van der Waals layered structures, notably the transitional metal dichalcogenides (TMDs) and TMD-based heterostructures, have recently attracted immense interest due to their unique physical properties and potential applications in electronics, optoelectronics, and energy harvesting. Despite the recent progress, it is still a challenge to perform comprehensive characterizations of critical properties of these layered structures, including crystal structures, chemical dynamics, and interlayer coupling, using a single characterization platform. In this study, we successfully developed a multimodal nonlinear optical imaging method to characterize these critical properties of molybdenum disulfide (MoS2) and MoS2-based heterostructures. Our results demonstrate that MoS2 layers exhibit strong four-wave mixing (FWM), sum-frequency generation (SFG), and second-harmonic generation (SHG) nonlinear optical characteristics. We believe this is the first observation of FWM and SFG from TMD layers. All three kinds of optical nonlinearities are sensitive to layer numbers, crystal orientation, and interlayer coupling. The combined and simultaneous SHG/SFG-FWM imaging not only is capable of rapid evaluation of crystal quality and precise determination of odd-even layers but also provides in situ monitoring of the chemical dynamics of thermal oxidation in MoS2 and interlayer coupling in MoS2-graphene heterostructures. This method has the advantages of versatility, high fidelity, easy operation, and fast imaging, enabling comprehensive characterization of van der Waals layered structures for fundamental research and practical applications.

In this study, we successfully developed a carbon dioxide (CO)-laser-assisted metal-organic chemical vapor deposition (LMOCVD) approach to fast synthesis of high-quality gallium nitride (GaN) epilayers on AlO [sapphire(0001)] substrates. By employing a two-step growth procedure, high crystallinity and smooth GaN epilayers with a fast growth rate of 25.8 μm/h were obtained. The high crystallinity was confirmed by a combination of techniques, including X-ray diffraction, Raman spectroscopy, transmission electron microscopy, and atomic force microscopy. By optimizing growth parameters, the ∼4.3-μm-thick GaN films grown at 990 °C for 10 min showed a smooth surface with a root-mean-square surface roughness of ∼1.9 nm and excellent thickness uniformity with sharp GaN/substrate interfaces. The full-width at half-maximum values of the GaN(0002) X-ray rocking curve of 313 arcsec and the GaN(101̅2) X-ray rocking curve of 390 arcsec further confirmed the high crystallinity of the GaN epilayers. We also fabricated ultraviolet (UV) photodetectors based on the as-grown GaN layers, which exhibited a high responsivity of 0.108 A W at 367 nm and a fast response time of ∼125 ns, demonstrating its high optical quality with potential in optoelectronic applications. Our strategy thus provides a simple and cost-effective means toward fast and high-quality GaN heteroepitaxy growth suitable for fabricating high-performance GaN-based UV detectors.

Traditional ceramic-based, high-temperature electrode materials (e.g., lanthanum chromate) are severely limited due to their conditional electrical conductivity and poor stability under harsh circumstances. Advanced composite structures based on vertically aligned carbon nanotubes (VACNTs) and high-temperature ceramics are expected to address this grand challenge, in which ceramic serves as a shielding layer protecting the VACNTs from the oxidation and erosive environment, while the VACNTs work as a conductor. However, it is still a great challenge to fabricate VACNT/ceramic composite structures due to the limited diffusion of ceramics inside the VACNT arrays. In this work, we report on the controllable fabrication of infiltrated (and noninfiltrated) VACNT/silicon composite structures via thermal chemical vapor deposition (CVD) [and laser-assisted CVD]. In laser-assisted CVD, low-crystalline silicon (Si) was quickly deposited at the VACNT subsurfaces/surfaces followed by the formation of high-crystalline Si layers, thus resulting in noninfiltrated composite structures. Unlike laser-assisted CVD, thermal CVD activated the precursors inside and outside the VACNTs simultaneously, which realized uniform infiltrated VACNT/Si composite structures. The growth mechanisms for infiltrated and noninfiltrated VACNT/ceramic composites, which we attributed to the different temperature distributions and gas diffusion mechanism in VACNTs, were investigated. More importantly, the as-farbicated composite structures exhibited excellent multifunctional properties, such as excellent antioxidative ability (up to 1100 °C), high thermal stability (up to 1400 °C), good high velocity hot gas erosion resistance, and good electrical conductivity (∼8.95 Sm at 823 K). The work presented here brings a simple, new approach to the fabrication of advanced composite structures for hot electrode applications.

increasing attention as an emerging class of new materials due to their unique aspects and new functionalities in optical, electrical, and chemical properties. [1] The design and fabrication of 2D semiconductor heterostructures stand as a major strategy for the development of various kinds of devices that range from electronics, [2] photovoltaics, [3] sensors, [4,5] spintronics, [6,7] and memories [8] with the intriguing possibility of enhanced performance. For example, TMD (MoS 2 /WS 2 ) heterostructures exhibit ultrafast charge transfer due to the close proximity of the two heterolayers and the collective motion of excitons at the interface, which enables novel 2D devices for optoelectronics and light harvesting. [9] In the TMD/graphene heterostructures, such hybrid systems exhibit obvious rectification and ambipolar properties with a high field-effect on-off ratio of >10 6 due to the strong interlayer coupling and large band offset between two layers, suggesting their great potentials for future novel optoelectronic devices. [10] Additionally, TMD/graphene heterostructures also demonstrate excellent chemical sensing capability and stability, which provide an essential sensing platform for wearable electronics. [5] To date, scale-up fabrication of transition metal dichalcogenide (TMD-) based 2D/2D or 2D/3D heterostructures with specific functionalities is still a great challenge. This study, for the first time, reports on the controllable synthesis of large-area and continuous 2D/3D semiconductor/metal heterostructures consisting of monolayer MoS 2 and bulk MoO 2 with unique electrical and optical properties via one-step, vapor-transport-assisted rapid thermal processing. The temperature-dependent electrical transport measurements reveal that the 2D/3D MoS 2 -MoO 2 heterostructure grown on SiO 2 /Si substrates exhibits metallic phase, while this heterostructure becomes a lowresistance semiconductor when it is grown on fused silica, which is attributed to the different degrees of sulfurization on different substrates, as being confirmed by surface potential analyses. Photoluminescence measurements taken on the MoS 2 -MoO 2 heterostructures reveal the simultaneous presence of both negative trions and neutral excitons, while only neutral excitons are observed in the monolayer MoS 2 . The trion-binding energy is determined to be ≈27 meV, and the trion signal persists up to 330 K, indicating significant stability at room temperature. This work not only provides a new platform for understanding the intriguing physics in TMD-based heterostructures but also enables the design of more complicated devices with potential applications in nanoelectronics and nanophotonics.To date, 2D semiconducting transition metal dichalcogenides (TMDs) and TMD-based heterostructures have gained

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