Undoped Cis Research Articles

Temperature dependence and valence band splitting of the photocurrent response in undoped p-type CuInSe2 layers

In this study, the photocurrent (PC) spectroscopy of undoped p-type CIS layers has been investigated at temperatures ranging from 10to293K. Three peaks, A, B, and C, corresponded to the intrinsic transition from the valence band states of Γ7(A), Γ6(B), and Γ7(C) to the conduction band state of Γ6, respectively. The crystal field splitting and the spin orbit splitting were found at 0.0059 and 0.2301eV, respectively, and the temperature dependence of the optical band gap could be expressed by using the empirical equation Eg(T)=Eg(0)−(8.57×10−4)T2∕(T+129). But the behavior of the PC was different from that generally observed in other semiconductors: the PC intensities decreased with decreasing temperature. From the relation of logJph vs 1∕T, where Jph is the PC density, the dominant level was observed at the higher temperatures. We suggest that in undoped p-type CIS layers, the trapping center limits the PC signal due to native defects and impurities with decreasing temperature.

Thermal conductivity of an oriented polyacetylene film

The temperature dependence of the thermal conductivity of an oriented undoped cis rich-polyacetylene film is reported in the temperature range 3–300K. The data are compared to the results previously obtained on non-oriented polyacetylene films. The thermal conductivity of the oriented specimen is 15 to 60 times higher than that of non-oriented polyacetylene, while the temperature dependences are quite similar. At room temperature, a value of ≈ 13 W/m-K is obtained, which is comparable to the thermal conductivity of stainless steel. The results indicate that the elongation of the polyacetylene film is mainly responsible for the conductivity enhancement.

Effect of interchain interactions on 13C nuclear magnetic resonance chemical shifts and electronic structures of polyacetylene in the solid state as studied by tight-binding molecular orbital theory

The 13C NMR chemical shifts of undoped cis and trans polyacetylenes in the solid state were calculated using the tight-binding sum-over-state theory within the CNDO/2 framework. In order to investigate the effect of the interchain interactions on the 13C NMR chemical shifts and electronic structures of the polymers, the calculation was carried out using the multichains models. The calculation for the seven-chains model explains reasonably the experimental data. From this, it was found that the interchain interactions greatly influence the electronic structures of polyacetylene chains.

Lattice Dynamical Calculations of Trans and Cis Polyacetylene

Abstract Lattice dynamical calculations of the lattice modes for both trans and cis polyacetylene were performed. Using rigid body approximation for the chain we obtained all 8 lattice frequencies and modes of motion for both trans and cis polyacetylene. The frequencies concerned with Raman active lattice modes are presented for the first time. We also performed far infrared transmission measurements of undoped trans and cis polyacetylene and confirmed the results obtained by Bechtold et al. Comparison of the lattice frequencies obtained by lattice dynamical calculations with the results obtained by far infrared measurement and inelastic neutron scattering is discussed.

Determination of 13C chemical shift tensor orientations in cis- and trans-polyacetylene by measurements of two-dimensional NMR powder patterns in rotating solids

The orientations of the 13C chemical shift tensors in undoped cis- and trans-polyacetylene have been determined from the chemical shift-dipolar two-dimensional powder patterns observed in rotating samples.

Effects of impurities and thermal isomerization on the electrical properties of undoped polyacetylene

Characterization and electrical properties of undoped cis -polyacetylene and annealed polyacetylene were studied by measuring the electrical conductivities, the rate of isomerization, the cis-trans content, wide angle X-ray diffraction (WAXD) and electron spin resonance(ESR). In cis -polyacetylene the electrical conductivity decrease with a decrease in the amount of residual catalysts. The electrical conductivity decrease with excess heating since the interchain cross-linking of the radicals was occurred, which also induced a decrease of crystallinity. The activation energy of cis-trans isomerization increased with an increase in the trans content. The crystal transition during the isomerization occurred following a certain amount of cis-trans isomerization in the polyacetylene chain. Two crystal structures were found in trans -polyacetylene. Secondary battery using polyacetylene as a cathode was also studied. The charge and discharge capability was improved by the decrease of residual catalyst.

Soliton spin density in polyacetylene studied by endor in pristine and 13C-enriched cis-rich sample

ENDOR measurements were performed on pristine and 13C-enriched undoped cis -rich (CH) X down to 4K. Temperature dependence of the spectra in 13C-enriched system revealed that the structures observed at higher temperatures, which were formerly assigned to proton or 13C hyperfine tensor components by Thomann et al. , grow into sharp single resonance lines as the temperature is lowered. The frequencies of those lines have been found to be related to proton and 13C nuclear frequencies. For example, the structure around 7.3 MHz, which was assigned to a 13C tensor component, is the second harmonic of 13C nuclear frequency. Comparison of the spectra between pristine and 13C-enriched system clearly showed that the intrinsic spin density of the soliton, confirmed by our previous ENDOR analysis in pristine system, is not observable in the latter system because of the line shape distortion due to the overlapping of those structurs due to nuclear frequencies.

Direct Observation of `Soliton-Like' Spin Density Distribution in UndopedCis-Rich (CH)xUsing ENDOR

Electron nuclear double resonance (ENDOR) measurements were performed on stretch oriented undoped cis -rich (CH) x at 4 K. The observed ENDOR line shape had spectral turning points and showed a clear anisotropy. The presence of the spin density which is comparable with the prediction of the soliton theory was directly confirmed from the maximum ENDOR frequency. The analysis of the observed line shape using a spectrum simulation method suggested that the spin density is of the form similar as those predicted by the soliton theory for finite on-site Coulomb energy. Especially the spectral turning points observed when the external magnetic field was parallel to the stretch direction of the sample were directly correlated to the existence of the negative spin density predicted by the theory.

Far infrared spectra of the lattice modes in trans and cis polyacetylene

Abstract Far infrared transmission spectra of undoped cis and trans polyacetylene are reported in the frequency range from 30 cm-1 to 450 cm-1. In both modifications two lines are found in the vicinity of 100 cm-1, which are attributed to lattice modes. Several additional lines are observed in cis polyacetylene.

Frequency dependent conductivity of lightly doped cis-polyacetylene: Role of multiply connected doped trans-polyacetylene regions

The evolution of the frequency and temperature dependence of the conductivity of cis-polyacetylene [ cis-(CH) x ] lightly doped with iodine is reported. As the dopant concentration is increased, the frequency-dependent contribution to the conductivity remains unchanged while the frequency-independent contribution increases dramatically. These results are inconsistent with the formation of small isolated metallic islands embedded in undoped cis-(CH) x . The data support the idea that even at the lightest dopant levels studied (d.c. conductivity increased by only one order of magnitude), the doped regions are multiply connected, not isolated. Comparison with the temperature- and frequency-dependent conductivities of cis and trans-(CH) x implies that the doped regions are isomerized to the trans configuration and that there may be some structural evolution in the undoped fibril cores, in agreement with earlier spectroscopic and structural studies.

Morphology of iodine-doped polyacetylene

Although the synthesis of polyacetylene powders was first carried out many years ago, Ito et al.1 have only recently obtained films which can be doped with various chemical species, such as I2, SbF5 or Na, K, giving p or n type semiconductors. These and other properties, such as its optical bandgap2 (Eg ≈ 1.5 eV) have made polyacetylene an interesting material for study3–6. Its mechanical stretching ability, for example, leads to partially oriented fibrils and an electrical conductivity anisotropy γ;¶/γ⊥≃10 has been measured7 with γ¶ reaching 3×103 Ω−1 cm−1 when highly doped with AsF5; roughly 12 orders of magnitude higher than the value measured on undoped cis (CH)x. Polyacetylene is available in two isomeric configurations (cis and trans, Fig. 1). More than 98% cis isomer is obtained if the synthesis is performed at low temperatures (−78 °C) but it is not thermodynamically stable and becomes trans on increasing the temperature, the total transformation occurring when T > 100°C. Any doping process induces a partial cis/trans isomerization8. The exact conduction mechanism is poorly known: the localization and nature of the doping species have been widely discussed. Here we investigate, using scanning electron microscopy, the behaviour of the morphology of (CH)x films when the dopant concentration varies. Starting from the fibrillar structure we have observed the formation of aggregates and a uniform melt of various globular features at high doping levels. Our results yield information on how the electrical conductivity takes place in these non-uniform materials.

THERMAL PROPERTIES OF DOPED AND UNDOPED (CH)x

Measurements of the thermal conductivity of undoped cis-(CH)x and trans-(CH)x and of iodine-doped cis-(CH)x in the temperature range between 60 K and 300 K are reported and compared to results of other authors. Adopting a very simple morphological model we estimate from our data the thermal conductivity of individual (CH)x-fibers. In addition, values for the specific heat of cis- and trans-(CH)x are determined and the activation energy for the cis-trans-isomerization is obtained from differential-scanning-calorimetry experiments.

High resolution 13C NMR in undoped cis- and trans-polyacetylene

We have applied 13C cross-polarization (CP) powder- and MAS-NMR to cis- and trans-polyacetylene. All three elements of the chemical shift tensor ( σ 11, σ 22, σ 33) were determined in p.p.m. with respect to TMS as: (a) Cis; σ 11 = −228, σ 22 = −139, σ 33 = −17; (b) Trans; σ 11 = −234, σ 22 = −146, σ 33 = −34.

P-π Electron Character Observed through the Anisotropy of ESR in Undoped (CH)x

ESR measurements were done in stretched films of undoped cis -rich (CH) x at X band. The anisotropy was observed in linewidth and g value. Both quantities showed maxima when the external field was parallel to the stretch direction of the film and showed minima when the field was perpendicular to this direction. Those features are reasonably explained by the anisotropic property of the carbon p -π orbital and strongly suggest that the ESR center has its origin in the p -π electron in undoped (CH) x .

Narrowing of Electron Spin Resonance Line of Undoped Polyacetylene

Narrowing mechanism of an ESR (Electron Spin Resonance) line in undoped cis- and trans-polyactylene, (CH) x , are discussed based on the temperature dependence of the ESR linewidths and line shapes. In a high temperature region, the classical thermal motion of a soliton is found to be inadequate to explain the observed motional narrowing, and it is found necessary to take a diffusive motion into account.

Specific heats of pure and doped polyacetylene

The temperature dependences (0.8 K ⩽ T < 7 K) of the specific heats of pure polyacetylene (both cis and trans isomers) and heavily doped metallic [CH(AsF 5) 0.12] X are reported. Results for undoped cis -(CH) X indicate behavior typical of crystalline polymers, whereas isomerization to the trans -form leads to a small term linear in temperature signifying increased disorder. Comparison of the data from cis and trans starting material indicates that isomerization is induced during doping. The increased coefficient of the linear term in the metallic polymer is discussed in terms of two contributions; an electronic term expected for a metal, and the increased effect of disorder in the doped polymer.