A. N. Novruzov

Computer Study of Physical Properties of Silicon Nanostructures

The method of molecular dynamics is applied to the study of variations in the physical properties of vitreous and amorphous silicon nanoparticles when heated from 300 to 1700 K. The nanoparticles consist of 300, 400, and 500 atoms. The energy and the average length of the Si-Si bond are calculated, and the average number of bonds per atom is determined. Thermally induced strains tend to change the distribution of the excess potential energy among the concentric layers in the nanoparticles. It is shown that, energetically, the most preferential layer is the middle spherical layer of the “warm” nanoparticle. The temperature behavior of the radial and tangential components of the atomic mobility coefficient in the concentric layers is considered. It is established that there is a liquid layer at the nanoparticle surface in the vicinity of the transition to melting. The vitrified Sin nanoparticles are kinetically more stable than the similar-sized amorphous particles.

The Structure of Water Clusters Interacting with Gaseous Acetylene

The interaction of water clusters with acetylene molecules at T = 230 K was studied by the molecular dynamics method. The structure of clusters was analyzed by constructing Voronoi polyhedra. Water clusters interacting with C2H2 molecules are characterized by a diversity of H-bond orientations, a more uniform distribution of H-bonds over the cluster volume, a larger number of bonds per atom, and smaller bond lengths. The spectrum of bond lengths broadens as the number of acetylene molecules interacting with the water cluster increases. C2H2 molecules have a pressing action on water clusters.

Computer simulation of the structure of water clusters containing absorbed ethane molecules

The structure of water clusters that have absorbed ethane molecules is studied by the molecular dynamics method. Structural analysis is performed by the construction of Voronoi polyheda for oxygen atoms and hybrid polyheda whose centers coincide with the centers of oxygen atoms and the faces are formed according to the positions of hydrogen atoms. The (H2O)20 cluster can retain no more than four ethane molecules remaining at the same time stable. When a water cluster adds more than four ethane molecules, the volumes of Voronoi polyheda acquire values close to the volume per molecule in the bulk liquid water. As the number of ethane molecules in a water cluster increases, the number of hydrogen atoms adjacent to oxygen, as well as the average number of units in cyclic formations composed of hydrogen atoms, also increases. In this case, the number of H-O-H angles formed by the nearest geometric neighbors close to 89° becomes dominant. The coefficient of nonsphericity reflecting the local arrangement of hydrogen atoms around the oxygen atoms decreases as the C2H6 molecules are added to water cluster and approaches to the value of this coefficient for the rhombic dodecahedron in the case of adsorption of six ethane molecules.

Simulation of dielectric properties and stability of clusters (H2O) i , CO2(H2O) i , and CH4(H2O) i

Molecular dynamics method is used for studying complex permittivity ɛ and the stability of individual water clusters as a function of the number of involved molecules (7 ≤ i ≤ 20) and also the corresponding characteristics of water aggregates with a captured CO2 or CH4 molecule. Absorption of the latter molecules leads to considerable changes in dielectric properties and stability of clusters. In particular, upon the addition of a CO2 molecule to a water cluster, the oscillation parameters of the real and imaginary parts of the permittivity change. Capture of a CH4 molecule by a water aggregate changes the ɛ(ω) dependence from the relaxation to resonance type. For i ≥ 15, the thermal stability of individual water clusters can be lower than that of aggregates CO2(H2O)i and CH4(H2O)i. The mechanical stability of (H2O)i ≥ 13 clusters can exceed that of heteroclusters under consideration. Clusters (H2O)i and CO2(H2O)i have approximately the same dielectric stability, whereas aggregates CH4(H2O)i exhibit lower stability with respect to electric perturbations.

Computer simulation of the adsorption of acetylene by disperse aqueous medium. IR spectra

Adsorption of acetylene molecules by water clusters at T 230 K was studied by the method of molecular dynamics. Addition of already two C2H2 molecules to (H2O)n clusters (10 ≤ n ≤ 20) makes them thermodynamically unstable. With an increase in the acetylene concentration in the disperse aqueous system, the IR absorption by the cluster system in the frequency range 0 ≤ ω ≤ 1000 cm−1 increases. Depending on the number of C2H2 molecules per water cluster, the IR reflection by cluster systems can either increase or decrease. The power of the thermal radiation emitted by the clusters considerably increases after the adsorption of C2H2 molecules and grows with an increase in the acetylene concentration in the disperse aqueous system.

Computer study of the absorption of ethane by aqueous ultradispersed medium: IR spectra

Absorption of ethane molecules by water clusters containing 10–20 molecules is studied by the molecular dynamics method. The (H2O)n (I), C2H6(H2O)n (II), and (C2H6)2(H2O)n (III) cluster systems are composed on the basis of specific statistical weights. Spectral characteristics of system and single clusters are determined in the frequency range of 0 ≤ ω ≤ 1000 cm−1. In this frequency range, both real and imaginary parts of dielectric permittivity decrease monotonically after the absorption of C2H6 molecules by an aqueous ultradispersed system. Integral coefficient of IR absorption increases, while average (over frequency) reflection coefficient decreases after the absorption of ethane molecules. The intensity of IR scattering by the systems of clusters containing C2H6 molecules lowers. Maximal values of radiation power for water clusters with various sizes are balanced with the capture of ethane molecules by the clusters, whereas oscillations in the size dependence of the density of electrons that are active with respect to IR radiation decrease.

Determination of Acetylene Solubility in Dispersed Water by Computer Simulation

The interaction of acetylene molecules with the (H2O)20 cluster was studied by molecular dynamics simulation. The behavior of the derivative of the chemical potential (∂μ/∂i)V,T (i is the number of acetylene molecules) shows that the cluster stability is retained when no more than two C2H2 molecules add to the cluster. The system composed of (C2H2)i(H2O)20 clusters has higher values of dielectric permittivity (both real and imaginary terms) than the (H2O)20 + n cluster system. The medium formed by water clusters with C2H2 molecules both absorbs and reflects IR radiation with a higher intensity as compared to a system of this type with pure water. The addition of C2H2 molecules to water clusters is also accompanied by an increase in the number of bands in an IR reflection spectrum. Adsorbed C2H2 molecules are aligned with the tangent to the water core of the cluster, thus impeding their penetration into the aggregate and, hence, decreasing the solubility of acetylene.

Computer study of the temperature dependence of physical properties of noncrystalline silicon nanoparticles

Variations in the structure and kinetic properties of vitreous and amorphous Si400 nanoparticles upon heating from 300 to 1700 K are studied by molecular dynamics. The nanoparticle density increases with temperature and approaches the density of bulk solid silicon. A transition from a unimodal to a bimodal distribution of bond lengths is observed upon heating. This transition is more pronounced in the case of the vitreous nanoparticle. The average bond length in the amorphous nanoparticle is, as a rule, larger than that in the vitreous one, and the average number of bonds per atom is lower than that in the vitreous nanoparticle for nearly all studied temperatures. Negative values of the excess potential energy correspond to middle concentric layers of nanoparticles. Liquid layers form in the surface region of nanoparticles in the vicinity of the melting transition. A kinetic test indicating the beginning of nanoparticle melting is formulated.

Computer simulation of oxygen and nitrate ion absorption by water clusters

Using the molecular dynamics method, the joint absorption of oxygen and nitrate ions by water clusters is studied in terms of the polarizable model of flexible molecules. Significant fluctuations are observed in the number of hydrogen bonds in the clusters during the addition of NO3− ions to water-oxygen aggregates. Dielectric permittivity noticeably changes upon the addition of O2 molecules to water clusters and nitrate ions to oxygen-containing water clusters. After the absorption of oxygen molecules and nitrate ions, water clusters markedly lose the ability to IR absorption. The Raman spectrum of a medium formed from disperse aqueous system, oxygen, and nitrate ions displays a greater number of bands than the spectrum of a system of pure water clusters.