J.F. Pérez-Robles

Characterization of sol–gel glasses with different copper concentrations treated under oxidizing and reducing conditions

Abstract Using the sol–gel process SiO 2 coating and powdered samples doped with different concentrations of copper were prepared. Two sets of samples were made: one was thermally treated in air atmosphere, and the other, was annealed under a CO atmosphere. Both sets of samples were characterized using X-ray diffraction, optical transmission spectroscopy, photoacoustic absorption spectroscopy (PAS) in the visible and near infrared region and electron paramagnetic resonance (EPR). It is found that the chemical state of the copper depends on the type of treatment, amount of copper and the temperature of the treatment. The formations of CuO or Cu colloidal particles occur after the thermal treatments in air or in the CO atmosphere, respectively. In both cases, copper is found in the Cu 2+ state in interstitial positions and probably substituting the Si atoms in the SiO 2 matrix. The evolution of the optical absorption spectrum as function of the Cu content is a consequence of the changes in the composition and size of the colloidal particles. The optical and the PAS measurements are found to be complementary tools to elucidate the origin of the blue band in the absorption spectrum of these doped samples.

Studies in Sol-Gel Glasses with Incorporation of Cu and Fe, using PAS, X-ray Diffraction and EPR

SiO2 Glasses with incorporation of either Cu or Fe were prepared by the sol-gel process and annealed in air at temperatures within the range of 100 to 500°C. The samples were characterized using Visible Photoacoustic Absorption Spectroscopy (PAS), X-ray powder diffraction and Electron Paramagnetic Resonance (EPR). Under the annealing both systems show significant changes in composition, producing an evolution in the optical absorption spectra. In both systems, the formation of the metal oxide particles is observed after annealing at or above 300°C. The optical and EPR spectra are interpreted in terms of the coordination of the metal ions using the ligand field theory. It is observed that the color of the samples has strong change with both the type of metal incorporated and with the annealing temperature.

Stabilization of copper-based colloidal particles in sol–gel SiO2 thin films

Abstract SiO 2 amorphous thin films containing two different concentrations of copper-based colloidal particles were prepared by the sol–gel method. Thermal treatments in air at 350°C result in the segregation of cupric oxide particles. Subsequent reducing treatments in a CO atmosphere at 300 and 500°C for one hour partially reduces the cupric oxide into particles with metallic copper as the main component. The optical properties of the reduced films were characterized measuring the optical absorption spectra as a function of the elapsed time after the reduction process. The absorption data were analyzed in terms of the Mie model for colloidal systems. The fitting of this model to the experimental data provides the temporal evolution of the volume fractions of the three components of the colloidal particles, that is; Cu, Cu 2 O and CuO. It is found that the exposure to air of the films treated at 300°C under the reducing atmosphere, results in a gradual reoxidation of the colloidal particles modifying their chemical composition and therefore the optical properties of the composite film. On the other hand, the reduction treatment at 500°C results in the stabilization of the composition of the colloidal particles, producing stable films with a high optical density.

Surface Enhanced Raman Scattering of graphite on metals

Surface Enhanced Raman Scattering of graphite deposited on Al, Cu and Fe surface has been observed. Ultrathin layers of graphite consisting of only a few carbon monolayers were deposited using a chemical method. Comparing the graphite Raman intensity from these ultrathin layers with those obtained in thicker carbon samples on glass substrates deposited using the sputtering method, the enhancement intensity factor has been determined.

Effect of gelation and ball milling on the optical absorption and emission of macrilon yellow dye molecules trapped in silica prepared by the sol-gel method

Abstract Organic dye molecules (macrilon yellow) were embedded into silicon dioxide matrix by the sol–gel method. Optical measurements at room temperature of absorption and emission were performed at different steps of the gelation process, starting when the suspension was liquid and finishing when it got the solid state. Optical absorption spectra show the dependence with gelation process and with the milling process. The intensity of the absorption band at 420 nm increases with the viscosity of the suspension. This effect is more important for the milled suspension, working with the same pigment concentration. The gelation and the milling processes also affect the emission spectra. The emission spectra has two bands centered at about 520 and 560 nm. During gelation the relative intensity of the band at about 560 nm increases respect the band at about 520 nm. The emission band of the sample made with milled suspension is three times more intense than that of the not milled suspension.

Optical methods of control and characterization of materials for infrared detectors

A brief review is made of some classic and novel experimental techniques allowing the determination of the main parameters of semiconducting materials with the application of purely optical excitation. Among them some modifications of the photothermoacoustical method (in particular, with the pulse excitation), plasma reflection, sample scanning with the intense light beam.