Neutral Oxygen Monovacancy Research Articles

Photoactivated processes in optical fibers: generation and conversion mechanisms of twofold coordinated Si and Ge atoms

In this work we present an extensive investigation of nanoscale physical phenomena related to oxygen-deficient centers (ODCs) in silica and Ge-doped silica by means of first-principles calculations, including nudged-elastic band, electron paramagnetic resonance parameters calculations, and many-body perturbation theory (GW and Bethe–Salpeter equation) techniques. We show that by neutralizing positively charged oxygen monovacancies we can obtain model structures of twofold Si and Ge defects of which the calculated absorption spectra and singlet-to-triplet transitions are in excellent agreement with the experimental optical absorption and photo-luminescence data. In particular we provide an exhaustive analysis of the main exciton peaks related to the presence of twofold defects including long-range correlation effects. By calculating the reaction pathways and energy barriers necessary for the interconversion, we advance a double precursory origin of the and Ge(2) centers as due to the ionization of neutral oxygen monovacancies (Si–Si and Ge–Si dimers) and as due to the ionization of twofold Si and Ge defects. Furthermore two distinct structural conversion mechanisms are found to occur between the neutral oxygen monovacancy and the twofold Si (and Ge) atom configurations. Such conversion mechanisms allow to explain the radiation induced generation of the ODC(II) centers, their photobleaching, and also their generation during the drawing of optical fibers.

Photosensitivity Color-Center Model for Ge-Doped Silica Preforms

A new photosensitivity physical model for Ge-doped silica preforms based on color-center photoreactions is presented. Simulation results are in close agreement with experimental results obtained by several condensed matter physics research groups working in this field, suggesting that the photoreactions of this model may, indeed, describe the physical processes involved in Ge-doped silica preform photosensitivity. The proposed photosensitivity model is defined by two differential equations that describe the temporal evolution of a set of color-center concentrations. The first is a modification of a very fast reversible reaction previously proposed by Fujimaki , where the reaction precursor has a different chemical structure (it is a neutral oxygen divacancy NODV unrelated to the previously proposed germanium lone pair center GLPC). The chemical structure of this precursor defect explains the generation of nonintrinsic neutral oxygen monovacancy (NOMV) color centers. These centers are transformed into GeE <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">'</sup> defects by means of a second nonlinear reaction. This justifies the slow increase in the absorption peak experimentally measured at 6.3 eV, which had no satisfactory explanation.

Preparation of thick, crack-free germanosilicate glass films by polyvinylpyrrolidone and study of the UV-bleachable absorption band

With addition of polyvinylpyrrolidone (PVP) transparent, stable GeO 2–SiO 2 sols containing up to 60–80 mol% GeO 2 were synthesized using tetraethyl orthogermanate (TEOG) and tetraethylorthosilicate (TEOS) as precursors for germania and silica, respectively. It was shown by TEM analyses that the PVP can be absorbed onto the colloidal particles providing steric hindrance for the combination and aggregation of particles. These sols were observed showing rapid increase in viscosity within both the early period and the end period of sol aging time, but exhibiting a viscosity value of about 28 mPa s within the rest aging time (45–90 h) satisfying well the requirements for the deposition of thick films by cycles of dip-coating operation. It was determined by TG-DTA and SEM analyses that the densification of GeO 2–SiO 2 gel material with PVP was much more retarded than the gel without PVP resulting in crack-free germansilicate films with a thickness of 3 μm. The crystallization behavior of germansilicate films was enhanced with the increase of GeO 2 content but glass films with a composition of 60GeO 2 · 40SiO 2 was obtained by sintered at 700 °C for 1 h and annealed at 550 °C under a flowing H 2/N 2 atmosphere for 2 h. FT-IR analyse showed that the heat treatment at 700 °C for 60 min was effective to remove the organics and hydroxyl groups in the germansilicate film. An intense 5 eV absorption band was distinctly observed in films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy (NODV) were detected near 375 and 276 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy (NOMV). A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 256 cm −1 for the 60GeO 2 · 40SiO 2 films implying the NOMV concentration in the films reached about 3.8 × 10 18 cm −3.

Preparation and optical properties of sol–gel neodymium-doped germania/γ-glycidoxypropyltrimethoxysilane organic–inorganic hybrid thin films

Germania/γ-glycidoxypropyltrimethoxysilane organic–inorganic hybrid spin-coating thin films doped with neodymium ions are prepared by a sol–gel technique and a spin-coating process. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide are used as matrix precursors. Thermal gravimetric analysis, UV–visible spectroscopy, and Fourier transform infrared spectroscopy are used to study the structural and optical properties of the hybrid thin films. The results indicate that films that are crack-free and have a high transparency in the visible and near-infrared range can be obtained; a strong UV absorption region at short wavelength ∼200 nm, accompanied with a shoulder peaked at ∼240 nm, due to the neutral oxygen monovacancy defects, is also identified. Upconversion emission properties of the transparent dried gel and the thin films heated at different heat treatment temperatures and doped with different neodymium ion concentrations are studied; a relatively strong room-temperature yellow to violet upconversion emission at 397 nm (4D3/2→4I13/2) is observed under a xenon lamp excitation with yellow light at the wavelength of 580 nm (4I9/2→4G5/2). The effect of Nd3+ doping concentration and heat treatment temperature on upconversion emission of the thin films is also studied. The mechanism of the upconversion is proposed.

Fabrication of sol–gel 70GeO2–30SiO2 thick films from TEOG and DEOS and investigation of the 5 eV band

Crack-free germanosilicate films, 3.6 µm-thick and containing up to 70 mol% germanium dioxide, which are inaccessible through the conventional sol–gel process, were fabricated using tetraethyl orthogermanate (TEOG) and diethylorthosilicate (DEOS) as precursors for germania and silica, respectively. The studies using viscosity, TEM and SEM revealed that DEOS contributed largely to stabilizing the GeO2–SiO2 sol and suppressing crack formation in thick films. XRD study showed that the films remained amorphous after being sintered at 600 °C in air for 60 min and annealed at 550 °C under a flowing H2/N2 atmosphere for 120 min. An intense absorption band at around 241 nm was distinctly observed in the films. The intensity of this absorption band was found to be effectively bleached by UV illumination. Weak photoluminescence emission bands which originated from the neutral oxygen di-vacancy were detected near 375 and 275 nm. Therefore, the 5 eV absorption band observed in this work was mainly caused by the neutral oxygen monovacancy. A saturated absorptivity change of the UV-bleachable band after prolonged illumination was found to be 389 cm−1 for 70GeO2–30SiO2 films.

Enhanced photosensitivity in sol–gel derived 20GeO2:80SiO2 thin films

We investigate the photosensitivity of binary 20GeO2:80SiO2 (germanosilicate) inorganic films. The samples have been fabricated by the sol–gel spin-coating method and the densification has been performed by rapid thermal annealing at various temperatures ranging from 500 °C to 1000 °C. The –OH absorption bands in the Fourier-transform infrared (FTIR) spectra and the refractive-index data show that the films annealed below 900 °C are porous and the films annealed at 900 °C and above are dense. An ultraviolet (UV) KrF laser at 248 nm has been used to induce the change in the refractive index of the samples. We have achieved a large refractive-index change (Δn) of 0.0098 after UV illumination in excess of 1 min for our dense germanosilicate films. This UV-induced refractive-index change is attributed to the formation of GeE’/SiE’ centers from Ge–Ge/Si–Si (neutral oxygen monovacancy) and Ge2+ centers and to the creation of oxygen deficiency related defects. From our experiments, the oxygen deficiency related defects correspond to the absorption band at 620–740 cm-1 in the FTIR spectra and these are the defects which make a large contribution to Δn. The attenuation coefficient of the as-deposited and UV-illuminated dense samples is about 0.42 dB/cm at 1550 nm. For porous samples, UV exposure has densified the samples to some extent.

Chemical nature of point defects at the (VO)2P2O7([formula omitted]) surface

Generally, the chemistry of VPO-based catalysts is controlled by the properties of surface defects. Therefore, the effect of isolated vacancies on the reactivity of vanadyl pyrophosphate, the main component of VPO catalysts, seems to be a natural starting point for a theoretical analysis to understand better the real material. Here, we report density functional calculations of the electronic structure for different clusters modeling the defective (VO)2P2O7(100) surface, where a neutral oxygen monovacancy is incorporated. This study is complementary to our previous investigations on the energetics and the Löwdin charge analysis of the same type of defects. The electronic structure of the defective (100) surface is discussed by means of the total and atomic (partial) density of states to show modifications of the electronic states caused by the differently located vacancies. A theoretical estimation of the density of states reveals evidence for the presence of color center-induced levels for high-coordinated vacant sites and their absence in the case of low-coordinated vacancies. It is shown that only in the case of high-coordinated oxygen vacancies the band gap is reduced relative to the clean surface. Specific features of the density of states projected onto different cationic and anionic surface sites agree with experimental evidence on their implications for catalytic performance.

Germania/ormosil hybrid materials derived at low temperature for photonic applications

We report on germania/organically modified silane (ormosil) hybrid materials produced by the sol–gel technique for photonic applications. Acid-catalyzed solutions of γ-glycidoxypropyltrimethoxysilane mixed with germanium isopropoxide have been used as precursors for the hybrid materials. Planar waveguide films with a thickness of about 2 μm have been prepared by a single spin-coating process and low-temperature heat treatment from these high germanium content hybrid materials. Atomic force microscopy, thermal gravimetric analysis, UV–visible spectroscopy, and Fourier-transform infrared spectroscopy have been used to investigate the optical and structural properties of the films. The results have indicated that a dense, low absorption, and high transparency (in the visible range) waveguide film could be achieved at a low temperature. A strong UV-absorption region at short wavelengths ∼200 nm, accompanied by a shoulder peaked at ∼240 nm, has been noticed due to the neutral oxygen monovacancy defects. The propagation mode and loss properties of the planar waveguide films have also been investigated by using a prism-coupling technique.

Mechanism of interconversion among radiation-induced defects in amorphous silicon dioxide.

We here present a series of ab initio quantum-chemical calculations on clusters of atoms modeling several oxygen-deficiency-related defects in amorphous silica and illustrate how these defect centers will change their atomic configurations upon photoionization. We first give theoretical evidence that structural conversion from a neutral oxygen monovacancy to a divalent Si defect is possible, explaining the observed photoluminescence properties associated with these defects.

Photosensitivity of germanium oxide and germanosilicate glass sol–gel films

Germanium oxide and germanosilicate glass films were fabricated using a sol–gel spin coating method. The formation and photobleaching of the optical absorption band of the films in the 5-eV region were measured as a function of annealing under a reducing atmosphere and ultraviolet (UV) illumination. A greater amplitude in the 5-eV absorption band was observed with increasing annealing time due to the formation of more germanium-related oxygen vacancies in the films. Also, the photobleaching of the absorption band increased with increasing illumination time. Although the amplitude of the absorption band in 50GeO 2–50SiO 2 glass films is greater than that in germanium oxide glass films because of higher annealing temperature, germanium oxide glass films are more photosensitive due to the greater photobleachable neutral oxygen monovacancy (NOMV) concentration than non-photobleachable neutral oxygen divacancy (NODV) concentration in the films.

Difference in the behavior of oxygen deficient defects in Ge-doped silica optical fiber preforms under ArF and KrF excimer laser irradiation

Photobleaching of optical absorption bands in the 5 eV region and the creation of others at higher and lower energy have been examined in the case of ArF (6.4 eV) and KrF (5 eV) excimer laser irradiation of 3GeO2:97SiO2 glasses. We report a difference in the transformation process of the neutral oxygen monovacancy and also of the germanium lone pair center (GLPC) into electron trap centers associated with fourfold coordinated Ge ions and Ge-E′ centers when we use one or the other laser. Correlations between absorption bands and electron spin resonance signals were made after different steps of laser irradiation. It was found that the KrF laser generates twice as many Ge-E′ centers as the ArF laser for the same dose of energy delivered. The main reason for this difference is found to be the more efficient bleaching of the GLPC (5.14 eV) by the KrF laser compared to that by the ArF laser.

Thermal equilibrium of Ge-related defects in a GeO2-SiO2 glass

In a 15.7 GeO2⋅84.3 SiO2 glass prepared by a vapor axial deposition method, optical absorptions at 4.92 and 5.08 eV were observed from 300 to 600 K. The results were then compared with electron spin resonance (ESR) measurement using the same samples. The absorption at 4.92 eV was found to increase with increasing temperature, while the optical absorption at 5.08 eV decreased with increasing temperature. These changes of optical absorption indicate that the concentration of neutral oxygen monovacancy is reduced at higher temperatures. The changes were quite reversible with the temperature. Concentration of Ge E′ center, estimated from ESR, increased with increasing temperature, which is in contrast with the decrease of neutral oxygen monovacancy. The present result strongly suggests the thermal equilibrium reaction between neutral oxygen monovacancy and Ge E′ center.

Preferred concentration enhancement of photobleachable defects responsible for 5 eV optical absorption band in SiO2:GeO2 glass preform by heating in a H2 atmosphere

Heat treatment of a 5GeO2-95SiO2 glass preform in a H2 atmosphere at 500 °C for 70 h increased intensities of an absorption band centered at 5 eV by a factor of ∼3. Intensities of photobleachable component, which is the precursor of UV-induced Ge E′ centers and assigned to a neutral oxygen monovacancy, of the 5-eV band were enhanced by a factor of ∼8 by heating. This increment is ∼3 times as large as that of UV-unbleachable component, which is assigned to Ge2+ centers coordinated by two oxygens (neutral oxygen divacancy).