V. M. Mikoushkin

Controlling graphite oxide bandgap width by reduction in hydrogen

Transformation of the chemical composition and electron structure of graphite oxide (GO) nanolayers as a result of their annealing in hydrogen has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that both the chemical composition and bandgap width of GO can be controlled by varying the temperature and duration of heat treatment. By this means, the properties of GO nanolayers can be smoothly changed from dielectric to semiconductor.

Fullerite C 60 as electron-beam resist for 'dry' nanolithography

The fullerite C60 modification induced by electron irradiation was studied using electron energy loss spectroscopy (EELS). We show that prolonged electron irradiation causes gradual transformation of electron and atomic structures of fullerite C60 films towards respective structures of amorphous carbon. Moreover, strongly modified parts of fullerite C60 films become thermally unevaporable. A possible mechanism of these effects is suggested. Finally, we propose a new original nanolithography technique based on two key ideas: the use of fullerite films as negative electron-beam resist and development of lithographic images by appropriate thermal treatment of these films.

Graphene hydrogenation by molecular hydrogen in the process of graphene oxide thermal reduction

Thermal reduction in molecular hydrogen of the graphene oxide films has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. The restoration process was revealed to be accompanied by hydrogenation due to collisionally induced interaction of molecular hydrogen with carbon atoms. One side hydrogenated graphene films consisting of 20 μm one monolayer flakes were fabricated on SiO2/Si surface with hydrogen concentration as far as 40 at. %, at which the 0.3 eV bandgap opening was observed. It was shown that both H-coverage and bandgap width of the films can be controlled by varying the temperature of the heat treatment.

Single and Collective Electron Excitations in the Solid C60F18

Abstract The electronic structure of the solid polycrystalline fluorinated fullerene C60F18 has been investigated by reflection electron‐energy‐loss spectroscopy for the first time. The elementary excitation spectrum of the solid C60F18 was compared with the respective spectrum of the solid C60, and an explanation of their significant similarity in the range of π→π* transitions was suggested.

Electronic Structure of Unoccupied States of Fluorinated Fullerenes C60F18 and C60F36

Comparative study of near edge X‐ray absorption fine structure spectra (NEXAFS) of fluorinated fullerenes C60Fx (x = 0, 18, 36) has been implemented. Local density of unoccupied states was obtained and an accurate boundary between π* and (π+σ)* states was determined. The experimental evidence was found that unoccupied π*‐ states of C60Fx are delocalized ones and form cluster shells that sequentially disappear in fluorination starting from the highest state. As a result, the density of the lowest π* ‐ state (LUMO) was revealed to remain being constant in fluorination despite the π ‐electron subsystem exhaustion.

Core electron level structure in C60F18 and C60F36 fluorinated fullerenes

The structure of C1s and F1s core electron levels in C60F18 and C60F36 fluorinated fullerenes has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that C1s levels of carbon atoms not bound to fluorine in these compounds are shifted down by 1.0 and 1.6 eV relative to the C1s level in the usual C60 fullerene, so that the binding energies of the core electron levels in C60F18 and C60F36 amount to Eb (C1s, C-C) = 285.7 and 286.3 eV, respectively. These values are characteristic and can be used for the identification of both homogeneous fluorinated fullerenes and combined materials comprising a mixture of various fluorinated fullerenes with each other and with different carbon-containing based materials.

Simulation of Fullerite C60 Polymerization Under Particle Beam Irradiation

ABSTRACT A model has been suggested, and the simulation of the fullerite C60 polymerization by electrons within the energy range 0.15 ÷ 1.5 keV has been carried out. The number of created intermolecular bonds was assumed to be proportional (with a coefficient k) to the number of excited molecules. Polarization, exchange, and muffin-tin size were taken into account for elastic scattering. Ionization of deep atom levels was treated as a binary collision, and the inelastic scattering by valence electrons was described in the framework of the dielectric formalism. The comparison of results of the simulation with experimental data gave the coefficient of proportionality k∼(2 ÷ 5) · 10−3.

Formation of GaAsN/GaN cluster nanostructures on the surface of GaAs by the implantation of low-energy nitrogen ions

A nitride nanolayer fabricated on a GaAs (100) surface by implanting ions N2+ (Ei = 1.5 keV) has been studied by high-resolution photoelectron spectroscopy with the use of synchrotron radiation. It has been found that, apart from the dominant GaN wide-gap semiconductor phase, an additional phase of the GaAs1 − xNx narrow-gap solid solution (x < 0.10) is present in the nitride layer. It has been shown that the nitride layer created by ion implantation is a nanostructure with an attribute of a system of quantum dots, since it consists of nanoclusters of the narrow-gap semiconductor in the wide-gap matrix.

Chemical effect of inert argon beam on nitride nanolayer formed by ion implantation into GaAs surface

The composition of a nitride nanolayer formed on a GaAs(100) surface by the implantation of ions with an energy of Ei = 2.5 keV and the chemical state of nitrogen in this layer have been studied by the method of Auger electron spectroscopy. It is established that, in addition to GaN, a GaAsN solid solution phase is formed in the ion-implanted layer. The energies of N KVV Auger electron transitions in these phases are determined as EA(GaN) = 379.8 ± 0.2 eV and EA(GaAsN) = 382.8 ± 0.2 eV (relative to the Fermi level), which allowed the distribution of nitrogen between these phases to be evaluated as [N(GaN)] = 70% and [N(GaAsN)] = 30%. It is established that an argon ion beam produces a chemical effect on the nitride layer, which is related to a cascade mixing of the material. Under the action of the argon ion bombardment, the distribution of nitrogen in the indicated phases changes to opposite. As a result a nitride nanolayer is formed in which the narrow-bandgap semiconductor (GaAsN) predominates rather than the wide-bandgap component (GaN).

The Effect of Dose of Nitrogen-Ion Implantation on the Concentration of Point Defects Introduced into GaAs Layers

Secondary-ion mass spectrometry and Rutherford proton backscattering have been used to measure the concentration profiles of nitrogen atoms and examine the defect structure of epitaxial GaAs layers implanted with 250-keV N+ ions at doses of 5 × 1014–5 × 1016 cm–2. It was found that no amorphization of the layers being implanted occurs at doses exceeding the calculated amorphization threshold, a concentration of point defects that is formed is substantially lower than the calculated value, and a characteristic specific feature of the defect concentration profiles is the high defect concentration in the surface layer.