Showing papers by "Anabela G. Rolo published in 2007"

Raman spectroscopy of optical phonons confined in semiconductor quantum dots and nanocrystals

Abstract: This paper gives an overview of theoretical and experimental results on Raman-active optical phonon modes confined in nearly spherical nanocrystals (NCs) of polar semiconductor materials with cubic crystal structure. A systematic consideration of the effect of reduced dimensionality on both electrons and phonons is presented. The theoretical approach is based on a continuum lattice dynamics model and the effective mass approximation for electronic states in the NCs. Calculated resonant and non-resonant Raman scattering spectra are compared to experimental data stressing the effects of phonon confinement, type of excitation, polaron correction to the exciton spectrum enhanced because of the reduced dimensionality, and individual features of the NC material. Copyright © 2007 John Wiley & Sons, Ltd.

X-ray diffraction and Raman study of nanogranular BaTiO3–CoFe2O4 thin films deposited by laser ablation on Si/Pt substrates

Abstract: Nanocomposite thin films composed by (BaTi03) 1-x -(CoFe 2 O 4 ) x with different cobalt ferrite concentrations (x) have been deposited by pulsed laser ablation on platinum covered Si(001) substrates. The films structure was studied by X-ray diffraction and Raman spectroscopy. It was found that the CoFe 2 O 4 phase unit cell was compressed along the growth direction of the films, and it relaxed with increasing x. The opposite behavior was observed in the BaTiO 3 phase where the lattice parameters obtained from the X-ray measurements presented a progressive distortion of its unit cell with increasing x. The presence of the strain in the films induced a blueshift of the Raman peaks of CoFe 2 O 4 that decreased with increasing CoFe 2 O 4 concentration. Cation disorder in the cobalt ferrite was observed for lower x, where the nanograins are more isolated and subjected to more strain, which was progressively decreased for higher CoFe 2 O 4 content in the films.

Nanogranular BaTiO3–CoFe2O4 thin films deposited by pulsed laser ablation

Abstract: Detailed structural and magnetic measurements were performed on nanostructured composite thin films of cobalt ferrite (CoFe2O4-magnetostrictive) dispersed in a barium titanate (BaTiO3-piezoelectric) matrix, with different CoFe2O4 concentrations (ranging from x=20% to x=70%). The films were deposited by laser ablation on platinum covered Si(100). Their structure was studied by x-ray diffraction and Raman spectroscopy. The magnetic properties were measured with a superconducting quantum inteference device magnetometer. The nanocomposite films were polycrystalline and composed by a mixture of tetragonal-BaTiO3 and CoFe2O4 with the cubic spinel structure. The lattice parameter of the CoFe2O4 phase varied from 8.26A (x=20%) to 8.35A (x=70%), and, compared with bulk CoFe2O4, it was under compressive stress that relaxed as its concentration progressively increased. In the tetragonal-BaTiO3 phase, the lattice parameter a was contracted relative to the bulk phase and decreases with x. The lattice parameter c incre...