Anvar Zakhidov

Doping effect of buckminsterfullerene in conducting polymer: Change of absorption spectrum and quenching of luminescene

Abstract Buckminsterfullerene(C 60 ) is found to be an effective dopant for poly(3-alkylthiophene). Inter-band absorption of poly(3-alkylthiophene) has been remarkably suppressed and blue shifted and new absorption peak evolve in the low energy range upon doping by C 60 . From a consideration of the energy diagrams, the electron transfer from poly)3-alkylthiophene) to C 60 is proposed to be favorable in energy at account of Jahn-Teller splitting of LUMO in C 60 −1 state or/and the Coulomb attraction of positively charged polaron P + to C 60 −1 . The suppression of photoluminescence(PL) in poly(3-alkylthiophene) upon doping by Buckminsterfullerene has also been observed confirming the doping effect. The nonradiative decay of excitons due to interaction with P + introduced by doping is discussed as the origin of such PL quenching.

Negative Poisson's ratios as a common feature of cubic metals

Poissons ratio is, for specified directions, the ratio of a lateral contraction to the longitudinal extension during the stretching of a material. Although a negative Poissons ratio (that is, a lateral extension in response to stretching) is not forbidden by thermodynamics, this property is generally believed to be rare in crystalline solids. In contrast to this belief, 69% of the cubic elemental metals have a negative Poissons ratio when stretched along the [110] direction. For these metals, we find that correlations exist between the work function and the extremal values of Poissons ratio for this stretch direction, which we explain using a simple electron-gas model. Moreover, these negative Poissons ratios permit the existence, in the orthogonal lateral direction, of positive Poissons ratios up to the stability limit of 2 for cubic crystals. Such metals having negative Poissons ratios may find application as electrodes that amplify the response of piezoelectric sensors.

Enhanced photoconductivity of C60 doped poly(3-alkylthiophene)

Abstract Photoconductivity of poly(3-alkylthiophene) is found to be remarkably enhanced upon C 60 doping. Excitation profile of photoconductivity shows two peaks in the spectral ranges around 650 nm and 350nm. The former peak is interpreted to be due to ππ ∗ interband excitation of poly(3-alkylthiophene) followed by electron transfer to C 60 , while the latter is due to the optical excitation of C 60 molecule, accompanied by hole transfer to poly(3-alkylthiophene). In both cases, positive polarons (P + ) photogenerated with increased efficiency in poly(3-alkylthiophene) chains are considered to be responsible for enhanced photoconductivity, since geminate recombination of those positive charges (P + ) in poly(3-alkylthiophene) with negative charges (P c − ) in C 60 is suppressed compared to intrachain P + P − recombination in undoped poly(3-alkylthiophene). The decay time of photoconduction becomes shorter upon C 60 doping.

Nanoimprinted Polymer Solar Cell

Among the various organic photovoltaic devices, the conjugated polymer/fullerene approach has drawn the most research interest. The performance of these types of solar cells is greatly determined by the nanoscale morphology of the two components (donor/acceptor) and the molecular orientation/crystallinity in the photoactive layer. A vertically bicontinuous and interdigitized heterojunction between donor and acceptor has been regarded as one of the ideal structures to enable both efficient charge separation and transport. Synergistic control of polymer orientation in the nanostructured heterojunction is also critical to improve the performance of polymer solar cells. Nanoimprint lithography has emerged as a new approach to simultaneously control both the heterojunction morphology and polymer chains in organic photovoltaics. Currently, in the area of nanoimprinted polymer solar cells, much progress has been achieved in the fabrication of nanostructured morphology, control of molecular orientation/crystallinity, deposition of acceptor materials, patterned electrodes, understanding of structure-property correlations, and device performance. This review article summarizes the recent studies on nanoimprinted polymer solar cells and discusses the outstanding challenges and opportunities for future work.

OBSERVATION OF INHIBITED SPONTANEOUS EMISSION AND STIMULATED EMISSION OF RHODAMINE 6G IN POLYMER REPLICA OF SYNTHETIC OPAL

We report the observation of inhibited spontaneous emission of organic dye rhodamine 6G infiltrated in a polymer replica of synthetic opal as a photonic crystal. The morphology-dependent resonances, superimposed on the broadband emission of rhodamine 6G due to spherical wavelength-sized microcavity enhancement of dye emission, have also been observed.

Laser-like emission in opal photonic crystals

In our studies of dye-infiltrated opal photonic crystals, we find a stimulated emission regime, which is characterized by highly efficient, directional laser-like emission and a complex finely structured spectrum. This regime can be interpreted as an onset of multimode, mirrorless laser oscillations that occur in the spectral range outside the photonic crystal stop bands. Such a phenomenon may be due to unusual optical feedback induced by multiple backscattering inside the excited region of the opals.

Structural Model for Dry-Drawing of Sheets and Yarns from Carbon Nanotube Forests

A structural model is developed for describing the solid-state transformation of a vertically oriented carbon multiwall nanotube (MWNT) forest to a horizontally oriented MWNT sheet or yarn. The key element of our model is a network of individual carbon nanotubes or small bundles interconnecting the array of main large-diameter MWNT bundles of the forest. The dry-draw self-assembly mechanism for MWNT sheet formation involves two principal processes that reconfigure the interconnection network: (1) unzipping by preferentially peeling off interconnections between the bundles in the forest and (2) self-strengthening of these interconnections by densification at the top and bottom of the forest during draw-induced reorientation of the bundles. It is shown that interconnection density is a key parameter that determines the ability of a MWNT forest to be dry-drawable into sheets and yarns. This model describes the principal mechanism of solid-state draw (confirmed by dynamic in situ scanning electron microscopy), the range of forest structural parameters that enable sheet draw, and observed dependencies of sheet properties on the parent MWNT forest structure.

Doping effect of buckminsterfullerene in poly(2,5‐dialkoxy‐p‐phenylene vinylene)

Photoluminescence has been markedly quenched and photoconductivity has been enhanced by more than one order of magnitude upon introduction of several mol% of buckminsterfullerene (C60) to poly(2,5‐dialkoxy‐p‐phenylene vinylene) (RO‐PPV), especially at excitations about 2.2 eV, corresponding to the band gap energy of RO‐PPV and also in bands at 1.8 and 3.5 eV, which correspond to optical excitation of C60 molecules, suggesting that photo‐induced charge transfer occurs between RO‐PPV and C60. On the other hand, absorption spectrum and electrical conductivity of RO‐PPV have been scarcely influenced by doping of small amount of C60, suggesting that the ground state charge transfer between C60 and RO‐PPV is not effective, contrary to the case of poly(3‐hexylthiophene). These results are discussed by taking relative electronic energy states of RO‐PPV and C60 into consideration. The photo‐excited exciton‐polaron (Ex‐P) in RO‐PPV is interpreted to migrate along about 100 monomer units along a polymer main chain i...

Evolution of superconductivity of KxC60 upon K-doping : microwave low-field signal and ESR study

Abstract By means of low magnetic-field micowave absorption (Low-Field Signal) and conventional ESR, we have studied the evolution of superconductivity in bulk KxC60 dependent on the variation of K- doping temperature and time. The first LFS as an occurrence of superconductivity is observed in a doping time as short as td = 0.5 h with a discrete Tc ∼ 19 K, and the volume of the superconducting phase increases on further dopings, while Tc changes slightly. The sample in a superconducting state exhibits strong ESR absorptions, which indicates the presence of a non-superconducting metallic phase coexisting with a superconducting phase in inhomogeneously doped KxC60. These phases are closely correlated and disappear at the same time by overdoping.

Electrical Conductivity and ESR Spectrum of Buckminsterfullerene-Doped Poly(3-alkylthiophene)

Buckminsterfullerene (C60) is effectively doped in poly(3-alkylthiophene). However, the characteristics of C60-doped poly(3-alkylthiophene) are different from the typical behavior observed in the case of conventional dopants. Electrical conductivity of poly(3-alkylthiophene) increases upon C60 doping and decreases again at higher concentration of C60. Its temperature dependence is also anomalous. The absorption spectrum changes markedly upon C60 doping. ESR linewidth decreases from 7.1 G to 1.9 G upon C60 doping. Spin density, evaluated by ESR, is enhanced upon C60 doping. These doping characteristics are tentatively explained by taking electronic energy diagrams of both poly(3-alkylthiophene) and C60 into consideration.