Showing papers in "Physics of the Solid State in 2013"

Classification of structural modifications of carbon

Abstract: New schemes have been proposed for the structural classification of carbon phases and nanostructures. These schemes are based on the types of chemical bonds formed in materials and on the number of nearest neighbors with which each atom forms covalent bonds. The classification schemes allow one to describe the already known phases and form the methodological basis for the prediction of new phases and nanostructures.

Structure of Nanodiamonds Prepared by Laser Synthesis

Abstract: A study is reported of nanodiamonds obtained by a new method—pulsed laser ablation of a specially prepared carbon target. In the mechanism employed to produce a diamond phase, this method is similar to that of detonation synthesis of nanodiamonds. The main structural characteristics of the material have been determined and compared with the corresponding characteristics of detonation nanodiamonds.

Atomistic simulation of the motion of dislocations in metals under phonon drag conditions

Abstract: The mobility of dislocations in the over-barrier motion in different metals (Al, Cu, Fe, Mo) has been investigated using the molecular dynamics method. The phonon drag coefficients have been calculated as a function of the pressure and temperature. The results obtained are in good agreement with the experimental data and theoretical estimates. For face-centered cubic metals, the main mechanism of dislocation drag is the phonon scattering. For body-centered cubic metals, the contribution of the radiation friction becomes significant at room temperature. It has been found that there is a correlation between the temperature dependences of the phonon drag coefficient and the lattice constant. The dependences of the phonon drag coefficient on the pressure have been calculated. In contrast to the other metals, iron is characterized by a sharp increase in the phonon drag coefficient with an increase in the pressure at low temperatures due to the α-∈ phase transition.

Formation of silver nanoparticles on the silicate glass surface after ion exchange

Abstract: It has been experimentally shown that water vapor thermal treatment of silicate glasses with silver ions introduced by ion exchange leads to the formation of a silver nanoparticle layer with a high packing density on the glass surface. The results of studying the morphology of samples by atomic force and electron microscopy and X-ray spectral analysis of the composition of nanoparticles, as well as the optical density and luminescence spectra in different stages of the treatment, are presented. Mechanisms explaining the processes responsible for silver nanoparticle formation upon water vapor thermal treatment on the glass surface after ion exchange are proposed.

Synthesis and electronic structure of nitrogen-doped graphene

Abstract: The crystalline and electronic structure of nitrogen-doped graphene (N-graphene) has been studied by photoelectron spectroscopy and scanning tunneling microscopy. Synthesis of N-graphene from triazine molecules on Ni(111) surface results in incorporation into graphene of nitrogen atoms primarily in the pyridinic configuration. It has been found that inclusions of nitrogen enhance significantly thermal stability of graphene on nickel. An analysis of the electronic structure of N-graphene intercalated by gold atoms has revealed that the pyridinic nitrogen culminates in weak p-type doping, in full agreement with theoretical predictions. Subsequent thermal treatment makes possible conversion of the major part of nitrogen atoms into the substitutional configuration, which involves n-type doping. It has been shown that the crystalline structure of the N graphene thus obtained reveals local distortions presumably caused by inhomogeneous distribution of impurities in the layer. The results obtained have demonstrated a promising application potential of this approach for development of electronic devices based on graphene with controllable type of conduction and carrier concentration.

Resonant diffraction of electromagnetic waves from solids (a review)

Abstract: The subject of the review is the diffraction of an electromagnetic wave on a periodic condensed matter where the Bragg condition is satisfied at the frequency of resonance excited in this medium. The Bragg systems known as resonant photonic crystals in general have been considered, and the propagation, reflection, transmission, and diffraction of electromagnetic radiation in different objects—(I) periodic quantum-well structures near the exciton resonance, (II) optical lattices of atoms cooled in a laser field, and (III) bulk crystals and multilayers with gamma-ray resonance intranuclear transitions—have been described in a unified context. The main attention has been paid to the steady-state linear diffraction, including resonant reflection and transmission, which is the best studied and allows a comparison of the three aforementioned systems with the aim of revealing specific characteristics and common features. A characteristic common property of the considered systems is the suppression of non-radiative channels and an inhomogeneous broadening of the resonance frequency. The second fundamental property of resonant photonic crystals is that the interaction of light with resonant excitation can occur in two regimes depending on the thickness of the sample. In the superradiant regime realized for a small number of resonant layers N (fine structures), the height of the peak and half-width of the reflection spectrum monotonically increase with increasing value of N. With a further increase in N, there occurs a transition to a photonic-crystal regime where the half-width of the reflection spectrum is saturated, which manifests itself in the form of an optical stop band. The theoretical description is illustrated by experimental spectra measured for all three resonant Bragg systems.

Sizes and fluorescence of cadmium sulfide quantum dots

Abstract: Cadmium sulfide quantum dots have been synthesized by wet chemical deposition from an aqueous solution. The sizes of the quantum dots determined by dynamic light scattering directly in the colloidal solution and by intermittent-contact atomic force microscopy in the dry sediment agree with each other. It has been found that splitting of the fluorescence peaks of the quantum dots can be affected by the disorder of the atomic structure of cadmium sulfide quantum dots.

Magnetic and electrical properties of the half-metallic ferromagnets Co2CrAl

Abstract: This paper presents the results of measurements of the magnetic and electrical properties of the ferromagnetic alloy Co2CrAl in two structural states: (i) after severe plastic deformation and (ii) after shortterm high-temperature annealing of the deformed specimens. The experiments have been performed at temperatures in the range from 2 to 900 K in magnetic fields H ≤ 50 kOe. The ferromagnetic Curie temperature TC and the paramagnetic Curie temperature Θ have been determined (TC = 305 K and Θ = 326 K), as well as the spontaneous magnetic moment μS and the effective magnetic moment μeff per molecule of the alloy (μS = 1.62 μB and μeff2 = 8.2 μB2). It has been shown that the magnetic crystalline anisotropy energy of the alloy is on the order of ∼5 × 105 erg/g. The specific features of the electrical properties are associated with the presence of an energy gap in the electronic spectrum near the Fermi level EF and with the change in the parameters of the energy gap as a function of the temperature.

Composition nanodefects in doped lithium fluoride crystals

Abstract: This paper presents the results of investigations into the optical properties of LiF crystals doped with oxides of different metals (Li, W, Ti, Fe). It has been proved that, during the growth of crystals, nanodefects containing polyvalent dopant ions and oxygen in different states (O2−, OH−, O2−) are formed in the crystalline matrix. It has been shown that these nanodefects are sinks of electronic excitations and determine the direction and efficiency of radiation-induced processes.

Atomic positions and diffusion paths of h and he in the α-Ti lattice

Abstract: The solution energy of H and He in various interstitial and substitution positions in the hcp lattice of α-Ti has been calculated based on the method of electron density functional. The lowest solution energy of He corresponds to the basal octahedral position and that of H corresponds to the octahedral position (next in energy is the tetrahedral position). The calculated vibration frequencies of H in various positions are used for identification of lines in the vibration spectrum obtained by the method of neutron inelastic scattering. Taking into account these spectra, it can be concluded that hydrogen atoms occupy in the hcp lattice of Ti both the octahedral and tetrahedral positions even at 600 K. The available experimental data do not contradict the conclusion that the octahedral position is more preferable in α-Ti. The energy barriers are estimated for various diffusion paths of H and He.

Magnetoelectric effect in a magnetostrictive-piezoelectric bilayer structure

Abstract: The results of theoretical and experimental investigations of the magnetoelectric effect in a magnetostrictive-piezoelectric bilayer structure have been presented. The expression for the magnetoelectric voltage coefficient in the region of electromechanical resonance has been derived based on the joint solution of the equations of motion for the magnetostrictive and piezoelectric media and the constitutive relations. The dependence of the magnitude of the effect on the thickness of the ferrite and piezoelectric layers has been analyzed. The experimental results for nickel-lead zirconate-titanate bilayer structures have been reported. The theoretical results agree perfectly with the experimental data.

Material constants of (Ba,Sr)TiO 3 solid solutions

Abstract: The phenomenological theory of solid solution is used to calculate the material constants (elastic compliances, susceptibilities, piezoelectric moduli, pyroelectric constants) of Ba x Sr1 − x TiO3 solid solutions over the entire range of concentrations x at room temperature. The phenomenological potential constants and the numerical values of the material constants are given for specific concentrations.

Radiation resistance of LaPO4 (monazite structure) and YbPO4 (zircon structure) from data of computer simulation

Abstract: The radiation resistance of the monazite LaPO4 and the compound YbPO4 (zircon structure type) has been investigated using the computer simulation. The number of Frenkel pairs, which are formed in the structure of these minerals after the passage of a primary knock-on thorium atom with an energy of 30 keV, has been calculated by the molecular dynamics method. The formation of Frenkel pairs and their recombination in the motion of recoil nuclei in the structure of the studied minerals have been discussed. It has been shown that the probability of the “survival” of Frenkel pairs in the LaPO4 monazite is significantly lower than in the YbPO4 compound. The tendency of these minerals toward amorphization under radiation damage has been described numerically. The obtained results have demonstrated that one of the main factors determining the radiation resistance of orthophosphates LnPO4 is the type of crystal structure, and the compounds with the monazite structure are more radiation resistant than the compounds with the zircon structure.

Effect of grain boundaries on the electron work function of nanocrystalline nickel

Abstract: The electron work function of nickel with various grain sizes has been studied. It has been shown that the work function decreases as the specific length of grain boundaries in nickel increases with decreasing average grain size. It has been found that the transformation of grain boundaries from a nonequilibrium to equilibrium state leads to an increase in the electron work function by 0.15 eV.

Contribution of thermal radiation in measurements of thermal conductivity of sandstone

Abstract: The effective thermal conductivity of sandstone at high pressures of up to 400 MPa and temperatures of 273–523 K has been studied. It has been shown that the degree of crystallization of rock-forming minerals substantially influences the temperature and pressure dependences of the thermal conductivity. The contribution of the radiation heat transfer in measurements of the thermal conductivity of sandstone at various temperatures has been analyzed taking into account the reflection and attenuation of the thermal radiation. The results of measuring the reflection and absorption spectra of the thermal radiation have been presented.

Kinetic properties of TiN thin films prepared by reactive magnetron sputtering

Abstract: Results of investigations of kinetic properties of TiN thin films prepared by dc reactive magnetron sputtering are presented. It is established that the TiN thin films are polycrystalline and possess semiconductor n-type conduction. The carrier concentration is ∼1022 cm−3, while electron scattering occurs at ionized titanium atoms.

Electrophysical properties and structural features of shungite (natural nanostructured carbon)

Abstract: This paper presents the results of investigations of the electrical conductive properties with a nanoscale locality at nanoampere currents and the results of an analysis of the correlation between the electrical conductivity and structural features of natural glassy carbon, i.e., shungite. The investigations have been performed using atomic force microscopy, electric force spectroscopy, scanning spreading resistance microscopy, X-ray spectroscopic analysis, and Raman spectroscopy. It has been found that there are differences in electrical conductive properties of the structurally similar shungite samples formed under different PT conditions. Based on the analysis of the structural parameters and specific features of the shungite compositions, it has been shown that the effect of intercalation of impurities into boundary layers of graphene sheets has the most significant influence on the electrical and physical properties of the shungites. The differences in types and values of conductivity of the shungite samples are determined by the different degrees of intercalation.

On the power-law pressure dependence of the plastic strain rate of crystals under intense shock wave loading

Abstract: The plastic deformation of metallic crystals under intense shock wave loading has been theoretically investigated. It has been experimentally found that the plastic strain rate \(\dot \varepsilon \) and the pressure in the wave P are related by the empirical expression \(\dot \varepsilon \) ∼ P4 (the Swegle-Grady law). The performed dislocation-kinetic analysis of the mechanism of the origin of this relationship has revealed that its power-law character is determined by the power-law pressure dependence of the density of geometrically necessary dislocations generated at the shock wave front ρ ∼ P3. In combination with the rate of viscous motion of dislocations, which varies linearly with pressure (u ∼ P), this leads to the experimentally observed relationship \(\dot \varepsilon \) ∼ P4 for a wide variety of materials with different types of crystal lattices in accordance with the Orowan relationship for the plastic strain rate \(\dot \varepsilon \) = bρu (where b is the Burgers vector). In the framework of the unified dislocation-kinetic approach, it has been theoretically demonstrated that the dependence of the pressure (flow stress) on the plastic strain rate over a wide range from 10−4 to 1010 s−1 reflects three successively developing processes: the thermally activated motion of dislocations, the viscous drag of dislocations, and the generation of geometrically necessary dislocations at the shock wave front.

Physical properties of glasses in the Ag 2 GeS 3 -AgBr system

Abstract: Glasses have been prepared by quenching melts in the Ag2GeS3-AgBr system in a range of 0–53 mol % AgBr. The concentration dependences of density, microhardness, glass transition temperatures, and crystallization of alloys have been established. The conductivity of glasses has been investigated by the dc probe method in a range of 240–420 K. The models of the drift motion of silver and halogen ions have been proposed.

Properties of the amorphous-nanocrystalline Gd 2 O 3 powder prepared by pulsed electron beam evaporation

Abstract: An amorphous-nanocrystalline Gd2O3 powder with a specific surface area of 155 m2/g has been prepared using pulsed electron beam evaporation in vacuum. The nanopowder consists of 20- to 500-nm agglomerates formed by crystalline nanoparticles (3–12 nm in diameter) connected by amorphous-nanocrystalline strands. At room temperature, the Gd2O3 nanopowder exhibits a paramagnetic behavior. The phase transformations occurring in the powder have been investigated using differential scanning calorimetry and thermogravimetry (40–1400°C). The amorphous phase of the nanopowder is thermally stable up to a temperature of 1080°C. It has been found that the amorphous phase has an inhibitory effect on the temperature of the polymorphic transformation from the cubic phase into the monoclinic phase. It has been revealed that, compared with the microcrystalline powder, the Gd2O3 nanopowder is characterized by a complete quenching of photoluminescence.

Kinetic stability of octagraphene

Abstract: The kinetic stability of octagraphene, i.e., a carbon atom monolayer in which the C-C bonds form octahedra and squares, has been studied by computer simulation. From the analysis of the molecular dynamics data and hypersurface of the potential energy of this metastable quasi-two-dimensional system, the main channel of its decomposition has been determined, the energy barrier height preventing the decomposition and the frequency factor in the Arrhenius formula for the decomposition rate have been found. It has been shown that the defects formed in octagraphene are not localized but induce avalanche-like damage of the whole structure. Therefore, despite a relatively low rate of defect formation, the lifetime of a macroscopic octagraphene sample at room temperature is insufficient for its practical use, although the mesoscopic samples can find their application.

Structural instability in BaZrO 3 crystals: Calculations and experiment

Abstract: The phonon spectrum of cubic barium zirconate is calculated from first principles using the density functional theory. An unstable R 25 phonon mode observed in the phonon spectrum indicates an instability of the BaZrO3 structure with respect to the oxygen octahedra rotations. It is shown that the symmetry of the ground-state structure of the crystal is I4/mcm. The local structure of BaZrO3 is studied by the EXAFS spectroscopy at the BaL III absorption edge at 300 K to search for the instability predicted by calculations. Anomalously high values of the Debye-Waller factor for the Ba-O atomic pairs (σ 1 2 ∼ 0.015 A2) are attributed to the appearance of this structural instability. The average amplitude of the octahedra rotations caused by thermal vibrations is estimated from the measured σ 1 2 value to be ∼4° at 300 K. The closeness of the calculated energies of various distorted phases resulting from the condensation of the R 25 mode suggests a possible formation of the structural glass state in BaZrO3 as the temperature is lowered. It explains the origin of the disagreement between the results of calculations and diffraction experiments.

Physical, mechanical, and tribological properties of quasicrystalline Al-Cu-Fe coatings prepared by plasma spraying

Abstract: The physical, mechanical, and tribological properties of quasicrystalline coatings based on the Al65Cu23Fe12 alloy prepared by plasma spraying have been investigated The specific features of the phase formation due to the competitive interactions of the icosahedral ψ and cubic β phases have been elucidated A correlation between the microhardness and the content of the icosahedral phase in the coating has been determined The decisive role of the quasicrystalline phase in the formation of high tribological characteristics of the coatings has been revealed and tested

Anomalous losses of lead in crystallization of the perovskite phase in thin PZT films

Abstract: A study has been performed of the mechanisms underlying evolvement of excess lead oxide from lead zirconate titanate films grown ex-situ in two different regimes by magnetron sputtering. In the first case, crystallization of the dense phase of perovskite passed through an intermediate “porous” phase, and in the second, straight through. It has been found that the anomalously large losses of lead are caused by migration of lead over interphase porous boundaries to the surface of the films and depend strongly on the regime of film preparation. The unusual variation of film composition with increasing annealing temperature has been discussed.

Quantum metal film in the dielectric environment

Abstract: A method has been proposed for self-consistent calculations of characteristics of a metal film in dielectrics. The most interesting (asymmetric) case of metal-dielectric sandwiches, where the dielectrics are different on both sides of the film, has been considered in terms of the modified Kohn-Sham method and the stabilized jellium model. The spectrum, electron work function, and surface energy of polycrystalline films placed in passive insulators have been calculated for the first time using Al and Na as an example. It has been found that the dielectric environment generally leads to a negative change in both the electron work function and the surface energy. In addition to the size changes, the shift of the work function is determined by the arithmetic mean of the dielectric constants of the surrounding media.

Specific features of the formation of arrays of silver clusters from a thin film on a SiO2 surface

Abstract: The decomposition of thin silver films into clusters on a SiO2 surface during thermal heating in vacuum has been studied. The experimental histograms of distribution of clusters formed from films of different thicknesses over their diameters have been constructed. It has been found that the decomposition of the films with thickness to 10 nm leads to the formation of monoclusters having one predominant diameter in the range from 15 to 20 nm. After a longer term annealing, the array of clusters almost does not change with time, and the predominant diameter is retained. From films with thicknesses ranging from 10 to 130 nm, an array of clusters with two predominant diameters containing monoclusters (40–80 nm) and globular clusters (400 nm) is formed. It is noted that, with increasing annealing time, the array is enlarged due to the coalescence process and is gradually evaporated; in this case, the predominant cluster diameters remain the same.

Molecular dynamics investigation of the diffusion permeability of triple junctions of tilt and mixed-type boundaries in nickel

Abstract: The diffusion permeability of triple junctions of high-angle tilt boundaries 〈111〉 and 〈100〉 and mixed-type boundaries in nickel has been investigated using the molecular dynamics method. It has been shown that the diffusion permeability of equilibrium triple junctions does not exceed the permeability of the grain boundaries forming them. The effective diffusion radius of the considered triple junctions and the width of the grain boundaries are determined.

Influence of chemical composition on the stability of YBa 2 Cu 3 O y in a humid atmosphere

Abstract: The influence of replacements of yttrium and barium in YBa2Cu3O y and doping of this compound with CeO2, ZrO2, and Pr2O3 on the capability of absorbing water from the annealing atmosphere at T = 200°C has been studied The complication of the chemical composition of the 123-type compounds leads to an enhancement of their stability to aggressive components of the gaseous phase (H2O, CO2) These compounds have a variable chemical composition both in oxygen and water and can exist in three modifications: the oxygen-ordered ortho-phase, the oxygen-disordered tetra-phase, and the pseudocubic phase with a variable water content, which at T = 200°C separates into an oxygen-deficient water-enriched phase and an oxygen-rich anhydrous phase

Computer study of the physical properties of a copper film on a heated graphene surface

Abstract: The structural, kinetic, and mechanical properties of a copper film deposited on single-layer and two-layer graphenes have been studied in a molecular-dynamics model in the temperature range 300 K ≤ T ≤ 3300 K. The film sizes are reduced in the “zigzag” direction more slowly than in the “armchair” direction. The differences have been found to appear in the behavior of copper atoms on single-layer and two-layer graphenes with increasing temperature. Copper atoms on the two-layer graphene have higher horizontal mobility over entire temperature range. However, Cu atoms on the single-layer graphene become more mobile in the vertical direction beginning from a temperature of ∼1500 K. The stress tensor components of the copper film characterizing the action of forces on the horizontal areas have a sharp extremum at T = 1800 K in the case of the single-layer graphene and are characterized by quite smooth behavior in the case of the two-layer graphene.

Organic light-emitting diodes based on polyvinylcarbazole films doped with polymer nanoparticles

Abstract: The optical and electrical properties of light-emitting diode structures with an active layer based on nanocomposite polyvinylcarbazole (PVK) films doped with nanoparticles of another light-emitting polymer, MEH-PPV, have been studied. It has been established that the size of MEH-PPV particles in the PVK matrix is of the order of 100 nm. The spectral range of photoluminescence of such structures can be changed by varying the ratio of PVK to MEH-PPV. The current-voltage characteristics of composite light-emitting diodes based on PVK: MEH-PPV films indicate p-type conductivity. It has been shown that a decrease in the MEH-PPV nanoparticle concentration in the PVK matrix shifts the threshold values of the bias for the onset of electroluminescence toward smaller values and makes the photoluminescence and electroluminescence spectra more similar to the spectrum of the white light-emitting diode. The influence of the form of the polymer and polymer nanoparticles on the mechanisms of injection and transport of charge carriers and the radiative recombination in the studied structures has been discussed.