Interband optical transition strengths in SiO2, GeO2, SnO2 and TiO2

An analysis of interband optical transition strengths for a series of MO₂ oxides is presented which gives clear evidence for a direct proportionality between transition strengths (as measured by the dispersion energy) and nearest-neighbor cation coordination number. It is also found that neither the extent of O2p orbital delocalization nor the d-electron configuration has a strong influence on transition strengths suggestin that these strengths can be viewed, to a good approximation, as intrinsic properties of valence s,p-electrons which are largely independent of band structure details.     

Raman study of rutile (TiO2) at high pressures

The Raman spectra and polymorphism of rutile have been investigated under hydrostatic pressures up to 90 kbar at room temperature. A transition previously observed in rutile at 30 kbar in a Drickamer-type cell under nonhydrostatic conditions was observed to begin at approximately 70 kbar in a 4:1 mixture of methanol and ethanol. The small amount (10–20%) of the high-pressure phase synthesized from rutile, however, did not increase even though the sample was left under pressure for a period of 1 month at ambient temperature. On the basis of factor group analysis, in situ powder x-ray diffraction data, and comparison of the Raman spectrum of the high-pressure modification with that of TiO₂-II (α-PbO₂-type structure synthesized from anatase powder at 40 kbar and 400°C), it is evident that a high pressures rutile transforms irreversibly to TiO₂-II.     

Electronic energy band calculation in β-PbO2 comparison with SnO2 and GeO2 energy band structures

The energy band structure of β-PbO₂ is determined semi-empirically by the Kohn-Korringa-Rostoker method. It is believed to be the first PbO₂ calculated band structure that appears in literature. The direct band gap value that is obtained directly from this method is 2.7eV. It is lowered to the value of 1.6 eV which seems to be the most likely value of β-PbO₂ energy gaps that were already obtained. This energy band structure is compared with those of other oxides whose lattices belong to the same space group (D¹⁴_4h).     

Raman spectra of CrO2 and MoO2 single Crystals

The Raman spectra of single crystals of CrO₂ and MoO₂ have been studied from 77°K to 500°K for a comparison with the heavily damped phonon features in metallic VO₂. The spectrum of MoO₂ is very similar to that of monoclinic VO₂ whereas CrO₂ exhibits phonon spectra similar to rutile with no appreciable change at the ferromagnetic c curie temperature. The results are discussed with respect to the various models proposed for the semiconductor-to-metal transition in VO₂.     

The influence of interfacial barrier engineering on the resistance switching of In2O3：SnO2/TiO2/In2O3：SnO2 device

The I-V characteristics of In2O3：SnO2/TiO2/In2O3：SnO2 junctions with different interracial barriers are inves- tigated by comparing experiments. A two-step resistance switching process is found for samples with two interfacial barriers produced by specific thermal treatment on the interfaces. The nonsynchronous occurrence of conducting filament formation through the oxide bulk and the reduction in the interracial barrier due to the migration of oxygen vacancies under the electric field is supposed to explain the two-step resistive switching process. The unique switching properties of the device, based on interracial barrier engineering, could be exploited for novel applications in nonvolatile memory devices.     

Infrared and Raman spectra of MnS2

The infrared and Raman spectra of the pyrite-type crystal MnS₂ are reported. Nine of the ten optically allowed $\text{q≌0}$ phonons are observed. A comparison of the phonon frequencies with those of FeS₂ allows us to conclude that the electrostatic forces as well as the covalent sulfur-sulfur bonds are about the same strength in the two compounds. The short-range forces between metal and sulfur ions in the two cases, however, are quite different.     

Raman and far-infrared spectra of NaCrS2

The Raman and far-infrared spectra of the layer structure compound NaCrS₂ have been investigated. Two Raman active modes have been identified and their observed symmetries are in accord with the R$\text{3}$m space group symmetry of NaCrS₂. Three of the four predicted infrared active modes have been observed. One of the infrared modes appears as a strong reststrahlen band indicating that the bonding in NaCrS₂ is partially ionic.     

Effect of pressure on Raman spectra of SnS2 single crystals

The 2H polytype of a SnS₂ layered crystal has been studied using Raman spectroscopy at pressures of up to 5 GPa in a diamond anvil cell. The Raman frequency of the intralayer mode increases linearly with increasing pressure at baric coefficients of 5.2 cm⁻¹/GPa for P<3 GPa and 3.4 cm⁻¹/GPa for P>3 GPa. This change in the baric coefficient for Raman scattering and the available data on X-ray measurements of the compressibility of 2H-SnS₂up to 10 GPa suggest that the crystal structure undergoes a transformation at about 3 GPa.     

Structural phase transitions in brookite-type TiO2 under high pressure

We have studied polycrystalline brookite TiO₂ using energy-dispersive X-ray diffraction at pressures up to 27.8 GPa and derived an ambient-pressure bulk modulus of 255 GPa using Birch-Murnaghan's equations of state with a fixed value of 4 as its first derivative. The transition from brookite-type to baddeleyite-type was observed to start at 15.8 GPa and finished at 22.8 GPa. Upon decompression, the α-PbO₂ structure appeared at 3.5 GPa and the baddeleyite-type structure remained down to 1.6 GPa, the lowest pressure in the present work.     

Infrared and Raman spectra of GeSe2

Far infrared room temperature reflectivity spectra have been measured with polarized light for single crystal GeSe₂ in the range 40–400 cm^-1 for all three principal directions. Altogether 15 “Reststrahlen” peaks were observed, seven for E∥a, six for E∥b and only two for E∥c. Optical parameters were calculated using both Kramers-Krönig integration and a fitting procedure. A nonpolarized Raman spectrum of GeSe₂ was also measured. 17 Raman active lines were observed. The strongest one was at 213cm^-1.     

Raman and infrared spectra of ZnSiAs2

The optical phonons at k = 0 of ZnSiAs₂ have been investigated by Raman scattering and infrared reflectivity measurements at 300 K. Eleven of thirteen expected optically active phonons have been observed and identified with respect to their symmetry types. The phonon frequencies appear in the range from 415 cm^-1 to 75 cm^-1 with predominant polar modes at 400 cm^-1 (gG₅), 389 cm^-1 (Γ₄) and 242 cm^-1 (Γ₄). The dielectric dispersion for E ⊥ c and E 6 c has been determined by Kramers-Kronig integrations.     

Raman spectroscopic and photoluminescence study of single-crystalline SnO2 nanowires

Single-crystalline SnO₂ nanowires with sizes of 4-14 nm in diameter and 100-500 nm in length were produced in a molten salt approach by using hydrothermal synthesized precursor. Structural characters of the nanowires were examined by X-ray diffraction and high-resolution electron transmission microscopy. Raman, photoluminescence and X-ray photoelectron spectra of the samples were examined under heat treatments. Three new Raman modes at 691, 514 and 358 cm⁻¹ were recorded and assigned. The former two are attributed to activation of original Raman-forbidden A_2uLO mode and the third is attributed to defects in small-sized nanowires. A strong photoluminescence is observed at about 600 nm, the temperature effects is examined and the origin of the PL process is discussed via X-ray photoelectron spectra.     

Raman spectra of K2CuF4

The crystal structure of K₂CuF₄ has been studied by means of Raman scattering. The structure is found to belong to the space group D_4h⁵ rather than D_4h¹⁷ of K₂NiF₄ between 2 and 800 K because of the distortion arising from the “orbital ordering effect”.     

Impurity transitions in the photoluminescence spectra of SnO2

The photoluminescence of SnO₂, a semiconductor with direct, dipole-forbidden gap, is investigated as a function of the lattice temperature and the excitation intensity. The new emission lines observed could be partly interpreted as donor-acceptor-pair recombination and as band-acceptor transition.     

SnO2/TiO2 mixed oxide particles synthesized in doped premixed H2/O2/Ar flames

SnO₂/TiO₂ mixed oxides with primary particle size ranging between 5 nm d_p 12 nm were synthesized by doping a H₂/O₂/Ar flame with Sn(CH₃)₄ and Ti(OC₃H₇)₄ co-currently. The effects of “flow coordinate,” concentration and flame configurations were investigated with respect to particle size and morphology of the generated mixed oxides. In situ characterization of the mixed oxides was performed using the particle mass spectrometer (PMS), while XRD, TEM, BET and UV–Vis were performed ex situ. Results obtained showed that primary particle size of mixed oxides can be controlled by varying experimental parameters. The mixed oxides have interesting properties compared to those of the pure oxides of TiO₂ and SnO₂, which were also synthesized in flames earlier. Band gap tuning opportunities are possible using mixed oxides.     

Two-photon spectroscopy of SnO2 under hydrostatic pressure

The pressure shift of S excitons in the rutile-type semiconductor tin oxide (SnO₂) is measured by two-photon absorption. From these data the pressure coefficients of the band gap (62.0 meV/GPa) and of the exciton binding energy (0.87 meV/GPa) are determined.     

Substrate effects on the oxygen gas sensing properties of SnO2/TiO2 thin films

In this study, SnO₂/TiO₂ thin films are fabricated on SiO₂/Si and Corning glass 1737 substrates using a R.F. magnetron sputtering process. The gas sensing properties of these films under an oxygen atmosphere with and without UV irradiation are carefully examined. The surface structure, morphology, optical transmission characteristics, and chemical compositions of the films are analyzed by atomic force microscopy, scanning electron microscopy and PL spectrometry. It is found that the oxygen sensitivity of the films deposited on Corning glass 1737 substrates is significantly lower than that of the films grown on SiO₂/Si substrates. Therefore, the results suggest that SiO₂/Si is an appropriate substrate material for oxygen gas sensors fabricated using thin SnO₂/TiO₂ films.     

Raman spectroscopic analysis of sulphur-doped TiO2 by co-grinding with TiS2

Doping sulphur into titania has been tried using TiS₂ as a doper based on the mechanically induced solid-state reaction between TiO₂ and TiS₂. The prepared samples have been characterized by X-ray diffraction (XRD), Raman spectroscopy and UV-Vis reflectance spectroscopy. Raman analysis, particularly has been proved to be effective in assessing the sulphur doping by correlating the oxygen deficiency of the doped oxide with the change of active Eg mode of rutile phase.     

不同晶粒尺寸SnO2纳米粒子的拉曼光谱研究

对晶粒尺寸在4—80nm范围的纯SnO₂纳米颗粒进行了拉曼散射研究.除了SnO₂本征拉曼振动峰外，还有几个新的拉曼振动峰和波长在700nm左右的一个发光很强而且峰宽很大的荧光峰被观察到.结果所示，当纳米颗粒尺寸减小时，纳米SnO₂颗粒的体相 特征拉曼峰变弱，而由缺陷，表面和颗粒尺寸引起的相关效应呈强势.晶粒尺寸在20nm左右是引起体相拉曼光谱变化的临界尺寸.晶粒尺寸在20nm以下，其体相拉曼峰的发生宽化和峰位移动，以及分别出现在位于571cm^-1 的表面振动峰，位于351cm-1 处的界面峰和与表面吸附水分子及氢氧基团的N系列拉曼峰是纳米SnO颗粒的主要特征.这些结果反映了纳米颗粒的微结构变化与颗粒尺寸和表面效应以及它们之间相互作用的信息. 关键词： ２')" href="#">纳米SnO_２ 拉曼光谱 荧光光谱 水分子的吸附     

The influence of hydrostatic pressure on Raman spectra of TIS and TlInSe2

First order Raman spectra of TIS and TlInSe₂ single crystals excited with 1.064μm line of the continuously operated YAG: Nd³⁺ laser have been investigated in equilibrium conditions under various hydrostatic pressures up to 1.08 × 10⁹ and 7.06 × 10⁸ Pa, respectively. Mode parameters γ_j = (1/ν_j)(dν_j/dP) were determined for all the Raman bands observed. Comparison of a set of these parameters in both crystals showed that the character of binding interatomic forces in these crystals appeared to be similar. For both crystals the intensity of Raman bands decreased with increasing the pressure.