Nikolai Sobolev - Academia.edu (original) (raw)

Papers by Nikolai Sobolev

physica status solidi (b), 2019

Low‐profile anodic alumina matrices of 1 μm thick with pore sizes of 105 and 160 nm are formed by... more Low‐profile anodic alumina matrices of 1 μm thick with pore sizes of 105 and 160 nm are formed by two‐step anodizing of Al layers and are used as templates for the deposition of multicomponent metal oxide films on their surfaces. The metal oxide systems of Sr2FeMoO6−δ, FexMoyOz, and SnxMoyOz composites with carbon nanotubes are synthesized by electrophoretic deposition, sol–gel method, and drop method using aqueous salt solutions and are annealed at 373–473 K for 1–2 h and at 773–1123 K for 2–10 h. The morphology, microstructure, and composition of the porous alumina matrices with multicomponent metal oxides films are examined by scanning electron microscopy, energy‐dispersive X‐ray microanalysis, and X‐ray phase analysis. The use of anodic alumina matrices allows reducing the grain dimensions and gives uniformity to the microstructure and properties of the metal oxide films. The carbon nanotubes increase the working surface of the functional layer and its electrical conductivity. T...

The composition of the films was measured by combined Rutherford backscattering spectrometry (RBS... more The composition of the films was measured by combined Rutherford backscattering spectrometry (RBS) and particle induced X-ray emission (PIXE). The Al content in the films could not be determined, due to the underlying Al2O3 substrate. The Co and Mn contents in the films turned out to be larger than in the corresponding PLD targets and amounted to about 9 at.%. The crystal structure of the films was characterized by X-ray diffraction (XRD) and reciprocal space mapping (RSM), which indicated the highly c-axis-oriented ZnO films. No second phases, especially, no Co nanocrystals were detected even for a very long signal accumulation. No magnetic resonance trace of Co nanocrystals [2] was detected, either. The critical electron concentration, at which the metal to insulator transition occurs, was estimated for a Co doped film to be nc = 4×10 cm [1]. Thus, we have a film with n < nc and another one with n > nc for each type of doping. A possible ferromagnetic response of charge carr...

physica status solidi (c), 2013

Ceramic piezomaterials based on Pb 0.75 Ba 0.15 Zr 0.53 Ti 0.47 O 3 (PBZT) system are characteriz... more Ceramic piezomaterials based on Pb 0.75 Ba 0.15 Zr 0.53 Ti 0.47 O 3 (PBZT) system are characterized by high values of the ferroelectric Curie temperature and polarization. They have widespread applications in electromechanical and electroacoustic transducers, bandwidth filters, transformers, frequency stabilized resonators, etc. Preparation of PBZT composites with nano-inclusions of Cu and Ni have been carried out by a complex powder technology including metallization procedures which were performed by the well-known chemical deposition method from standard solutions of copper and nickel. Studies of hydrostatic density and porosity of obtained samples as well as their microstructural investigations have yielded a larger density of PBZT-metal composites (5.3-7.3 g/cm 3) as compared with pure PBZT (4.7 g/cm 3). Measurements of dielectric properties of samples in the frequency range from 20 Hz-1 MHz have shown that the Curie temperature increases in PBZT (Ni) and decreases in PBZT(Cu) in comparison with pure PBZT. The complex dielectric permittivity of all investigated composites at higher temperatures and low frequencies is mainly caused by the high electrical conductivity. Only at the highest frequency used (1 MHz) the real part of the complex permittivity is caused by a resonant soft mode and could be fitted with the Curie-Weiss law. The activation energy of ferroelectric domain mobility in PBZT (Cu, Ni) composites is lower than in pure PBZT. This can be explained by a decrease of the domain size in composites.

Journal of Alloys and Compounds, 2013

ChemSusChem, Jan 5, 2018

A green, template-free easy-to-implement strategy giving access to holey g-C₃N₄ nanosheets doped ... more A green, template-free easy-to-implement strategy giving access to holey g-C₃N₄ nanosheets doped with carbon is described. The protocol involves heating dicyandiamide with β-cyclodextrin (βCD) prior to polymerization. By this approach, the local symmetry of g-C3N4 skeleton is broken, thus yielding CxGCN (x corresponds to the initial amount of βCD used) with porous and a distorted structure. The electronic, emission, optical and textural properties of the best-performing material, C2GCN, are thus greatly modified as compared to bulk g-C₃N₄ (GCN). The spectroscopic and luminescent features of C2GCN show the characteristic π-π* electronic transition of GCN, accompanied by much stronger n-π* electronic transitions due to the porous and defect network. These new electronic transitions, along with the presence of additional carbon synergistically contribute to enhance visible light absorption and to restrain the recombination of electron-hole pairs. Steady-state and time-resolved photolum...

Journal of Physics Condensed Matter, Apr 21, 2003

ABSTRACT

Radiat Eff Defect Solid, 1979

The production of point defects and the amorphous phase in silicon during the implantation of var... more The production of point defects and the amorphous phase in silicon during the implantation of various mass ions at 77 K and 300 K has been studied as a function of ion fluence using electron paramagnetic resonance (EPR). The EPR spectra of the centres Si-P3 (neutral tetra-vacancy), Si-P6 (silicon di-interstitial), Si-A5 (probably another self-interstitial defect), Si-B3 (of unknown structure) were

Physica B: Condensed Matter, 1999

ABSTRACT

Thin Solid Films, 2013

Epitaxial zinc ferrite thin films are grown on (001) SrTiO 3 substrates by pulsed laser depositio... more Epitaxial zinc ferrite thin films are grown on (001) SrTiO 3 substrates by pulsed laser deposition. The magnetic anisotropy of the films is examined by superconducting quantum interference device magnetometry and ferromagnetic resonance, with various in-plane and out-of-plane orientations of the magnetic field. The volume magnetization of the films is found to be around 55 kA/m. The existence of a cubic magnetic anisotropy is shown, with the easy axes oriented along the [111] crystallographic directions. The films show ferrimagnetic properties with a Curie temperature higher than 350 K. The nature of a strong "easy-plane"-type bimodal anisotropy in the films is discussed.

Physics of the Solid State, 2005

Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molec... more Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low temperatures (<350 ° C) and studied using photoluminescence and atomic force microscopy. It is found that the germanium layer undergoes a morphological transformation when its thickness becomes equal to approximately five monolayers: an island relief transforms into a smooth undulating relief.

physica status solidi (b), 2013

ABSTRACT The Sr2FeMoO6−δ compound attracts the attention of researchers due to a high Curie tempe... more ABSTRACT The Sr2FeMoO6−δ compound attracts the attention of researchers due to a high Curie temperature, large values of negative magnetoresistance at room temperature and a practically 100% spin polarization of conduction electrons. We have studied the role of grain boundaries on the electrical transport in Sr2FeMoO6−δ in applied magnetic fields up to 8 T. The compound was synthesized out of partially reduced SrFeO3−x, SrMoO4−y precursors (sample I). At the first oxidation stage upon annealing at 700 K in argon with a partial oxygen pressure p(O2) = 10 Pa for 15 h (sample II), the internal structure of the Sr2FeMoO5.82 grains did not change. Sample II exhibits a mixed type of electrical conduction. In a magnetic field B its resistivity decreases without changing the mixed regime of charge transport, only shifting the temperature of minimum resistivity (TmB) to lower values. At temperatures above TmB the conductivity is predominantly metallic, whereas below TmB down to 4.2 K it is of semiconductor type. The increase of the annealing time up to 30 h (sample III) brings about an increase of the resistivity and the appearance of semiconductor-type conductivity at T = 300–4.2 K, which indicates the formation of a continuous insulating interlayer between the grains. In a magnetic field, the resistivity decreases, with the conductivity changing from the semiconducting to a mixed one. In the latter case the charge transport can occur both through point metallic contacts and by means of spin-dependent tunneling across dielectric interlayers between Sr2FeMoO5.82 grains.

Physica B: Condensed Matter, 1999

We report four new EPR spectra found in high-pressure synthetic diamonds with high nitrogen conte... more We report four new EPR spectra found in high-pressure synthetic diamonds with high nitrogen content, grown using nickel, and annealed at 16003C. Analyzing the complex spectra around g+2 two trigonal and two orthorhombic defects, all with S" , were identi"ed from the angular dependence and the di!erent saturation behavior of the transitions. Besides optical and EPR investigations reveal several centers which have been previously ascribed to defects containing nickel and nitrogen. However, nitrogen is not involved in the new centers, since no evidence of an N hyper"ne structure could be found. Possible relations of the new defects with Ni are discussed.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1996

ABSTRACT

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2001

Different Ge/Si superlattices were irradiated with 150 keV Ar ions at room temperature with fluen... more Different Ge/Si superlattices were irradiated with 150 keV Ar ions at room temperature with fluences in the range 1012 to 5×1015cm−2. Defect production was studied with Rutherford backscattering/channeling spectrometry and X-ray diffraction (XRD). The evolution of the damage with ion fluence reveals the existence of three distinct regimes. During the first regime the concentration of defects increases slowly until the

Materials Science and Engineering: C, 2003

Si 5 Ge 5 superlattices (SL) were treated by 80-ns pulses of a ruby laser in a wide range of ener... more Si 5 Ge 5 superlattices (SL) were treated by 80-ns pulses of a ruby laser in a wide range of energy densities. The induced structural and electronic changes were monitored in situ by time-resolved reflectivity (TRR) and ex situ by scanning electron microscopy (SEM), Raman scattering and atomic force microscopy (AFM). The SL starts to melt at energy densities typical of bulk Ge (less than 0.4 J/cm 2). At R 0.7 J/ cm 2 , a self-organization phenomenon is observed: a system of quasiregular rectangular grains with linear dimensions of about 100 nm is developed on the sample surface.

Materials Science and Engineering: B, 2008

ABSTRACT The influence of defects on the optical properties of a single Ge quantum well deposited... more ABSTRACT The influence of defects on the optical properties of a single Ge quantum well deposited on a Si substrate and on a diode structure containing a Si/Ge multilayer structure was investigated. In order to change the density of optically active defects, the as-grown samples were exposed to post-growth treatments: atomic hydrogen passivation and irradiation with 2.0 MeV protons to fluences in the range 2×1012 to 1×1014 cm−2. The optical and structural properties were investigated by photoluminescence and X-ray diffraction and reflection measurements. An unexpectedly high radiation hardness was observed for the as-grown Ge quantum wells.

Journal of Luminescence, 2006

In this work we studied the influence of high-energy proton irradiation on the optical and struct... more In this work we studied the influence of high-energy proton irradiation on the optical and structural properties of an Si/Ge superlattice (SL) with embedded Ge quantum dots (QDs). The presence of QDs in the as-grown samples was established by transmission electron microscopy and photoluminescence (PL). The samples were irradiated with 2.0MeV protons to fluences in the range 2×1012–2×1014cm–2. The structural

Journal of Crystal Growth, 1996

ABSTRACT

physica status solidi (b), 2019

Low‐profile anodic alumina matrices of 1 μm thick with pore sizes of 105 and 160 nm are formed by... more Low‐profile anodic alumina matrices of 1 μm thick with pore sizes of 105 and 160 nm are formed by two‐step anodizing of Al layers and are used as templates for the deposition of multicomponent metal oxide films on their surfaces. The metal oxide systems of Sr2FeMoO6−δ, FexMoyOz, and SnxMoyOz composites with carbon nanotubes are synthesized by electrophoretic deposition, sol–gel method, and drop method using aqueous salt solutions and are annealed at 373–473 K for 1–2 h and at 773–1123 K for 2–10 h. The morphology, microstructure, and composition of the porous alumina matrices with multicomponent metal oxides films are examined by scanning electron microscopy, energy‐dispersive X‐ray microanalysis, and X‐ray phase analysis. The use of anodic alumina matrices allows reducing the grain dimensions and gives uniformity to the microstructure and properties of the metal oxide films. The carbon nanotubes increase the working surface of the functional layer and its electrical conductivity. T...

The composition of the films was measured by combined Rutherford backscattering spectrometry (RBS... more The composition of the films was measured by combined Rutherford backscattering spectrometry (RBS) and particle induced X-ray emission (PIXE). The Al content in the films could not be determined, due to the underlying Al2O3 substrate. The Co and Mn contents in the films turned out to be larger than in the corresponding PLD targets and amounted to about 9 at.%. The crystal structure of the films was characterized by X-ray diffraction (XRD) and reciprocal space mapping (RSM), which indicated the highly c-axis-oriented ZnO films. No second phases, especially, no Co nanocrystals were detected even for a very long signal accumulation. No magnetic resonance trace of Co nanocrystals [2] was detected, either. The critical electron concentration, at which the metal to insulator transition occurs, was estimated for a Co doped film to be nc = 4×10 cm [1]. Thus, we have a film with n < nc and another one with n > nc for each type of doping. A possible ferromagnetic response of charge carr...

physica status solidi (c), 2013

Ceramic piezomaterials based on Pb 0.75 Ba 0.15 Zr 0.53 Ti 0.47 O 3 (PBZT) system are characteriz... more Ceramic piezomaterials based on Pb 0.75 Ba 0.15 Zr 0.53 Ti 0.47 O 3 (PBZT) system are characterized by high values of the ferroelectric Curie temperature and polarization. They have widespread applications in electromechanical and electroacoustic transducers, bandwidth filters, transformers, frequency stabilized resonators, etc. Preparation of PBZT composites with nano-inclusions of Cu and Ni have been carried out by a complex powder technology including metallization procedures which were performed by the well-known chemical deposition method from standard solutions of copper and nickel. Studies of hydrostatic density and porosity of obtained samples as well as their microstructural investigations have yielded a larger density of PBZT-metal composites (5.3-7.3 g/cm 3) as compared with pure PBZT (4.7 g/cm 3). Measurements of dielectric properties of samples in the frequency range from 20 Hz-1 MHz have shown that the Curie temperature increases in PBZT (Ni) and decreases in PBZT(Cu) in comparison with pure PBZT. The complex dielectric permittivity of all investigated composites at higher temperatures and low frequencies is mainly caused by the high electrical conductivity. Only at the highest frequency used (1 MHz) the real part of the complex permittivity is caused by a resonant soft mode and could be fitted with the Curie-Weiss law. The activation energy of ferroelectric domain mobility in PBZT (Cu, Ni) composites is lower than in pure PBZT. This can be explained by a decrease of the domain size in composites.

Journal of Alloys and Compounds, 2013

ChemSusChem, Jan 5, 2018

A green, template-free easy-to-implement strategy giving access to holey g-C₃N₄ nanosheets doped ... more A green, template-free easy-to-implement strategy giving access to holey g-C₃N₄ nanosheets doped with carbon is described. The protocol involves heating dicyandiamide with β-cyclodextrin (βCD) prior to polymerization. By this approach, the local symmetry of g-C3N4 skeleton is broken, thus yielding CxGCN (x corresponds to the initial amount of βCD used) with porous and a distorted structure. The electronic, emission, optical and textural properties of the best-performing material, C2GCN, are thus greatly modified as compared to bulk g-C₃N₄ (GCN). The spectroscopic and luminescent features of C2GCN show the characteristic π-π* electronic transition of GCN, accompanied by much stronger n-π* electronic transitions due to the porous and defect network. These new electronic transitions, along with the presence of additional carbon synergistically contribute to enhance visible light absorption and to restrain the recombination of electron-hole pairs. Steady-state and time-resolved photolum...

Journal of Physics Condensed Matter, Apr 21, 2003

ABSTRACT

Radiat Eff Defect Solid, 1979

The production of point defects and the amorphous phase in silicon during the implantation of var... more The production of point defects and the amorphous phase in silicon during the implantation of various mass ions at 77 K and 300 K has been studied as a function of ion fluence using electron paramagnetic resonance (EPR). The EPR spectra of the centres Si-P3 (neutral tetra-vacancy), Si-P6 (silicon di-interstitial), Si-A5 (probably another self-interstitial defect), Si-B3 (of unknown structure) were

Physica B: Condensed Matter, 1999

ABSTRACT

Thin Solid Films, 2013

Epitaxial zinc ferrite thin films are grown on (001) SrTiO 3 substrates by pulsed laser depositio... more Epitaxial zinc ferrite thin films are grown on (001) SrTiO 3 substrates by pulsed laser deposition. The magnetic anisotropy of the films is examined by superconducting quantum interference device magnetometry and ferromagnetic resonance, with various in-plane and out-of-plane orientations of the magnetic field. The volume magnetization of the films is found to be around 55 kA/m. The existence of a cubic magnetic anisotropy is shown, with the easy axes oriented along the [111] crystallographic directions. The films show ferrimagnetic properties with a Curie temperature higher than 350 K. The nature of a strong "easy-plane"-type bimodal anisotropy in the films is discussed.

Physics of the Solid State, 2005

Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molec... more Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low temperatures (<350 ° C) and studied using photoluminescence and atomic force microscopy. It is found that the germanium layer undergoes a morphological transformation when its thickness becomes equal to approximately five monolayers: an island relief transforms into a smooth undulating relief.

physica status solidi (b), 2013

ABSTRACT The Sr2FeMoO6−δ compound attracts the attention of researchers due to a high Curie tempe... more ABSTRACT The Sr2FeMoO6−δ compound attracts the attention of researchers due to a high Curie temperature, large values of negative magnetoresistance at room temperature and a practically 100% spin polarization of conduction electrons. We have studied the role of grain boundaries on the electrical transport in Sr2FeMoO6−δ in applied magnetic fields up to 8 T. The compound was synthesized out of partially reduced SrFeO3−x, SrMoO4−y precursors (sample I). At the first oxidation stage upon annealing at 700 K in argon with a partial oxygen pressure p(O2) = 10 Pa for 15 h (sample II), the internal structure of the Sr2FeMoO5.82 grains did not change. Sample II exhibits a mixed type of electrical conduction. In a magnetic field B its resistivity decreases without changing the mixed regime of charge transport, only shifting the temperature of minimum resistivity (TmB) to lower values. At temperatures above TmB the conductivity is predominantly metallic, whereas below TmB down to 4.2 K it is of semiconductor type. The increase of the annealing time up to 30 h (sample III) brings about an increase of the resistivity and the appearance of semiconductor-type conductivity at T = 300–4.2 K, which indicates the formation of a continuous insulating interlayer between the grains. In a magnetic field, the resistivity decreases, with the conductivity changing from the semiconducting to a mixed one. In the latter case the charge transport can occur both through point metallic contacts and by means of spin-dependent tunneling across dielectric interlayers between Sr2FeMoO5.82 grains.

Physica B: Condensed Matter, 1999

We report four new EPR spectra found in high-pressure synthetic diamonds with high nitrogen conte... more We report four new EPR spectra found in high-pressure synthetic diamonds with high nitrogen content, grown using nickel, and annealed at 16003C. Analyzing the complex spectra around g+2 two trigonal and two orthorhombic defects, all with S" , were identi"ed from the angular dependence and the di!erent saturation behavior of the transitions. Besides optical and EPR investigations reveal several centers which have been previously ascribed to defects containing nickel and nitrogen. However, nitrogen is not involved in the new centers, since no evidence of an N hyper"ne structure could be found. Possible relations of the new defects with Ni are discussed.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1996

ABSTRACT

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2001

Different Ge/Si superlattices were irradiated with 150 keV Ar ions at room temperature with fluen... more Different Ge/Si superlattices were irradiated with 150 keV Ar ions at room temperature with fluences in the range 1012 to 5×1015cm−2. Defect production was studied with Rutherford backscattering/channeling spectrometry and X-ray diffraction (XRD). The evolution of the damage with ion fluence reveals the existence of three distinct regimes. During the first regime the concentration of defects increases slowly until the

Materials Science and Engineering: C, 2003

Si 5 Ge 5 superlattices (SL) were treated by 80-ns pulses of a ruby laser in a wide range of ener... more Si 5 Ge 5 superlattices (SL) were treated by 80-ns pulses of a ruby laser in a wide range of energy densities. The induced structural and electronic changes were monitored in situ by time-resolved reflectivity (TRR) and ex situ by scanning electron microscopy (SEM), Raman scattering and atomic force microscopy (AFM). The SL starts to melt at energy densities typical of bulk Ge (less than 0.4 J/cm 2). At R 0.7 J/ cm 2 , a self-organization phenomenon is observed: a system of quasiregular rectangular grains with linear dimensions of about 100 nm is developed on the sample surface.

Materials Science and Engineering: B, 2008

ABSTRACT The influence of defects on the optical properties of a single Ge quantum well deposited... more ABSTRACT The influence of defects on the optical properties of a single Ge quantum well deposited on a Si substrate and on a diode structure containing a Si/Ge multilayer structure was investigated. In order to change the density of optically active defects, the as-grown samples were exposed to post-growth treatments: atomic hydrogen passivation and irradiation with 2.0 MeV protons to fluences in the range 2×1012 to 1×1014 cm−2. The optical and structural properties were investigated by photoluminescence and X-ray diffraction and reflection measurements. An unexpectedly high radiation hardness was observed for the as-grown Ge quantum wells.

Journal of Luminescence, 2006

In this work we studied the influence of high-energy proton irradiation on the optical and struct... more In this work we studied the influence of high-energy proton irradiation on the optical and structural properties of an Si/Ge superlattice (SL) with embedded Ge quantum dots (QDs). The presence of QDs in the as-grown samples was established by transmission electron microscopy and photoluminescence (PL). The samples were irradiated with 2.0MeV protons to fluences in the range 2×1012–2×1014cm–2. The structural

Journal of Crystal Growth, 1996

ABSTRACT