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A new technique for determining the point spread function, which is required for measuring the surface potential using Kelvin probe microscopy (KPM), is presented. The method involves using a silicon carbide substrate coated with single-layer and bilayer graphene as a test structure and obtaining KPM potential profiles in different directions on the surface. This makes it possible to determine the KPM point spread function, which can be used to perform deconvolution and accurately recover the surface potential.

Young's modulus of tapered mixed composition (zinc-blende with a high density of twins and wurtzite with a high density of stacking faults) gallium phosphide (GaP) nanowires (NWs) was investigated by atomic force microscopy. Experimental measurements were performed by obtaining bending profiles of as-grown inclined GaP NWs deformed by applying a constant force to a series of NW surface locations at various distances from the NW/substrate interface. Numerical modeling of experimental data on bending profiles was done by applying Euler-Bernoulli beam theory. Measurements of the nano-local stiffness at different distances from the NW/substrate interface revealed NWs with a non-ideal mechanical fixation at the NW/substrate interface. Analysis of the NWs with ideally fixed base resulted in experimentally measured Young's modulus of 155 +/- 20 GPa for ZB NWs, and 157 +/- 20 GPa for WZ NWs, respectively, which are in consistence with a theoretically predicted bulk value of 167 GPa. Thus, impacts of the crystal structure (WZ/ZB) and crystal defects on Young's modulus of GaP NWs were found to be negligible.

AbstractThe properties of the hybrid energy sources “piezoelectric nanogenerator-solar cell” based on GaAs nanowires with the wurtzite crystal structure were investigated. Measurements were performed by the bending of the nanowire by the probe of the atomic force microscope with simultaneous recording of short circuit current in dark and illuminated conditions. We show that a piezoelectric current pulse of ~10 pA arises in the “nanowire-probe” circuit during the deformation of nanowire by the AFM probe. Under laser illumination, the value of the pulse increases by two orders of magnitude as a result of the piezophototronic effect. Deformation of the nanowire boosts the photocurrent by 40% up to 0.5 nA.

In this work, various regimes of local anodic oxidation (LAO) of MoSe2 were studied. Here we show that there is a certain set of oxidation parameters that results in the anisotropic oxidation of MoSe2. In this mode, LAO leads to the formation of oxidized triangles. The triangles have the same orientation on the surface of the flakes, which indicates that MoSe2 is oxidized mainly along the crystallographic directions of the zigzag edges. These results can be useful for determining crystallographic directions of zigzag/armchair edges and the degree of single-crystallinity of MoSe2 flakes.

In this work, the stability of short-wavelength spatial modes in quarter-disk cavities was studied. It was found that in such resonators, short-wave modes (with wavelengths λ smaller than the radius R of the quarter-disk λ<0.1 lack total internal reflection modes.

Research regarding ways to increase solar cell efficiency is in high demand. Mechanical deformation of a nanowire (NW) solar cell can improve its efficiency. Here, the effect of uniaxial compression on GaAs nanowire solar cells was studied via conductive atomic force microscopy (C-AFM) supported by numerical simulation. C-AFM I&ndash, V curves were measured for wurtzite p-GaAs NW grown on p-Si substrate. Numerical simulations were performed considering piezoresistance and piezoelectric effects. Solar cell efficiency reduction of 50% under a &minus, 0.5% strain was observed. The analysis demonstrated the presence of an additional fixed electrical charge at the NW/substrate interface, which was induced due to mismatch between the crystal lattices, thereby affecting the efficiency. Additionally, numerical simulations regarding the p-n GaAs NW solar cell under uniaxial compression were performed, showing that solar efficiency could be controlled by mechanical deformation and configuration of the wurtzite and zinc blende p-n segments in the NW. The relative solar efficiency was shown to be increased by 6.3% under &minus, 0.75% uniaxial compression. These findings demonstrate a way to increase efficiency of GaAs NW-based solar cells via uniaxial mechanical compression.

Efficiency of in situ AlGaAs and GaP and ex situ nitride surface passivation of p+ GaAs nanowires was studied. The efficiency was estimated by comparing of the photoluminescence intensity of the passivated nanowires with the unpassivated nanowire. The AlGaAs and nitride passivation lead to the increasing of the PL intensity by three orders of magnitude while the GaP passivation increases PL intensity only by one order. Photodegradation of the passivated NWs under intensive laser illumination was observed. AlGaAs, GaP and nitride passivated NWs photodegrade after one-minute exposure under laser power densities of 500, 300 and 30 kW/cm2, respectively.

This work presents the results of an investigation of in plane horizontal GaAs nanowires by Scanning Probe Microscopy (SPM) methods. Topography and a local conductivity map of the horizontal nanowire with axial p-n junction are obtained. The distribution of doping and position of p-n junctions in nanowires are visualized. The I-V curves of differently doped parts of nanowires were measured. These data can be important for the understanding of the doping incorporation mechanism of GaAs nanowires.

Optomechanical resonators formed by the surface of an emitting semiconductor laser and the reflective surface of the probe of scanning probe microscope (SPM) were studied. The use of scanning probe microscopy techniques allowed studying the properties of such optomechanical resonators in details. The effect of optical cooling of an SPM probe in an optomechanical resonator was investigated by measuring the spectra of thermal vibrations of an SPM probe. Minimal achievable temperatures of SPM probe cooling were experimentally established.

Energy harvesting is an area that presents the greatest potential for powering wireless low-energy electronics. Development of semiconductor-based energy harvesting is of interest. Herein we focus on studying two different energy harvesting mechanisms in indium phosphide. Piezoelectric harvesting was checked by bending vertical InP nanowires with conductive atomic force microscope (AFM) probe and simultaneously detecting current passing through the probe. No current pulses were observed that we associate with surface trapping of piezoinduced carriers. Triboelectric generation was studied by creating frictional contact between AFM tip and metal-dielectric-semiconductor (MIS) interface formed by InP layer covered with native oxide. Current peaks of 320 pA amplitude corresponding to the current density around 2.3 kA/m2 were observed under sliding reciprocating motion of the tip during AFM scanning. Achieved current density was higher than in polymer- and silicon-based triboelectric structures. The open-circuit voltage value exceeded 15 mV and the output electric power was 35 W/m2.

Usually electromagnetic modes inside a laser resonator are a matter of the theoretical studies. In a sense we manage "to have a look into a whispering gallery mode (WGM) resonator" and observe how the resonator modes arrange in reality. The picture occurs to be quite different from the commonly used Bessel modes in a disk resonator. A chance to explore optical modes inside a resonator appears in a WGM laser with a cleaved cavity. The flat laser facet gives an opportunity to study both far and near field patterns formed by different modes. In this research we use a high resolution technique of detection of laser emission based on an atomic force microscope, which allowed us to visualize even high Q modes normally sealed inside the resonator. This information was completed with spatially resolved emission spectra and far-field patterns measured using an infrared camera. The analysis of the obtained results using both wave and geometrical optics approaches and finite elements simulations showed that emission of the studied devices is governed by a few low order optical modes experiencing a small number of reflections from the resonator walls. These modes can be considered as counter propagating Gaussian beams and their interference at the laser facet was also observed in the experiment. This work showed that, contrary to conventional ridge or surface emitting lasers, in such deformed disk resonators outputs of different optical modes are spatially separated and can be studied individually along the cleaved facet of the laser.

Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al$_{x}$Ga$_{1-x}$As (0$\le$x$\le$0.45) and Ga$_{x}$In$_{1-x}$As (0$\le$x$\le$1) alloys is pinned at the same position of 4.8$\pm$0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al$_{x}$Ga$_{1-x}$As and Ga$_{x}$In$_{1-x}$As nanowires leads to the accumulation of an excess arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires., Comment: 7 pages, 3 figures

In this manuscript, we present the study of elastic properties of InP nanowires with help of scanning probe microscope in advanced PeakForce Tapping® regime. The measuring method was developed in order to investigate the Young's modulus of these cone-shaped structures with significant accuracy. The difference in InP elasticity for wurtzite phase and zinc- blende phase was revealed. It was shown that elastic modulus of InP nanowires significantly increases from 60 GPa to more than 100 GPa when diameter of a nanowire is reduced below 50 nm. The core-shell model for InP nanowire was used for the explanation of this effect.

Scanning probe microscopy allows to study surface processes in semiconductor heterostructures. This work shows the investigation of topography and surface potential distribution in AlGaAs/GaAs p-n diode after applied reverse voltage. Formation of oxide on the surface in the area of p-n junction was found due to increasing of the applied reverse voltage. Formed oxide increases a breakdown voltage by 20%.

Using the methods of scanning probe microscopy, I-V characteristics of individual InxGa1-xAs nanowires with different In content (x) were measured. A sharp decrease in the conductivity was observed at x=0.7. It is shown that type of contact between nanowire and probe changes from ohmic at x=1 to Schottky at x=0.85. These changes were explained by the formation of surface conductive channel, induced by surface Fermi level pinning in conduction band for x>0.85.

This work presents the results of an investigation of thin layers of MoSe2 on graphene by Scanning Probe Microscopy (SPM) methods. Dependences of the surface potential and work function on the number of monolayers of the structure are presented. The dependence of the surface potential on air humidity is shown. The band structure and doping of the MoSe2 monolayer on graphene was determined. These data can be important for detecting of the number of MoSe2 layers and for designing nanodevices, because the surface potential has a strong influence on the operation of such devices.

The method of measuring nanowires elastic modulus by scanning probe microscopy is presented. This method uses the measurement of nanowire bending profiles in the precise force control mode. The possibilities of the method are demonstrated by measuring the Young's modulus of thin tapered InP nanowires.

Optical and electrical properties of MoSe2 monolayer transferred to graphene/SiC substrate are studied by scanning probe microscopy and photoluminescence spectroscopy. By Kelvin probe microscopy it was measured a work function of the MoSe2 monolayer (4.3 eV) and it was revealed the n-type of doping. With the increasing of MoSe2 layers quantity the work function also increases which leads to the reducing of the Schottky barrier height between the SPM probe and the layers. Monolayer graphene quenches photoluminescence of the MoSe2 monolayer while the bilayer graphene does not perturb the photoluminescence.

In this work we demonstrate the possibility of studying the current-voltage characteristics for single vertically standing semiconductor nanowires on standard AFM equipped by current measuring module in PeakForce Tapping mode. On the basis of research of eight different samples of p-doped GaAs nanowires grown on different GaAs substrates, peculiar electrical effects were revealed. It was found how covering of substrate surface by SiOx layer increases the current, as well as phosphorous passivation of the grown nanowires. Elimination of the Schottky barrier between golden cap and the top parts of nanowires was observed. It was additionally studied that charge accumulation on the shell of single nanowires affects its resistivity and causes the hysteresis loops on I-V curves.

The control of nanowire-based device performance requires knowledge about the transport of charge carriers and its limiting factors. We present the experimental and modeled results of a study of electrical properties of GaAs nanowires (NWs), considering their native oxide cover. Measurements of individual vertical NWs were performed by conductive atomic force microscopy (C-AFM). Experimental C-AFM observations with numerical simulations revealed the complex resistive behavior of NWs. A hysteresis of current-voltage characteristics of the p-doped NWs as-grown on substrates with different types of doping was registered. The emergence of hysteresis was explained by the trapping of majority carriers in the surface oxide layer near the reverse-biased barriers under the source-drain current. It was found that the accumulation of charge increases the current for highly doped p+-NWs on n+-substrates, while for moderately doped p-NWs on p+-substrates, charge accumulation decreases the current due to blocking of the conductive channel of NWs.

In this paper a probe of an atomic force microscope (AFM) served as a detector of optical radiation. We investigated absorption of optical radiation in the visible range by commercially available Si and Si3N4 probes. It has been shown that the radiation in the far field is mainly absorbed in the probe tip. By scanning a laser beam with a diameter of ~ 0.7 microns by the AFM probe, a lateral resolution of ~ 1.5 microns was demonstrated. Numerical modeling of evanescent wave absorption by AFM probes showed enhancement of the absorption in the probes as compared with a flat surface. A Si3N4 probe more effectively absorbs the evanescent radiation.

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

Current–voltage (I–V) characteristics of vertical p-GaAs nanowires (NWs) covered by different surface passivation materials were experimentally measured by conductive atomic force microscopy (C-AFM). The obtained I–V curves for individual NWs with a diameter of 100 nm covered with AlGaAs, GaN, GaP or InP shell layers were compared to analyse the influence of surface passivation on the density of surface states and choose the most beneficial passivating material for technological applications. We have found the absence of a Schottky barrier between the golden catalytic cap on the top of a NW and the nanowire situated below and covered with an ultrathin GaP passivating layer. It was suggested that passivating material can arrange the heterostructure configuration with the GaAs NW near the Au cap. The latter mechanism was proposed to explain a strong energy barrier found in nanowires covered with InP passivation. AlGaAs passivation affected the forward threshold voltage of nanowires for NWs, which was measured simultaneously with the resistivity of each individual vertical structure from an array by means of AFM in the regime of measuring the I–V curves and onefold calculations. We made an attempt to develop the methodology of measurement and characterization of electric properties of passivated NWs.