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Impact of Defects on Electronic Properties of Heterostructures Constructed From Monolayers of Transition Metal Dichalcogenides
摘要： Electronic properties of heterostructures composed of two single molecular layers (monolayers) of MoS2, WS2, WSe2, and MoSe2 are ab initio simulated with an emphasis to the stacking peculiarities and an influence of point defects in their lattices. MoS2/MoSe2, MoS2/WS2, WS2/WSe2, and MoSe2/WSe2 heterostructures with the monolayers shifted like in the bulk material have been found to behave like semiconductors with the energy gaps of 0.88, 1.25, 1.06, and 1.07 eV, respectively. Such heterostructures possess indirect gaps in contrast to individual monolayers, while direct-gap character is preserved in two layer stacking variants in WS2/WSe2 heterostructures and in MoS2/MoSe2 heterostructure with mirror stacking of the monolayers. Vacancies and Te atoms substituting other chalcogen atoms reduce the band gaps. The calculated orbital compositions of first direct band gap transitions in the defect-free heterostructures and those with the point defects have demonstrated d-electrons of Mo or W atoms to be mainly involved in the transitions.
关键词： defects,two-dimensional crystals,transition metal dichalcogenides,heterostructures,electronic structure
更新于2025-09-23 15:22:29
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Sb <sub/>2</sub> Te <sub/>3</sub> growth study reveals: Formation of Nanoscale Charge Carrier Domains Is an Intrinsic Feature Relevant for Electronic Applications
摘要： Sb2Te3 exhibits a plethora of fundamentally relevant electronic phenomena enabling electronic phase change memory cells, thermoelectric devices and three-dimensional topological insulator structures. Thus, the controlled growth of nanostructures and thin films with well-defined electronic properties is of uttermost importance. Previously, our group observed symmetric infrared domains in hexagonal Sb2Te3 nanoplatelets from a solvothermal chemical synthesis. The relative optical contrast observed was indirectly linked to the formation of regions with different defect densities (charge carrier concentrations). This raises two major questions, which we answer in this study: Is the domain formation restricted to the specific platelet growth process? No! Do the infrared spectra of both domains really follow a ‘Drude-like’ free charge carrier response? Yes! By controlling the initial water concentration, we promote the growth of the nanoplatelets in c-direction and tune the morphology from platelet-like to octahedra-like. Although the growth mode changes from spiral growth to layer-by-layer, similar infrared domains are identified using scattering-type scanning near-field optical microscopy (s-SNOM). Furthermore, we also reproduced the formation of symmetric infrared domains in thin, high quality crystalline films grown using molecular beam epitaxy (MBE). Normalized infrared near-field spectra of smaller Sb2Te3 nanoparticles reveal a relative shift of the plasma frequency in both domains. These findings demonstrate that the formation of domains with different charge carrier properties is an intrinsic material property of Sb2Te3 and might strongly influence all of its electronic applications.
关键词： infrared near-field microscopy,Antimony telluride,domain formation,van der Waals materials,transition metal dichalcogenides,epitaxial growth
更新于2025-09-23 15:21:21
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Selective Transfer of Rotationally Commensurate MoS <sub/>2</sub> from an Epitaxially Grown van der Waals Heterostructure
摘要： Large-scale synthesis of high quality two-dimensional (2D) semiconductors are critical for their incorporation in emerging electronic and optoelectronic technologies. In particular, chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) via van der Waals epitaxy on epitaxial graphene (EG) leads to rotationally commensurate TMDs in contrast to randomly aligned TMDs grown on amorphous oxide substrates. However, the interlayer coupling between TMDs and EG hinders the investigation and utilization of the intrinsic electronic properties of the resulting TMDs, thus requiring their isolation from the EG growth substrate. To address this issue, we report here a technique for selectively transferring monolayer molybdenum disulfide (MoS2) from CVD-grown MoS2-EG van der Waals heterojunctions using copper (Cu) adhesion layers. The choice of Cu as the adhesion layer is motivated by density functional theory calculations that predict the preferential binding of monolayer MoS2 to Cu in contrast to graphene. Atomic force microscopy and optical spectroscopy confirm the large-scale transfer of rotationally commensurate MoS2 onto SiO2/Si substrates without cracks, wrinkles, or residues. Furthermore, the transferred MoS2 shows high performance in field-effect transistors with mobilities up to 30 cm2/Vs and on/off ratios up to 106 at room temperature. This transfer technique can likely be generalized to other TMDs and related 2D materials grown on EG, thus offering a broad range of benefits in nanoelectronic, optoelectronic, and photonic applications.
关键词： molybdenum disulfide,van der Waals epitaxy,two-dimensional semiconductors,field-effect transistors,copper adhesion layers,transition metal dichalcogenides,chemical vapor deposition
更新于2025-09-23 15:21:21
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Flexoelectricity in Monolayer Transition Metal Dichalcogenides
摘要： Flexoelectricity, the coupling effect of the strain gradient and charge polarization, is an important route to tune electronic properties of low-dimensional materials. Here our extensive first-principles calculations reveal that structural wrinkling and corrugation will cause significant flexoelectricity in transition metal dichalcogenide (TMD) monolayers. The flexoelectricity is induced by the strain gradients created along the finite thickness of the wrinkled TMD monolayers and becomes more dominant in determining out-of-plane polarizations with decreasing wavelengths of the TMD wrinkles. According to the first-principles calculations and whole structural symmetry, a theoretical model is developed to describe the total out-of-plane polarizations and flexoelectric effect of the wrinkled TMD monolayers. The unveiled flexoelectricity in monolayer TMDs highlights a potential for their application in energy conversion devices.
关键词： energy conversion,monolayer,strain gradient,flexoelectricity,polarization,transition metal dichalcogenides
更新于2025-09-23 15:21:21
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Moire intralayer excitons in a MoSe <sub/>2</sub> /MoS <sub/>2</sub> heterostructure
摘要： Spatially periodic structures with a long range period, referred to as moir′e pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra and interlayer excitons of transition metal dichalcogenides heterostructures. Here, we report on the impact of the moir′e pattern formed in a MoSe2/MoS2 heterobilayer encapsulated in hexagonal boron nitride. The periodic in-plane potential results in a splitting of the MoSe2 exciton and trion in both emission and absorption spectra. The observed energy difference between the split peaks is fully consistent with theoretical predictions.
关键词： valley polarization,van der Waals heterostructures,moir′e pattern,moir′e excitons,Transition metal dichalcogenides
更新于2025-09-23 15:21:21
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Van der Waals junction field effect transistors with both n- and p-channel transition metal dichalcogenides
摘要： Two-dimensional (2D) transition metal dichalcogenides (TMDs)-based van der Waals (vdW) PN junctions have been used for heterojunction diodes, which basically utilize out-of-plane current across the junction interface. In fact, the same vdW PN junction structure can be utilized for another important device application, junction field effect transistors (JFETs), where in-plane current is possible along with 2D–2D heterojunction interface. Moreover, the 2D TMD-based JFET can use both p- and n-channel for low voltage operation, which might be its unique feature. Here we report vdW JFETs as an in-plane current device with heterojunction between semiconducting p- and n-TMDs. Since this vdW JFET would have low-density traps at the vdW interface unlike 2D TMD-based metal insulator semiconductor field effect transistors (MISFETs), little hysteresis of 0.0–0.1 V and best subthreshold swing of ~100 mV/dec were achieved. Easy saturation was observed either from n-channel or p-channel JFET as another advantage over 2D MISFETs, exhibiting early pinch-off at ~1 V. Operational gate voltage for threshold was near 0 V and our highest mobility reaches to ~>500 cm2/V·s for n-channel JFET with MoS2 channel. For 1 V JFET operation, our best ON/OFF current ratio was observed to be ~104.
关键词： n-channel,field effect transistors,van der Waals junction,transition metal dichalcogenides,p-channel
更新于2025-09-23 15:21:21
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Photogenerated-Carrier Separation and Transfer in Two-Dimensional Janus Transition Metal Dichalcogenides and Graphene van der Waals Sandwich Heterojunction Photovoltaic Cells
摘要： Two-dimensional (2D) Janus transition metal dichalcogenides (JTMDs) show direct band gaps and strong visible-light absorption with promising applications in photovoltaic (PV) cells. Here, we investigate the electronic structures and dynamics of photogenerated carriers in 2D JTMDs and graphene van der Waals sandwich heterojunction (G/JTMDs/G) photovoltaic cells by using first-principles calculations. We find that the intrinsic built-in electric field in JTMDs results in an asymmetry potential, which can be used to effectively enhance the separation and transfer of photogenerated carriers from JTMDs to different graphene layers with a preferred direction within hundreds of femtoseconds in the G/JTMDs/G heterostructures. Furthermore, the photogenerated electrons (holes) can transfer from monolayer MoSSe (MoSeTe) to the graphene sheets by the Se side with lower (higher) potential, while the transfer of the photogenerated holes (electrons) is prohibited due to the large separation between donor and acceptor states.
关键词： First-principles calculations,van der Waals heterostructures,Janus transition metal dichalcogenides,Photogenerated carriers,Two-dimensional materials,Graphene,Charge transfer,Photovoltaic cells
更新于2025-09-23 15:21:01
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Ultrafast Electron Cooling and Decay in Monolayer WS <sub/>2</sub> Revealed by Time- and Energy-Resolved Photoemission Electron Microscopy
摘要： A comprehensive understanding of the ultrafast electron dynamics in two-dimensional transition metal dichalcogenides (TMDs) is necessary for their applications in optoelectronic devices. In this work, we contribute a study of ultrafast electron cooling and decay dynamics in the supported and suspended monolayer WS2 by time- and energy-resolved photoemission electron microscopy (PEEM). Electron cooling in the Q valley of the conduction band is clearly resolved in energy and time, on a timescale of 0.3 ps. Electron decay is mainly via defect trapping process on a timescale of several picoseconds. We observed that the trap states can be produced and increased by laser illumination under ultra-high vacuum, and the higher local optical-field intensity led to the faster increase of trap states. The enhanced defect trapping could significantly modify the carrier dynamics and should be paid attention to in photoemission experiments for two-dimensional materials.
关键词： transition metal dichalcogenides,defect trapping,ultrafast dynamics,energy-resolved,electron cooling,photoemission electron microscopy
更新于2025-09-23 15:21:01
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Strongly Coupled Coherent Phonons in Single-Layer MoS <sub/>2</sub>
摘要： We present a transient absorption setup combining broadband detection over the visible-UV range with high temporal resolution (～20fs) which is ideally suited to trigger and detect vibrational coherences in different classes of materials. We generate and detect coherent phonons (CPs) in single layer (1L) MoS2, as a representative semiconducting 1L-Transition Metal Dichalcogenide (TMD), where the confined dynamical interaction between excitons and phonons is unexplored. The coherent oscillatory motion of the out-of-plane A'1 phonons, triggered by the ultrashort laser pulses, dynamically modulates the excitonic resonances on a timescale of few tens fs. We observe an enhancement by almost two orders of magnitude of the CP amplitude when detected in resonance with the C exciton peak, combined with a resonant enhancement of CP generation efficiency. Ab Initio calculations of the change in 1L-MoS2 band structure induced by the A'1 phonon displacement confirm a strong coupling with the C exciton. The resonant behavior of the CP amplitude follows the same spectral profile of the calculated Raman susceptibility tensor. These results explain the CP generation process in 1L-TMDs and demonstrates that CP excitation in 1L-MoS2 can be described as a Raman-like scattering process.
关键词： exciton-phonon interaction,Coherent phonons,transient absorption spectroscopy,transition metal dichalcogenides,ab-initio calculation
更新于2025-09-23 15:21:01
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2D Material Optoelectronics for Information Functional Device Applications: Status and Challenges
摘要： Graphene and the following derivative 2D materials have been demonstrated to exhibit rich distinct optoelectronic properties, such as broadband optical response, strong and tunable light–mater interactions, and fast relaxations in the flexible nanoscale. Combining with optical platforms like fibers, waveguides, grating, and resonators, these materials has spurred a variety of active and passive applications recently. Herein, the optical and electrical properties of graphene, transition metal dichalcogenides, black phosphorus, MXene, and their derivative van der Waals heterostructures are comprehensively reviewed, followed by the design and fabrication of these 2D material-based optical structures in implementation. Next, distinct devices, ranging from lasers to light emitters, frequency convertors, modulators, detectors, plasmonic generators, and sensors, are introduced. Finally, the state-of-art investigation progress of 2D material-based optoelectronics offers a promising way to realize new conceptual and high-performance applications for information science and nanotechnology. The outlook on the development trends and important research directions are also put forward.
关键词： transition metal dichalcogenides,optoelectronics,MXene,van der Waals heterostructures,information devices,graphene,black phosphorus,2D materials
更新于2025-09-23 15:21:01