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Transition metal nanoparticle decoration is widely used to enhance the hydrogen storage properties of carbon materials. However, little efforts have been devoted to unveil structural interaction between nanoparticles and substrate and understand the underlying kinetic mechanism for hydrogen sorption. In this work, a suite of TiO2 nanoparticles decorated graphene oxide (GO) composites is fabricated and characterized to determine the interactions between material structure and hydrogen storage kinetics. EELS and XPS results show that interactions between nanoparticles and GO cause changes of the chemical states of C–O, C O and C–OH groups; furthermore, reactions of C–OH, HO–C O and C–O–C groups with TiO2 nanoparticles create C–Ti and Ti–O–C bonding. Decoration of TiO2 nanoparticles improves the capacity of GO by 2.3×, and 80% of the adsorption is reversible. By means of semi-empirical kinetic analysis, it is determined that hydrogen adsorption is controlled by two-dimensional diffusion regardless of layer spacing; while desorption is controlled by multiple diffusion processes and is sensitive to layer spacing. Collectively, these new findings deepen the understanding of transition metal nanoparticles decorated GO materials in the aspects of nanoparticle incorporation and hydrogen storage kinetic mechanism. In particular, oxygen groups enhance nanoparticle decoration, while high layer spacing improves desorption kinetics and reversibility.

In this work, two kinds of novel manganese decorated (G + Mn) and manganese-vanadium co-decorated (G + MnV) graphene composites are synthesized by in situ wet chemical reduction, and their hydrogen storage properties and microstructures are characterized by Sievert-type adsorption apparatus, BET, SEM, TEM/STEM, EDX and EELS. Compared with pristine graphene, Mn decoration marginally increases the hydrogen adsorption capacity of graphene at room temperature and 4 MPa hydrogen pressure from 0.25 wt% to 0.36 wt%. On the other hand, the co-decoration of Mn and V increases the room temperature hydrogen storage capacity of graphene significantly to 1.81 wt% under 4 MPa hydrogen pressure, which is 1.56 wt% higher than the capacity of pristine graphene. The microstructures and valence states of the decorated Mn and Mn-V nanoparticles are investigated by TEM, EDX and EELS analyses, and strong interactions between the decorated nanoparticles and graphene are observed. Based on the results from structural analyses, potential enhancement mechanisms are suggested in terms of the catalytic effects of nanoparticles on graphene hydrogen adsorption. Given the relatively low cost of Mn and V metals compared to noble metals such as Pd, Pt and Au, these results demonstrate a low cost and effective way to significantly enhance the room temperature hydrogen adsorption properties of graphene for potential hydrogen storage applications.

GaAs grown with MBE is the basis for many useful optoelectric devices. Measurements are presented of the smoothing of patterned and randomly roughened GaAs surfaces during homoepitaxy over a large range of Ga flux, substrate temperatures, arsenic fluxes, and Bi surfactant. The bulk of these measurements were taken by in-situ elastic light scattering or ex-situ AFM. These measurements provide experimental support for a non-linear continuum growth model that has been derived analytically from basic atomic level phenomena that occur in epitaxial film growth. During epitaxial growth the smoothing is observed to change in nature as the surface amplitude decreases. One of the regimes of smoothing is associated with the linear smoothing coefficients from the physically based non-linear continuum growth equation. The temperature and growth rate dependence of the smoothing coefficients are presented and found to be in good agreement with predictions from the continuum growth model. A key parameter in the continuum growth equation, the density of atomic steps, is measured independently using AFM. The step density, which agrees with theoretical predictions, is used to compute smoothing coefficients and is shown to be in agreement with the light scattering measurements. Complex shapes are observed for epitaxial growth on patterned GaAs substrates. Two characteristic surface morphologies were observed. The first is characterized by downward V-shaped cusps and rounded mounds caused by non-linear smoothing. The second morphology is similar, however the symmetry of the surface structure was inverted. This surface morphology has not been previously observed in GaAs. Step edge attachment was found to be the driving mechanism that produced both of these morphologies. Bismuth is observed to act as a surfactant in GaAs homoepitaxy. While Bi assisted growth is found to decrease the overall surface roughness, it is also found to alter the characteristics of the surface morphology. Notably, roughness at low spatial frequency was increased with the addition of Bi, while at high spatial frequency roughness was decreased. Significant changes to the shape evolution of patterned substrate are also observed when Bi is added to GaAs epitaxial growth.

The photochemical reaction of Cl₂ gas with (100) GaAs was studied in this work. Thermal effects due to light induced heating of the GaAs substrate were isolated from photochemical effects. Light induced heating was calculated and the corresponding photothermal Cl₂ etching of GaAs was accounted for. The temperature dependence of the photochemical etch rate was examined. The light-induced etch rate potentially follows an Arrhenius temperature dependence with an activation energy of 0.161 eV. Near 200°C the light-induced etch rate does not increase with increasing temperature. The photochemical etch rate was found to depend linearly on intensity. A possible explanation for the temperature dependence of the photochemical etch rates is presented. It is proposed that the desorption of Ga Cl₃ is the important mechanism in the photochemical Cl₂-GaAs etch. GaCl₃ is photodesorbed with illumination, which increases the rate of etching.

Logic circuits and the ability to amplify electrical signals form the functional backbone of electronics along with the possibility to integrate multiple elements on the same chip. The miniaturization of electronic circuits is expected to reach fundamental limits in the near future. Two-dimensional materials such as single-layer MoS2 represent the ultimate limit of miniaturization in the vertical dimension, are interesting as building blocks of low-power nanoelectronic devices, and are suitable for integration due to their planar geometry. Because they are less than 1 nm thin, 2D materials in transistors could also lead to reduced short channel effects and result in fabrication of smaller and more power-efficient transistors. Here, we report on the first integrated circuit based on a two-dimensional semiconductor MoS2. Our integrated circuits are capable of operating as inverters, converting logical “1” into logical “0”, with room-temperature voltage gain higher than 1, making them suitable for incorporation into digital circuits. We also show that electrical circuits composed of single-layer MoS2 transistors are capable of performing the NOR logic operation, the basis from which all logical operations and full digital functionality can be deduced.

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials, have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we have used molecular beam epitaxy (MBE) to grow atomically thin MoSe2 on GaAs(111)B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates, allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe2. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film.