Phase Transformation of Metastable Cu2ZnGeO4 with a Wurtz-Kesterite Structure at Elevated Temperatures.

The thermal stability and high-temperature phase transformation of metastable Cu2ZnGeO4 were investigated in an Ar atmosphere by thermogravimetry, differential thermal analysis, and high-temperature X-ray diffraction. Three Cu-deficient CuI2-xZnGeO4-2/x phases with a wurtzite-related structure were observed, with varying amounts of copper deficiency. The metastable Cu2ZnGeO4 was stable at approximately 275 °C and transformed into intermediate phases. The intermediate phases had a wurtz-kesterite structure with a small number of copper and oxygen vacancies, which later transformed into a high-temperature phase at approximately 425 °C. The crystal structure of the high-temperature phase was assumed to be a deficient wurtzite-related structure with hexagonal closely packed oxygen and deficient copper sites on the order of tens of a percent. The high-temperature phase decomposed into stable Cu2O, GeO2, and Zn2GeO4 phases above 550 °C. The mechanism for the formation of the phase with a large amount of copper deficiency is discussed, leading to an understanding of the formation process for the copper-deficient phase of complex compounds containing monovalent copper.

Wurtzite-Derived Quaternary Oxide Semiconductor Cu2ZnGeO4: Its Structural Characteristics, Optical Properties, and Electronic Structure.

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 ß-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.

Formation of Amorphous H3Zr2Si2PO12 by Electrochemical Substitution of Sodium Ions in Na3Zr2Si2PO12 with Protons.

The sodium ions in Na3Zr2Si2PO12 (NASICON) were substituted with protons using an electrochemical alkali-proton substitution (APS) technique at 400 °C under a 5% H2/95% N2 atmosphere. The sodium ions in NASICON were successfully substituted with protons to a depth of <400 µm from the anode. Completely protonated NASICON, i.e., H3Zr2Si2PO12, was obtained to a depth <40 µm from the anode, although complete protonation of NASICON cannot be achieved by ion exchange in aqueous acid. H3Zr2Si2PO12 was amorphous, whereas the partially protonated NASICON was crystalline, and its unit cell volume decreased with an increase in the extent of substitution. Amorphous H3Zr2Si2PO12 was prepared by pressure-induced amorphization of the NASICON framework, in which an internal pressure of ∼3.5 GPa was induced by the substitution of large sodium ions with small protons during APS at 400 °C.

First-Principles Study of CuGaO2 Polymorphs: Delafossite α-CuGaO2 and Wurtzite ß-CuGaO2.

The electronic structures of delafossite α-CuGaO2 and wurtzite ß-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 ß-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 ß-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 ß-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.

Structural and thermal properties of ternary narrow-gap oxide semiconductor; wurtzite-derived ß-CuGaO2.

The crystal structure of the wurtzite-derived ß-CuGaO2 was refined by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Its structural characteristics are discussed in comparison with the other I-III-VI2 and II-VI oxide semiconductors. The cation and oxygen tetrahedral distortions of the ß-CuGaO2 from an ideal wurtzite structure are small. The direct band-gap nature of the ß-CuGaO2, unlike ß-Ag(Ga,Al)O2, was explained by small cation and oxygen tetrahedral distortions. In terms of the thermal stability, the ß-CuGaO2 irreversibly transforms into delafossite α-CuGaO2 at >460 °C in an Ar atmosphere. The transformation enthalpy was approximately -32 kJ mol(-1), from differential scanning calorimetry. This value is close to the transformation enthalpy of CoO from the metastable zincblende form to the stable rock-salt form. The monovalent copper in ß-CuGaO2 was oxidized to divalent copper in an oxygen atmosphere and transformed into a mixture of CuGa2O4 spinel and CuO at temperatures >350 °C. These thermal properties indicate that ß-CuGaO2 is stable at ≤300 °C in both reducing and oxidizing atmospheres while in its metastable form. Consequently, this material could be of use in optoelectronic devices that do not exceed 300 °C.

Wurtzite-derived ternary I-III-O2 semiconductors.

Ternary zincblende-derived I-III-VI2 chalcogenide and II-IV-V2 pnictide semiconductors have been widely studied and some have been put to practical use. In contrast to the extensive research on these semiconductors, previous studies into ternary I-III-O2 oxide semiconductors with a wurtzite-derived ß-NaFeO2 structure are limited. Wurtzite-derived ß-LiGaO2 and ß-AgGaO2 form alloys with ZnO and the band gap of ZnO can be controlled to include the visible and ultraviolet regions. ß-CuGaO2, which has a direct band gap of 1.47 eV, has been proposed for use as a light absorber in thin film solar cells. These ternary oxides may thus allow new applications for oxide semiconductors. However, information about wurtzite-derived ternary I-III-O2 semiconductors is still limited. In this paper we review previous studies on ß-LiGaO2, ß-AgGaO2 and ß-CuGaO2 to determine guiding principles for the development of wurtzite-derived I-III-O2 semiconductors.

Wurtzite CuGaO2: a new direct and narrow band gap oxide semiconductor applicable as a solar cell absorber.

An oxide semiconductor ß-CuGaO2 with a wurtzite-derived ß-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 ß-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.

Enhanced Valley Splitting of Exciton Emission in Colloidal PbSe Quantum Dots When the Interdot Distance Coincides with Onset of Förster Resonance Energy Transfer.

Photoluminescence (PL) emission of colloidal PbSe/CdSe core/shell quantum dots (QDs, CdSe shell thickness: 0.2 nm) at the lowest exciton state was investigated at room temperature and varying inter-QD distance (L = 7-240 nm) by changing the QD concentration. A distinct enhancement of the valley splitting of PbSe QDs was observed upon reducing L. Simultaneously, there was a redshift in the emission due to Förster resonance energy transfer (FRET), when the L value was still sufficiently large (7 nm ≤ L ≤ 50 nm) so that the wave functions of different QDs do not overlap. The enhanced valley splitting under no apparent external field is quite interesting as a method to control the valley splitting. The electronic coupling leading to FRET may enhance the valley splitting, because it occurs in an identical range of L.

Colloidal Zn(Te,Se)/ZnS Core/Shell Quantum Dots Exhibiting Narrow-Band and Green Photoluminescence.

Colloidal CdSe quantum dot (QD) phosphors have attracted considerable attention as green and red phosphors for blue backlight downconversion in next-generation liquid-crystal displays because of their excellent emission features including tunable emission wavelength and narrow emission bands. Alternatives to CdSe, which do not contain toxic cadmium, are strongly desired to ensure safety and reduce the environmental load of consumer products. Herein, we synthesized colloidal Zn(Te,Se)/ZnS core/shell QDs and demonstrated narrow-band green photoluminescence (PL) emission. A full width at half-maximum of 30 nm was achieved for PL emission at 535 nm from Zn(Te0.77Se0.23)/ZnS core/shell QDs with a core QD diameter of 4.3 nm. This emission characteristic was as good as that of CdSe QDs.