Z. Yin

Raman scattering study on anatase TiO2 nanocrystals

Titanium dioxide (TiO2 ) nanocrystals are prepared by a hydrolysis process of tetrabutyl titanate. Nanocrystal samples with various sizes of 6.8-27.9 nm are obtained after annealing from 100 to 650 °C. The crystal structures and the average particle sizes are examined using x-ray diffraction. Raman scattering was employed to investigate the evolution of the anatase phase in the nanocrystals during annealing. Phonon confinement and non-stoichiometry effects are responsible for the blueshift and broadening of the lowest-frequency Eg Raman mode. The influence of interfacial vibrations on the Raman linewidth is also discussed.

Roles of defects and grain sizes in photoluminescence of nanocrystalline SrTiO3

Nanocrystalline SrTiO3 was prepared by a stearic acid sol-gel technique. X-ray measurements showed that the sample obtained is of good quality and the grain sizes range from 26 to 120 nm. One blue photoluminescence (PL) band at around 470 nm was observed at room temperature. Investigation of the dependence of the visible emission band on annealing temperatures and grain sizes showed that the effects of grain size and dielectric confinement play important roles, and that oxygen vacancies may act as the radiative centres responsible for the observed visible emission band. The mechanism for the luminescence is explained within the framework of self-trapped excitons.

Large third-order optical nonlinearity in SrBi2Ta2O9 thin films by pulsed laser deposition

Well-crystallized SrBi2Ta2O9 (SBT) thin films with good surface morphology were prepared on quartz substrates by the pulsed laser deposition technique at a deposition temperature of 750 °C. The third-order nonlinear optical properties of the films were measured by the Z-scan technique. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. A nonlinear refractive index as high as 1.9×10−6 esu was displayed in the SBT thin film. These results show that SBT ferroelectric thin films have potential applications in nonlinear optics.

Structural and phonon characteristics of PbxLa1−xTiO3 nanocrystals prepared by hydrothermal technique

Abstract Pb 1− x La x TiO 3 (PLT) nanocrystals of various La concentrations were prepared by hydrothermal technique. Raman spectroscopy, X-ray diffraction (XRD) and thermal gravity analysis (TGA) were used to investigate the lattice structures, phonon properties and volatile behaviors. With increasing La concentration, most of Raman modes shift downward. A soft mode E(1TO) at 82 cm −1 shifts and disappears at 60 cm −1 when the La concentration reaches 14 mol.%. This weaker softening property of the soft mode is attributed to the decrease of the tetragonality by La doping. Grain sizes of the PLT nanocrystals from 12 to 28 nm were obtained at various annealing temperatures by X-ray measurement, via the Sherrer formula. The saturation for grain growing becomes more appreciable with increased La doping, owing to more ‘pinning centers’ inside. When the La concentration is over 12 mol.%, after 1000 o C heat treatment a PbTi 3 O 7 impurity phase appears in PLT nanocrystals due to PbO volatilization. The volatile temperature for PbO from PbTiO 3 lattice decreases with increasing La concentration.

Electronic band structure of ferroelectric SrBi2Ta2O9

The electronic band structures of both the ferroelectric and paraelectric phases of SrBi2Ta2O9 (SBT) were calculated, by first-principle methods. We find that the band gap in the ferroelectric phase increases by 0.7 eV, compared with the paraelectric phase. The edges of the valence band and the conduction band are mainly derived, respectively, from Bi 6s and O 2p states, and from Ta 5d and Bi 6p states. The partial Bi–O covalency indirectly strengthens the Ta–O hybridization, favoring stabilization of the ferroelectric phase. We attribute the SBT fatigue-free property to weak pinning of its domains by the trapped centers formed by both Bi+3 at the top of the valence band and Ta+5 at the bottom of the conduction band, and the high nonlinear refractive index in SBT to a large contribution from Ta 5d states.

Phase character and structural anomaly of Bi4Ti3O12 nanoparticles prepared by chemical coprecipitation

Abstract Nanoparticles of Bi4Ti3O12 have been prepared by chemical coprecipitation. The phase structure was investigated by X-ray diffraction (XRD) and refined using the Rietveld method. Orthorhombic Bi4Ti3O12 particles were obtained after heating at 700 and 800 °C; however, for the particles calcined at 500–650 °C, the results suggested the presence of a high-temperature tetragonal phase at room temperature. This structural anomaly was further confirmed by differential thermal analysis (DTA).

Blue-Light Emission from Nanocrystalline La0.5Na0.5TiO3

Nanocrystalline La0.5Na0.5TiO3 powders were prepared by the stearic acid-gel method, and the grain sizes could be controlled by varying annealing temperatures. The sizes were determined to be from 14 to 50 nm using Scherrers formula. A very strong blue-light emission band around 480 nm was clearly observed in the nano-sized La0.5Na0.5TiO3 with a grain size of 14 nm at room temperature. Also, nano-size effects on the luminescence properties were investigated. These results could be explained with the aid of a self-trapped exciton model.