D. G. Gromov

Kinetics of the melting-dispersion process in copper thin films

The kinetics of a melting-dispersion process in copper thin films is investigated at different thicknesses of the films. It is shown that the film initially melts in local regions and then the melting front propagates over the sample. Melting of copper thin films of the same thickness can occur within different time periods depending on the temperature (from almost instantaneous melting at higher temperatures to melting proceeding over the course of a few hours at lower temperatures). The dependence of the activation energy for the melting-dispersion process on the film thickness is determined and explained in terms of hydrodynamics. The mechanism of the melting-dispersion process is considered.

The effect of gold on modern bimetallic Au–Cu/MWCNT catalysts for the oxy-steam reforming of methanol

Copper and gold doped copper catalysts supported on multi-walled carbon nanotubes were prepared by wet impregnation and deposition–precipitation methods, respectively. The catalysts were tested in the oxy-steam reforming of methanol (OSRM) and characterized by XRD, SEM-EDS, TOF-SIMS, thermo-gravimetric analysis and temperature-programmed desorption of ammonia. The reactivity results showed the promotion effect of gold on the activity and selectivity of the copper catalysts in the OSRM. The formed Cu–Au alloy as an active phase was responsible for the activity and selectivity improvement of the bimetallic catalysts in the oxy-steam reforming of methanol. The formation of an Au–Cu alloy was confirmed by the XRD, TOF-SIMS and SEM-EDS techniques. The reducibility and acidity of the tested catalysts are important factors, which influence the activity of the copper and gold–copper catalysts.

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

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.

Investigation of the early stages of condensation of Ag and Au on the amorphous carbon surface during thermal evaporation under vacuum

The formation of arrays of Ag and Au nanoparticles from the vapor deposition phase on an unheated thin-film amorphous carbon substrate has been systematically investigated using transmission electron microscopy. It has been found that there is a significant dependence of the size of particles and the density of their location on the surface on the amount of evaporated silver or gold. In particular, for a gold sample weighing ∼2 mg, the predominant diameter of clusters is ∼9 nm at a surface distribution density of ∼10000 μm−2, whereas during evaporation of a larger sample weighing ∼11 mg, the predominant diameter of clusters is ∼35 nm at a surface distribution density of ∼200 μm−2. A phenomenological description of the formation of an array of nanoparticles through thermal evaporation and condensation under vacuum onto an unheated surface has been presented. The specific features of the process have been considered. It has been concluded that the nucleation of a silver or gold particle on the surface gives rise to a flow of vaporous silver or gold, which is directed to this particle and, thus, maintains it in a hot state and determines its growth.

The factors that determine the temperature of fusion of Cu and Ni thin films on inert surfaces

A method for calculating the temperature of fusion of thin films depending on their thickness was suggested. It was shown for the example of copper and nickel films that the main factors that determined a substantial decrease in the temperature of fusion of thin films compared with massive materials were the different heats of fusion of thin films, the temperature dependence of the heat of fusion, and the tendency of thin-film systems toward decreasing the absolute surface energy at the expense of decreasing the specific surface energy and surface area. The dependences of the temperature of fusion on the thickness of copper and nickel films were calculated. Fusion and dispersion processes were studied as depending on nickel film thickness on the surfaces of Al2O3 and SiO2. The calculation results obtained for copper and nickel were in close agreement with experimental data.

Investigation of transport mechanisms in Bi doped Ge2Sb2Te5 thin films for phase change memory application

The influence of Bi doping on the charge carrier transport mechanism in GST225 thin films was investigated. The three regions with different current-voltage dependencies were established. The energy diagrams for Bi doped GST225 thin films for different regions were analyzed. Analysis of experimental data showed that space charge limited current is the most possible explanation for the nonlinear I-V dependence in the middle electrical field strength (103 < E < 104 V/cm). Position of the trap levels (Et) controlling transport mechanism, and density of traps (Nt) were estimated with using of Rose and Lampert theories. It was established that Bi doping can significantly change I-V characteristic, resistivity, mobility gap, Urbach energy, density distribution of localized states, and activation energy of conductivity. The most pronounced modification of current-voltage characteristic and parameters of the thin films was established for GST225 + 0,5 wt. % Bi. Thus, doping of Ge2Sb2Te5 by Bi expands the range of material properties, which is important for the optimization of PCM technology.

Piezoelectric energy nanoharvester based on an array of ZnO whisker nanocrystals and a flat copper electrode

A piezoelectric low-power nanogenerator based on an array of ZnO whisker nanocrystals and a movable flat top electrode has been developed. It has been shown that this element generates ac current under the effect of mechanical vibrations with a frequency up to 100 Hz, which makes it possible to accumulate the capacitor charge. The investigation of the sample on a vibration stand under the acceleration of 9.8 m/s2 has demonstrated that the stable variable signal from 50 mV to 1.5 V appears at a frequency of 20–120 Hz. The experimental dependence of the generated voltage on the weight of the load applied to the top electrode is obtained. The appearance of the ac signal is explained in the context of the classical piezoelectric effect, namely, the ZnO whisker nanocrystals in the starting state are already bent, and the different-sign charges are generated under the effect of mechanical vibrations during bending and unbending.

Specific features of the structure and properties of carbon nanocolumns formed by low-temperature chemical vapor deposition

The process of the formation of column-shaped carbon nanostructures by plasma-enhanced chemical vapor deposition is explored. Carbon nanocolumns are formed at 250°C. The structure and properties of the structures are studied by atomic-force microscopy, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The electrical properties of the structures are investigated.

Specifics of low-temperature melting and disintegration into drops of silver thin films

The behavior of silver thin films on SiO2, amorphous carbon, and Al2O3 surfaces upon thermal vacuum heating has been studied. It has been shown that Ag thin films decay into drops at heating. This process does not have strongly defined temperature and takes place at temperatures that are essentially lower than the reference melting point of silver. At the same time, it shows a cumulative character. Decay of a silver thin film of 12-nm thickness on the SiO2 surface occurs instantly at the temperature about 230°C, while, at a temperature of 120°C, the process of decaying into drops starts after 11 h. It has been speculated that disintegration of the film goes through the stage of melting caused by the surface. The influence of the substrate surface on the disintegration process of films with different thicknesses has been studied. It has been shown that the evaporation process plays an essential role in the decay of thin films.

Use of thin film of a Co15Ti40N35 alloy for CVD catalytic growth of carbon nanotubes

It is shown that multiwalled carbon nanotubes can be grown on the catalytic surface of a Co–Ti–N alloy with low (~10 at %) cobalt content by the conventional method of chemical deposition from acetylene. Adding nitrogen to the composition of the Co–Ti contributes the formation of the TiN compound and extrusion of Co onto the surface where it makes a catalytic effect for CNT growth. It was found that the tubes begin growth at a temperature of 400°C. It is shown by studies using Raman spectroscopy that the quality of CNT improves with increasing temperature.