Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have shown promise for a variety of optoelectronic applications due to a wide range of optical, electrical, and mechanical properties. ...Large-area chemical vapor deposition (CVD)-grown TMDC flakes could be useful in such devices. However, the defects present in large-area TMDC flakes can significantly influence carrier dynamics and transport properties. Here, the ultrafast carrier dynamics of monolayer tungsten disulfide (WS2) covering a large area of the substrate was explored using transient absorption spectroscopy. By monitoring the transient optical response, exciton trapping by oxygen-induced defects has been identified in monolayer WS2. We observe excitation-density-dependent exciton decay dynamics for both band-edge and above band-edge excitations due to exciton–exciton annihilation. Our results demonstrate the impact of defect states on carrier recombination in CVD-grown TMDCs, which could pave the way for utilizing such materials in optoelectronic device applications.
The electronic structure and dynamics of 2D transition metal dichalcogenide (TMD) monolayers provide important underpinnings both for understanding the many-body physics of electronic quasi-particles ...and for applications in advanced optoelectronic devices. However, extensive experimental investigations of semiconducting monolayer TMDs have yielded inconsistent results for a key parameter, the quasi-particle band gap (QBG), even for measurements carried out on the same layer and substrate combination. Here, we employ sensitive time- and angle-resolved photoelectron spectroscopy (trARPES) for a high-quality large-area MoS2 monolayer to capture its momentum-resolved equilibrium and excited-state electronic structure in the weak-excitation limit. For monolayer MoS2 on graphite, we obtain QBG values of ≈2.10 eV at 80 K and of ≈2.03 eV at 300 K, results well-corroborated by the scanning tunneling spectroscopy (STS) measurements on the same material.
Transition metal dichalcogenides are known to possess large optical nonlinearities, and driving these materials at high intensities is desirable for many applications. Understanding their optical ...responses under repetitive intense excitation is essential to improve the performance limit of these strong-field devices and to achieve efficient laser patterning. Here, we report the incubation study of monolayer MoS2 and WS2 induced by 160 fs, 800 nm pulses in air to examine how their ablation threshold scales with the number of admitted laser pulses. Both materials were shown to outperform graphene and most bulk materials; specifically, MoS2 is as resistant to radiation degradation as the best of the bulk thin films with a record fast saturation. Our modeling provides convincing evidence that the small reduction in threshold and fast saturation of MoS2 originate from its excellent bonding integrity against radiation-induced softening. Sub-ablation damages, in the form of vacancies, strain, lattice disorder, and nanovoids, were revealed by transmission electron microscopy, photoluminescence, Raman, and second harmonic generation studies, which were attributed to the observed incubation in 2D materials. For the first time, a sub-ablation damage threshold is identified for monolayer MoS2 to be 78% of the single-shot ablation threshold, below which MoS2 remains intact for many laser pulses. Our results firmly establish MoS2 as a robust material for strong-field devices and for high-throughput laser patterning.
Utilization of the excess energy of photoexcitation that is otherwise lost as thermal effects can improve the efficiency of next-generation light-harvesting devices. Multiple exciton generation (MEG) ...in semiconducting materials yields two or more excitons by absorbing a single high-energy photon, which can break the Shockley–Queisser limit for the conversion efficiency of photovoltaic devices. Recently, monolayer transition metal dichalcogenides (TMDs) have emerged as promising light-harvesting materials because of their high absorption coefficient. Here, we report efficient MEGs with low threshold energy and high (86%) efficiency in a van der Waals (vdW) layered material, MoS2. Through different experimental approaches, we demonstrate the signature of exciton multiplication and discuss the possible origin of decisive MEG in monolayer MoS2. Our results reveal that vdW-layered materials could be a potential candidate for developing mechanically flexible and highly efficient next-generation solar cells and photodetectors.
Atomically thin two-dimensional transition-metal dichalcogenides (TMDCs) have attracted much attention recently due to their unique electronic and optical properties for future optoelectronic ...devices. The chemical vapor deposition (CVD) method is able to generate TMDCs layers with a scalable size and a controllable thickness. However, the TMDC monolayers grown by CVD may incorporate structural defects, and it is fundamentally important to understand the relation between photoluminescence and structural defects. In this report, point defects (Se vacancies) and oxidized Se defects in CVD-grown MoSe2 monolayers are identified by transmission electron microscopy and X-ray photoelectron spectroscopy. These defects can significantly trap free charge carriers and localize excitons, leading to the smearing of free band-to-band exciton emission. Here, we report that the simple hydrohalic acid treatment (such as HBr) is able to efficiently suppress the trap-state emission and promote the neutral exciton and trion emission in defective MoSe2 monolayers through the p-doping process, where the overall photoluminescence intensity at room temperature can be enhanced by a factor of 30. We show that HBr treatment is able to activate distinctive trion and free exciton emissions even from highly defective MoSe2 layers. Our results suggest that the HBr treatment not only reduces the n-doping in MoSe2 but also reduces the structural defects. The results provide further insights of the control and tailoring the exciton emission from CVD-grown monolayer TMDCs.
Single‐layered MoS2 is a naturally stable material. Integrating spin, valley, and circularly polarized photons is an interesting endeavor to achieve advanced spin‐valleytronics. In this study, ...room‐temperature ferromagnetism in MoS2 induced by the magnetic proximity effect (MPE) of yttrium iron garnet (YIG) and the antiferromagnetic coupling at the interface is demonstrated. Insulating YIG without charge carriers is an excellent magnetic candidate featuring a long spin diffusion length and remarkable surface flatness, enabling long‐range magnetic interactions with MoS2. Spin‐resolved photoluminescence spectroscopy and magnetic circular dichroism (MCD) reveal that the spin‐polarized valleys of MoS2 can achieve sustained ferromagnetism even at room temperature. The bandgap‐sensitivity of MCD further demonstrates the extent of antiferromagnetic coupling between the MPE‐induced moments of MoS2 and YIG. This work provides a layer‐selected approach to study magnetic interactions/configurations in the YIG/MoS2 bilayer and highlights the role of MoS2 in achieving the MPE toward high temperature.
Room‐temperature ferromagnetism of MoS2 induced by the magnetic proximity effect of yttrium iron garnet and the antiferromagnetic coupling at the interface is demonstrated, which has been shown strongly related to the spin‐transfer across the interface of the heterostructure, supported by both spectroscopy and first‐principles calculation. This observation provides a route for MoS2‐based spin‐valleytronics toward room‐temperature applications.
A moiré superlattice formed in twisted van der Waals bilayers has emerged as a new tuning knob for creating new electronic states in two-dimensional materials. Excitonic properties can also be ...altered drastically due to the presence of moiré potential. However, quantifying the moiré potential for excitons is nontrivial. By creating a large ensemble of MoSe2/MoS2 heterobilayers with a systematic variation of twist angles, we map out the minibands of interlayer and intralayer excitons as a function of twist angles, from which we determine the moiré potential for excitons. Surprisingly, the moiré potential depth for intralayer excitons is up to ∼130 meV, comparable to that for interlayer excitons. This result is markedly different from theoretical calculations based on density functional theory, which show an order of magnitude smaller moiré potential for intralayer excitons. The remarkably deep intralayer moiré potential is understood within the framework of structural reconstruction within the moiré unit cell.
The properties of van der Waals heterostructures are drastically altered by a tunable moiré superlattice arising from periodically varying atomic alignment between the layers. Exciton diffusion ...represents an important channel of energy transport in transition metal dichalcogenides (TMDs). While early studies performed on TMD heterobilayers suggested that carriers and excitons exhibit long diffusion, a rich variety of scenarios can exist. In a moiré crystal with a large supercell and deep potential, interlayer excitons may be completely localized. As the moiré period reduces at a larger twist angle, excitons can tunnel between supercells and diffuse over a longer lifetime. The diffusion should be the longest in commensurate heterostructures where the moiré superlattice is completely absent. Here, we experimentally demonstrate the rich phenomena of interlayer exciton diffusion in WSe
/MoSe
heterostructures by comparing several samples prepared with chemical vapor deposition and mechanical stacking with accurately controlled twist angles.
2D materials have great potential for not only device scaling but also various applications. To prompt the development of 2D electronics and optoelectronics, a better understanding of the limitation ...of materials is essential. Material failure caused by bias can lead to variations in device behavior and even electrical breakdown. In this study, the structural evolution of monolayer MoS
with high bias is revealed via in situ transmission electron microscopy at the atomic scale. The biasing process is recorded and studied with the aid of aberration-corrected scanning transmission electron microscopy. The effects of electron beam irradiation and biasing are also discussed through the combination of experiments and theory. It is found that the Mo nanoclusters result from disintegration of MoS
and sulfur depletion, which are induced by Joule heating. The thermal stress can also damage the MoS
layer and form long cracks in both in situ and ex situ biasing cases. Investigation of the results obtained with different applied voltages helps to further verify the mechanism of evolution and provide a comprehensive study of the function of biasing.
•Single crystalline AlN films were obtained on sapphire with MoS2 as buffer layer.•The dislocation density of AlN films were decreased using the inserted MoS2 layer.•The growth temperature is reduced ...to 300 °C, a record low for epitaxial AlN growth.•The finding is important for GaN industry, as AlN often serves as a seed layer.•The finding is beneficial to the exploration for the application of 2-D materials.
AlN films are commonly deposited on sapphire substrates. However, the growth process usually requires high temperature and multi-steps to improve the film quality, due to the large lattice mismatch between the film and substrate. Here we demonstrate that using monolayer MoS2 grown on sapphire as a template, single crystalline AlN films with high crystallinity can be obtained at a much lower growth temperature using helicon sputtering system. The x-ray rocking curve of AlN films prepared on MoS2/sapphire template at 400 °C shows a full width at half maximum of 0.050°, showing a dramatic reduction compared to that of 0.803° for those grown directly on sapphire substrate. The estimated dislocation density of AlN on MoS2/sapphire is decreased to 7.7x107 cm−2, which is comparable to that prepared by metal-organic chemical vapor deposition at high temperatures. The small lattice mismatch between MoS2 and AlN, as well as the polar crystal of monolayer MoS2 apparently improve the quality of AlN films and facilitate AlN formed at lower temperature.
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