V. B. Kulik

Structural changes in the surface of a heterogeneous nanocrystalline body (sandstone) under the friction

The structure of a ~30 nm thick surface layer of a heterogeneous nanocrystalline solid body (sandstone) before and after the friction was investigated using photoluminescence and Raman spectroscopy. Before the friction, this layer contained nanocrystals of quartz, anatase, feldspar, and montmorillonite. The friction caused a sharp decrease in the concentration of nanocrystals of quartz and feldspar.

Thermal expansion of the skeleton of chain molecules in polymer crystals

The expansion of the carbon skeleton of molecules in crystallites and the longitudinal contraction of the crystallite lattice in poly(ethylene) (PE) and poly(caproamide) (PCA) with an increase in the temperature are measured using IR and Raman spectroscopy and x-ray diffraction. The thermal expansion of the carbon skeleton due to transverse vibrations is theoretically calculated within the atomic chain model. The theoretical and experimental data on the thermal expansion of the carbon skeleton are in good agreement.

Observation of a compressed state of the quantum wire material in porous silicon by the method of Raman scattering

An experiment on Raman scattering in porous silicon containing a regular system of quantum wires reveals a shift of the phonon frequencies of the silicon lattice toward higher energies. The effect is interpreted as a manifestation of a compressed state of the material in the quantum wires. It is shown that the observed deformation plays a large role in the formation of the structure of porous silicon.

Enhancement of Raman scattering intensity in porous silicon

An enhancement in inelastic light scattering intensity from porous-silicon quantum wires has been discovered. It is shown that this effect is caused by a decrease in the absorption coefficient of the optical medium formed by quasi-one-dimensional structures, with the crystal structure of the wires themselves remaining unchanged.

Stresses induced in polymer crystals by the atomic-molecular internal dynamics

The temperature dependences of the dynamic tensile stresses on the chemical bonds of the skeleton of chain molecules and the bonds between molecules in poly(ethylene) and polyamide 6 crystals are measured using X-ray diffraction and Raman spectroscopy in the temperature range 5–500 K. The relations of the dynamic stresses to the quantum characteristics of the vibrational dynamics of polymer molecules are derived. The contributions from the zero-point and thermal vibrations of molecules to the stresses are determined. It is revealed that the dynamic tensile stresses on the intermolecular bonds at the melting temperatures of the crystals approach the tensile strength of these bonds.

Raman spectroscopy of nanocrystals in rock

Nanocrystals were detected and identified in rocks by the method of Raman spectroscopy. The experiments showed that the Raman scattering spectra of fine-lamellar arkosic sandstone exhibit bands corresponding to lattice vibrations of anatase, α-quartz, and plagioclase. In all spectra of the rock, the bands are displaced towards high frequencies as compared with their position in spectra of single crystals and widen on the same side. These results show that, in all of the studied places of the sample, the particles of anatase, quartz, and plagioclase have nanometer sizes, namely, of the order of 10 nm in anatase and quartz and about 20 nm in plagioclase. Moreover, in different places of the sample, not only the shape and position of the bands under study but also their intensity vary, the latter being directly proportional to the concentration of nanocrystals.

Detailing of deformation processes in polymeric crystals

Structural changes in polymer crystals (polyethylene, polyimide, and others) have been studied using the X-ray diffraction and Raman spectroscopy methods under different influences: tensile loading along the chain molecule axis and heating from 90 to 350 K. An increase in the molecule axial length under loading and a decrease in the molecule axial length upon heating have been identified and measured using X-ray diffraction. A decrease in the skeletal vibration frequency during loading and heating has been identified and measured using Raman spectroscopy, which indicates an increase in the molecule contour length in both cases. A technique for determining the change in the polyethylene molecule contour length in the crystal from the measured change in the skeletal vibration frequency has been justified. The contributions of two components, namely, skeletal (carbon–carbon) bond stretching and the change (an increase during stretching and a decrease during heating) in the angle between skeletal bonds, to the longitudinal deformation of polyethylene crystals, have been quantitatively estimated. It has been shown that the negative thermal expansion (contraction) of the polymer crystal is caused by the dominant contribution of the decrease in the bond angle.

Effect of water on the α-β phase transition in a surface quartz layer

The temperature dependence of the α-phase concentration in surface layers and in the bulk of quartz plates cut out at a distance of ∼2 mm from the natural growth surface of druses extracted at the Dodo deposit in the Polar Urals has been studied using infrared and Raman spectroscopy. It has been found that, in the bulk of the sample, the temperature dependence behaves as expected for a first-order phase transition; more specifically, below 800 K, it remains unchanged and, at high temperatures, approaches zero. In surface layers with thicknesses of ∼0.15 and ∼0.8 μm, the α-phase concentration decreases monotonically by approximately 10% with an increase in the temperature to 780 K. The temperature dependence of the α-phase concentration in the layer at a depth of ∼6 μm passes through two minima, namely, at ∼370 and ∼570 K, at which the concentration of this phase decreases by about one half. This is accompanied by an increase in the concentration of the β-phase. The revealed behavior of the α-phase concentration with an increase in the temperature has been assigned to the influence of water on crystal lattice distortions near growth dislocations. At 370 K, free water evaporates from grain boundaries, and at 570 K, the water bound by hydrogen bonds to the SiOH groups. The evaporation of water affects stresses at grain boundaries, and it is this factor that brings about a change of the α-phase concentration. It has been demonstrated that tensile stresses generated with increasing temperature in a near-surface quartz layer to ∼0.8 μm thick can reach ∼170 MPa. The stresses create microcracks, which culminate in destruction of the sample. The generation of the tensile stresses is explained by an increase in the volume of the microcrystal layer located at a depth from ∼1 to ∼8 μm from its surface as a result of the increase in the β-phase concentration in it.

Spectra of second-order Raman scattering in porous silicon

Spectra of second-order Raman scattering in porous silicon are investigated. A band shift towards lower energies in second-order spectra is observed, as well as the correlation between the values of band shift in first-and second-order spectra. It is demonstrated that the observed effect cannot be interpreted using the conventional concepts of the mechanisms of scattering in microcrystalline samples. An interpretation of the revealed effect is suggested.

Anomalous polarization of Raman scattering spectra from porous silicon

A violation of the polarization selection rules for Raman scattering is observed in porous silicon. This effect is caused by a weak disorientation of the quasi-one-dimensional silicon wires, with the crystal structure of the wires themselves and the macroscopic homogeneity of the material in optical experiments remaining intact.