Influence of Exposure to a Wet Atmosphere on the UV-Sensing Characteristics of ZnO Nanorod Arrays.

Zinc oxide is a promising material for the creation of various types of sensors, in particular UV detectors. In this work, arrays of ordered nanorods were grown by chemical vapor deposition. The effect of environmental humidity on the sensing properties of zinc oxide nanorod arrays was investigated, and a prototype UV sensor using indium as an ohmic contact was developed. UV photoresponses were measured for the samples stored in dry and wet atmospheres. The increase in sensitivity and response of the ZnO nanorod arrays was observed after prolonged exposure to a wet atmosphere. A model was proposed to explain this effect. This is due to the formation of hydroxyl groups on the surface of zinc oxide nanorods, which is confirmed by FTIR spectroscopy data. For the first time, it has been shown that after storage in a wet atmosphere, the sensory properties of the structure remain stable regardless of the ambient humidity.

An Experimental Study of Dislocation Dynamics in GaN.

The dynamics of dislocations introduced through indentation or scratching at room temperature into a few GaN layers that were grown using the HVPE, MOCVD and ELOG methods and had different dislocation densities were studied via the electron-beam-induced current and cathodoluminescence methods. The effects of thermal annealing and electron beam irradiation on dislocation generation and multiplication were investigated. It is shown that the Peierls barrier for dislocation glide in GaN is essentially lower than 1 eV; thus, it is mobile even at room temperature. It is shown that the mobility of a dislocation in the state-of-the-art GaN is not entirely determined by its intrinsic properties. Rather, two mechanisms may work simultaneously: overcoming the Peierls barrier and overcoming localized obstacles. The role of threading dislocations as effective obstacles for basal plane dislocation glide is demonstrated. It is shown that under low-energy electron beam irradiation, the activation energy for the dislocation glide decreases to a few tens of meV. Therefore, under e-beam irradiation, the dislocation movement is mainly controlled by overcoming localized obstacles.

Electrical and Recombination Properties of Polar Orthorhombic κ-Ga2O3 Films Prepared by Halide Vapor Phase Epitaxy.

In this study, the structural and electrical properties of orthorhombic κ-Ga2O3 films prepared using Halide Vapor Phase Epitaxy (HVPE) on AlN/Si and GaN/sapphire templates were studied. For κ-Ga2O3/AlN/Si structures, the formation of two-dimensional hole layers in the Ga2O3 was studied and, based on theoretical calculations, was explained by the impact of the difference in the spontaneous polarizations of κ-Ga2O3 and AlN. Structural studies indicated that in the thickest κ-Ga2O3/GaN/sapphire layer used, the formation of rotational nanodomains was suppressed. For thick (23 µm and 86 µm) κ-Ga2O3 films grown on GaN/sapphire, the good rectifying characteristics of Ni Schottky diodes were observed. In addition, deep trap spectra and electron beam-induced current measurements were performed for the first time in this polytype. These experiments show that the uppermost 2 µm layer of the grown films contains a high density of rather deep electron traps near Ec - 0.3 eV and Ec - 0.7 eV, whose presence results in the relatively high series resistance of the structures. The diffusion length of the excess charge carriers was measured for the first time in κ-Ga2O3. The film with the greatest thickness of 86 µm was irradiated with protons and the carrier removal rate was about 10 cm-1, which is considerably lower than that for ß-Ga2O3.

Multiple Resistive Switching Mechanisms in Graphene Oxide-Based Resistive Memory Devices.

Among the different graphene derivatives, graphene oxide is the most intensively studied material as it exhibits reliable and repeatable resistive switching. The operative mechanisms that are responsible for resistive switching are being intensively investigated, and three models explaining the change in the resistive states have been developed. These models are grounded in the metallic-like filamentary conduction, contact resistance modification and the oxidation of/reduction in the graphene oxide bulk. In this work, using Al/GO/n-Si structures, we demonstrate that all three of these operative mechanisms can simultaneously participate in the resistive switching of graphene oxide. Multiple point-like conduction channels in the graphene oxide films were detected by the electron beam-induced current (EBIC) technique. At the same time, large areas with increased conductivity were also revealed by EBIC. An analysis of these areas by Raman spectroscopy indicates the change in the graphene oxide bulk's resistive properties. The EBIC data along with the measurements of the capacitance-voltage characteristics provided strong evidence of the involvement of an aluminum/graphene oxide interface in the switching processes. In addition, by using Al/GO/n-Si structures, we were able to identify unique local properties of the formed conductive channels, namely the change of the charge state of a conductive channel due to the creation of negatively charged traps and/or an increase in the GO work function.

Binder-Free MnO2/MWCNT/Al Electrodes for Supercapacitors.

Recently, significant progress has been made in the performance of supercapacitors through the development of composite electrodes that combine various charge storage mechanisms. A new method for preparing composite binder-free MnO2/MWCNT/Al electrodes for supercapacitors is proposed. The method is based on the original technique of direct growth of layers of multi-walled carbon nanotubes (MWCNTs) on aluminum foil by the catalytic pyrolysis of ethanol vapor. Binder-free MnO2/MWCNT/Al electrodes for electrochemical supercapacitors were obtained by simply treating MWCNT/Al samples with an aqueous solution of KMnO4 under mild conditions. The optimal conditions for the preparation of MnO2/MWCNT/Al electrodes were found. The treatment of MWCNT/Al samples in a 1% KMnO4 aqueous solution for 40 min increased the specific capacitance of the active material of the samples by a factor of 3, up to 100-120 F/g. At the same time, excellent adhesion and electrical contact of the working material to the aluminum substrate were maintained. The properties of the MnO2/MWCNT/Al samples were studied by electron probe microanalysis (EPMA), Raman spectroscopy, cyclic voltammetry (CV), and impedance spectroscopy. Excellent charge/discharge characteristics of composite electrodes were demonstrated. The obtained MnO2/MWCNT/Al electrodes maintained excellent stability to multiple charge-discharge cycles. After 60,000 CVs, the capacitance loss was less than 20%. Thus, this work opens up new possibilities for using the MWCNT/Al material obtained by direct deposition of carbon nanotubes on aluminum foil for the fabrication of composite binder-free electrodes of supercapacitors.

Electrochemical Improvement of the MWCNT/Al Electrodes for Supercapacitors.

An original technique of chemical deposition (CVD) by catalytic pyrolysis of ethanol vapor was used to directly grow multiwall carbon nanotubes (MWCNTs) layers on aluminum foil. The grown nanotubes had excellent adhesion and direct electrical contact to the aluminum substrate. This material was perfect for use in electrochemical supercapacitors. In this work, the possibility of a significant increase in the specific capacity of MWCNTs by simple electrochemical oxidation was investigated. The optimal conditions for improving the characteristics of the MWCNT/Al electrodes were found. Electrochemical treatment of MWCNT/Al electrodes in a 0.005 M Na2SO4 solution at a potential of 4-5 V for 20-30 min increased the specific capacity of MWCNTs from 30 F/g to 140 F/g. The properties of modified nanotubes were investigated by X-ray photoelectron spectroscopy, cyclic voltammetry (CV), and impedance spectroscopy. A significant increase in the concentration of oxygen-containing functional groups on the surface of MWCNTs was found as a result of electrochemical oxidation. The modified MWCNT/Al electrodes maintained excellent stability to multiple charge-discharge cycles. After 20,000 CVs, the capacity loss was less than 5%. Thus, the results obtained significantly expanded the possibilities of using MWCNT/Al composite materials obtained by the method of direct deposition of carbon nanotubes on aluminum foil as electrodes for supercapacitors.

Prediction of betavoltaic battery output parameters based on SEM measurements and Monte Carlo simulation.

An approach for a prediction of (63)Ni-based betavoltaic battery output parameters is described. It consists of multilayer Monte Carlo simulation to obtain the depth dependence of excess carrier generation rate inside the semiconductor converter, a determination of collection probability based on the electron beam induced current measurements, a calculation of current induced in the semiconductor converter by beta-radiation, and SEM measurements of output parameters using the calculated induced current value. Such approach allows to predict the betavoltaic battery parameters and optimize the converter design for any real semiconductor structure and any thickness and specific activity of beta-radiation source.