Using high-temperature annealing of thin gold nanofilms deposited onto the (001) surface of doped p-GaAs crystal with an ultrathin oxide layer, the nanoclusters of gold (Au2Ga alloy) are fabricated. The gold clusters have the wedge shapes with rectangular bases elongated in [110] direction at GaAs(001) surface. This assertion is confirmed by the data of diagnostics of Au/p-GaAs(001) structures. Anisotropic plasmons localized on equally oriented wedge-shaped Au (Au2Ga) clusters are investigated with the optical reflectance anisotropy spectroscopy and spectroscopy of polarized light reflection. It is shown that the spectral peak at the energy about 0.9 eV in the near infrared range is associated with plasmons polarized along the longest sides of clusters in crystallographic direction [110]. Another peak—at the energy of 1.8 eV—is due to plasmons having polarization in direction [11̅0] .

With a decrease in the thickness of the walls separating the space of pores in porous semiconductors, the potential energy of interaction between an electron and a donor (or a hole and an acceptor) can become greater than the kinetic energy of a free charge carrier. As a consequence, such interlayers lose their conductivity and transit into the dielectric state (Mott phase transition). With regard to the conditions of electrochemical pore formation, this means that as the pore channels approach each other during anodic etching to a distance at which the current flow through the wall that separates them stops, the potential of its surface ceases to be determined by the external electric bias and the electrochemical process, that leads to a further decrease in the thickness of such a wall, stops. Expressions are obtained for the limiting thickness of the walls of pores formed in degenerate semiconductors of n- and p-type conductivity. In contrast to the well-known model that relates the loss of conductivity by pore walls to the combination of space charge layers, the proposed model allows a consistent explanation for the experimental data for both n- and p-type silicon with doping levels above 10 18 cm -3 . Keywords: thickness limitation, pore formation, silicon, donor, acceptor.

Wedge-shaped nanoclusters of gold (Au2Ga) are fabricated by high-temperature annealing of a gold nanofilm deposited onto (001) surface of p-doped GaAs crystal with a very thin overlayer of natural oxide. The data of diagnostics confirm the presence in prepared Au/p-GaAs(001) structures of the wedge-shaped Au-intermetallic nanoclusters elongated in [110] direction at GaAs surface. A crystallographic model of the wedge-shaped Au (Au2Ga) nanoclusters conditioned by GaAs(001) surface is discussed in relation with their physicochemical nature. Anisotropic plasmons localized on equally oriented Au-based nanoclusters are detected optically with the reflectance anisotropy spectroscopy and investigated thoroughly with the spectroscopy of polarized light reflection. It is proved experimentally and theoretically that the inhomogeneously broadened infrared spectral peak at the energy about 1.1 eV is associated with plasmons polarized along the wedge-shaped clusters in [110] crystal direction. Another peak - at the energy approximately of 1.8 eV - is due to plasmons having orthogonal polarization in direction [110].

This work demonstrates how to create the structures Au/GaAs with perfect on-surface gold nanoclusters. In doing so, used is covering the GaAs substrate with chemically stable atomic monolayers of sulphur to prevent subsequently a chemical reaction of Au with GaAs. The structures Au/S/GaAs with monolayers of chemisorbed sulphur atoms are fabricated, characterized and studied by polarized reflection spectroscopy. The anisotropy of on-surface gold nanoclusters is established, and the anisotropic plasmons localized in Au clusters are investigated using the spectra of polarized reflection and interpreted theoretically.

The principal role of chemical passivation of GaAs surface in the formation on it of oriented anisotropic nanoclusters of gold is discussed. The nanoclusters are fabricated by thermal annealing of a gold film deposited onto GaAs(001) surface passivated as a preliminary by a monolayer of nitrogen or sulfur atoms. These atoms, bonded chemically to gallium atoms of the crystal surface, form a crystal lattice and prevent the chemical interaction of Au with GaAs. As a result of annealing, the arrays of anisotropic (elongated) nanoclusters of chemically pure Au oriented preferably in crystal [11̅0] direction are formed on passivated GaAs(001) surface. The presence of strong anisotropy and orientation of Au clusters on passivated GaAs surfaces is established by the methods of probe diagnostics and of optical reflectance anisotropy spectroscopy and polarized reflection spectroscopy. Using an optical model of plasmonic polarizability of elongated Au spheroids, it is shown that the spectral features observed in polarized reflection originate from anisotropic plasmons of Au nanoclusters polarized mainly in direction [11̅0] of crystal.

With a decrease in the thickness of the walls separating the space of pores in porous semiconductors, the potential energy of interaction between an electron and a donor (or a hole and an acceptor) can become greater than the kinetic energy of a free charge carrier. As a consequence, such interlayers lose their conductivity and transit into the dielectric state (Mott phase transition). With regard to the conditions of electrochemical pore formation, this means that as the pore channels approach each other during anodic etching to a distance at which the current flow through the wall that separates them stops, the potential of its surface ceases to be determined by the external electric bias and the electrochemical process, that leads to a further decrease in the thickness of such a wall, stops. Expressions are obtained for the limiting thickness of the walls of pores formed in degenerate semiconductors of n- and p-type conductivity. In contrast to the well-known model that relates the loss of conductivity by pore walls to the combination of space charge layers, the proposed model allows a consistent explanation for the experimental data for both n- and p-type silicon with doping levels above 10^18 cm^-3.

The principal role of chemical passivation of GaAs surface in the formation on it of oriented anisotropic nanoclusters of gold is discussed. The nanoclusters are fabricated by thermal annealing of a gold film deposited onto GaAs(001) surface passivated as a preliminary by a monolayer of nitrogen or sulfur atoms. These atoms, bonded chemically to gallium atoms of the crystal surface, form a crystal lattice and prevent the chemical interaction of Au with GaAs. As a result of annealing, the arrays of anisotropic (elongated) nanoclusters of chemically pure Au oriented preferably in crystal [110] direction are formed on passivated GaAs(001) surface. The presence of strong anisotropy and orientation of Au clusters on passivated GaAs surfaces is established by the methods of probe diagnostics and of optical reflectance anisotropy spectroscopy and polarized reflection spectroscopy. Using an optical model of plasmonic polarizability of elongated Au spheroids, it is shown that the spectral features observed in polarized reflection originate from anisotropic plasmons of Au nanoclusters polarized mainly in direction [110] of crystal. Keywords: semiconductor surface, nitride passivation, gold nanoclusters, anisotropic plasmons, polarized reflectance.

Disperse composite materials based on silicon monoxide and carbon (SiO/C) have been obtained by thermal treatment of a powder mixture consisting of 40 wt % SiO and 60 wt % CF0.8. Annealing has been performed in the argon atmosphere at temperatures 1000–1250°C. It has been established using electron microscopy and Raman scattering that, at T ≥ 1100°C, silicon carbide is formed in the solid-phase product, including in the form of nanowhiskers of cubic modification. Based on the data on the decrease of the reaction mixture weight, the composition of the formed products is calculated as a function of the annealing temperature. The anodes prepared from the composites obtained at a temperature above 1100°C demonstrate a sharp decrease in the capacitance and in the Coulomb efficiency. It is shown that the observed changes are determined by an increase in the oxygen concentration in the matrix surrounding silicon precipitates, which have been formed as a result of SiO disproportionation, rather than by the formation of SiC. It is established that an optimal annealing temperature provides the highest values of the electrode capacitance, the Coulomb efficiency of the first cycle, and the ability to operate at high current densities is T = 1050°C.

The processes of the disproportionation of solid-phase silicon monoxide, accompanied by the formation of nanocrystalline silicon precipitates in the medium of amorphous suboxide SiOx (initial composition of SiO0.9), are studied. Based on the data of X-ray diffraction analysis and transmission electron microscopy, the dynamics of changes in the amount, concentration, and size of phase precipitates of silicon with the temperature of isochronous annealing increasing from 800 to 1200°C is traced. It is found that, with the total mass of the precipitated silicon steadily increasing, the number of its crystallization centers per unit volume nonmonotonically depends on temperature. The activation energy of the diffusion of silicon atoms in the SiOx matrix is determined to be Ea1 = 1.64 eV and the activation energy of their transfer from precipitates formed to the SiOx growth medium is Ea2 = 2.38 eV. The anisotropic deformation of silicon crystallites precipitated during the disproportionation of SiO is revealed for the first time. This phenomenon is attributed to the difference between the specific volumes of the phases being separated and to the anisotropy of the growth rate of silicon precipitates formed in a solid amorphous medium.

A principal role of chemical passivation of GaAs surface in the formation of oriented anisotropic on-surface nanoclusters of gold is discussed. The gold nanoclusters are fabricated by thermal annealing of the gold film deposited onto GaAs(001) surface passivated preliminary by an atomic monolayer of nitrogen or sulphur. These atoms forming a crystal lattice are bonded chemically to gallium atoms of the crystal surface to prevent chemical interaction of Au with GaAs. Annealing Au on passivated GaAs(001) surface results in formation of arrays of anisotropic (elongated) nanolusters of chemically pure Au oriented preferably in crystal direction. The presence of strong anisotropy and orientation of Au clusters on passivated GaAs surfaces is established by the methods of probe diagnostics and the optical techniques of reflectance anisotropy spectroscopy and polarized reflection spectroscopy. Within an optical model of plasmon polarizability of elongated Au spheroids, it is shown that the spectral features observed in polarized reflection originate from anisotropic plasmons polarized in Au nanoclusters predominantly in direction on GaAs(001) surface.

In this work, the processes of disproportionation of solid-phase silicon monoxide, accompanied by the formation of nanocrystalline silicon precipitates in the medium of amorphous SiOx suboxide (initial composition SiO0.9), have been studied. Based on the data of X-ray diffraction analysis and transmission electron microscopy, the dynamics of changes in the amount, concentration and size of phase precipitates of silicon with an increase in the temperature of isochronous annealing from 800 °C to 1200 °C is traced. It was found that with a monotonic increase in the total mass of the precipitated silicon, the number of its crystallization centers per unit volume nonmonotonically depends on temperature. The activation energy of diffusion of silicon atoms in the SiOx matrix was determined to be Ea1= 1.64 eV, and the activation energy of their transfer from the formed precipitates to the growth medium of SiOx was Ea2 = 2.38 eV. Anisotropic deformation of silicon crystallites precipitated during the disproportionation of SiO has been revealed for the first time. This phenomenon is associated with the difference in the specific volumes of the separated phases and the anisotropy of the growth rate of silicon precipitates formed in a solid amorphous medium.

Dispersed composite materials based on silicon monoxide and carbon (SiO/C) were obtained as a result of heat treatment of a powder mixture consisting of 40 wt% SiO and 60 wt% CF0.8. Annealing was carried out in an argon atmosphere at temperatures 1000 − 1250°C. Using electron microscopy and Raman scattering, it was found that at Т>=1100°C, silicon carbide appears in the solid-phase product, including in the form of cubic nanowhiskers. Based on the data on the weight loss of the reaction mixture, the composition of the resulting products was calculated as a function of the annealing temperature. Anodes made of composites obtained at temperatures above 1100°C demonstrate a sharp drop in capacitance and Columbic efficiency. It is shown that the observed changes are caused not so much by the formation of SiC as by an increase in the oxygen content in the matrix surrounding the silicon precipitates, which were formed as a result of disproportionation of SiO. It was found that the optimal annealing temperature, which provides the highest values of capacity, the initial coulombic efficiency and the ability to operate at high speed current densities is Т =1150°С.

The Schottky nanostructures Au/GaAs with Au nanoclusters are prepared by annealing of thin gold films deposited on nitridized GaAs(001) surface. The nanostructures are diagnosed nanoscopically and investigated by optical reflection anisotropy spectroscopy. The Au nanoclusters arrays of two types are found to be formed respectively under and over GaAs surface. The energies of local plasmons of the arrays are detected at 1.6 and 2.15 eV, respectively. The latter plasmons possess in-surface anisotropy which causes in optical anisotropy spectra the intensive resonant feature at energy about 2 eV. The plasmon anisotropy is studied comprehensively, and the results are interpreted theoretically.

The effect of annealing temperature in argon atmosphere on the ability of Si-C nanocomposites to reversibly insert lithium was investigated. It was found that the higher the annealing temperature during the formation of the composite, the lower is the capacitance of the electrode made from it. X-ray diffraction analysis and transmission electron microscopy reveal that the reason of the capacitance decrease is formation at T  1100°C of silicon carbide of cubic modification -SiC, inactive with respect to the formation of lithium alloys or intercalates.

It is suggested to form porous silicon-carbon nanocomposites via thermal reduction of carbon monofluoride by silicon. For this purpose a mixture of powders of nanocrystalline silicon and fluorocarbon is subjected to cold compaction and the resulting pellets are annealed in an inert atmosphere at T = 800 degrees C. The density, porosity, structure, composition, and electrical resistivity of thus produced Si-C materials have been studied in detail in relation to the content of the monofluoride in the starting mixture. It was shown that the materials obtained have a hierarchical porous structure constituted by silicon nanoparticles in a shell of finely dispersed carbon. The shells contacting with each other form a carbon matrix providing a high electrical conductivity of the material. The composite was used to fabricate negative electrodes of lithium-ion batteries with increased storage capacity. The electrochemical characteristics of Si-C nanocomposite anodes of varied composition were analyzed and those with high carbon content demonstrated the best performance. (C) 2020 Elsevier B.V. All rights reserved.

Influence exerted by the temperature of annealing in the atmosphere of argon on the ability of Si‒C nanocomposites to enable a reversible introduction of lithium has been studied. It was found that the higher the annealing temperature in the formation of a composite, the lower the capacity of the electrode fabricated from this composite. X-ray diffraction analysis and scanning electron microscopy demonstrated that the capacity decreases because silicon carbide of cubic modification β-SiC inactive toward formation of lithium alloys or intercalates is formed at T ≥ 1100°C.

A procedure is developed for controlled creation of Au nanoclusters by annealing of a gold film deposited onto GaAs(0 0 1) crystal surface. The nanoclusters of Au are formed at GaAs surfaces covered by either a natural oxide layer or a monolayer of gallium nitride. Surface morphology of the Au/GaAs structures with Au nanoclusters is characterized by scanning probe diagnostics and localized plasmons of the nanoclusters are investigated by optical reflection spectroscopy. In annealing Au film dissimilar gold nanoclusters are found to occur on oxidized or nitridized GaAs(0 0 1) surface via chemical transformation or recrystallization of Au film, respectively. Gold nanoclusters of the two types cause resonant peaks in optical reflectance spectra at the energies of 1.6 eV and 2.15 eV. Using the data of optical spectroscopy and their theoretical analysis we assign the former peak to localized plasmons of prolate Au nanoclusters buried into GaAs crystal near its surface. Another peak at 2.15 eV is attributed to plasmons of oblate Au nanoislands appearing on nitride overlayer which prevents any chemical contact of Au with GaAs bulk. The asserted existence of Au nanoclusters in the bulk of GaAs crystal near its oxidized surface is expected to be helpful in elucidating the nature and structure of Ohmic Au-GaAs contacts.

A study of the processes of the thermal carbonization of silicon monoxide in the presence of nonstoichiometric carbon monofluoride demonstrated that raising the annealing temperature of mixtures of SiO and CFx powders in a quasi-closed volume to 1000°C and higher leads to the formation of whisker-like SiC nanocrystals. It is found that, in parallel with the known crystallization of SiC nanowires as a result of the interaction of SiO vapor with carbon monoxide, the previously undescribed interaction of CO with gas-phase silicon difluoride SiF2 takes part in their formation. At temperatures below 1200°C, this reaction is dominant and makes the most pronounced contribution to the yield of SiC nanowires.

When studying the processes of thermal carbonization of silicon monoxide in the presence of non-stoichiometric carbon monofluoride, it was found that increasing the annealing temperature of mixtures of SiO and CFx powders in a quasiclosed volume to 1000°C and higher leads to the release of whisker-like SiC nanocrystals. The studies showed that in parallel with the known crystallization of SiC nanowires as a result of the interaction of SiO vapors with carbon monoxide, previously undescribed interaction of CO with gas-phase silicon difluoride SiF2 takes part in their formation. At temperatures below 1200°C, this reaction is dominant, making the largest contribution to the yield of SiC nanowires.