Band gap engineering, band edge emission, and p-type conductivity in wide-gap LaCuOS1–xSex oxychalcogenides

The preparation of LaCuOS1–xSex solid solutions (x = 0.0, 0.25, 0.5, 0.75, and 1.0) was attempted to control their energy gap and band edge emission energy. X-ray diffraction analysis revealed that the lattice constant of LaCuOS1–xSex increased linearly with increasing x, indicating the formation of a complete solid solution in the LaCuOS–LaCuOSe system. The energy gap estimated from the diffuse reflectance spectra varied continuously from ~3.1 eV for x = 0 to ~2.8 eV for x = 1. The sharp emission near the absorption edge was observed in all samples at room temperature under ultraviolet light irradiation. p-type electrical conduction in these materials was confirmed by Seebeck measurements, and the conductivity was enhanced by substitution of Sr for La. These results demonstrated that the formation of the solid solutions enabled band gap engineering in LaCuOS1–xSex oxychalcogenides keeping their band edge emission feature and p-type conductivity

Thermoelectric properties of layered oxyselenides La1–xSrxCuOSe (x = 0 to 0.2)

Thermoelectric properties of layered oxyselenides La1–xSrxCuOSe (x = 0.00 to 0.20) were investigated to evaluate the potential as thermoelectric material. Temperature dependence of the electrical conductivity and Seebeck coefficient measured in a temperature range of 373 to 673 K indicated that nondoped LaCuOSe was a p-type degenerate semiconductor due to Cu vacancies, while Sr-doped materials with x = 0.05 to 0.20 were p-type metals. The electrical conductivity increased and Seebeck coefficient decreased with increasing Sr concentration up to x = 0.10 in La1–xSrxCuOSe, suggesting that the effective hole carriers increase with increasing Sr content up to x = 0.10. Thermoelectric power factors were drastically enhanced by the Sr doping, and the value reached 1.0–1.4×10–4 W m–1 K–2 for La0.95Sr0.05CuOSe. Thermal conductivities measured for the materials with x = 0.00 and 0.05 were 2.1 W m–1 K–1 and 2.3 W m–1 K–1 at room temperature, respectively. These results lead to an estimation of Z value of 4.4×10–5 K–1 for La0.95Sr0.05CuOSe

Valence-band structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), studied by ultraviolet photoemission spectroscopy and energy-band calculations

To examine the electronic structure of the valence band, ultraviolet photoemission spectra of a series of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), were measured. The measurements were conducted using He II, He I, and Ne I excitation lines to observe the excitation energy dependence of the spectral shape. Energy-band calculations based on a full-potential linearized augmented plain-wave method were performed. The calculated density of states and partial density of states were compared to the observed photoemission spectra. Five bands were observed in the valence band of LaCuOCh, and Ne I radiation remarkably enhanced two of them. The energy dependence of the photoionization cross section of atomic orbitals indicated that the two enhanced bands were due to the Ch p states. Energy calculations were used to assign the remaining bands. The electronic structure of LaCuOCh was further discussed using molecular-orbital diagrams to visualize the (La2O2)2+ and (Cu2Ch2)2– layers as large donor-acceptor pairs. The energy-band calculation and molecular-orbital diagram analyses suggested that the main difference among the valence-band structures of LaCuOCh (Ch=S, Se, and Te) originates from the variations in the energy position of the Ch p bands. The observed spectra are consistent with the results of the band calculations and clearly show the energy variations in the Ch p bands with respect to spectral shape and excitation energy dependence

NOTE ON LOWER BOUNDS OF ENERGY GROWTH FOR SOLUTIONS TO WAVE EQUATIONS

In this note we study lower bounds of energy growth for solutions to wave equations which are compact in space perturbations of the wave equation ∂^2_tu−Δu=0. Assuming that there exists a null bicharacteristic (x(t),ξ(t)), parametrized by the time t, such that x(t) remains inside a ball and ξ(t) outside a ball for t≥0 we prove that the solution operator R(t) is bounded from below by constant times √ in the operator norm. We apply this result to examples constructed by the same idea as in Colombini and Rauch [1] and show that there exist compact in space perturbations which cause exp(ct^α) growth of the energy for any given 0≤α≤1

Thermoelectric properties of delafossite-type layered oxides AgIn1–xSnxO2

The thermoelectric properties of delafossite-type layered oxides AgIn1–xSnxO2 that consist of alternating layers of Ag and In1–xSnxO2 were investigated to elucidate their potential as a thermoelectric material. Polycrystalline materials of the AgIn1–xSnxO2 were prepared by a cation exchange reaction between NaIn1–xSnxO2 and AgCl. The solubility limit of the Sn atoms on the In sites was approximately x=0.05. The electrical conductivity and Seebeck coefficient were measured between 373 and 673 K in air. Undoped AgInO2 was an n-type semiconductor with conductivities of 10–4–10–2 –1 cm–1, and the electron carriers were generated via the formation of oxygen vacancies. AgIn0.95Sn0.05O2 was an n-type degenerate semiconductor with conductivities of 100–101 –1 cm–1 where the Sn atoms acted as electron donors. This drastic increase in the electrical conductivity increased the thermoelectric power factor by approximately two orders of magnitude to 10–6–10–5 W m–1 K–2

Segmental isotopic labeling of a 140 kDa dimeric multi-domain protein CheA from Escherichia coli by expressed protein ligation and protein trans-splicing

Segmental isotopic labeling is a powerful labeling tool to facilitate NMR studies of larger proteins by not only alleviating the signal overlap problem but also retaining features of uniform isotopic labeling. Although two approaches, expressed protein ligation (EPL) and protein trans-splicing (PTS), have been mainly used for segmental isotopic labeling, there has been no single example in which both approaches have been directly used with an identical protein. Here we applied both EPL and PTS methods to a 140 kDa dimeric multi-domain protein E. coli CheA, and successfully produced the ligated CheA dimer by both approaches. In EPL approach, extensive optimization of the ligation sites and the conditions were required to obtain sufficient amount for an NMR sample of CheA, because CheA contains a dimer forming domain and it was not possible to achieve high reactant concentrations (1–5 mM) of CheA fragments for the ideal EPL condition, thereby resulting in the low yield of segmentally labelled CheA dimer. PTS approach sufficiently produced segmentally labeled ligated CheA in vivo as well as in vitro without extensive optimizations. This is presumably because CheA has self-contained domains connected with long linkers, accommodating a seven-residue mutation without loss of the function, which was introduced by PTS to achieve the high yield. PTS approach was less laborious than EPL approach for the routine preparation of segmentally-isotope labeled CheA dimer. Both approaches remain to be further developed for facilitating preparations of segmental isotope-labelled samples without extensive optimizations for ligation. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10858-012-9628-3) contains supplementary material, which is available to authorized users