S. G. Yastrebov

Analysis of size-distribution function of metallic nanoclusters in hydrogenated amorphous carbon matrix

A method is proposed for mathematical processing of TEM images of nanocomposites with a large amount of non-uniform noise. The approach is 2D spline filtering of the image using the values of average background intensity. The method has been applied to TEM images of amorphous carbon doped with copper. A size distribution function of copper clusters has been obtained which is in a good accordance with earlier data. A dependence of cluster-to-cluster distance on cluster size has been found. This dependence validates the hypothesis of fluctuation growth of the clusters at lower metal contents and their coalescence at higher contents.

Optical properties of amorphous carbon films deposited by magnetron sputtering of graphite

The thermal stability of amorphous carbon (a-C and a-C:H) has been studied by ellipsometry and spectrophotometry in the visible and near-UV range (1.5–5.6 eV). The dielectric function of amorphous carbon has been derived and analyzed using the Kramers-Kronig technique. The conventional analytical approach is shown to be insufficient for the analysis of thermally treated samples. The fundamental absorption edge is analyzed with respect to collective effects in nanoscale fragments of the graphite-like component of the amorphous carbon structure. Two types of graphite-like clusters contributing to the spectral dependence of the fundamental absorption edge and modified by thermal treatment are revealed.

Magnetic properties of iron-modified amorphous carbon

The structure and magnetic properties of films of iron-modified amorphous carbon (a-C:Fe) prepared by magnetron cosputtering of iron and graphite targets are studied. X-ray diffraction measurements show that iron enters the samples in the form of Fe nanocrystals that are typically about 20 nm in size and also forms nanocrystals of hexagonal iron carbide. The temperature dependences of the magnetization, measured under cooling in zero and nonzero magnetic fields, are studied. At temperatures T ≲ 8 K, a magnetic transition, which provides evidence for the onset of magnetic ordering in the material, is observed to occur. The magnetization isotherms obtained in the 8-to 20-K temperature range are in agreement with this observation. It is shown that a modified version of Langevin’s formalism adequately describes the observed features of a-C:Fe film magnetization.

Raman spectra of iron-modified amorphous carbon

Raman spectra of amorphous carbon films containing encapsulated iron (a-C:Fe) have been measured in the frequency range 200–1000 cm−1. The concentration of encapsulated iron atoms (3, 26, 38, and 54 at. %) was controlled by changing the relative areas of iron and graphite targets during the film deposition by RF magnetron sputtering and checked by Rutherford backscattering. The Raman spectra of a-C:Fe films display a series of almost equidistant bands spaced by approximately 110 cm−1. This character of the spectrum is explained in terms of the atomic vibrations in short carbon nanotubes formed during the introduction of iron into an amorphous carbon matrix.

Photoluminescence from amorphous carbon grown by laser ablation of graphite

Photoexcitation and photoluminescence (1–7 eV) spectra of amorphous carbon films grown by laser ablation of a graphite target have been measured. The experimental data obtained are analyzed using the sum rule, and the energy dependence of the effective electron density of states is determined. Characteristic threshold energies in the effective density of states are revealed at ∼1.4 and ∼4 eV, and the conclusion is made that the energy distribution of the density of states is nonuniform, which is attributed to different contributions from the σ and π states of electrons. The temperature of the electron system excited by light is estimated to exceed the lattice temperature by a factor of ∼40. This circumstance is attributed to heating of electrons by light.

Optical properties of interstellar medium

The spectra of normalized extinction of electromagnetic radiation by the interstellar medium have been analyzed in terms of a model representing the total extinction as a sum of three absorption coefficients. The first coefficient is related to optical transitions of the π-π* type and the second is related to those of the σ-σ* type in amorphous carbon. The third term represents two absorption features occurring in the region of the π-π* type transitions in the amorphous phase, which are satisfactorily described by the Gaussian contours peaked at 3.2 eV (385.6 nm) and 5.67 eV (217.5 nm). These contours are compared to published data on the absorption spectra of a copolymer composed of alternating triphenylamine and fluorene molecules. The results of this analysis confirm the hypothesis concerning the most probable chemical composition and physical state of particles of the interstellar medium.

Size distribution of cobalt nanoclusters in an amorphous carbon matrix

Results obtained in a study of electron micrographs of cobalt-doped amorphous hydrogenated carbon are presented. The micrographs were obtained by transmission electron microscopy, including high-resolution electron microscopy. Layers of amorphous carbon were grown by magnetron cosputtering of a graphite and a cobalt target in an atmosphere of argon-hydrogen plasma. It is shown that nanosize crystalline clusters are formed in the process. The influence exerted by 1-h thermal annealing at 800°C in an atmosphere of argon on the size distribution of nanoclusters was studied. It is shown that the distribution function is described by a Gaussian curve that is peaked at ∼7 nm for as-grown samples and is strongly broadened upon annealing, with the peak position shifted to larger sizes and the curve exhibiting deviations from the Gaussian shape. A characteristic structure, attributed to nanosize carbon capsules that envelop the clusters, is seen at cluster boundaries in the high-resolution mode.

Allotropic composition of amorphous carbon

Using the concept of an inhomogeneous broadening of spectral lines of the basic oscillators responsible for forming the spectrum, the experimental dependences of the dispersion of the imaginary part of permittivity are analyzed for amorphous carbon. It turned out that four types of oscillators contribute to this dependence. The first three types represent the electron transitions from the energy-spectrum ground state for π and σ electrons of amorphous carbon to an excited state. The fourth type is related to the absorption of electromagnetic radiation by free charge carriers. The absolute values of squared plasma frequencies of oscillators are estimated, and, using them, the relative fraction of sp2-bonded atoms forming the amorphous-carbon skeleton is calculated. This estimate agrees closely with the theoretical predictions for amorphous carbon of the same density as the material under study. The dependence of the relative fraction of sp2-bonded atoms contained in amorphous hydrogenised carbon on annealing temperature is determined. The developed method is also applied to the analysis of the normalized curve for the light extinction in the interstellar medium. The contribution to the extinction of two varieties of interstellar matter is detected.

Spectral ellipsometry of a nanodiamond composite

Optical properties of a nanodiamond composite were analyzed by methods of spectral ellipsometry in the range of photon energies 1.4–5 eV, which are characteristic of π-π* transitions in amorphous carbon. The nanocomposite was synthesized by molding nanodiamond powder with subsequent binding of diamond nanoparticles by pyrocarbon formed as a result of the heterogeneous chemical reaction of methane decomposition. The dispersion curves of the imaginary and real parts of the dielectric function were reconstructed. It is shown that the imaginary part of the dielectric function can be represented as the sum of two components generated by the two types of π-π* optical transitions. The maximum contribution of the transitions of the first and second types manifests itself at energies of 2.6 and 5.6 eV, respectively, which correspond to peaks in optical density at 2.9 and 6.11 eV. It was established that the main specific features of the normalized optical density of the nanodiamond composite almost coincide with those for poly(para-phenylenevinylene). It was found that the energy of a σ + π plasmon of the pyrocarbon component of the nanodiamond composite is 24.2 eV. On the basis on this value, the pyrocarbon density matrix was estimated to be 2 g/cm3. Within the concepts of optimum filling of an elementary volume by carbon atoms in an amorphous material with such a density, the allotropic composition of the pyrocarbon matrix was restored.

Density of states in amorphous carbon and its modification by annealing

The effect of annealing on the electron density of states in amorphous carbon a-C and amorphous hydrogenated carbon a-C:H has been studied. a-C and a-C:H layers were grown by magnetron sputtering of a graphite target in, respectively, argon and argon-hydrogen plasmas. Optical transmission spectra were studied experimentally in the range 1.5–5.6 eV, and ellipsometric parameters were measured at the He-Ne laser wavelength. The spectral dependence of the imaginary part of the dielectric function was reconstructed. A model describing the optical response of amorphous carbon was developed on the basis of the hypothesis that there are fluctuations of the sp2 fragment sizes in the allotropic composition of amorphous carbon. The optical gap Eg in both types of material is accounted for by the presence of fragments of critical size. Experimental data were used to reconstruct, with the use of model parameters, the energy dependence of the density of states in the ground and excited bands, and the plasma frequencies for electrons involved in optical transitions are found. It is shown that the bands of both the ground and excited states are inhomogeneously broadened sets of levels, which are symmetrical about the Fermi level. The behavior of the model parameters Eg and EG (energy corresponding to the peak of the Gaussian distribution) and the plasma frequency with annealing temperature shows, for materials of both types, a substantial rise in the size and number of critical fluctuations with increasing temperature.