Yu. Ya. Gafner

Formation of an icosahedral structure during crystallization of nickel nanoclusters

The crystallization of nickel nanoclusters is investigated using a molecular dynamics simulation with tight-binding potentials. The formation of a cluster structure depends on the cooling conditions. Slow cooling results in the formation of a face-centered cubic structure, whereas fast cooling, according to the data obtained in 40% of the simulation experiments, leads to the formation of an icosahedral structure. The molecular dynamics simulation experiments demonstrate the possibility of controlling the formation of a structure of nickel nanoclusters during crystallization.

Possible Mechanisms of Increase in Heat Capacity of Nanostructured Metals

The problem of anomalously high experimental values of the heat capacity of metallic nanoclusters has been analyzed in terms of the thermodynamics of the surfaces, as well as based on the data of computer experiment. The heat capacity of ideal face-centered cubic (fcc) palladium clusters with a diameter of 6 nm in the temperature range of 150–300 K has been investigated using the molecular dynamics method with several tight-binding potentials. It has been found that, at a temperature T = 150 K, the heat capacity of a Pd nanoparticle exceeds the heat capacity of the bulk material by 12–16%. Based on the results of the theoretical treatment, computer simulation, and analysis of experimental data, it has been concluded that an increase in the heat capacity of the compacted nanomaterial is not determined by the high heat capacity of individual clusters. Apparently, the significant increase in the heat capacity of compact nanomaterials can be explained either by their disordered state or by the high content of different types of impurities, mainly hydrogen.

Formation of structural modifications in copper nanoclusters

The melting and crystallization of copper nanoclusters are investigated using the molecular dynamics simulation with tight-binding potentials. The formation of a cluster structure depends on the conditions used for cooling from the liquid phase. Slow cooling results predominantly in the formation of a face-centered cubic structure, whereas rapid cooling in the majority of cases leads to the formation of an icosahedral structure. Therefore, the simulation performed has demonstrated the possibility of controlling the formation of a structure of copper nanoclusters during crystallization.

Preparation of nickel nanopowder through evaporation of the initial coarsely dispersed materials on an electron accelerator

The experimental studies have shown that, during evaporation of coarsely dispersed carbonyl nickel, a nanodispersed powder of high purity can be obtained, which is promising for use in different technological applications. The process has a high efficiency and a relatively high output (of the order of 100 g/h). The specific feature of the synthesis is the separation of obtained nanoparticles into two fractions with average sizes of approximately 100 and 200 nm. The computer analysis of the system evolution has demonstrated that this separation can occur as a result of the aggregation of the initial clusters of sufficiently large sizes.

Effect of temperature on structural transformations of nickel nanoclusters

The gas-phase condensation of nickel nanoclusters is simulated by the molecular dynamics method with the use of tight-binding potentials. It is revealed that subsequent heating of the synthesized clusters to temperatures of 400–500 K leads to a substantial improvement of their internal structure with a hexagonal close-packed phase predominating. Upon heating of the nanoparticles above the melting point and subsequent gradual cooling, the formation of a cluster structure depends strongly on the cooling rate. The inference is made that heating of the nanoclusters synthesized from a gas phase can be used for the controlled formation of nickel nanoparticles with a predicted structure.

Analysis of the applicability of Ni, Cu, Au, Pt, and Pd nanoclusters for data recording

The applicability of individual Ni, Cu, Au, Pt, and Pd nanoclusters as data bits in next generation memory devices constructed on the phase-change carrier principle is studied. To this end, based on the modified tight-binding potential (TB-SMA), structure formation from the melt of nanoparticles of these metals to 10 nm in diameter was simulated by the molecular dynamics method. The effect of various crystallization conditions on the formation of the internal structures of Ni, Cu, Au, Pt, and Pd nanoclusters is studied. The stability boundaries of various crystalline isomers are analyzed. The obtained systematic features are compared for nanoparticles of copper, nickel, gold, platinum, and palladium of identical sizes. It is concluded that platinum nanoclusters of diameter D > 8 nm are the best materials among studied metals for producing memory elements based on phase transitions.

Role of “magic” numbers in structure formation in small silver nanoclusters

The molecular dynamics method with the modified tight-binding (TB-SMA) potential has been used to study thermal stability of the initial fcc phase in perfect silver clusters to 2 nm in diameter. Dimensional boundaries of nanoparticles, at which the internal atomic configuration changes upon heating, have been determined using the molecular dynamics simulation. It has been shown that the temperature factor can cause the transition from the initial fcc phase to other structural modifications, including those with pentagonal symmetry, in small Ag clusters. It has been demonstrated that “magic” numbers play an important role in the formation of the internal structure of silver clusters.

Effect of disorder on the structure of small aluminum clusters during heat treatment

The boundaries of thermal stability of the initial face-centered cubic (fcc) phase in perfect aluminum clusters with a diameter up to 3 nm have been investigated by the molecular dynamics method using a modified tight-binding (TB-SMA) potential. Based on the performed computer analysis, it has been concluded that, in most cases, for aluminum clusters with sizes up to D = 2.5 nm, the most stable configuration is the structure with pentagonal symmetry. With a further increase in the number of atoms, the fcc structure becomes more stable. The influence of the degree of disorder of nanocompacted aluminum particles up to 4 nm in diameter on the formation of a crystal structure during heat treatment has been analyzed. It has been shown that, under the effect of the temperature factor, the clusters undergo a transition from the initial fcc phase to other structural modifications, including those with pentagonal symmetry, even in the case of clusters with fairly large sizes.

A study of melting of various types of Pt–Pd nanoparticles

The melting processes of various Pt–Pd nanoparticles (binary alloy, core–shell, D ≤ 4.0 nm) with different percent platinum atom content are investigated via the molecular dynamics using the embedded atom method potential in order to establish the thermal stability of simulated particle structure. In accordance with the data obtained, the most thermally stable are Pt–Pd nanoalloys with a diameter above 2.0 nm and core–shell Pd@Pt particles. As is shown, heating of binary Pt–Pd cluster alloys with the particle diameters less than 2.0 nm may cause the transition to pentagonal symmetry structures and core–shell-like complex formations.

Thermal stability of Pt nanoclusters interacting to carbon sublattice

The catalytic activity of Pt clusters is dependent not only on the nanoparticle size and its composition, but also on its internal structure. To determine the real structure of the nanoparticles used in catalysis, the boundaries of the thermal structure stability of Pt clusters to 8.0 nm in diameter interacting with carbon substrates of two types: a fixed α-graphite plane and a mobile substrate with the diamond structure. The effect of a substrate on the processes melting of Pt nanoclusters is estimated. The role of the cooling rate in the formation of the internal structure of Pt clusters during crystallization is studied. The regularities obtained in the case of “free” Pt clusters and Pt clusters on a substrate are compared. It is concluded that platinum nanoparticles with diameter D ≤ 4.0 nm disposed on a carbon substrate conserve the initial fcc structure during cooling.