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We have synthesized inverse-perovskite-type oxysilicides and oxygermanides represented by R3SiO and R3GeO (R = Ca and Sr) and studied their characteristics in the search for nontoxic narrow band gap semiconductors. These compounds exhibit a sharp absorption edge around 0.9 eV and a luminescence peak in the same energy range. These results indicate that the obtained materials have a direct-band electronic structure, which was confirmed by hybrid DFT calculations. These materials, made from earth abundant and nontoxic elements and with a relatively light electron/hole effective mass, represent strong candidates for nontoxic optoelectronic devices in the infrared range.

We report the growth of large Cl-doped and Br-doped SnS single crystals from a molten Sn-based flux. Compared with the small and lamellar undoped SnS crystals, the addition of SnCl2 or SnBr2 haloge...

It is critical to reveal how crystalline polarity correlates with the electronic transport properties in polar semiconductors, such as zinc oxide (ZnO), for the development of materials with properties tailored for specific optoelectronic applications. In this paper, we investigate the structure–property relationships in ZnO films by means of tailoring the polarity of homostructural ZnO bilayers that are subjected to an external substrate bias (Vsub) during deposition. Since the probability of incorporating defects and impurities in ZnO depend on the crystalline facet, the polarity of ZnO should be a critical parameter in determining electronic conductivity. The electric conductivity and Hall mobility of the ZnO films deposited under positive Vsub were much higher than those deposited under negative Vsub. Further investigations of the homostructural ZnO bilayer films, where different signs and magnitudes of Vsub were applied, revealed that the polarity is responsible for electronic conductivity of the ZnO...

The quaternary I2–II–IV–O4 semiconductor, Cu2ZnGeO4, with a wurtz-kesterite structure and 1.4 eV energy band gap has been synthesized for the first time via ion exchange of precursor Na2ZnGeO4. Its crystal structure was refined by Rietveld analysis, and the structural distortion was quantitatively evaluated based on the cation tetrahedral tilting and angle distortion indexes. The tetrahedral distortion in Cu2ZnGeO4 was smaller than in Ag2ZnGeO4 but larger than in β-CuGaO2, suggesting an indirect band gap of Cu2ZnGeO4. Density functional theory calculations using the functional of the local density approximation with corrections for on-site Coulomb interactions indicated that Cu2ZnGeO4 is an indirect semiconductor as expected from its structural feature. However, the energy difference between the direct and indirect band gaps is very small, suggesting that Cu2ZnGeO4 shows strong light absorption near the band edge.

The electronic structures of delafossite α-CuGaO2 and wurtzite β-CuGaO2 were calculated based on density functional theory using the local density approximation functional including the Hubbard correction (LDA+U). The differences in the electronic structure and physical properties between the two polymorphs were investigated in terms of their crystal structures. Three major structural features were found to influence the electronic structure. The first feature is the atomic arrangements of cations. In the conduction band of α-CuGaO2 with a layered structure of Cu2O and Ga2O3, Cu and Ga states do not mix well; the lower part of the conduction band mainly consists of Cu 4s and 4p states, and the upper part consists of Ga 4s and 4p states. By contrast, in β-CuGaO2, which is composed of CuO4 and GaO4 tetrahedra, Cu and Ga states are well-mixed. The second feature is the coordination environment of Cu atoms; the breaking of degeneracy of Cu 3d orbitals is determined by the crystal field. Dispersion of the Cu 3d valence band of β-CuGaO2, in which Cu atoms are tetrahedrally coordinated to oxygen atoms, is smaller than those in α-CuGaO2, in which Cu atoms are linearly coordinated to oxygen atoms; this results in a larger absorption coefficient and larger hole effective mass in β-CuGaO2 than in α-CuGaO2. The interatomic distance between Cu atoms-the third feature-also influences the dispersion of the Cu 3d valence band (i.e., the effective hole mass); the effective hole mass decreases with decreasing interatomic distance between Cu atoms in each structure. The results obtained are valuable for understanding the physical properties of oxide semiconductors containing monovalent copper and silver.

Among the langasite family, Ca3TaAl3Si2O14 (CTAS) is particularly promising for high temperature sensor applications due to its relatively high resistivity. In this work, CTAS single crystals have been grown by the Czochralski technique under various oxygen partial pressures using Ir and Pt crucibles. Colorless CTAS crystals were grown for the first time; however, the elimination of related defects did not influence the electrical properties. Furthermore, dielectric and piezoelectric properties were independent of growth conditions. Instead, the electrical resistivity was found to be remarkably higher the lower the oxygen partial pressure, reaching a value of ∼7 × 1010 Ω cm at 400 °C, which is 3 orders of magnitude higher than that of disordered LTGA. Therefore, grown CTAS crystals are promising for high temperature piezoelectric sensor applications.

An oxide semiconductor β-CuGaO2 with a wurtzite-derived β-NaFeO2 structure has been synthesized. Structural characterization has been carried out by Rietveld analysis using XRD and SAED, and it was shown that the lattice size is very close to that of zinc oxide. The optical absorption spectrum indicated that the band gap is 1.47 eV, which matches the band gap required to achieve the theoretical maximum conversion efficiency for a single-junction solar cell. The thermoelectromotive force indicated p-type conduction in its intrinsic state. Density functional theory calculations were performed to understand the electronic structure and optical properties of the semiconductor. These calculations indicated that β-CuGaO2 is a direct semiconductor and intense absorption of light occurs near the band edge. These properties render this new material promising as an absorber in solar cells.

International audience; Functional silica nanoparticles and in particular luminescent silica nanoparticles constitute very promising candidates for many applications in the field of biotechnology, theranostics, and photonics. However, optimizing the design of such materials requires a deep understanding of their physicochemical properties. In this article are reported extended investigations on luminescent Cs2[Mo6Br14]@SiO2 nanoparticles prepared by a water-in-oil microemulsion technique. We bring here new insights into the structure of such nanoparticles and its interplay with their optical properties. The structural interactions between the cluster units and the silica matrix were investigated and are discussed in details on the basis of FE-SEM, HAADF-STEM, ICP-OES, BET, 29Si MAS NMR, and photoluminescence studies. As part of the risk evaluation before potential applications, the toxicity of the nanoparticles both on plants and on human cells was evaluated.

ZnO films on nonseeded substrates with controllable microstructures and crystal orientations were fabricated with a spin-spray method employing trisodium citrate as an additive. The microstructure of the films was made to change from a rod array to a dense film by increasing the trisodium citrate concentration in the solution. This change in structure was accompanied by a weakening of the 002 peak in the XRD pattern. After postdeposition heating, the ZnO rod arrays showed a decrease in visible luminescence, whereas dense ZnO films showed sharp UV luminescence and a weak broad visible luminescence, thus demonstrating a difference in the defect structure of the samples. We tuned the microstructures of ZnO films on nonseeded substrates by using the spin-spray method and analyzed the formation mechanism.

A zinc oxide (ZnO) rod array was deposited on a nonseeded glass substrate at 90 °C by using a spin-spray technique. This technique is a solution process using selective heterogeneous nucleation of ZnO crystals on the substrate, and it eliminates homogeneously nucleated particles that degrade film quality. These features are an obvious progress for solution routes which essentially involves heterogeneous nucleation on seed layer and homogeneous nucleation in solution. In this study, we prepared a ZnO rod array by spin-spray on a nonseeded glass substrate and analyzed the formation mechanism.

The electronic states of zinc magnesium oxide (Zn1−xMgx)O thin films were determined exactly by hard X-ray photoemission spectroscopy (HX-PES) using synchrotron radiation. The Zn 2p core level was shifted to a higher binding energy along with widening of the bandgap by alloying with MgO, whereas the shift of the O 1s peak was less than that of the Zn 2p peak. Density functional theory (DFT) calculations revealed that the electronic state of an O ion bonding with an adjacent Mg ion is remarkably different from that not bonding with a Mg ion. As a result, the energy shift observed in the O 1s peak results from a combination of the expansion of the bandgap energy and the chemical shift due to a change in the ionicity. The Fermi level is always situated just below the conduction band; this suggests that a shallow donor can be added, even in the alloy film with very high MgO fraction, for example, x = 0.47.

An etching effect of N-plasma on a GaN buffer layer was found in the initial stage of InN growth by plasma-assisted molecular beam epitaxy. We proposed to predeposit 1–2 monolayers (ML) of In (referred to as “In insertion”) on the GaN buffer layer to protect it from etching, thus preserving the flat buffer surface for InN growth. Atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) were carried out to evaluate the GaN buffers and InN films and compare the effect of the In insertion. It has been shown that the In insertion significantly improves the structural quality and optical property of InN. A flatter InN surface, narrower XRD full-width at half-maximum, sharper InN/GaN interface, and stronger PL were observed in optimal samples with 1.8 ML In insertion. The effect of In insertion is discussed in terms of buffer surface protection and enhancement of surface migration of In adatoms.

Iron(III)-doped TiO(2) nanopowders, with controlled iron to titanium atomic ratios (R(Fe/Ti)) ranging from nominal 0 to 20%, were synthesized using oxidative pyrolysis of liquid-feed metallorganic precursors in a radiation-frequency (RF) thermal plasma. The valence of iron doped in the TiO(2), phase formation, defect structures, band gaps, and magnetic properties of the resultant nanopowders were systematically investigated using Mössbauer spectroscopy, XRD, Raman spectroscopy, TEM/HRTEM, UV-vis spectroscopy, and measurements of magnetic properties. The iron doped in TiO(2) was trivalent (3+) in a high-spin state as determined by the isomer shift and quadrupole splitting from the Mössbauer spectra. No other phases except anatase and rutile TiO(2) were identified in the resultant nanopowders. Interestingly, thermodynamically metastable anatase predominated in the undoped TiO(2) nanopowders, which can be explained from a kinetic point of view based on classical homogeneous nucleation theory. With iron doping, the formation of rutile was strongly promoted because rutile is more tolerant than anatase to the defects such as oxygen vacancies resulting from the substitution of Fe(3+) for Ti(4+) in TiO(2). The concentration of oxygen vacancies reached a maximum at R(Fe/Ti) = 2% above which excessive oxygen vacancies tended to concentrate. As a result of this concentration, an extended defect like crystallographic shear (CS) structure was established. With iron doping, red shift of the absorption edges occurred in addition to the d-d electron transition of iron in the visible light region. The as-prepared iron-doped TiO(2) nanopowders were paramagnetic in nature at room temperature.

N−F−codoped TiO2 (NFT) powders, prepared by spray pyrolysis (SP), were further characterized by ultraviolet−visible (UV−Vis) absorption spectroscopy and photoluminescence (PL) spectra. The UV−Vis spectra indicated that the NFT powders could absorb not only ultraviolet light like pure TiO2 powder but also part of the visible-light spectrum (λ < 550 nm). The PL spectra provided confirmation that four electronic energy states exist between the valence band and conduction band of N−F−codoped TiO2 that were attributed to F center, F+ center, an origin-unidentified energy state, and an impurity energy state formed by doped N atoms. Acetaldehyde decomposition was used as a probe reaction to evaluate the photocatalytic properties of these NFT powders. As a result, we found that the photocatalytic activity of the NFT powder prepared at the SP temperature of 1173 K was superior to that of commercial P25 under both UV and Vis irradiation. Moreover, trichloroethylene and toluene were selected as the other two target ...

Novel photocatalytic materials, yellow-colored N−F-codoped TiO2 (NFT) powders, were synthesized from a mixed aqueous solution containing TiCl4 and NH4F by spray pyrolysis (SP). The resulting powders were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), NH3-temperature-programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and N2 adsorption. FE-SEM images and N2 adsorption confirm that the NFT powders possessed spherical particles with a highly porous surface morphology. The doped N and F concentrations in the NFT powders depended significantly on the SP temperature and ranged from 0.10 to 0.26 and from 0.11 to 0.40 atomic % (at. %), respectively. NH3-TPD measurement indicates that the surface of the NFT powder was strongly acidic. Moreover, by investigating the characteristics of the TiO2 precursor hydrolysis and the thermal decomposition behaviors of the other components in the starti...