a 2H semiconducting to a distorted transient structure (2Hd) and orthorhombic Td conducting phase in vertical 2H-MoTe2- and Mo1?xWxTe2-based resistive random access memory (RRAM) devices. RRAM programming voltages are tunable by the transition metal dichalcogenide thickness and show a distinctive trend of requiring lower electric fields for Mo1?xWxTe2 alloys versus MoTe2 compounds. Devices showed reproducible resistive switching within 10?ns between a high resistive state and a low resistive state. Moreover, using an Al2O3/MoTe2 stack, On/off current ratios of 106 with programming currents lower than 1?μA were achieved in a selectorless RRAM architecture. The sum of these findings demonstrates that controlled electrical state switching in two-dimensional materials is achievable and highlights the potential of transition metal dichalcogenides for memory applications.
关键词： Transition metal dichalcogenides,electric-field-induced structural transition,MoTe2,Mo1?xWxTe2,RRAM
更新于2025-09-04 15:30:14