F Clemente

OH-dependence of ultraviolet emission in porous silica

Abstract Photoluminescence and Raman measurements have been performed on sol–gel synthesized porous silica monoliths. Porous silica and polyethilenglycole/silica hybrids thermally treated at different temperatures have been analyzed. By exciting at 5.6 eV a structured photoluminescence emission centered at 3.7 eV is found in the whole set of samples. Hydration treatment of the pore surfaces increases the H-bonding interaction between silanols, inhibits the isolated silanols vibrations and changes the photoluminescence spectrum increasing the emission peaked at 3.7 eV.

Ultraviolet excitation fine tuning of luminescence bands of oxygen-deficient centers in silica

The photoluminescence (PL) activity of oxygen deficient centers in commercial silica samples has been reconstructed by exciting in the spectral range of their optical absorption band around 5 eV. The fine tuning of the excitation was provided by a tunable pulsed optical parametric oscillator seeded by a Nd:yttrium–aluminum–garnet laser. The experimental results, analyzed in the spectral moment framework, indicate the presence of different centers whose spectral features are modulated by structural disorder typical of the glassy matrix. The photoluminescence excitation spectra at different emission energies evidence the composite structure of the luminescence bands around 3.1 and 4.2 eV, usually ascribed to triplet and singlet transitions of the same oxygen deficient center. Besides the well-known emission bands centered at 3.15 and 4.20 eV, the contribution of a new PL band below 3 eV is reported. Time resolved photoluminescence measurements of the low energy band give an estimate of its spectral characte...

Ultraviolet excitation of photoluminescence of porous silica under vacuum conditions

Abstract The optical properties of porous sol–gel silica are investigated by measuring the absorbance spectrum in the vacuum ultraviolet energy range and the excited photoluminescence. The emission spectrum shows a luminescence with two maxima at about 3.5 eV and

Excitation pattern of the blue emission in Ge-doped silica

Abstract The luminescence of Ge-doped glassy SiO 2 has been investigated at room temperature using a deuterium lamp as the excitation source in the ultraviolet and vacuum ultraviolet energy range. Two emission bands are observed around 3 and 4 eV as the excitation increases from 4.5 to 8 eV. The peak shift and bandwidth broadening of the emission around 3 eV indicate the overlapping of several bands. We carried out a Gaussian best-fit approach of the detected spectra and extracted the excitation spectra of three distinct emission: a 2.9 eV band with two excitation channels at 4.7 and 5.3 eV and no contribution above 6 eV and the well-known composite β-band (3.1 eV). The latter is resolved into two Gaussian components whose excitation spectra present three main excitations around 5.1, 6.5 and 7.3 eV.

Surface morphology of electrochemically etched porous GaP

The mechanisms of porous gallium phosphide formation by anodic etching are studied. Gallium phosphide porous samples <100> oriented were prepared in sulfuric acid solution with different concentrations of NaF. The current-voltage characteristic curve depends on the NaF concentrations and shows the typical behavior for porous semiconductors formation. Four regions can be distinguished in the I-V curve: a pore formation zone, a dielectric layer growth, a transition region in which the two processes compete for the control of the surface morphology and finally the GaP surface electropolishing. The oxide formation shifts to higher potential and the pore formation zone is widened by adding fluorides in the etching solution. Raman spectroscopy is applied to investigate the surface morphology of samples prepared in different anodizing current density conditions and in different acid solutions. As regards the dielectric growth, the direct observation of the sample surface and the analysis of the vibrational spectra indicate that in different potential regimes two chemically different oxides can be formed.