Luis A. O. Nunes

Green photoluminescence from Er-containing amorphous SiN thin films

Green light emission at room temperature was achieved from nonhydrogenated amorphous silicon–nitrogen (a-SiN) thin films. The films were deposited by cosputtering a silicon target covered with metallic erbium platelets in an Ar+N2 atmosphere. According to the deposition conditions, the nitrogen concentration [N] reached ∼40 at. % rendering an optical gap of approximately 3.5 eV while the Er concentration [Er] was estimated to be ∼10 at. % in the present films. The high [Er] associated to the optical band gap allows the direct excitation of Er3+ ions. This optical excitation is more efficient at low temperatures as a consequence of the reduction in nonradiative processes, and when exciting the samples with the 488.0 nm line of an Ar+ laser which is in resonance with the 4F7/2Er3+ energy level. In addition to light emission at ∼520 and ∼545 nm, transitions in the infrared energy region could be easily verified in as-deposited samples.

Erbium luminescence from hydrogenated amorphous silicon-erbium prepared by cosputtering

Hydrogenated amorphous silicon with small amounts of erbium (Er/Si concentration ∼5 at. %) was prepared by radio frequency sputtering from a Si target partially covered by tiny metallic Er chunks. Four sets of samples were studied: nonintentionally contaminated hydrogenated and nonhydrogenated amorphous silicon-erbium (a-SiEr:H and a-SiEr); nitrogen doped a-SiEr(N):H and oxygen contaminated a-SiEr(O):H. Samples from the first two sets present only faint 1.54 μm photoluminescence characteristic from Er3+ ions even at 77 K. Samples from the other sets show this luminescence at 77 K as deposited, without any further annealing step. Thermal annealing up to 500 °C increases the photoluminescence intensity, and room temperature emissions become strong enough to be easily detected. These results indicate that in an amorphous silicon environment the chemical neighborhood of the Er3+ ions is crucial for efficient 1.54 μm emission. Raman scattering from both as-deposited and annealed samples showed that network dis...

Optical spectroscopy of Er3+ and Yb3+ co-doped fluoroindate glasses

Optical absorption, Stokes, and anti-Stokes photoluminescence were performed on Er3+–Yb3+ co-doped fluoroindate glasses. For compounds prepared with a fixed 2 mol % ErF3 concentration and YbF3 contents ranging from 0 to 8 mol %, important upconversion processes were observed as a function of temperature and photon excitation energy. Based on the experimental data, two mechanisms for the upconversion (or anti-Stokes photoluminescence) processes were identified and analyzed in detail. At high Yb contents, the upconversion mechanisms are mostly determined by the population of the 2F5/2 levels of Yb3+ ions (or 4I11/2 levels of Er3+ ions, by energy transfer) regardless of the photon excitation energy and temperature of measurement. Moreover, green and red light emission have similar intensities when a large Yb3+ content is present.

Eu3+ and Gd3+ spectroscopy in fluoroindate glasses

Luminescence data for Eu3+ and Gd3+ in fluoroindate glasses are compared to those of a fluorozirconate glass. Emission is observed from Eu3+5DJ (J=0, 1, 2 and 3) and Gd3+6P12 excited-state levels and the results put in evidence Eu-Eu and Gd-Eu energy transfer processes. Vibronic bands related to a 320 cm−1 vibrational mode could be observed for Eu3+ luminescent transitions with ΔJ=0,1 and 2 and also for the 6P72→8S72 transition of Gd3+. Lanthanide ion site symmetry is closer to an inversion center in fluoroindate glasses than it is in fluorozirconate.

Optical properties of pure, Nd3+-doped and Pr3+-doped fluoroindate glasses

Abstract Fluoroindate glasses doped with Pr3+ or Nd3+ are promising materials for the development of fluoride fibers, amplifiers and lasers. The optical properties of an InF3SrF2ZnF2GdF3NaF composition pure and doped up to 3 mol% have been systematically studied. The extended UV transmission of this glass allows the identification of most of the 8 S → 6 P and 8 S → 6 I transitions of Gd ions. The optical transitions of Pr3+ and Nd3+ have been identified in absorption between 250 and 10 000 nm. Fluorescence and excitation spectra and lifetime have been measured at 300 K. Most of the transitions have been identified.

V2O5/TiO2 catalyst xerogels: Method of preparation and characterization

This work describes a modified sol-gel method for the preparation of V2O5/TiO2 catalysts. The samples have been characterized by N2 adsorption at 77 K, X-ray Diffractometry (XRD), Scanning Electronic Microscopy (SEM/EDX) and Fourier Transform Infrared Spectroscopy (FT-IR). The surface area increases with the vanadia loading from 24 m2 g−1 for pure TiO2 to 87 m2 g−1 for 9 wt% of V2O5. The rutile form is predominant for pure TiO2 but becomes enriched with anatase phase when vanadia loading is increased. No crystalline V2O5 phase was observed in the diffractograms of the catalysts. Analysis by SEM showed heterogeneous granulation of particles with high vanadium dispersion. Two species of surface vanadium were observed by FT-IR spectroscopy: a monomeric vanadyl and polymeric vanadates. The vanadyl/vanadate ratio remains practically constant. Ethanol oxidation was used as a catalytic test in a temperature range from 350 to 560 K. The catalytic activity starts around 380 K. For the sample with 9 wt% of vanadia, the conversion of ethanol into acetaldehyde as the main product was approximately 90% at 473 K.

Blue and green upconversion in Er3+-doped fluoroindate glasses

Abstract The upconversion properties of Er3+ in fluoroindate bulk glasses (composition: 40InF3−20ZnF2−16BaF2−(20−x)SrF2−2GaF3−2NaF−xErF3 with x=1,2,3 and 4 mole %) are investigated, following 4 F 9 2 excitation with a red krypton laser. A strong green and a weaker blue luminescence is observed at room temperature corresponding to emissions from the thermally coupled 4 S 3 2 and 2 H 11 2 bands and the 2 H 9 2 level respectively. Resonant energy transfer processes involving two excited erbium ions and a two-step absorption process are proposed to explain the upconversion phenomena. The emission intensities depend on the excitation power as Pexcn with 1.5≤n≤1.7 for the green and 1.6≤n≤1.9 for the blue emission. The decay times and the relative intensities of the luminescence are also studied as a function of Er3+ concentration.

V2O5/TiO2 Catalytic Xerogels Raman and EPR Studies

Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V2O5/TiO2 catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm−1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm−1 and 770 to 850 cm−1. The Raman spectra do not exhibit characteristic peaks of crystalline V2O5. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V4+ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V4+ ions are located in sites with octahedral symmetry substituting for Ti4+ ions in the rutile structure. Magnetically interacting V4+ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V2O5, a partial oxidation of V4+ to V5+ is apparent from the EPR results.

INFRARED SPECTROSCOPY OF ER-CONTAINING AMORPHOUS SILICON THIN FILMS

Hydrogenated amorphous SiEr (a-SiEr:H) thin films were deposited by cosputtering. Oxygen and nitrogen were employed as impurity enhancers of Er3+ emission of light at 1540 nm, and photoluminescence, infrared absorption, and Raman spectroscopies were performed as a function of various annealing temperatures. As-deposited O-contaminated and N-doped a-SiEr:H samples exhibit Er3+ related photoluminescence, low intensity at room temperature, and maximum intensity after thermal annealing at ∼500 °C. In addition to the enhancement of the Er3+ emission of light, thermal annealing provokes the outdiffusion of hydrogen bonded to silicon atoms. The experimental data suggest that both hydrogen and thermal treatments improve the Er3+ related photoluminescence by decreasing the occurrence of nonradiative processes.

CARACTERIZAÇÃO TEXTURAL E ESTRUTURAL DE V 2 O 5 /TiO 2 OBTIDOS VIA SOL-GEL: COMPARAÇÃO ENTRE SECAGEM CONVENCIONAL E SUPERCRÍTICA

This work describes a modified sol-gel method for the preparation of V2O5/TiO2 catalysts. The samples have been characterized by N2 adsorption at 77K, x-ray diffractometry (XRD) and Fourier Transform Infrared (FT-IR). The surface area increases with the vanadia loading from 24 m2 g-1, for pure TiO2, to 87 m2 g-1 for 9wt.% of V2O5. The rutile form is predominant for pure TiO2 but became enriched with anatase phase when vanadia loading is increased. No crystalline V2O5 phase was observed in the catalysts diffractograms. Two species of surface vanadium observed by FT-IR spectroscopy a monomeric vanadyl and polymeric vanadates, the vanadyl/vanadate ratio remains practically constant.