ABSTRACT
Density-functional-theory (DFT) calculations within the generalised gradient approximation (GGA) were used to examine the behaviour of point defects in the cubic BVIO3 perovskite-type oxide, ReO3. Energies of reduction and of hydration were calculated, and the results are compared with literature data for ABO3 perovskite oxides. The activation energies of migration were determined for O2-, H+, Li+, Na+, K+ and H3O+. An occupied A site in ReO3 is found to be beneficial to oxide-ion migration by a vacancy mechanism as well as to proton migration by a Grotthuss mechanism. Na+, K+ and H3O+ exhibit activation energies of migration higher than 2 eV, whereas Li+ is characterised by a very low migration barrier of 0.1 eV. Reasons for this behaviour are discussed. Our results suggest that H+, O2-, and especially Li+, are highly mobile ions in ReO3.
ABSTRACT
Although tungsten trioxide (WO3) has been extensively studied since its electrochromic properties were first discovered, the mechanism responsible for the coloration or bleaching effect is still disputed. New insights into the coloration mechanism of electrochromic, nanocrystalline WO3 are provided in this paper by studying thin WO3 films combining the electrochemical and spectroscopic techniques. By employing in situ UV-Vis transmission spectroscopy at a fixed spectral band pass during electrochemical experiments, such as cyclic voltammetry, a two-step insertion process for both protons and lithium ions is identified, of which one step exhibits a significantly higher coloration efficiency than the other. To obtain a better understanding of the insertion process AxWO3 (A = H, Li, ) thin films were studied at different stages of intercalation using UV-Vis and X-ray photoelectron spectroscopy. The results show that the first step of the intercalation process represents the reduction from initial W(6+) to W(5+) and the second step the reduction of W(5+) to W(4+). We found that the blue coloration of this nanocrystalline tungsten trioxide is mainly due to the presence of W(4+) rather than that of W(5+).
ABSTRACT
The conductivity and charge carrier concentration of single GaN nanowires (NWs) doped with different concentrations of Ge were determined by four-point resistivity and temperature-dependent Seebeck coefficient measurements. We observed high carrier concentrations ranging from 9.1 × 10(18) to 5.5 × 10(19) cm(-3), well above the Mott density of 1.6 × 10(18) cm(-3), and conductivities up to 625 S cm(-1) almost independent of the NW diameter. The weak temperature dependence of the conductivity between 2 and 10 K could be assigned to the formation of an impurity band. For the sample with the highest conductivity metallic behaviour was found, indicated by a positive temperature coefficient of the resistivity. The near band edge emission analyzed by micro-photoluminescence spectroscopy showed only a small increase of the peak width up to 70 meV and no spectral shift for carrier concentrations up to 5.5 × 10(19) cm(-3). The latter was attributed to the simultaneous influence of band filling, band gap renormalization, and strain.
