T. L. Makarova

Magnetic Properties of Carbon Structures

Magnetic properties of the main allotropic modifications of carbon (diamond, graphite, nanographite, nanotubes, and fullerenes) are described. Properties of nanocarbon are considered from the standpoint of the interrelation between structural imperfection and magnetic ordering. Experimental data on high-temperature ferromagnetism in carbon structures and some theoretical models of magnetic carbon are reported.

Electrical and optical properties of pristine and polymerized fullerenes

The band structure, optical, photoelectric and transport properties of solids composed of fullerene molecules linked by van der Waals bonds or forming polymers are considered. Particular attention is concentrated on different polymerization mechanisms.

Light-induced transformation of C60 films in the presence and absence of oxygen

The composition and structure of C60 fullerite films prepared by discrete evaporation in quasiclosed volume, as well as their changes induced by laser irradiation, have been studied by ellipsometry and Rutherford backscattering. The starting film has a 150-Å thick stable top layer and a carbon to oxygen ratio of 10:1. Exposure of a film both to vacuum and to air results in formation of an insoluble photo-transformed phase, but in the second case the change in the refractive index implies the appearance of compounds with oxygen. The material does not undergo complete polymerization, although all structural changes cease at an irradiation dose of 104 photons per fullerene molecule. Treatment of the polymerized phase with organic solvents produces a porous structure, with the voids totaling 48% in the case of exposure in vacuum, and 30% when exposed in air.

Composition and porosity of multicomponent structures: porous silicon as a three-component system

It is shown for the system porous silicon (por-Si)-silicon (Si) that effective nondestructive investigation of the interfacial morphology of layered semiconductor systems and of the composition of multicomponent layers by ellipsometry and Rutherford backscattering is possible. Both methods were used to determine the percentage composition of the main components of por-Si: crystal silicon, silicon oxide, and voids (porosity). It is shown that por-Si obtained by pulse-anodization contains a substantial quantity of silicon oxide. It is also shown that spectral ellipsometry can be used to determine the specific ratio of individual layers or components of multilayer and multicomponent systems (provided that the spectral dispersion of the optical constants of these components is known).

Optical and structural properties of fullerene films doped with cadmium telluride

Composite thin-film structures based on C60/CdTe were prepared by discrete vacuum evaporation in a quasi-closed volume and by vacuum evaporation from a Knudsen cell. The synthesized fullerene samples contained cadmium telluride with a content ranging from 1 to 50 wt %. The morphology and composition of the films were controlled using scanning electron microscopy and energy-dispersive X-ray analysis. The optimum geometry, the total energy, and the spectrum of excited states of the fullerene-cadmium telluride molecular complex were calculated by quantum-chemical methods. The spectral dependences of the photo-luminescence, Raman scattering, extinction coefficient, and refractive index were measured for different compositions. It was found that, in contrast to the spectra of pure fullerene, the luminescence and absorption spectra of the fullerene doped with cadmium telluride exhibit an additional peak in the wavelength range of 600–620 nm. These data were interpreted as the existence of dipole-allowed transitions in the spectrum of excited singlet states of the fullerene due to the interaction with cadmium telluride. The composite films had an increased resistance to degradation under the action of oxygen and water vapor.

Oriented growth of oxygen-free C60 crystallites on silicon substrates

This is the first report of preparation of C60 fullerite films on silicon substrates coated by a layer of natural oxide. The crystallites are about 1 µm in size. The films were found to have an enhanced stability to atmospheric oxygen. The films were obtained by a modified method of discrete evaporation in a quasi-closed volume. The principal features of the method are the quasi-equilibrium condensation conditions, a high substrate temperature (up to 300 °C), and alternation of deposition with recrystallization in a single technological cycle. The method is characterized by high film condensation rates (up to 2000 Å/min) and an economical expenditure of the starting material. A study has been made of the surface structure and morphology, and of the depth profile of the film optical constants and elemental composition.

Optical and electrical properties of C60Tex films

The structure, composition, and electrical and optical properties of thin tellurium-intercalated fullerene films C60Tex are investigated. The samples of compositions from C60Te0.1 to C60Te6 are prepared by thermal evaporation. The sample composition and the impurity distribution are controlled by the Rutherford backscattering technique. The Raman vibrational spectra indicate changes in the symmetry of a C60 molecule: the strain of the molecule increases with a decrease in the tellurium concentration and decreases as the tellurium impurity concentration increases. The evolution of the optical absorption spectra and the electrical conductivity suggests that intercalation of a tellurium impurity leads to modification of the electronic structure of the material. This process is accompanied by a shift and change in shape of the optical absorption edge and a change in the electrical conductivity of films by several orders of magnitude depending on the composition. The electrical conductivity is minimum at a low tellurium impurity content.

Detection of impurity diamagnetic susceptibility and its behavior in n-Ge:As in the region of the insulator–metal phase transition

The method of superconducting quantum interference device (SQUID) magnetometry is used to measure and study low-temperature (T ≤ 100K) susceptibility in a series of samples of heavily doped Ge:As samples on the insulator side of the insulator–metal phase transition. Subtracting the known values of the magnetic susceptibility of the lattice from the measurement results, the values of the impurity magnetic susceptibility of the system are obtained. Using the method of electron spin resonance, the paramagnetic component of the impurity susceptibility is determined. Subtraction of the paramagnetic component from the total impurity susceptibility is used to obtain, for the first time, the values of the impurity diamagnetic susceptibility (~5 × 10–8 cm3/g). The obtained result is consistent with estimates obtained for the localization radius of an electron at an As donor. Lowering the temperature to T ≤ 4 K leads to an increase in the diamagnetic susceptibility, which is consistent with the observed increase in the paramagnetic susceptibility. The observed effect is accounted for by the transition of impurity electrons from the singlet state to the triplet one.

Analysis of spectral features of the optical constants of fullerene and halogen-fullerene films near the absorption edge

This paper reports on measurements of IR spectra and of the extinction coefficient in the 1.2-to 4.2-eV range made on C60Brx films and compared with those for pure C60 films. The modification of the electronic spectrum near the absorption edge is connected with differences in the film structure and with suppression of charge-transfer excitons. The C60Brx films exhibit additional absorption below the fundamental absorption edge of C60.

Low-temperature variation of magnetic order in a nonmagnetic n-Ge:As semiconductor in the vicinity of the metal-insulator phase transition

The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K).