Chia-Jyi Liu

Optical and electrical properties of undoped ZnO films

Optical and electrical properties of undoped ZnO films were investigated in this study. We find that the conductivity increased with increasing the ratio of (blue luminescence+green luminescence) to ultraviolet luminescence [i.e., (BL+GL)∕UVL] defect emission intensities, due to the increase of the singly ionized oxygen vacancy. We deduce that the BL and GL are related to the oxygen-vacancy deep level and the yellow luminescence is attributed to the band-acceptor transition. We also find that poorer crystal quality may lead to the reduction of the optical transmittance. This suggests that these defects and the crystal quality interact to change the conductivity and optical transmittance of ZnO. The spectroscopic correlations may be used as a predictive tool to identify the quality of ZnO.

Improvement of the thermoelectric characteristics of Fe-doped misfit-layered Ca3Co4−xFexO9+δ (x=0, 0.05, 0.1, and 0.2)

The authors report measurements of the electrical resistivity, Seebeck coefficient, and thermal conductivity for a series of misfit-layered oxides Ca3Co4−xFexO9+δ (x=0, 0.05, 0.1, 0.15, 0.2) prepared by solid state reaction. Structural parameters are refined with superspace group of X2∕m(0b0)s0 using powder x-ray diffraction data. With partial substitution of Fe+2 for Co+3, the resistivity decreases, while the thermopower increases simultaneously. The x=0.05 sample exhibits a higher figure of merit (Z=3.01×10−4K−1) than that of Ca3Co4O9+δ (0.33×10−4K−1) at 300K, indicating much improvement of thermoelectric characteristics via partial substitution of Fe for Co.

Thermoelectric power and conductivity of different types of polypyrrole

We have measured the thermoelectric power and conductivity as a function of temperature of a wide range of polypyrrole samples, including a film of soluble polypyrrole synthesized chemically, and wrinkled films synthesized using indium–tin oxide electrodes; other samples investigated include high-conductivity polypyrrole films synthesized at different temperatures and current densities, films grown on nonconducting substrates, and polypyrrole gas sensors. The thermoelectric powers are remarkably similar and metal-like for the medium and high conductivity samples but show nonzero extrapolations to zero temperature for wrinkled samples. The temperature dependence of conductivity tends to be greater for samples of lower conductivity. In contrast to polyaniline and polyacetylene, a crossover to metallic sign for the temperature dependence of conductivity at higher temperatures is not observed in any of our samples; the fluctuation-induced tunnelling and variable-range hopping expressions account for nearly all our conductivity data except for low-temperature anomalies.

High thermoelectric figure-of-merit in p-type nanostructured (Bi,Sb)2Te3 fabricated viahydrothermal synthesis and evacuated-and-encapsulated sintering

High ZT values of nanostructured Bi2−xSbxTe3 for energy conversion are fabricated by hydrothermal methods followed by cold-pressing and sintering in an evacuated and encapsulated ampoule. We show that the sample with a nominal composition of x = 1.55 exhibits a dimensionless figure of merit ZT = 1.65 at 290 K and 1.75 at 270 K with significant 60–70% improvement of that of the commercial state-of-the-art Bi2Te3 materials around room temperature. The significant ZT improvement arises from the much-reduced thermal conductivity. The low thermal conductivity is mainly due to the increased phonon scattering in the nanostructured materials.

High-temperature transport properties of Ca0.98RE0.02MnO3−δ (RE=Sm, Gd, and Dy)

We report measurements of electrical resistivity and thermopower on CaMnO3−δ and Ca0.98RE0.02MnO3−δ (RE=Sm, Gd, and Dy) prepared by solid state reaction. CaMnO3−δ exhibits nonmetal-like temperature dependence of resistivity while metal-like temperature dependence of thermopower. This inconsistency can be explained by the extrinsic carriers arising from oxygen defects using two-band model. Ca0.98RE0.02MnO3−δ exhibits metal-like temperature dependence in both resistivity and thermopower. The transition to metal-like behavior resembles the case in degenerate semiconductors and can be attributed to an impurity band formation with characteristic of hybridized Mn 3d eg and O 2p states due to electron doping via partial substitution of lanthanides for Ca2+ and oxygen defects.

Effects of Na content on the luminescence behavior, conduction type, and crystal structure of Na-doped ZnO films

This study investigates the effect of Na content on the structural, optical, and electrical properties of sol-gel Na-doped ZnO films using x-ray diffraction, photoluminescence, and conductivity measurements. It is shown that a p-type conversion of the Na-doped ZnO film might be due to a combined effect of the increased substitutional-Na density and the decreased oxygen-vacancy (VO) density. However, excess Na incorporation into ZnO shows an ambiguous carrier type due to the increase in the donorlike VO density. These results indicate that compensation effects limit the hole concentration in the Na-doped ZnO films. In addition, when more Na is substituted into the ZnO system, the difference in the ionic radii of Zn2+ and Na+ starts playing an increasingly important role, causing the presence of tensile stress in the Na-doped ZnO film.

Comparison of electronic transport in polyaniline blends, polyaniline and polypyrrole

Abstract The conductivity of PAni/PETG copolyester blends shows the expected reduction compared to unblended PAni, but PAni/PMMA (and PAni/PVC blends at lower temperatures) show a larger conductivity. SEM images indicate that the different behaviour of the blends appears to be associated with greater granularity retained by the PAni particles dispersed in PETG. The conductivities of the more highly conducting PAni blends, like PAni-CSA and its blends with PMMA measured by other authors, show a change to metallic sign for the temperature dependence near room temperature — simple models combining hopping/tunnelling with metallic conduction are consistent with the conductivity except at very low temperatures. The thermopower of PAni blends shows a remarkable similarity for blends with different insulating polymers and widely varying conductivity. For polypyrrole samples of very different conductivity, the change to metallic sign for conductivity near room temperature does not occur, indicating a much smaller ‘metallic’ resistance contribution. Nevertheless, the thermopower of polypyrrole, like that of PAni and PAni blends, is of metallic size and generally increases with temperature, as expected for metallic diffusion thermopower.

Conductivity and thermopower of blends of polyaniline with insulating polymers (PETG and PMMA)

We have measured the temperature dependence of the conductivity and thermoelectric power of conducting polyaniline (PAni) dispersed in PETG copolyester and in poly(methylmethacrylate) (PMMA). Near room temperature, the PAniPMMA blends (like unblended PAni) show a change to metallic sign for the conductivity temperature dependence, whereas this sign change does not occur for the PAniPETG blends. As temperature decreases, the PAniPMMA blends show a much smaller decrease of conductivity than the PAniPETG blends and even unblended PAni. A simple model involving metallic conduction in addition to tunnelling between metallic particles can give a good account of all the conductivity data. The thermopower of all the blends is small and increases as temperature increases.

X-ray absorption spectroscopy studies of Fe-doped misfit-layered Ca3Co4−xFexO9+δ (x=0, 0.05, 0.1, and 0.15)

We have carried out O K- and Co L2,3-edge x-ray absorption studies on misfit-layered oxides of polycrystalline Ca3Co4−xFexO9+δ (x=0,0.05,0.1,0.15). The analyses of integrated absorption intensity show that the number of Co 3d unoccupied states decreases upon partial substitution of Fe for Co, which correlates well the variation trend of the room-temperature resistivity with x. Nevertheless, the number of O 2p unoccupied states increases with x. Both Hall and thermopower measurements indicate that the majority carrier is of hole type for Ca3Co4−xFexO9+δ. Therefore, the decrease of O 2p occupancy should be responsible for the increase of the hole carrier concentration upon partial substitution of Fe for Co.

Effect of Mn doping on the physical properties of misfit-layered Ca3Co4O9+δ

We have carried out an x-ray absorption near-edge structure (XANES) study on a series of misfit-layered cobaltites Ca3Co4−xMnxO9+δ (x = 0, 0.05, 0.1, 0.15). In this system, the average valences of Co and Mn, according to Co and Mn K-edge measurements are about 3.50+ and 3+, respectively. Both the trivalent and tetravalent Co are of low-spin configurations as confirmed by our magnetization measurements. The variations of absorption intensities of Co L2,3-edge and O K-edge XANES with respect to the Mn doping concentration (x) are analysed. Co L2,3-edge results show that the Co 3d unoccupied states decrease with x, which correlate well with the trend of variation of thermoelectric power. O K-edge results indicate that the number of O 2p unoccupied states decreases with x, which implies a decrease in the majority charge carriers (hole) concentration upon partial substitution of Mn for Co.