A. S. Fedorov

Theoretical Study of the Lithium Diffusion in the Crystalline and Amorphous Silicon as well as on its Surface

Using the PAW DFT-GGA method and numerical solving of master equation the diffusion rates of lithium atoms inside both crystal and amorphous silicon of LixSi (x= 0..0.5) composition have been calculated for different temperatures. It is shown the diffusion rate for amorphous silicon is ~10 times greater than that for the crystal silicon. For both structures the rate is increased by 1.5-2 orders of magnitude while the lithium concentration is increased up to 0.5 value. This should result in that the LixSi/Si interface will be sharp. This fact has been further confirmed using molecular dynamic calculations based on Angular Dependent Potential (ADP) model. Also binding energies of Li atoms lying on different sites of Si (001) surface as well as the potential barriers for the atom jumps both along the surface and in the subsurface layers have been calculated. The data show the Li atoms move along the surface very easily but their jumps into subsurface layers are very difficult due to the high potential barrier values.

Thermodynamic Stability and Electron Structure of Polymeric Sandwich Complexes of Porphyrins with Different Metals

The thermodynamic stability of different conformers of the polymeric sandwich structures of metalloporphyrins (MeP) is studied by means of quantum chemistry. The possibility of forming stable layered BaP, SrP, ScP, YP, and ZrP structures with shielded and retarded conformation is demonstrated. Shielded conformers are preferable in the case of SrP, BaP, and ScP complexes, while retarded conformers are most advantageous for YP and ZrP complexes. Based on the results from calculating the electron structure of the investigated compounds, we find that SrP and BaP are semiconductors and ScP, YP, and ZrP are electrical conductors

Selective synthesis of higher manganese silicides: a new Mn 17 Si 30 phase, its electronic, transport, and optical properties in comparison with Mn 4 Si 7

The electronic structure, transport and optical properties of thin films of Mn4Si7 and Mn17Si30 higher manganese silicides (HMS) with the Nowotny “chimney-ladder” crystal structure are investigated using different experimental techniques and density functional theory calculations. Formation of new Mn17Si30 compound through selective solid-state reaction synthesis proposed and its crystal structure is reported for the first time, the latter belonging to I-42d. Absorption measurements show that both materials demonstrate direct interband transitions around 0.9xa0eV, while the lowest indirect transitions are observed close to 0.4xa0eV. According to ab initio calculations, ideally structured Mn17Si30 is a degenerate n-type semiconductor; however, the Hall measurements on the both investigated materials reveal their p-type conductivity and degenerate nature. Such a shift of the Fermi level is attributed to introduction of silicon vacancies in accordance with our DFT calculations and optical characteristics in low photon energy range (0.076–0.4xa0eV). The Hall mobility for Mn17Si30 thin film was found to be 25xa0cm2/Vxa0s at Txa0=xa077xa0K, being the highest among all HMS known before. X-ray photoelectron spectroscopy discloses a presence of plasmon satellites in the Mn4Si7 and Mn17Si30 valence band spectra. Experimental permittivity spectra for the Mn4Si7 and Mn17Si30 compounds in a wide range (0.076–6.54xa0eV) also indicate degenerate nature of both materials and put more emphasis upon the intrinsic relationship between lattice defects and optical properties.

Quantum-chemical Research of Endohedral Yttrium Metallofullerenes

Endohedral fullerenes are an interesting class of fullerenes because electron transfer from encaged metal atom to carbon cage has been known to occur and this oftentimes alters the electronic and magnetic properties of the fullerenes [1]. Particularly, endohedral yttrium-fullerenes have been obtained by several research groups [2, 3]. In 1995, Takata and co-workers performed synchrotron X-ray studies on a powder of Y@C82 to confirm the endohedral nature of EMFs for the first time [3]. However, it is still not clear whether the sample contained a pure Y@C82 isomer or if it was a mixture of two or more regioisomers. Nowadays, the structures of new EMFs can be routinely predicted from the first principles (knowing only a formula of the molecule) with the high reliability rivaling that of singlecrystal X-ray diffraction studies [4]. Potential applications of endodedral fullerenes were also predicted on the basis of their peculiar electronic, physical and chemical properties, including superconductors [5], metallofullerene lasers [5] ferroelectric materials [6, 7], nanomemory devices [5] quantum computers, etc. [8].

Molecular dynamical modelling of endohedral fullerenes formation in plasma

The initial stages of fullerene and endohedral metallofullerene (EMF) synthesis in carbon-helium plasma at 1500 K and 2500 K have been simulated with quantum chemical molecular dynamics (MD) based on density-functional tight-binding (DFTB). The cases of formation of large (>100 atoms) sp2-carbon clusters with scandium atoms inside were observed. These clusters are considered as precursors of fullerenes or EMFs, and thus it is shown that formation of EMFs can be explained within the framework of shrinking hot giant mechanism. Also, the dependence of formation rates on plasma parameters, including temperature, buffer gas and metal atoms concentrations, has been studied.

Theoretical study of the thermodynamic stability and electronic structure of thin films of 3 C , 2 H , and 2 D silicon carbides

Silicon carbide is among the most common materials used in semiconductor engineering. Silicon carbide thin films are attractive from the standpoint of designing devices based on heterojunctions. This is due to a characteristic feature of this compound, such as polytypism, leading to the difference in the physical properties and also hampering the preparation of high-quality material samples. In this work, the thermodynamic stability and electronic structure of thin films based on the polytypes 3C, 2H, and 2D with a thickness of a few nanometers have been studied.