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We study structural and chemical transformations induced by focused laser beam in GaAs nanowires with axial zinc-blende/wurtzite (ZB/WZ) heterostucture. The experiments are performed using a combination of transmission electron microscopy, energy-dispersive X-ray spectroscopy, Raman scattering, and photoluminescence spectroscopy. For the both components of heterostructure, laser irradiation under atmospheric air is found to produce a double surface layer which is composed of crystalline arsenic and of amorphous GaO$_{x}$. The latter compound is responsible for appearance of a peak at 1.76 eV in photoluminescence spectra of GaAs nanowires. Under increased laser power density, due to sample heating, evaporation of the surface crystalline arsenic and formation of $\beta$-Ga$_{2}$O$_{3}$ nanocrystals proceed on surface of the zinc-blende part of nanowire. The formed nanocrystals reveal a photoluminescence band in visible range of 1.7-2.4 eV. At the same power density for wurtzite part of the nanowire, total amorphization with formation of $\beta$-Ga$_{2}$O$_{3}$ nanocrystals occurs. Observed transformation of WZ-GaAs to $\beta$-Ga$_{2}$O$_{3}$ nanocrystals presents an available way for creation of axial and radial heterostuctures ZB-GaAs/$\beta$-Ga$_{2}$O$_{3}$ for optoelectronic and photonic applications., Comment: 12 pages, 5 figures

Signal measured by electrostatic force gradient microscopy (EFGM) is the z-gradient of the electrostatic force acting between the probe of an atomic force microscope (AFM) and the surface under study. A model is presented for calculating the z-gradient of the electrostatic interaction of the AFM probe with local charges in a dielectric layer at the surface. In the EFGM mode, the interaction of charges with only the probe tip apex should be taken into account. In this approach, a simplified expression can be derived for calculating the z-gradient of the electrostatic force. The model makes it possible to estimate the lateral resolution limit for EFGM imaging of individual charges and to simulate experimental EFGM images as a function of the tip-surface distance and the tip radius. The adequacy of the model was confirmed by quantitative simulation of the experimental EFGM images of local charges injected from the AFM probe into a planar array of Si nanocrystals in a thin SiO2 layer on a Si substrate.

Application of Kelvin probe force gradient microscopy (KPFGM) to visualize the local charge dissipation in thin dielectric layers is considered. By this method, the local charge behavior in nano thin SiO2, Si3N4, and LaScO3 dielectric layers has been studied. Local charging of the layers has been performed at the point contact with a conductive probe. KPFGM potential images reveal variations of the surface potential in the locally charged areas, which makes it possible to detect the injected charge and to study its behavior. Special experiments on the SiO2 layers with embedded Si-nanocrystals, when lateral spreading of injected charge had been suppressed, permitted to demonstrate high (better than 20 nm) lateral resolution of KPFGM observations. A simple electrostatic model has been developed to estimate the total amount of injected charge. The obtained estimations made it possible to control charge retention in the dielectric layer and possible leaks into the substrate. The studied dielectric layers demo...