The Role of GaN in the Heterostructure WS2/GaN for SERS Applications.

In the application of WS2 as a surface-enhanced Raman scattering (SERS) substrate, enhancing the charge transfer (CT) opportunity between WS2 and analyte is an important issue for SERS efficiency. In this study, we deposited few-layer WS2 (2-3 layers) on GaN and sapphire substrates with different bandgap characteristics to form heterojunctions using a chemical vapor deposition. Compared with sapphire, we found that using GaN as a substrate for WS2 can effectively enhance the SERS signal, with an enhancement factor of 6.45 × 104 and a limit of detection of 5 × 10-6 M for probe molecule Rhodamine 6G according to SERS measurement. Analysis of Raman, Raman mapping, atomic force microscopy, and SERS mechanism revealed that The SERS efficiency increased despite the lower quality of the WS2 films on GaN compared to those on sapphire, as a result of the increased number of transition pathways present in the interface between WS2 and GaN. These carrier transition pathways could increase the opportunity for CT, thus enhancing the SERS signal. The WS2/GaN heterostructure proposed in this study can serve as a reference for enhancing SERS efficiency.

Solid Phase Epitaxy of Single Phase Two-Dimensional Layered InSe Grown by MBE.

Single-phase two-dimensional (2D) indium monoselenide (γ-InSe) film is successfully grown via solid phase epitaxy in the molecular beam epitaxy (MBE) system. Having high electron mobility and high photoresponsivity, ultrathin 2D γ-InSe semiconductors are attractive for future field-effect transistor and optoelectronic devices. However, growing single-phase γ-InSe film is a challenge due to the polymorphic nature of indium selenide (γ-InSe, α-In2Se3, ß-In2Se3, γ-In2Se3, etc.). In this work, the 2D α-In2Se3 film was first grown on a sapphire substrate by MBE. Then, the high In/Se ratio sources were deposited on the α-In2Se3 surface, and an γ-InSe crystal emerged via solid-phase epitaxy. After 50 min of deposition, the initially 2D α-In2Se3 phase was also transformed into a 2D γ-InSe crystal. The phase transition from 2D α-In2Se3 to γ-InSe was confirmed by Raman, XRD, and TEM analysis. The structural ordering of 2D γ-InSe film was characterized by synchrotron-based grazing-incidence wide-angle X-ray scattering (GIWAXS).

Temperature Effect of van der Waals Epitaxial GaN Films on Pulse-Laser-Deposited 2D MoS2 Layer.

Van der Waals epitaxial GaN thin films on c-sapphire substrates with a sp2-bonded two-dimensional (2D) MoS2 buffer layer, prepared by pulse laser deposition, were investigated. Low temperature plasma-assisted molecular beam epitaxy (MBE) was successfully employed for the deposition of uniform and ~5 nm GaN thin films on layered 2D MoS2 at different substrate temperatures of 500, 600 and 700 °C, respectively. The surface morphology, surface chemical composition, crystal microstructure, and optical properties of the GaN thin films were identified experimentally by using both in situ and ex situ characterizations. During the MBE growth with a higher substrate temperature, the increased surface migration of atoms contributed to a better formation of the GaN/MoS2 heteroepitaxial structure. Therefore, the crystallinity and optical properties of GaN thin films can obviously be enhanced via the high temperature growth. Likewise, the surface morphology of GaN films can achieve a smoother and more stable chemical composition. Finally, due to the van der Waals bonding, the exfoliation of the heterostructure GaN/MoS2 can also be conducted and investigated by transmission electron microscopy. The largest granular structure with good crystallinity of the GaN thin films can be observed in the case of the high-temperature growth at 700 °C.

Complex Optical Conductivity of Two-Dimensional MoS2: A Striking Layer Dependency.

The complex optical conductivities of two-dimensaionl (2D) materials are fundamental for extended applications of related optoelectronic devices. Here, we systematically investigate the layer-dependent evolutions in the complex optical conductivities of 1-6 layer 2D MoS2 over an ultrawide spectral range (0.73-6.42 eV) by spectroscopic ellipsometry. We identify five feature peaks (A-E) in the optical conductivity spectra, which present interesting layer dependencies due to the scaling effect. Results suggest that the center energies of peaks A and B are nearly layer-independent, while those of peaks C and D exhibit redshifts as the layer increases. We interpret these layer-dependent evolutions as the competition between the decreasing exciton effect and the prominent band shrinkage with the increasing layer number. Additionally, the applicability of the classical slab model and the surface current model in evaluating the optical conductivities of 2D MoS2 with different layers is discussed from an experimental perspective.

Efficiency improvement of InGaP/GaAs/Ge solar cells by hydrothermal-deposited ZnO nanotube structure.

In this paper, a zinc oxide (ZnO) nanotube, fabricated by the hydrothermal growth method on triple-junction (T-J) solar cell devices to enhance efficiency, is investigated. Compared to those of bare T-J solar cells (without antireflection (AR) coating) and solar cells with Si3N4 AR coatings, the experimental results show that the T-J solar cells, which use a ZnO nanotube as an AR coating, have the lowest reflectance in the short wavelength spectrum. The ZnO nanotube has the lowest light reflection among all experimental samples, especially in the range of 350 to 500 nm from ultraviolet (UV) to visible light. It was found that a ZnO nanotube can enhance the conversion efficiency by 4.9%, compared with a conventional T-J solar cell. The Si3N4 AR coatings also enhance the conversion efficiency by 3.2%.The results show that a cell with ZnO nanotube coating could greatly improve solar cell performances.

Defects in semipolar (1122) ZnO grown on (112) LaAlO3/(La,Sr)(Al,Ta)O3 substrate by pulsed laser deposition.

The microstructure of semipolar [Formula: see text] ZnO deposited on (112) LaAlO3/(La,Sr)(Al,Ta)O3 was investigated by transmission electron microscopy. The ZnO shows an in-plane epitaxial relationship of [Formula: see text] with oxygen-face sense polarity. The misfit strain along [Formula: see text] and [Formula: see text] is relieved through the formation of misfit dislocations with the Burgers vectors [Formula: see text] and [Formula: see text], respectively. The line defects in the semipolar ZnO are predominantly perfect dislocations, and the dislocation density decreases with increasing ZnO thickness as a result of dislocation reactions. Planar defects were observed to lie in the M-plane and extend along 〈0001〉, whereas basal stacking faults were rarely found.