T. S. Zhuravleva

Study of the bromination of pitch-based carbon fibres

Abstract The changes in the structure and properties of pitch-based carbon fibres (CF) brominated for different periods of time (up to 144 h) and the state of bromine in these fibres have been studied. Electrical conductivity in the temperature range from 10 to 300 K, IR and Raman spectra, and thermogravimetric analysis data are presented. The structural modifications in CF upon bromination, as indicated by these data, include the growth of the fine-crystalline phase, the degradation of the graphite structure, the emergence and growth of an amorphous phase, and an increase in the number of graphite layers adjacent to bromine. If the bromination time does not exceed 96 h, the process leads to the intercalation of bromine, i.e., the formation of charge-transfer complexes between the carbon network and bromine. More prolonged bromination results in the formation of C-Br chemical bonds.

Pulsed photoconductivity in composite polyaniline-polyvinyl alcohol films

Kinetics of delay photoconductivity induced by the ns-laser pulses is studied for films of polyaniline (PANI) and polyaniline-polyvinyl alcohol (PANI-PVA) composite in the temperature range from 110 to 300 K. Kinetic curves are fitted well by the stretched-exponential function with the temperature and time dependent parameters. Obtained results are discussed from the point of view of charge transfer in disordered systems.

Charge Carrier Photogeneration and Transport in Organic Photoconducting Polymer Nanocomposites Based on Polyimides

ABSTRACT On the base of photo- and thermostable polyimides (PI) and carbazolyl-containing polymers (CzCP) possessing predominantly p-photoconductivity and doped with perylenediimide derivative (PDID) as an acceptor with n-conductivity, the efficient bipolar polymer nanocomposites are developed. The efficient bipolar transport is obtained for microsegregated triple composites (PI-CzCP-PDID and others) in which there are p- and n-microregions with efficient charge photogeneration (quantum yield up to 0.6) at the extended microphase interface and high photoelectrical sensitivity (up to 400 m2/J) in the UV and visible ranges. The photovoltaic effect is investigated for sandwich cells with (Al, ITO) electrodes. The best parameters are found for triple composite films (0.7–1 μm thickness).

Temperature dependence of conductivity in carbon fibrils

An anomalous temperature dependence of electric resistivity in carbon fibrils in the range from 11 to 350 K is considered. The fibrils are prepared from the polyacrylonitrile fibrils by heat-treatment. A heterogeneous model of conductivity is developed and a fit to the experimental dependencies of resistivity is obtained. The adequacy of this model to a fibril conductivity problem is explained.

Synthesis of poly-p-xylylene + CdS nanocomposites and studing of their surface structure

Thin-film polymeric nanocomposites based on a poly-p-xylylene matrix (PPX) and nanoparticles of cadmium sulfide (CdS) have been obtained using solid-phase cryochemical synthesis on optical quartz substrates. The optimal conditions for the formation of PPX + CdS nanostructured polymeric films have been determined. The topography and surface characteristics of PPX + CdS nanocomposites have been studied by scanning atomic force microscopy (AFM). The characteristics of the morphological and structural changes of the film surfaces have been found at different CdS nanoparticle concentrations. These changes point to a reorganization of the PPX + CdS polymeric matrix. It is shown that at 2–10 vol % CdS concentration, the elements of the surface’s structural organization are polymeric nanofibers, parallel to the substrate plane, and a small number of nanoglobules, locally positioned directly on the nanofiber surfaces. Ordering of the surface structure accompanied by the formation of a periodic surface relief in the shape of nanoglobules was observed at a CdS concentration of 13.5 vol %. A mechanism of the formation of nanostructured PPX + CdS films is proposed. Based on the AFM experimental data, one can assume that during co-condensation of a p-xylylene monomer, CdS nanoparticles initiate radical polymerization of the PPX matrix, when the substrate is heated from 77 K to 300 K. The type of surface structure (fibrillar, fibrillar + globular and globular) depends on the nanoparticle concentration in the polymeric matrix.

Effect of Filler Concentration and Film Thickness on Structure and Optical Properties of Poly(p-Xylylene)−Cadmium Sulphide Nanocomposites

The effect of filler concentration (C ≈ 0–100 vol %) and film thickness (d ≈ 0.02, 0.2, 0.5, and 1.0 μm) on the optical absorption spectra and surface morphology of thin nanocomposite films based on poly(p-xylylene) and cadmium sulphide (PPX–CdS) has been studied. The PPX–CdS films are prepared by low-temperature vapor deposition polimerization on quartz and silicon substrates. A nonmonotonic dependence of the absorption spectrum red shift on the filler concentration is revealed. The red shift of the spectrum reaches a maximum at some critical concentration of the filler C0. It is observed that the value of the critical concentration C0 increases with the film thickness and equals C0 ≈ 11, 30, and 50 vol % for d ≈ 0.02, 0.5, and 1 μm, correspondingly. The average size of the nanoparticles is evaluated by an analysis of the absorption spectra (via estimating the exciton-peak wavelength). It is found that the shift in the absorption spectra is determined by a variation in the nanoparticle size. Atomic force microscopy reveals the effect of the filler content on the surface morphology of the polymer matrix in nanocomposite films and their surface roughness. The size distribution of the polymeric grains is evaluated. The most significant changes in the absorption spectra and surface morpology of the composites with a variation in the filler content are observed in the filler concentration range from 0 to C0. The absorption spectra of the composites with the filler concentration above C0 are similar, which can be attributed to the almost equal nanoparticle size in these nanocomposites. The correlation between changes in the matrix morphology and filler optical properties is established.

Effect of biogenic photochromic electron acceptors on chlorophyll fluorescence

It is shown that the photophysical properties of chlorophyll a (Chl) depend on the nature and relative amounts of 2-methyl-1,4-naphthoquinone (MNQ) and nicotinamide adenine dinucleotide phosphate (NADP). Photoinduced charge separation occurs in aqueous ethanol solutions of Chl (1 × 10−5 M) and NADP (5 × 10−6–5 × 10−4 M), resulting in the dynamic quenching of Chl fluorescence. Coordination interaction between Chl and NADP is established at an NADP concentration of ≥5 × 10−4 M. The nonlinear Stern-Volmer dependence in this range is due to the input from static quenching. It is shown that the quenching of Chl fluorescence in an MNQ solution at Chl and MNQ concentrations of 1 × 10−5 M and 6.7 × 10−5–1 × 10−4 M, respectively, is described by a linear dependence in the Stern-Volmer coordinates; no complex formation is observed for Chl and MNQ under these conditions, and electron transfer is of the dynamic type. Static or mixed-type energy transfer from MNQ to Chl dominates at elevated MNQ concentrations.

Surface topography and the spectral and photoelectric properties of intercalated molybdenum disulfide films

The structural and photoelectric properties of nanodispersed molybdenum disulfide films intercalated with cationic organic compounds (rhodamine 6G, oxazine, triethylbenzylammonium, hexadecyltrimethylammonium, and polyvinylpyrrolidone) and hydroxocomplexes of Zn, Co, Ni, and Cd are studied. It is found that the greatest differences between the absorption spectra of MoS2 films intercalated with compounds of various natures are observed in the range of 500–850 nm, and the conductivities (dark and photo-) of MoS2 films intercalated with Ni or Co hydroxocomplexes rise by one or more orders of magnitude, respectively, compared to films of intercalated molybdenum disulfide. The correlation between photoconductivity and the corresponding values of dark conductivity is determined, proving that charge transport (and not generation processes) in films is the limiting step for photocurrent. It is concluded that highly conductive polycrystal films of intercalated MoS2 with conductivities close to those in MoS2 monocrystals are of interest for practical use in optoelectronics.

Impulse photoconductivity in EB films of various morphology

Experimental data related to the impulse photocurrent in emeraldine base (EB) films of various morphology in a wide temperature range are considered. The rate constants for the recombination of the photoinduccd charges are estimated; their dependence on the material morphology is explained. The temperature dependence of photoconductivity is discussed.

Kinetic model of delay photoconductivity in EB films

A simple kinetic model of photocurrent relaxation in emeraldine base (EB) films is considered. Parameters of a distribution of electrons in traps are evaluated depending on temperature and time after ns-laser excitation. It is shown that carrier traps in EB films are formed by fluctuations of mutual arrangement of polymer fragments.