Due to its electronic-grade quality and potential for scalability, two-dimensional (2D) MoS2 synthesized by chemical vapor deposition (CVD) has been widely explored for electronic/optoelectronic applications. As 2D MoS2 can be considered a 100% surface, its unique intrinsic properties are inevitably altered by the substrate upon which it is grown. However, systematic studies of substrate-layer interactions in CVD-grown MoS2 are lacking. In this study, we have analyzed built-in strain and charge doping using Raman and photoluminescence spectroscopy in 2D MoS2 grown by CVD on four unique substrates: SiO2/Si, sapphire, Muscovite mica, and hexagonal boron nitride. We observed decreasing strain and charge doping in grown MoS2 as the substrates become less rough and more chemically inert. The possible origin of strain was investigated through atomic force microscopy roughness measurements of the as-grown layer and substrate. Our results provide direction for device optimization through careful selection of the growth substrate and pave the way for further investigations to unravel the complex nature of the 2D monolayer-substrate interface.
Integrating plasmonic materials into semiconductor media provides a promising approach for applications such as photosensing and solar energy conversion. The resulting structures introduce enhanced light-matter interactions, additional charge trap states, and efficient charge-transfer pathways for light-harvesting devices, especially when an intimate interface is built between the plasmonic nanostructure and semiconductor. Herein, we report the development of plasmonic photodetectors using Au@MoS heterostructures-an Au nanoparticle core that is encapsulated by a CVD-grown multilayer MoS shell, which perfectly realizes the intimate and direct interfacing of Au and MoS. We explored their favorable applications in different types of photosensing devices. The first involves the development of a large-area interdigitated field-effect phototransistor, which shows a photoresponsivity ∼10 times higher than that of planar MoS transistors. The other type of device geometry is a Si-supported Au@MoS heterojunction gateless photodiode. We demonstrated its superior photoresponse and recovery ability, with a photoresponsivity as high as 22.3 A/W, which is beyond the most distinguished values of previously reported similar gateless photodetectors. The improvement of photosensing performance can be a combined result of multiple factors, including enhanced light absorption, creation of more trap states, and, possibly, the formation of interfacial charge-transfer transition, benefiting from the intimate connection of Au and MoS.
There are emerging opportunities to harness diverse and complex geometric architectures based on nominal two-dimensional atomically layered structures. Herein we report synthesis and properties of a new core-shell heterostructure, termed Au@MoS, where the Au nanoparticle is snugly and contiguously encapsulated by few shells of MoS atomic layers. The heterostructures were synthesized by direct growth of multilayer fullerene-like MoS shell on Au nanoparticle cores. The Au@MoS heterostructures exhibit interesting light-matter interactions due to the structural curvature of MoS shell and the plasmonic effect from the underlying Au nanoparticle core. We observed significantly enhanced Raman scattering and photoluminescence emission on these heterostructures. We attribute these enhancements to the surface plasmon-induced electric field, which simulations show to mainly localize within the MoS shell. We also found potential evidence for the charge transfer-induced doping effect on the MoS shell. The DFT calculations further reveal that the structural curvature of MoS shell results in a modification of its electronic structure, which may facilitate the charge transfer from MoS to Au. Such Au@MoS core-shell heterostructures have the potential for future optoelectronic devices, optical imaging, and other energy-environmental applications.
Molybdenum disulfide (MoS 2 ) has been recognized as a promising cost-effective catalyst for water-splitting hydrogen production. However, the desired performance of MoS 2 is often limited by insufficient edge-terminated active sites, poor electrical conductivity, and inefficient contact to the supporting substrate. To address these limitations, we developed a unique nanoarchitecture (namely, winged Au@MoS 2 heterostructures enabled by our discovery of the "seeding effect" of Au nanoparticles for the chemical vapor deposition synthesis of vertically aligned fewlayer MoS 2 wings). The winged Au@MoS 2 heterostructures provide an abundance of edge-terminated active sites and are found to exhibit dramatically improved electrocatalytic activity for the hydrogen evolution reaction. Theoretical simulations conducted for this unique heterostructure reveal that the hydrogen evolution is dominated by the proton adsorption step, which can be significantly promoted by introducing sufficient edge active sites. Our study introduces a new morphological engineering strategy to make the pristine MoS 2 layered structures highly competitive earth-abundant catalysts for efficient hydrogen production.
Bulk and nanoscale molybdenum trioxide (MoO 3 ) has shown impressive technologically relevant properties, but deeper investigation into 2D MoO 3 has been prevented by the lack of reliable vapor-based synthesis and doping techniques. Herein, the successful synthesis of high-quality, few-layer MoO 3 down to bilayer thickness via physical vapor deposition is reported. The electronic structure of MoO 3 can be strongly modified by introducing oxygen substoichiometry (MoO 3−x ), which introduces gap states and increases conductivity.A dose-controlled electron irradiation technique to introduce oxygen vacancies into the few-layer MoO 3 structure is presented, thereby adding n-type doping. By combining in situ transport with core-loss and monochromated low-loss scanning transmission electron microscopy-electron energy-loss spectroscopy studies, a detailed structure-property relationship is developed between Mo-oxidation state and resistance. Transport properties are reported for MoO 3−x down to three layers thick, the most 2D-like MoO 3−x transport hitherto reported. Combining these results with density functional theory calculations, a radiolysis-based mechanism for the irradiation-induced oxygen vacancy introduction is developed, including insights into favorable configurations of oxygen defects. These systematic studies represent an important step forward in bringing few-layer MoO 3 and MoO 3−x into the 2D family, as well as highlight the promise of MoO 3−x as a functional, tunable electronic material.
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