T. Zhou

Growth and luminescence characterization of large-scale zinc oxide nanowires

Large-scale zinc oxide (ZnO) nanowires were grown via a simple chemical reaction involving water vapour. Electron microscopy observations reveal that the ZnO nanowires are single crystalline and grow along the c-axis ([001]) direction. Room temperature photoluminescence measurements show a striking blue emission at 466 nm along with two other emissions in the ultraviolet and yellow regions. Annealing treatment of the as-grown ZnO nanowires results in an apparent reduction of the intensity of the blue emission, which indicates that the blue emission might be originating from the oxygen or zinc defects generated in the process of growth of the ZnO nanowires.

Superconducting Phases in Potassium-Intercalated Iron Selenides

The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in A(x)Fe(2-y)Se(2) (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, K(x)Fe(2)Se(2)(NH(3))(y) (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K(0.3)Fe(2)Se(2)(NH(3))(0.47) corresponds to the 44 K phase with lattice constant c = 15.56(1) Å and K(0.6)Fe(2)Se(2)(NH(3))(0.37) to the 30 K phase with c = 14.84(1) Å. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH(3) has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K(0.3)Fe(2)Se(2) and K(0.6)Fe(2)Se(2) SC phases. K(0.3)Fe(2)Se(2)(NH(3))(0.47) and K(0.6)Fe(2)Se(2)(NH(3))(0.37) stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of T(c) with doping is not dome-like.

Growth and optical characterization of Ga2O3 nanobelts and nanosheets

Monoclinic Ga2O3 nanobelts and nanosheets are obtained by a simple chemical route involved with H2O at 950 °C in Ar atmosphere. Electron microscopy observations reveal that the as-synthesized Ga2O3 nanobelts and nanosheets are structurally uniform, single crystalline, and most of them are free of defects and dislocations. The nanobelts are growing along with [001] facets, and the nanosheets are stacked up by (011) facets. The Raman scattering spectrum of Ga2O3 nanostructures shows a 30 cm−1 redshift at high wave numbers in comparison with that of bulk Ga2O3 powder. The photoluminescence spectrum reveals that there exists a stable blue emission band centered at 460 nm, which is mainly attributed to the oxygen vacancies in the Ga2O3 nanostructures.

Aligned silica nanofibres

Large-scale highly aligned silica nanofibres were produced on silicon substrates. The diameters of these nanofibres vary from 50 to 100 nm and the lengths are up to several millimetres. Transmission electron microscopy and selected-area electron diffraction reveal that the fibres are in an amorphous state. The growth of silica nanofibres is probably controlled by a vapour-solid process. Gas-phase SiO seems to play the key role in the synthesis, serving as a transportation medium.

Exploring FeSe-based superconductors by liquid ammonia method

Our recent progress on the preparation of a series of new FeSe-based superconductors and the clarification of SC phases in potassium-intercalated iron selenides are reviewed here. By the liquid ammonia method, metals Li, Na, Ca, Sr, Ba, Eu, and Yb are intercalated in between FeSe layers and form superconductors with transition temperatures of 30 K~46 K, which cannot be obtained by high-temperature routes. In the potassium-intercalated iron selenides, we demonstrate that at least two SC phases exist, KxFe2Se2(NH3)y (x ≈ 0.3 and 0.6), determined mainly by the concentration of potassium. NH3 has little, if any, effect on superconductivity, but plays an important role in stabilizing the structures. All these results provide a new starting point for studying the intrinsic properties of this family of superconductors, especially for their particular electronic structures.

New Layered Iron Sulfide NaFe1.6S2: Synthesis and Characterization

Na was intercalated between [Fe2S2] layers for the first time, giving a novel compound NaFe(1.6)S2. This material adopts a CaAl2Si2-type structure with ~20% iron vacancies and represents the first layered compound in a ternary Na-M-X (M = Fe, Co, Ni; X = S, Se) system. First-principles calculations reveal that phonon dynamics is an important factor for it to prefer the CaAl2Si2-type rather than the ThCr2Si2-type structure. It features a magnetic transition at 205 K and is a narrow-band-gap semiconductor.

Structures and Physical Properties of Layered Oxyselenides Ba2MO2Ag2Se2 (M = Co, Mn)

Two new layered oxyselenides, Ba2MO2Ag2Se2 (M = Co, Mn), have been successfully synthesized via solid-state reaction. It is found that these two compounds, consisting of the infinite MO2 square planes and antifluorite-type Ag2Se2 layers separated by barium, possess new structural features while keeping I4/mmm symmetry. A detailed calculation on the discrete coordination of transition metals by oxygen in the two compounds and Ba2ZnO2Ag2Se2 revealed quite different energy landscapes. The calculated results indicate that the manganese compound favors adoption of the I4/mmm space group, while the cobalt compound could be at the boundary of the transition between the I4/mmm and Cmca phases. In Ba2CoO2Ag2Se2, the coexistence of a large barium ion and a Ag2Se2 layer expands the oxide layer significantly and results in the largest Co-O bond length in the square-planar sheet ever reported. Ba2CoO2Ag2Se2 is near-stoichiometric, whereas Ba2MnO2Ag2Se2 contains 7% silver vacancies, which is explained by the mixed valence of the manganese ion between 2+ and 3+. In Ba2CoO2Ag2Se2, the zero-field-cooled and field-cooled susceptibilities bifurcate at 159 K, located between two antiferromagnetic (AFM) transitions. Meanwhile, Ba2MnO2Ag2Se2 shows high-temperature Curie-Weiss behavior, followed by a low-temperature AFM transition with TN = 32 K. They both exhibit semiconducting behavior with resisitivities of about 10(5)Ω cm at room temperature. The optical band gaps are determined to be 1.49 and 1.18 eV for Ba2CoO2Ag2Se2 and Ba2MnO2Ag2Se2, respectively. Band structure calculations reveal that Ba2CoO2Ag2Se2 is a direct-gap semiconductor, with a calculated band gap of 1.147 eV; however, Ba2MnO2Ag2Se2 failed to reproduce the semiconducting behavior within an A-type AFM model.

Nonlinear electrical and dielectric properties of (Ca, Ta)-doped TiO2 varistors

(Ca, Ta)-doped TiO2 varistors with high nonlinear coefficients were obtained by ceramic sintering process. The nonlinear electrical and dielectric properties of the samples doped with 1.0 mol% Ca and different concentrations of Ta (0.05-2.0 mol%) were investigated. Especially, the effects of Ta dopant on the nonlinear and dielectric properties of the (Ca, Ta)-doped TiO2 varistors were studied in detail. When the concentration of Ta is 0.5 mol%, the sample possesses the highest nonlinear coefficient (16.6). By analogy to a, grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained.

Crystal structures of two novel borate compounds MgInBO{sub 4} and MgIn{sub 7/8}B{sub 7/8}O{sub 29/8}

Two novel borate compounds MgInBO4 and MgIn7/8B7/8O29/(8), have been synthesized via solid-state reactions, and their crystal structures have been solved and refined from powder X-ray diffraction data. The compound MgInBO4, which was obtained at 1190 degrees C, belongs to the warwickite family. It crystallizes in the Pnma space group (no. 62) with a=9.5443(1) angstrom, b=3.2771(1) angstrom, c=9.5228(1) angstrom, and Z=4. The fundamental building units are liner -Mg(In)O-6-[In(Mg)](2)O-10-Mg(In)O-6- chains and isolated BO3 triangles. The low-temperature phase, MgIn7/8B7/8O29/8, whose crystal structure is solved ab initio by the charge-flipping method with standard chemical formula MgInBO4, was prepared at 1080 degrees C. It crystallizes in the P12(1)/n1 space group (no. 14) with a=17.0976(1) angstrom, b=3.2504(1) angstrom, c=5.3387(1) angstrom, beta=96.0829(3)degrees, and Z=4. The structure of MgIn7/8B7/8O29/8 contains[In(2)/Mg(2)](2)O-10 groups, -MgO6-InO6- infinite ribbons and isolated BO3 triangles. The experiments and the differential thermal analysis (DTA) show the decompositions of MgInBO4 and MgIn7/8B7/8O29/8 happen at about 1220 degrees C and 1180 degrees C, respectively. The comparative crystal chemistry from MgIn7/8B7/8O29/8 to MgInBO4 has been discussed. Infrared spectra and UV-vis diffuse reflectance spectra of MgInBO4 and MgIn7/8B7/8O29/8 were measured