M.S. Zhang

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.

Nonlinear optical absorption in undoped and cerium-doped BaTiO3 thin films using Z-scan technique

BaTiO3 and BaTiO3:Ce thin films with good surface morphology and tetragonal perovskite structure were fabricated on quartz substrates by pulsed-laser deposition technique at a deposition temperature of 750 °C. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties of the films were investigated using Z-scan method. The films display large nonlinear absorption effects. The two-photon absorption coefficients of the BaTiO3 and BaTiO3:Ce films were determined to be 51.7 and 59.3 cm/GW, respectively. All the results show that undoped and cerium-doped BaTiO3 ferroelectric thin films have potential applications in absorbing-type optical devices.

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.

Optical properties of ferroelectric (Pb, La)(Zr, Ti)O3 thin films grown by pulsed laser deposition

Abstract Ferroelectric Pb0.95La0.05Zr0.53Ti0.47O3 (PLZT) thin films were prepared on quartz substrates by pulsed laser deposition (PLD) technique at a deposition temperature of 750°C. The crystalline structure of the film was evaluated by X-ray diffraction. The dispersion of refractive indices and optical nonlinearities of the PLZT thin films were investigated by optical transmittance and Z-scan techniques, respectively. The film exhibited strong third-order nonlinear optical effect. A nonlinear refractive index as high as 1.3×10−6 esu was displayed in the PLZT thin film. These results show that PLZT 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.

Epitaxial growth of optical waveguiding LiTaO3 films by excimer laser ablation

Waveguiding epitaxial LiTaO3 films have been prepared on sapphire substrates by the excimer laser ablation technique. The as-grown films were characterized by Raman scattering, XRD and SEM techniques, which revealed that epitaxial (001)-oriented and (012)-oriented LiTaO3 films with small surface roughness were achieved on sapphire (001) and (012) substrates, respectively. Optical waveguiding properties were demonstrated by m-line measurement of TM and TE multimodes.

Formation of copper electrodeposits on an untreated insulating substrate

We report here the electrodeposition of copper on an insulating glass substrate without introducing additives into the electrolyte and/or metallic clusters on the surface of the substrate. The deposit morphology, which varies from compact film to dense-branching patterns, can be achieved by changing the concentration and pH of the electrolyte, and the electric current in electrodeposition. The grain size of the electrodeposits is analysed for various growth conditions. We find that finer copper grains can be easily achieved at large electric current, high pH and low electrolyte concentration. We explain, using the theory of nucleation, why a metallic layer may develop horizontally over an insulating glass plate.

Manipulation of electrical properties of graphene by Fe 3 O 4 nanoparticles

Half metal magnetite (Fe3O4) is a widely studied material as it presents unique properties such as theoretically 100 % spin polarization at the Fermi level, metal to insulator transition (Verwey transition) at 120 K, ferrimagnetic nature with 858 K Curie temperature, and multiferroicity at low temperatures, all of these crucial issues for both material science and engineering[1]. Graphene, a single atomic layer of graphitic carbon, is a very promising spin channel material owing to the achievement of room-temperature spin transport with long spin diffusion lengths of several micrometers[2]. Fe3O4 nanoparticles and graphene heterostructures are very interesting in novel electrical and magnetotransport properties. However, most studies focus on enhancing Lithium storage rate[3]. The effect of electrical and transport properties of graphene by attachment/decoration of Fe3O4 nanoparticles has yet remained unknown.