Hot-injection synthesis and characterization of quaternary Cu2ZnSnSe4 nanocrystals

Cu₂ZnSnSe₄ (CZTSe) is one of promising materials in the use of absorber layers of solar cells. It contains earth-abundant elements of zinc and tin, a near-optimal direct band gap of ∼ 1.5 eV, as well as a large absorption coefficient ∼ 10⁴ cm-1. The CZTSe nanocrystals in oleylamine (OLA) was successfully prepared via hot-injection method. The characterization of its structure, composition, morphology and absorption spectra were done using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV-vis absorption spectra. The results revealed that the monodispersed nanocrystals were single phase polycrystalline within the range of 15-20 nm. Optical measurements showed a direct band gap of 1.52 eV, which was optimal for low cost solar cells. The capping property of OLA was also demonstrated by examining Fourier Transform Infrared Spectroscopy (FTIR) feature peaks of CZTSe and OLA, respectively.     

Promotion effect of nanosized Pt,RuO2 and NiOx loading on visible light-driven photocatalysts K4Ce2M10O30 (M = Ta,Nb) for hydrogen evolution from water decomposition

Metal oxide photocatalysts K₄Ce₂M₁₀O₃₀ (M = Ta, Nb) capable of responding to visible light were synthesized by conventional high temperature solid-state reaction. The photocatalysts have an appropriate band gap energy ca. 1.8–2.3 eV and excellent chemical potential level to evolve H₂ from aqueous solutions containing a sacrificial electron donor (Na₂SO₃) under visible light irradiation (λ>420 nm) without any co-catalyst. When they were loading with Pt, RuO₂ and NiO_x, the activities for evolving H2 were prompted markedly. By SEM and TEM investigations, it can be seen that these loading K₄Ce₂M₁₀O₃₀ (M = Ta, Nb) in diameter of about 10–30nm particles, especially the NiO_x loading even formed double layered structure with metal nickel (Ni) and metal oxide (NiO). The reasons for the increasing activities after these loading electron migrating from the conduction band of K₄Ce₂M₁₀O₃₀ (M =Ta, Nb) to the Pt, RuO₂ and NiO_x nanoparticles, which function as H₂ production sites on the surface of catalysts. The same phenomenon appears on the solid solution K₄Ce₂Ta_{10 –x}Nb_xO30 (x = 0–10) with loading RuO₂.     

XPS study of CZTSSe monograin powders

The surface and bulk composition of Cu₂ZnSn(Se_xS_1-x)₄ (CZTSSe) monograin powders were investigated by X-ray photoelectron spectroscopy (XPS). The concentration depth profiling of CZTSSe monograin powders was obtained by Ar⁺ ion etching.According to the XPS spectra of CZTSSe monograin powder, the binding energies of Zn 2p_3/2, Cu 2p_3/2, Sn 3d_5/2, S 2p_3/2 and Se 3d_5/2 core levels after surface cleaning are located at 1021.6 eV, 932.4 eV, 486.1 eV, 161.5 eV, 53.9 eV, respectively. From XPS depth profile analysis, Cu deficiency and the excess of chalcogenides on the powder crystals surface were observed.     

Room temperature ferromagnetism in Ni doped In2O3 nanoparticles

In the present work, Ni doped In₂O₃ nanoparticles were prepared using simple co-precipitation method. From the x-ray diffraction analysis it is observed that all samples exhibit single phase polycrystalline nature. All the diffraction lines correspond to the bixbyite type cubic structure. A UV visible analysis reveals that optical band gap decreases from 4.63 to 3.84 eV with Ni doping. DC magnetization measurements reveal that Ni doped In₂O₃ nanoparticles exhibit room temperature ferromagnetism.     

Abnormal dielectric behaviors in Mn-doped CaCu3Ti4O12 ceramics and their response mechanism

Mn-doped CaCu₃Ti₄O₁₂ (CCTO) polycrystalline ceramics have been prepared by the conventional solid state sintering. Our results indicate that 10% Mn doping can decrease the dielectric permittivity in CaCu₃Ti₄O₁₂ by about 2 orders of magnitude (from 10⁴ to 10²). The grain and grain boundary activation energies show an obvious increase from 0.054 eV to 0.256 eV, and decrease from 0.724 eV to 0.258 eV with increasing the Mn doping concentration, respectively, which may be caused by the variation of Cu and Ti valence states in the CCTO samples evidenced by the X-ray absorption spectra. The similar grain and grain boundary activation energies result in invalidation of the internal boundary layer capacitance effect for the 10% Mn-doped CCTO sample, and thus result in the dramatic decrease of dielectric permittivity.     

The Study of Hole Distribution in High-Tc Tl- and Hg-based 1212 Type Cuprates via High-Resolution X-Ray Absorption Near Edge Structure

High resolution O K-edge and Cu L₃-edge X-ray absorption near-edge-structure (XANES) spectra of the high-T_c cuprates of (Tl_0.5Pb_0.5)Sr₂(Ca_1–xY_x)Cu₂O₇ (Tl-1212) and (Hg_0.5Pb_0.5)Sr₂(Ca_1–xY_x)Cu₂O₇ (Hg-1212) in powder form were measured using a bulk sensitive total-fluorescence-yield technique. Near the O 1s edge, the pre-edge peak with maxima at 528.3 eV is ascribed to the transitions to O 2p holes located in the CuO₂ planes. The intensity of this pre-edge peak increases with increasing doping level of Ca²⁺ into the Y³⁺ sites in Tl-1212 and Hg-1212. In the Cu L-edge absorption spectra, high-energy shoulders at around 933 eV are attributed to the transitions to the Cu(2p_3/2)^–13d¹⁰L states in the CuO₂ layers, where L denotes the O 2p ligand hole. The behavior of these shoulders in Tl-1212 and Hg-1212 correlates with that of the pre-edge peak at 528.3 eV in the O K-edge absorption spectra. The results can lead us to understand the hole distribution in high-T_c cuprates which will give a direction to find new high-T_c materials.     

Temperature-dependent high-resolution X-ray photoelectron spectroscopic study on Ge1Sb2Te4

Oxygen-free and amorphous Ge₁Sb₂Te₄ thin film was obtained in an ultra-high vacuum and then annealed in situ to the stable-phase temperature. High-resolution X-ray photoelectron spectroscopy using synchrotron radiation was performed on the film at the different annealing temperatures of 100, 130, 150, 180, and 250 °C. The Te 4d, Sb 4d, and Ge 3d shallow core levels as well as the valence-band spectra were acquired. In the shallow core-level spectra, we observed distinguishable changes in the Sb 4d and Ge 3d levels as the film phase changed. As the temperature increased, a higher binding-energy (BE) component appeared at the Sb 4d level, the intensity of the component increased, and the spin-orbit split feature was enhanced at the Ge 3d level. In the valence-band spectra, a slight increase was observed at 0-1, ~ 3, ~ 9, and ~ 12 eV BE, and a decrease, at ~ 1.5 and ~ 4.5 eV BE. The energy resolution employed in this study was about 150 meV.     

Study on microstructure and properties of centrifugal casting 35Cr45NiNb+MA furnace tubes during service

In the present study, the microstructure evolution of 35Cr45NiNb+MA ethylene cracking furnace tubes during service and its effect on the properties were investigated. According to the results, in the early stage of service, the skeletal M₇C₃ and vermicular NbC were transformed into blocky M₂₃C₆ and G phase (Ni₁₆Nb₆Si₇), respectively, accompanied with many dispersed M₂₃C₆ secondary carbides. With the extension of service time, M₂₃C₆ carbides on the grain boundaries were transformed into M₇C₃ with high stacking fault structure and coarsened, and the blocky G phase was transformed into granular NbC. The yield strength and ultimate tensile strength of the aging furnace tubes decreased by 9%-18%, yet the elongation after fracture decreased significantly from 10.0%-14.0% to 3.0%-5.0%. The hardness of the carburized zone of the carburized tubes increased by 10%-17%, and the rupture time decreased by 45%-75% under the test condition of 1100℃ and 16MPa. Finally, the evolution map was summarized.     

Carbide transformation in carburised zone of 25Cr35NiNb+MA alloy after high-temperature service

Microstructure evolution due to carburising of a 25Cr35NiNb+MA ethylene pyrolysis furnace tube was investigated after service for approximately 4 years. The microstructure of 25Cr35NiNb+MA alloy was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy (TEM). The results revealed that M₂₃C₆ carbides of the 25Cr35NiNb+MA alloy had coarsened and transformed to M₇C₃ carbides with a heavily faulted structure during carburising. The content of M₇C₃ carbide increased closer to the inner wall. A large number of dislocations had been observed surrounding the carbides and some formed dislocation walls. The carbide transformation mechanism and the effect of the dislocation walls are discussed.     

Microstructure evolution and mechanical properties of 316H weld metal during aging

ABSTRACT

A standard 316H welding wire was used to obtain 6% δ-ferrite in the weld metal by the automatic tungsten inert gas welding. The effect of aging time at 750°C on microstructure and mechanical properties of the weld metal were analysed. The results showed that as the aging time increased, the δ-ferrite fraction decreased, and finally, δ-ferrite was no longer seen, the M₂₃C₆ fraction first increased and then decreased, and the σ phase fraction increased. Finally, the fractions of M₂₃C₆ and σ phase kept stable, but the size increased. Accordingly, as the aging time increased, the yield and tensile strengths first decreased significantly, then increased obviously and finally decreased slightly again, while the elongation and impact energy decreased all the time.     

Ab initio calculations of electronic structures of LiYF4 crystals containing F-type color centers

The electronic structures of LiYF₄ crystals containing F, F⁻, and M color centers (F₂ center) with the lattice structure optimized are studied within the framework of the density functional theory. The calculation indicated that F, F⁻, and M color centers have donor energy levels in the forbidden band. The electronic transition energies from the donor level to the bottom of the conduction band are 3.74 eV, 2.85 eV, and 2.42 eV, respectively, which correspond to the 331 nm, 436 nm, and 513 nm absorption bands. It is predicted that the absorption bands observed at 330 nm, 440 nm, and 505 nm could arise from the F, F⁻, and M centers, respectively, in LiYF₄ crystals.     

Te concentration dependent photoemission and inverse-photoemission study of FeSe1?xTex

We have characterized the electronic structure of FeSe_1−xTe_x for various x values using soft x-ray photoemission spectroscopy (SXPES), high-resolution photoemission spectroscopy (HRPES) and inverse photoemission spectroscopy (IPES). The SXPES valence band spectral shape shows that the 2 eV feature in FeSe, which was ascribed to the lower Hubbard band in previous theoretical studies, becomes less prominent with increasing x. HRPES exhibits systematic x dependence of the structure near the Fermi level (E_F): its splitting near E_F and filling of the pseudogap in FeSe. IPES shows two features, near E_F and approximately 6 eV above E_F; the former may be related to the Fe 3d states hybridized with chalcogenide p states, while the latter may consist of plane-wave-like and Se d components. In the incident electron energy dependence of IPES, the density of states near E_F for FeSe and FeTe has the Fano lineshape characteristic of resonant behavior. These compounds exhibit different resonance profiles, which may reflect the differences in their electronic structures. By combining the PES and IPES data the on-site Coulomb energy was estimated at 3.5 eV for FeSe.     

First results from a dispersive EXAFS beamline developed at INDUS-2 synchrotron source at RRCAT, Indore, India

A dispersive extended X-ray absorption fine structure (EXAFS) beamline employing an elliptically bent crystal and a position-sensitive CCD detector has been developed at the BL-8 bending magnet port of INDUS-2 synchrotron source at RRCAT, Indore, India. The beamline enables simultaneous measurement of X-ray absorption spectra over the range of 5–20 keV in a short span of time. The beamline performance has been investigated by measuring the absorption spectra of metal foils and oxide powders. The EXAFS measurements on crystalline Pb₅Ge₃O₁₁ samples were performed for the first time in literature. The average Pb–O bond length obtained from these measurements is found to agree well with the theoretically estimated value.     

2D and 3D Crystal Forms of the C36 Fullerenes: Geometrical and Electronic Structure

Abstract

In the present work calculations of geometrical structure and electronic properties of the covalent crystal consisted of hexagonal close packaged layers of C₃₆ fullerenes in 2D & 3D cases are carried out. The analysis of the stability and formation paths of such crystal according to PM3 semiempirical method was performed. Extended Huckel theory has been applied to electron spectra calculations. Comparison of the structure with similar fullerene and nanotube structures was carried out and effect of geometry to electron properties is discussed. In the work the values of 6.695 and 6.763 Å for the lattice constants were obtained with good agreement with experiment (Piskoti, C.; Yarger, J.; Zettl, A. C₃₆, a new carbon solid. Nature 1998, 393, 771), and value of 1.50 eV in 2D case and 1.88 eV in 3D case was obtained for bandgap.     

Nanostructured optical waveguide films of TiO2 and WO3−x for photonic gas sensors

Pulsed laser deposition technique was used to fabricate successfully nanostructured optical waveguide films of TiO₂ and WO_3 − x for gas-sensing applications. High-quality mode spectra, consisting of well-defined bright mode lines were observed in the samples with smooth surface. Films' structural, morphological and optical properties as well as the chemical composition were investigated by using x-ray diffraction, scanning electron and atomic force microscopy, UV-VIS-NIR, m-line and x-ray photoelectron spectroscopy, respectively.Gas sensitivity was investigated at room temperature using m-line spectroscopy technique on basis of reversible variations of the refractive index of the samples under gas exposure. Testing scheme recording the bright m-line shift as optical response was proposed.     

Gamma rays interactions with WO3-doped lead borate glasses

Optical and FT infrared spectral properties of tungsten ions in a host lead borate glass with composition PbO 55%, B₂O₃ 45% (wt%) were studied. The same spectral properties were re-measured after subjecting the samples to successive gamma irradiation. The work was undertaken to justify the state of tungsten ions in such glass system by combined spectral investigations. Optical and FTIR spectral studies were confirmed by investigating electron spin resonance (ESR) of the undoped and WO₃-doped samples before and after gamma irradiation. The optical spectrum of the undoped glass exhibits strong and wide UV absorption bands, which are related to the combined UV spectra of trace iron impurities (Fe³⁺ ions) and that from divalent lead (Pb²⁺) ions. Optical studies of WO₃-doped sample indicate the presence of tungsten ions mostly in the hexavalent W⁶⁺ state. The presence of tungsten ions as structural groups was obtained by comparing the FTIR spectra of the undoped and WO₃-doped samples. ESR spectra confirm the optical and FTIR spectral studies. The studied host lead borate glass has been found to show obvious shielding behavior towards successive gamma irradiation as revealed by the constancy of optical absorption spectral curves.     

Structural and Photocatalytic Activity of Mesoporous N-Doped TiO2 with Band-to-Band Visible Light Absorption Ability

White colored N-doped TiO₂ and a neat TiO₂ powder were synthesized via sol–gel method. Prepared samples were characterized by means of x-ray diffractions, Brunauer–Emmet–Teller and Barrett–Joyner–Halenda methods, x-ray photoelectron spectroscopy, ultraviolet-visible analysis, scanning electron microscopy, and energy dispersive x-ray spectroscopy. Both of the N-doped TiO₂ and neat TiO₂ consisted of anatase phase of titania with mesoporous nature and according to XPS analysis prepared N-doped TiO₂ is a substitutional nitrogen containing sample. The band gap of N-doped TiO₂ and neat TiO₂ were estimated from ultraviolet-visible spectroscopy data to be 2.7 and 3.2 eV, respectively. Prepared substitutional N-doped TiO₂ featured steep light absorption edge with an approximately parallel characteristic of its absorption edge to that neat TiO₂. This is due to its band-to-band visible light absorption ability. Synthesized N-doped TiO₂ had a large surface area value of 193 m²/g and high photon absorption ability causing superior photocatalytic properties towards Congo red azo dye compared to neat TiO₂ either under ultraviolet or visible light illumination.     

Selective-syntheses, characterizations and photocatalytic activities of nanocrystalline ZnTa2O6 photocatalysts

Nanocrystalline ZnTa₂O₆ photocatalysts with different crystal structures were prepared via a simple and facile sol-gel method in a temperature range of 650-950 °C. The absorption edges and particle sizes of the samples were located at about 285 nm (corresponding a band gap of 4.35 eV) and ranged from 25 to 150 nm, respectively. The photocatalytic activities of the samples were tested by the degradation of methyl orange under UV light irradiation. The results indicated that the crystal structure of ZnTa₂O₆ was a main factor for the different photocatalytic activities of the ZnTa₂O₆ samples. Moreover, the effects of crystallinities and surface areas of the obtained samples on the catalytic activities were also discussed.     

Powder X-Ray Reference Patterns of Sr2RGaCu2Oy (R = Pr,Nd, Sm,Eu, Gd,Dy, Ho,Er, Tm,and Y)

X-Ray Rietveld refinements were conducted on a series of eleven lanthanide phases, Sr₂RGaCu₂O_y (2112 phase, R = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er, Tm, and Yb) that are structurally related to the high T_c superconductor Ba₂YCu₃O₇ (213). In the 2112 structure, instead of square planar Cu-O chains, tetrahedral GaO4 chains were found to run in a zig-zag fashion along the diagonal of the basal 213 ab-direction. Reference powder patterns for these compounds were prepared by using the Rietveld decomposition technique. The unit cell volume of these compounds follows the expected trend of the lanthanide contraction. The lattice parameters range from a = 22.9694(3) Å, b = 5.5587(2) Å, and c = 5.44743(7) Å for R = Pr, to a = 22.8059(2) Å, b = 5.46031(5) Å, and c = 5.37773(5) Å for R = Yb. An electon diffraction study of the Sm- and Er-analogs showed characteristic diffuse streaks along the b-axis, suggesting some disorder within the GaO₄ chains.     

The effect of WO3 source stability on the properties of films deposited by electron beam deposition

This study investigates the impact of high temperature and vacuum on the properties of WO₃ powder during electron beam deposition and evaluates the consequential effects on the as-deposited films. Therein, the grain size and the crystallinity of the source were observed to increase and become non-stoichiometric tungsten oxide (WO_3−x) due to the high temperature and vacuum during the first deposition. As a result of these changes in the source, the optical band gap, E_g, of the deposited film decreased from 3.11 eV to 3.07 eV, and the absorbance was observed to increase. The coloration efficiencies of the deposited films decreased from 23 to 16 cm² C⁻¹. WO₃-incorporated carbon nanotubes (WO₃/CNT) were observed in the source after electron beam deposition if there were some initial carbon impurities in the source prior to deposition.