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Delayed Charge Recombination by Opena??Shell Organics: Its Application in Achieving Superb Photodetectors with Broadband (400a??1160 nm) Ultrahigh Sensitivity and Stability
摘要： Monolayer transition-metal dichalcogenides have inspired worldwide efforts in optoelectronic devices but real applications are hindered with their reduced optical absorption due to their atomically ultrathin signature. In this study, by utilizing their biradical nature such as excellent absorption coefficient, broad bandwidth from the ultraviolet to near-infrared region, and small triplet–singlet energy gap, a series of helicene 5,14-diaryldiindeno[2,1-f:1′,2′-j]picene (DDP) derivatives (1ab, 1ac, and 1bb) are integrated with monolayer MoS2 for extraordinary photodetector performance and outstanding stability. Via comprehensive time-resolved studies, the interfacial charge-transfer process from the DDPs to the MoS2 layer is evidenced by the stabilized exciton property of the organics (1ac)/MoS2 heterostructure. Significantly, the 1ac/MoS2 photodetector exhibits an ultrahigh photoresponsivity of 5 × 107 A W?1 and a fast response speed of 45 ms due to the highly efficient photoexcited carrier separation and the matched type-II energy band alignment. The biradical 1ac/MoS2 hybrid photodetector shows no sign of degradation after one-month operation. The results pave a new avenue for biradical based high-performance and super-broadband optoelectronic devices.
关键词： photodetectors,hybrid structures,MoS2,biradical organic molecules,transition metal dichalcogenides
更新于2025-09-23 15:19:57
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Rational Control of WSe<sub>2</sub> Layer Number via Hydrogen-Controlled Chemical Vapor Deposition
摘要： Transition metal dichalcogenides are a promising family of materials for electronics and optoelectronics, in part due to their range of bandgaps that can be modulated by layer number. Here, we show that WSe2 can be selectively grown with one, two, or three layers, as regulated by a one-step hydrogen-controlled chemical vapor deposition (H-CVD) process involving cyclical pulses of H2 flow. The physical and vibrational properties of the resulting mono-, bi-, and tri-layer WSe2 films are characterized by atomic force microscopy and Raman spectroscopy. Modifying the H-CVD process to include more than three H2 pulses results in thicker WSe2 films, however the thickness of these films is not well controlled and feature small, bulk-like pyramidal islands. Transmission electron microscopy analysis reveals that most of these islands exhibit a spiral structure and appear to be grown via screw-dislocation-driven growth, similar to other works. Therefore, the H-CVD process is demonstrated to be a powerful tool for controlling the layer thickness of WSe2, but its practicality is limited to the few-layer regime.
关键词： Chemical vapor deposition,Transition metal dichalcogenides,Screw-dislocation-driven growth
更新于2025-09-23 15:19:57
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Digital holography for non-invasive quantitative imaging of two-dimensional materials
摘要： Digital holography has found applications in many walks of life, from medicine to metrology, due to its ability to measure complex fields. Here, we use the power of digital holography to quantitatively image two-dimensional Transition Metal Dichalcogenides (TMDs) such as MoS2 and WS2 placed on a SiO2/Si substrate and determine their complex refractive indices or layer thicknesses. By considering the different refractive indices of the TMDs as they are thinned down from bulk to monolayers and by holographically capturing both the amplitude and the phase of reflected light, single atomic layers of TMDs, about 0.7 nm thick, can be resolved. Using holography, we also predict the number of layers contained within a thick TMD flake, which shows agreement with results obtained using Atomic Force Microscopy (AFM). A Bland–Altman analysis was performed to compare our experimental results with the standard AFM measurements, yielding a limit of agreement <5 nm for samples with thicknesses ranging from 15 to 60 nm. Our technique is non-contact, non-invasive, does not require scanning, and produces a field of view of a few hundred micrometers by a few hundred micrometers in a single capture. To further our study, we also perform simulations to demonstrate how the thickness of the SiO2 layer and the laser wavelength are critical in optimizing the amplitude and phase response of a two-dimensional material. These simulations can be used as a roadmap to determine the ideal wavelength and SiO2 layer thickness that should be used to accurately determine the refractive index or thickness of any given sample.
关键词： digital holography,Atomic Force Microscopy,layer thicknesses,Bland–Altman analysis,Transition Metal Dichalcogenides,two-dimensional materials,complex refractive indices
更新于2025-09-23 15:19:57
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van der Waals Epitaxial Growth of Atomically Thin 2D Metals on Dangling-Bond-Free WSe <sub/>2</sub> and WS <sub/>2</sub>
摘要： 2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe2 or WS2 as the growth substrate, the synthesis of atomically thin 2D metallic MTe2 (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe2/WSe2 (WS2) vertical heterojunctions is reported. Comparison with the growth on the SiO2/Si substrate under the same conditions reveals that the utilization of the dangling-bond-free WSe2 or WS2 as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe2 (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.
关键词： van der Waals epitaxy,field-effect transistors,2D materials,chemical vapor deposition,transition metal dichalcogenides
更新于2025-09-19 17:15:36
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Engineering Point Defect States in Monolayer WSe <sub/>2</sub>
摘要： Defect engineering is a key approach for tailoring the properties of the emerging two-dimensional semiconductors. Here, we report an atomic engineering of the W vacancy in monolayer WSe2 by single potassium atom decoration. The K decoration alters the energy states and reshapes the wave-function such that previously hidden mid-gap states become visible with well-resolved multiplets in scanning tunneling spectroscopy. Their energy levels are in good agreement with first principle calculations. More interestingly, the calculations show that an unpaired electron donated by the K atom can lead to a local magnetic moment, exhibiting an on-off switching by the odd-even number of electron filling. Experimentally the Fermi level is pinned above all defect states due to the graphite substrate, corresponding to an off state. The close agreement between theory and experiment in the off state, on the other hand, suggest a possibility of gate-programmable magnetic moments at the defects.
关键词： mid-gap defect states,spin splitting,defect engineering,local magnetic moment,transition metal dichalcogenides
更新于2025-09-19 17:15:36
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Two-dimensional (PEA) <sub/>2</sub> PbBr <sub/>4</sub> perovskite single crystals for a high performance UV-detector
摘要： Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDCs), the complement of 2D semiconducting TMDCs, have attracted extensive attentions in recent years because of their versatile properties such as superconductivity, charge density wave, and magnetism. To promote the investigations of their fantastic properties and broad applications, the preparation of large-area, high-quality, and thickness-tunable 2D MTMDCs has become a very urgent topic and great efforts have been made. This topical review therefore focuses on the introduction of the recent achievements for the controllable syntheses of 2D MTMDCs (VS2, VSe2, TaS2, TaSe2, NbS2, NbSe2, etc.). To begin with, some earlier developed routes such as chemical vapor transport, mechanical/chemical exfoliation, as well as molecular beam epitaxy methods are briefly introduced. Secondly, the scalable chemical vapor deposition methods involved with two sorts of metal-based feedstocks, including transition metal chlorides and transition metal oxidations mixed with alkali halides, are discussed separately. Finally, challenges for the syntheses of high-quality 2D MTMDCs are discussed and the future research directions in the related fields are proposed.
关键词： metallic transition metal dichalcogenides,synthesis,chemical vapor deposition,two dimensional
更新于2025-09-19 17:15:36
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Thickness-controlled synthesis of CoX2 (X = S, Se, Te) single crystalline 2D layers with linear magnetoresistance and high conductivity.
摘要： Two-dimensional (2D) materials especially transition metal dichalcogenides (TMDs) have drawn intensive interests owing to their plentiful properties. Some TMDs with magnetic elements (Fe, Co, Ni, etc.) are reported to be magnetic theoretically and experimentally, which undoubtedly provide a promising platform to design functional devices and study physical mechanisms. Nevertheless, plenty of the theoretical TMDs remain unrealized experimentally. In addition, the governable synthesis of these kinds of TMDs with desired thickness and high crystallinity poses a tricky challenge. Here, we report a controlled preparation of CoX2 (X = S, Se, Te) nanosheets through chemical vapor deposition (CVD). The thickness, lateral scale and shape of the crystals show great dependence with temperature and the thickness can be controlled from monolayer to tens of nanometers. Magneto-transport characterization and Density Function Theory (DFT) simulation indicate CoSe2 and CoTe2 are metallic. Besides, unsaturated and linear magnetoresistance have been observed even up to 9 Tesla. The conductivity of CoSe2 and CoTe2 can reach 5 × 106 and 1.8 × 106 S/m respectively, which is pretty high and even comparable with silver. These cobalt-based TMDs show great potential to work as 2D conductors and also provide a promising platform for investigating their magnetic properties.
关键词： Transition metal dichalcogenides,Magnetoresistance,Conductivity,Two-dimensional materials,Chemical vapor deposition
更新于2025-09-19 17:13:59
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Enhanced Absorption in MoS <sub/>2</sub> /Hg <sub/>0.33</sub> Cd <sub/>0.66</sub> Te Heterostructure for Application in Solar Cell Absorbers
摘要： The heterostructure of mercury cadmium telluride (Hg0.33Cd0.66Te) and monolayer MoS2 with Van der Waals (VdW) interaction is suggested for tuning the optical absorption in visible region. The calculations are performed using density functional theory (DFT). From the results, it is found that for molybdenum disulfide (MoS2) absorption is high in the ～390–430 nm range of visible region and for mercury cadmium telluride (Hg0.33Cd0.66Te) absorption is high in the ～310–350 nm. However, absorption in the MoS2/Hg0.33Cd0.66Te heterostructure increases with the wavelength and also shifts towards the red region of the visible spectrum, resulting into a well known red-shift phenomenon. Moreover, higher absorption is seen (entirely in visible region) in the heterostructure in the desired visible region range (λ ～640–710 nm) of the spectrum required for application in optoelectronic devices. The heterostructure results into semi-metallic or negligible indirect bandgap creation in contrast to direct bandgap observed in individual MoS2 and Hg0.33Cd0.66Te structures. The heterostructure can find application in Schottky barrier solar cell absorbers.
关键词： Optoelectronics,solar cells,transition metal dichalcogenides,absorption coefficient,refractive index
更新于2025-09-19 17:13:59
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Waveguide-integrated van der Waals heterostructure photodetector at telecom wavelengths with high speed and high responsivity
摘要： Intensive efforts have been devoted to the exploration of new optoelectronic devices based on two-dimensional transition-metal dichalcogenides (TMDCs) owing to their strong light–matter interaction and distinctive material properties. In particular, photodetectors featuring both high-speed and high-responsivity performance are of great interest for a vast number of applications such as high-data-rate interconnects operated at standardized telecom wavelengths. Yet, the intrinsically small carrier mobilities of TMDCs become a bottleneck for high-speed application use. Here, we present high-performance vertical van der Waals heterostructure-based photodetectors integrated on a silicon photonics platform. Our vertical MoTe2–graphene heterostructure design minimizes the carrier transit path length in TMDCs and enables a record-high measured bandwidth of at least 24 GHz under a moderate bias voltage of –3 V. Applying a higher bias or employing thinner MoTe2 flakes boosts the bandwidth even to 50 GHz. Simultaneously, our device reaches a high external responsivity of 0.2 A W–1 for incident light at 1,300 nm, benefiting from the integrated waveguide design. Our studies shed light on performance trade-offs and present design guidelines for fast and efficient devices. The combination of two-dimensional heterostructures and integrated guided-wave nano photonics defines an attractive platform to realize high-performance optoelectronic devices, such as photodetectors, light-emitting devices and electro-optic modulators.
关键词： transition-metal dichalcogenides,photodetectors,silicon photonics,van der Waals heterostructures,two-dimensional materials
更新于2025-09-19 17:13:59
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Recent progress in ultrafast lasers based on 2D materials as a saturable absorber
摘要： Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.
关键词： saturable absorber,MXene,antimonene,bismuthene,graphene,ultrafast lasers,transition metal dichalcogenides,black phosphorus,2D materials,topological insulators
更新于2025-09-19 17:13:59