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Synthesis of Monolayer MoSe2 with Controlled Nucleation via Reverse-Flow Chemical Vapor Deposition
摘要： Two-dimensional (2D) layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have attracted considerable interests because of their intriguing optical and electronic properties. Controlled growth of TMDC crystals with large grain size and atomically smooth surface is indeed desirable but remains challenging due to excessive nucleation. Here, we have synthesized high-quality monolayer, bilayer MoSe2 triangular crystals, and continuous thin films with controlled nucleation density via reverse-flow chemical vapor deposition (CVD). High crystallinity and good saturated absorption performance of MoSe2 have been systematically investigated and carefully demonstrated. Optimized nucleation and uniform morphology could be achieved via fine-tuning reverse-flow switching time, growth time and temperature, with corresponding growth kinetics proposed. Our work opens up a new approach for controllable synthesis of monolayer TMDC crystals with high yield and reliability, which promote surface/interface engineering of 2D semiconductors towards van der Waals heterostructure device applications.
关键词： controlled growth,transition metal dichalcogenides (TMDCs),MoSe2,reverse-flow chemical vapor deposition (CVD)
更新于2025-09-16 10:30:52
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Tantalum disulfide quantum dots: preparation, structure, and properties
摘要： Tantalum disulfide (TaS2) two-dimensional film material has attracted wide attention due to its unique optical and electrical properties. In this work, we report the preparation of 1 T-TaS2 quantum dots (1 T-TaS2 QDs) by top-down method. Herein, we prepared the TaS2 QDs having a monodisperse grain size of around 3 nm by an effective ultrasonic liquid phase exfoliation method. Optical studies using UV-Vis, PL, and PLE techniques on the as-prepared TaS2 QDs exhibited ultraviolet absorption at 283 nm. Furthermore, we found that dimension reduction of TaS2 has led to a modification of the band gap, namely a transition from indirect to direct band gap, which is explained using first-principle calculations. By using quinine as reference, the fluorescence quantum yield is 45.6%. Therefore, our results suggest TaS2 QDs have unique and extraordinary optical properties. Moreover, the low-cost, facile method of producing high quality TaS2 QDs in this work is ideal for mass production to ensure commercial viability of devices based on this material.
关键词： Transition metal dichalcogenides,First-principle,Quantum dots,Ultrasonic method,Modulating bandgap
更新于2025-09-16 10:30:52
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Ultrafast Response of a Hybrid Device Based on Strongly Coupled Monolayer WS <sub/>2</sub> and Photonic Crystals: The Effect of Photoinduced Coulombic Screening
摘要： Quantum interactions between transition metal dichalcogenides (TMDs) and optical cavities are rapidly becoming an appealing research topic since these interactions underly a broad spectrum of optical phenomena. Here, we fabricate a simple device in which coherent strong coupling interactions occur between a photonic crystal (PhC) slab and monolayer tungsten disulfide (WS2). Both steady-state angle-resolved spectroscopy and transient absorption microscopy (TAM) are employed to explore the coupling behavior of this device. Specifically, anticrossing dispersions are observed in the hybrid device, indicating a Rabi splitting of 40.2 meV. A newly formed spectral feature emerges in the transient absorption (TA) spectrum of this polariton device under near-resonant excitation, which is subsequently evidenced to be a signature of the upper hybrid exciton–polariton state. Moreover, by carefully analyzing the ultrafast responses of both bare WS2 and the WS2-PhC polariton device excited both off resonance and near resonance, it is found that nonequilibrium thermal decay induces Coulombic screening in the monolayer WS2, which has a major impact on the formation of exciton–polariton. The results of this work could not only improve the current understanding of photophysics in the strong light–matter coupling regime but also lay the foundation for tailoring the development of TMD-based coherent devices.
关键词： strong coupling,transition metal dichalcogenides,photonic crystals,Rabi splitting,transient absorption microscopy,Coulombic screening
更新于2025-09-16 10:30:52
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Observation of charge transfer in mixed-dimensional heterostructures formed by transition metal dichalcogenide monolayers and PbS quantum dots
摘要： We report an experimental study on charge transfer properties of mixed-dimensional heterostructures formed by zero-dimensional PbS quantum dots and two-dimensional transition metal dichalcogenides. Monolayers of MoSe2 and MoS2 were fabricated by mechanical exfoliation and chemical vapor deposition techniques, respectively. PbS quantum dots with diameters of 2.3 and 5 nm were synthesized by a hot-injection method and characterized by optical absorption spectroscopy and ultraviolet photoemission spectroscopy. The quantum dots were deposited on the MoS2 and MoSe2 monolayers to form heterostructures. Photoluminescence and transient absorption measurements were performed on the heterostructures as well as individual materials to reveal their photocarrier dynamics. We found that the holes excited in MoSe2 can efficiently transfer to both 2.3- and 5-nm PbS quantum dots, while electrons in these quantum dots cannot transfer to MoSe2. Similar charge transfer properties were observed between MoS2 and the 5-nm PbS quantum dots, while no charge transfer was observed between MoS2 and the 2.3-nm quantum dots. These results provide useful information for understanding the physical mechanism of charge transfer in mixed-dimensional heterostructures and for developing PbS quantum-dot-based mixed-dimensional materials.
关键词： PbS quantum dots,mixed-dimensional heterostructures,transition metal dichalcogenides,photocarrier dynamics,charge transfer
更新于2025-09-12 10:27:22
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Role of graphene and transition metal dichalcogenides as hole transport layer and counter electrode in solar cells
摘要： Photovoltaic (PV) technology got much attention in the past few decades in developing advanced and environment friendly solar cells (SCs). However, high cost, unstable nature, and low efficiency are major limitations towards commercialization of SCs. To overcome the issues, two-dimensional materials (2DMs) have been exploited due to low cost, high catalytic activity, fast charge separation, and better electrochemical performance. The review emphasis on (a) the electrochemical performance of graphene and transition metal dichalcogenides (TMDCs) as a hole transport layer (HTL) in SCs and (b) to explore low-cost and effective counter electrode (CE) based on graphene and TMDCs for dye-sensitized solar cell (DSSC). The review presents a comparative analysis of 2DMs as HTL and CE to attain highly efficient and low-cost PV devices. Multiple combinations of the material with graphene, graphene oxide (GO), reduced graphene oxide (rGO), tungsten disulfide (WS2), molybdenum disulfide (MoS2) as HTL, and CE material in PV cells are discussed and comparatively analyzed. Numerous strategies are briefly discussed to enhance the efficiency of SCs by utilizing graphene and TMDCs based HTL and CEs. The review focuses on the recent progress in developing low-cost and highly efficient PV devices by using 2DMs. Our study reveals that GO/PEDOT:PSS demonstrate a maximum power conversion efficiency (PCE) of 13.1% when fabricated at different revolutions. Moreover, our statistical analysis unveils that efficiency of the cell can be enhanced by optimizing the layer thickness, which provide a route to develop highly efficient and better performance SCs that can be exploited for future commercial applications.
关键词： solar cell,transition metal dichalcogenides,electrochemical characterization,efficiency,graphene
更新于2025-09-12 10:27:22
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A room-temperature polariton light-emitting diode based on monolayer WS2
摘要： Exciton polaritons that arise through the strong coupling of excitons and cavity photons are used to demonstrate a wide array of fundamental phenomena and potential applications that range from Bose–Einstein-like condensation to analogue Hamiltonian simulators and chip-scale interferometers. Recently, the two-dimensional (2D) transition metal dichalcogenides (TMDs), because of their large exciton binding energies, oscillator strength and valley degree of freedom, have emerged as a very attractive platform to realize exciton polaritons at elevated temperatures. Achieving the electrical injection of polaritons is attractive both as a precursor to realizing electrically driven polariton lasers as well as for high speed light-emitting diodes (LEDs) for communication systems. Here, we demonstrate an electrically driven polariton LED that operates at room temperature using monolayer tungsten disulfide (WS2) as the emissive material. The extracted external quantum efficiency is ~0.1% and is comparable to recent demonstrations of bulk organic and carbon nanotube-based polariton electroluminescence (EL) devices. The possibility to realize electrically driven polariton LEDs in atomically thin semiconductors at room temperature presents a promising step towards achieving an inversionless electrically driven laser in these systems as well as for ultrafast microcavity LEDs using van der Waals (vdW) materials.
关键词： room temperature,transition metal dichalcogenides,exciton polaritons,van der Waals materials,electrically driven polariton LED
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Excitonic Effects in Single Layer MoS <sub/>2</sub> Probed by Broadband Two-Dimensional Electronic Spectroscopy
摘要： Atomically thin Transition-metal dichalcogenides (TMDs) have come into the spotlight in optoelectronics thanks to their outstanding physical properties. In single-layer (1L) TMDs strong quantum confinement effects cause a weak screening of Coulomb, so that the excitons created by photo-excitation have large binding energy, up to several hundred meVs. While the steady-state properties of TMDCs have been studied in detail by linear optical techniques, the recent application of time-resolved nonlinear spectroscopy (mainly ultrafast pump-probe) has enabled the study of excited-state dynamics on femtosecond timescales opening up questions about the mechanisms of exciton relaxations and exciton-exciton interactions. Among TMDs, the spectrum of 1L MoS2 is characterized by two peaks of excitonic nature in the visible spectral region (1.9 eV and 2.05 eV), the so-called A and B excitons. They arise from optical transitions between the spin?orbit split top valence band and the bottom conduction band, around K and K’. Here we use two-dimensional electronic spectroscopy (2DES) to track the sub-ps excitonic interactions within 1L MoS2. We report 2DES measurements obtained on a chemical vapor deposition grown 1L MoS2 sample, at 77K, using sub 20-fs broadband pulses. Thanks to its unique combination of high temporal and spectral resolution, 2DES provides a series of excitation/detection correlation energy maps at different delays T, simultaneously covering the A and B excitons.
关键词： excitonic effects,MoS2,Transition-metal dichalcogenides,two-dimensional electronic spectroscopy
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Spin-Layer- and Spin-Valley-Locking Due to Symmetry in Differently-Stacked Tungsten Disulfide Bilayers
摘要： The coupling of the spin and valley degrees of freedom and valley-selective optical selection rules in transition-metal dichalcogenides (TMDCs) monolayers (ML) has been a major research topic in recent years. In contrast, valleytronic properties of TMDC bilayers have not been in the focus so much by now. In our contribution, we study the effect of the relative layer alignment in TMDC homo-bilayer samples on their polarization-dependent optical properties. To this end, CVD-grown WS2 bilayer samples have been prepared that during synthesis favour either the inversion symmetric AA’ or AB stacking without inversion symmetry. For the optical studies, the bilayer samples were transferred either onto a bare SiO2 or a few-layer h-BN buffer. To verify the difference in symmetry for these bilayer configurations, second-harmonic-generation (SHG) raster-scans have been performed, confirming inversion symmetry for the AA’ configuration. Subsequently, a detailed analysis of reflection contrast and photoluminescence (PL) spectra under different polarization conditions has been performed. In photoluminescence, we find a circular and linear dichroism for both stacking configurations that is more pronounced for the AB stacked sample. Furthermore, a lifting of degeneracy and a small shift (~10 meV) of the excitons for different stacks relative to each other is found. Amended theoretical calculations based on our recently developed approach combining density-functional theory (DFT), the gap equations (GE) and the Dirac-Bloch equations (DBE), show that the spin–valley locking is maintained in the AB-stacked WS2 bilayer, whereas the inversion symmetric AA’ bilayer shows a spin–layer-locking instead. Differences in the optical selection rules together with the substrate dependent renormalizations lead to the observed dichroism. Furthermore, the observed lifting of the degeneracy between different stacking configurations is confirmed by theory.
关键词： spin-valley locking,optical properties,transition-metal dichalcogenides,bilayer stacking,spin-layer-locking
更新于2025-09-12 10:27:22
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The 2D Materials Used for Nanodevice Applications: Utilizing Aggressively Scaled Transistors
摘要： CCUR R ENT NANODEVICES FACE many difficulties, one of which is the well-known issue of short-channel effects (SCEs). SCEs cause significant degradation in device characteristics. To overcome this issue, 2D materials have been studied since 2000. This article reveals the motivation for research on 2D materials for device application.
关键词： transition metal dichalcogenides,nanodevices,short-channel effects,MOSFET,2D materials,graphene
更新于2025-09-12 10:27:22
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ZrS3/MS2 and ZrS3/MXY (M Mo, W; X, Y S, Se, Te; X?≠?Y) type-II van der Waals hetero-bilayers: Prospective candidates in 2D excitonic solar cells
摘要： Excellent photovoltaic abilities in a 2D excitonic solar cell based on staggered type-II van der Waals (vdW) hetero-bilayers comprising of semiconducting ZrS3 monolayer and monolayers of MS2 & MXY (M=Mo, W; X, Y=S, Se, Te; X≠Y) are reported herewith, using DFT-D2 and HSE06 functional. Studies on vdW hetero-bilayers of MX3/MX2 and MX3/M’Y2 have so far been conveniently avoided on account of their large lattice mismatch. The present work is the first attempt to address such hetero-bilayers constituted by monolayers of transition metal dichalcogenides and trichalcogenides. The nature of the band edges in ZrS3 and MS2 monolayers induces high electron and hole mobility in these individual monolayers, respectively, which has been combined synergistically in the hetero-bilayers consisting of them. The Power Conversion Efficiency (PCE) in ZrS3/MoS2, ZrS3/WS2, ZrS3/MoSeTe, ZrS3/WSTe, and ZrS3/WSeTe hetero-bilayers, calculated within the Anderson-limit, are found to reach as high as ~12%, 8%, 16%, 14%, and 14% respectively. The PCE of the hetero-bilayers reported herewith are much higher than the efficiency in MoS2/p-Si heterojunction solar cells (5.23%) and comparable to that of the theoretically proposed PCBM fullerene/BCN system (10?20%) and g-SiC2-based systems (12?20%) and the recently predicted TiNF/TiNBr (18%), TiNCl/TiNBr (19%), TiNF/TiNCl (22%) bilayer solar cell systems.
关键词： 2D materials,transition metal dichalcogenides,photovoltaic,van der Waals hetero-bilayers,transition metal trichalcogenides,power conversion efficiency
更新于2025-09-11 14:15:04