A.A. Zakhidov

Marked Enhancement of Photoconductivity and Quenching of Luminescence in Poly(2,5-dialkoxy-p-phenylene vinylene) upon C60 Doping

The absorption spectrum and electrical conductivity of poly(2,5-dioctyloxy-p-phenylene vinylene) (OO-PPV) are hardly influenced by doping of a small amount of C60, suggesting that C60 is not an effective dopant for OO-PPV. On the other hand, photoluminescence has been markedly quenched and photoconductivity has been enhanced by several orders of magnitude upon introduction of several mol% of C60, suggesting photo-induced charge transfer between OO-PPV and C60. These results are discussed in terms of relative electronic energy states of OO-PPV and C60. The photoexcited exciton-polaron (Ex-P) is speculated to migrate along 50~100 monomer units of the polymer main chain in its lifetime and dissociate when encountered with C60.

The absorption spectrum and a cyclic voltammogram of C60-doped poly(3-alkylthiophene)

The absorption spectrum and a cyclic voltammogram for poly(3-hexylthiophene) changed upon C60 doping but those for poly(3-octadecylthiophene) did not. This suggests that C60 is an effective dopant for poly(3-hexylthiophene) in the ground state but not for poly(3-octadecylthiophene). The threshold of cyclic voltammetry has shifted to a higher potential in C60-doped poly(3-hexylthiophene). This is explained by the lowering of the valence band top due to the electron transfer from poly(3-hexylthiophene) to C60. The shift depends on alkyl chain length and decreases with alkyl chain length in accordance with the results of photoemission spectroscopy.

Evolution of superconductivity in Ca doped C60 prepared from azide. Low-field microwave absorption and ESR

Abstract Superconducting (SC) behavior of Ca doped C 60 prepared by calcium azide thermal decomposition is studied by low-field signal (LFS) microwave absorption and conventional ESR. SC phase is obtained for the sample prepared with nominal composition of Ca 6 C 60 and its volume fraction is significantly enlarged by annealing at 350–400°C for 2 h. It is suggested that the SC compound may have two SC phases with slightly different transition temperatures, T C1 (8.2–8.4 K) and T C2 (8.7–8.8 K). The SC 2 phase with higher T C2 is degraded by annealing at 450–500°C and converts to SC 1 . Degradation of SC 2 at high temperature is interpreted to be caused either by (1) thermal reaction of some intercalated Ca with quartz glass and/or (2) dissociation of (Caue5f8N) y or N y clusters in the octahedral sites of the Ca 5 (Caue5f8N) y C 60 or Ca 5 N y C 60 lattice.

A new metastable superconductor Li2Na1NyC60 prepared from azides

Abstract A new metastable superconductor with nominal composition of Li 2 Na 1 N y C 60 ( y unknown) is prepared by thermal decomposition of lithium azide (LiN 3 ) and sodium azide (NaN 3 ). It is suggested that the superconducting phase in this compound is created by intercalation of the neutral spacer such as NaN 3 . The supeconductivity is examined by a low-field signal of microwave absorption (LFS) and by SQUID. The critical temperature is estimated to be 12 K from SQUID and 14 K from LFS measurement, and the shielding fraction is over 9% at 5 K. Superconducting parameters, such as critical temperature, upper and lower critical fields, coherence length and penetration depth are evaluated and found to be distinct from those in other fulleride superconductors.

Computer simulation of photoinduced conductivity in oriented polymer/fullerene films

Abstract Monte Carlo computer simulations for photoconducting polymers with nondegenerated ground states/ fullerene films have been performed. The dependencies of photoconductivity and concentrations of photocarriers on fullerene concentrations have been studied. The enhancement of photoconductivity and the persistent conductivity, which have been observed in experiments, were confirmed by computer simulations.

Electron spin resonance spectra of KxC60 films prepared by electrochemical doping

We have found that K-doped C60 films can be prepared by the electrochemical doping technique using a solid electrolyte. From these films, we obtained four electron spin resonance (ESR) peaks at a temperature below 50 K. The intensity of one peak in the high field region was extremely sensitive to slight changes in temperature below 10 K and in microwave power. Spin-lattice relaxation time gives a major effect on the power dependence of the increasing rate of the intensity. The ESR center giving this anomalous peak is tentatively interpreted to be related to the superconducting state.

Photogeneration and recombination in conducting polymers: Computer simulation in oriented 2D networks

Abstract A computer simulation of photogeneration and recombination of self-trapped charge carriers is performed at account of interchain processes in single and two-component oriented 2D polymeric networks. Decay kinetics and steady-state concentrations of polarons n p and bipolarons n BP are studied as a function of photoexitation rate h for two polarizations of incident light. We find n ∼ h 0.5 in good agreement with analytical results for 1D case. We observed also the increase of n BP with increase of conjugation length 1 c . Preliminary results for two-component networks can be used for interpretation of some experimental data on photoinduced absorption in cis-trans (CH)x alloys.

Superconductivity of CaxC60 prepared by thermal decomposition of azide: ESR and LFS microwave absorption study

Summary form only given. We have prepared C/sub ax/C/sub 60/ by thermal decomposition of CaN6 and studied the X-evolution of its superconducting properties by low field (LFS) microwave absorption and conventional ESR. Superconducting phase is found at x=5 and also x=6 (with lower fraction) with Tc = 8.4-8.8K. The results of the annealing at different temperature (300-450 /spl deg/C) and of aging effect on the intensity of LFS (and hence the amount of superconducting fraction) and ESR g-value and linewidths are presented. Preliminary results on the C/sub 13/ isotope effect on superconducting T/sub c/ in C/sub a5/C/sub 60/ are also discussed.

Kinetics of one-dimensional diffusion-limited reactions in (CH)/sub x/ chain in the presence of particle input and external electric field: computer simulation

Summary form only given. Diffusion-limited reactions of A + B /spl rarr/ 0 type on one-dimensional lattice ( 10E5 sites with PBC ) in the presence of particle source ( photogeneration of unlike charged solitons ) has been studied by using of proposed biased random walking model. The particles distribution has been studied by using of pair-correlation functions. The dependences of steady-state concentrations and relaxation times on the system parameters - rate of photogeneration, external electric field have been obtained. The phenomenon of stochastic aggregation of unlike charged particles has been found in case of random initial particles distribution. The results have been compared with analytical and experimental data on photoinduced conductivity in oriebted (CH)/sub x/.

Thermal stability of sodium-nitrogen-C/sub 60/ superconductor prepared from Na-AZIDE

Abstract Thermal stability of superconducting sodium-nitrogen-C 60 compound prepared from sodium azide has been studied after high temperature treatment by means of low-field signal of microwave absorption(LFS). The LFS amplitude decreased in a short time by high temperature (350∼400°C) annealing. The critical temperature Tc decreased smoothly from about 16K down to zero, indicating continuous change of superconducting composition and the average lattice constant.