Non-bridging Oxygen Hole Centers Research Articles

Mechanochemical immobilization of lead smelting slag using quartz sand

The lead smelting industry has produced a large amount of heavy metal-containing solid wastes, and lead leaching from these wastes under acidic conditions poses a great threat to the environment. In this study, lead slag was effectively stabilized by a mechanochemical reaction through a co-grinding operation with quartz sand (SiO2) in a planetary ball mill. The synthetic precipitation leaching procedure (SPLP) was used to examine the stabilization efficiency of lead slag by ball milling, and the results demonstrated that the lead immobilization for smelting slag with initial lead leaching concentration of 1503 mg/L could reach 99.99% under the conditions of 1000 rpm and 35:1 charge ratio when the ball milling time was 2 h, which was under the permitted concentration of lead according to the Chinese standards for hazardous wastes (<5 mg/L, GB5085.3-2007). Kinetic studies found that the rotation speed of the ball mill and the charge ratio between balls and powder were two key factors affecting stabilization efficiency. The stabilization rate increased with the rotation speed of the ball mill and the charge ratio, and fitted well with the Delogu Model. X-ray diffraction (XRD) characterization revealed that the characteristic peaks for both PbO and PbS disappeared after long-time grinding with SiO2. Mechanistic studies suggested that É centers (≡Si·) and non-bridging oxygen hole center radicals were generated due to the mechanical energy from ball milling with SiO2, which provide the possibility for the formation of Si–O–Pb and could effectively stabilize Pb species in the lead slag.

γ-Irradiation effects in borosilicate glass studied by EPR and UV–Vis spectroscopies

Assessing the behavior of borosilicate glass used for nuclear waste immobilization under irradiation is essential for determining the long-term behavior of the high-level waste. In this work, the γ irradiation induced structural properties of the borosilicate glass have been studied via ultraviolet–visible (UV–Vis) spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. The optical band gap (Eg) and the Urbach energy (Eu) deduced from the UV–Vis spectra of borosilicate glasses. The Eg decreased and the Eu increased with the increase of absorbed dose, which indicated the increase of non-bridging oxygen and disorder in the glass network. The EPR results revealed that there formed the following paramagnetic defects: hole center associated with boron (BOHC), hole centers near alkaline ions (HC1 center), E′ centers (Si), peroxy radical (Oxy defect), non-bridging oxygen hole centers (NBOHC), BO3− center, Si − BOHC (BOSi) and B–O–B–iso center (HOHOBOH-O·-BOHOH).

Unique silica polymorph obtained under electron irradiation

High purity synthetic silica glass (Suprasil F300) samples were densified by High Pressure/High Temperature (HP-HT) using three different pressure/temperature/duration values. Their relaxation process was studied by applying 2.5 MeV electron irradiation with doses varying from 1 MGy up to 11 GGy. At very high dose (11 GGy), all the densified silica samples exhibit the same density value (∼2.26 g/cm3) regardless of the densification conditions, referring to an equilibrium value known as an amorphous silica metamict phase. In detail, the HP-HT silica samples were progressively dedensified from 1 GGy irradiation dose until reaching this equilibrium. Whatever the initial topological disorder revealed through Raman spectra, all silica samples present identical Raman spectra at 11 GGy, with high intensities of D1 and D2 components, attesting they reach one unique silica polymorph. Regarding the irradiation at high temperature (600 MGy, 600 K), the results show that the thermal annealing during irradiation “accelerates” somehow the relaxation kinetics of both macroscopic density and vibrational signature. Furthermore, it seems that such high irradiation temperatures lead to increase the Non-Bridging Oxygen Hole Center point defects generation.

Influence of N-Methylaniline on physicochemical and optical properties of γ-Al2O3 nanoparticles

Influence of N-Methylaniline (NMA) on the physicochemical and optical properties of γ-Al2O3 nanoparticles were investigated using XRD, FTIR, UV–vis DRS, TEM, AFM, PL, TGA and the results were reported here. The TEM results were showed that the surface morphology of NMA capped γ-Al2O3 nanoparticles has sphere like nanostructures. The size of the nanoparticles was controlled by the concentration of NMA and thus modified the geometry of γ-Al2O3 nanoparticles from primary (∼8 nm) to spherical nanoparticles (∼30-50 nm). XRD results were confirmed the enhancement of crystallite size of γ-Al2O3 proportionally with the NMA molar concentration. FTIR results were confirmed the formation of NMA capped γ-Al2O3 nanospheres by N-H stretching and O-H bending vibrations. The PL results were indicated that, the emission intensities at 589 nm (yellow emission) and 713 nm (orange-red emission) were reduced after the capping of NMA for the defects center and non-bridging oxygen hole center (NBOHC).

Orange Persistent Luminescence and Photodarkening Related to Paramagnetic Defects of Nondoped CaO-Ga2O3-GeO2 Glass

Orange persistent luminescence and photodarkening phenomena were observed after ultraviolet (UV) excitation in calcium-gallium-germanate (CGG) glass with a composition of 3CaO-1Ga2O3-3GeO2 prepared by a melt-quenching method. The CGG glass was characterized by using X-ray diffraction, differential scanning calorimetry, and Raman scattering analyses. Also, the optical and electronic properties of the CGG glass were investigated. Based on the electron paramagnetic resonance (EPR) spectra, it is found that new defects, NBOHC (nonbridging oxygen hole center) and E’(Ge) (unpaired electron at Ge ion bonding three oxygen), were created after UV excitation. From the good agreement between the time dependence of the photodarkening absorption intensity and EPR intensity of the NBOHC, the origin of the photodarkening phenomenon is found to be caused by NBOHC. Also, from the relationship between the time dependence of persistent luminescence intensity and the time-derivative EPR intensities of NBOHC and E’(Ge) center...

Near-IR- and UV-femtosecond laser waveguide inscription in silica glasses

The influence of laser parameters on silica based waveguide inscription is investigated by using femtosecond laser pulses at 1030 nm (near-IR) and at 343 nm (UV). Negative phase contrast microscopy technique is used to measure the refractive index contrast for different photo-inscribed waveguides and shows the effects of both laser wavelength and scanning speed. In particular, UV photons have a higher efficiency in the waveguide production process as also confirmed by the lower optical losses at 1550 nm in these waveguides. These measurements are combined with micro-Raman and photoluminescence techniques, highlighting that laser exposure induces both structural modification of the silica and point defects generation. The contribution of induced defects to the total refractive index change is singled out by applying two different thermal treatments on the waveguide. The first, up to 500 °C, is able to remove the most of the induced non-bridging-oxygen-hole-centers (NBOHCs) while the second up to 750 °C erases almost all absorbing induced defects, highlighting the strong contribution of additional defects, not yet identified.

Efficient sensitized photoluminescence of Er silicate in silicon oxide films embedded with amorphous silicon clusters, part II: photoluminescence

We report on the sensitized photoluminescence (PL) of Er silicates in a-Si-embedded Er silicate films. A two-step annealing process is utilized, where the first step determines the distribution and the sizes of the a-Si embedded structure and the second step modifies the crystal quality and the phase composition of Er silicates as well as the concentration of sensitizers. The determination of the annealing temperatures and the annealing time for each step requires an overall consideration of these factors. Optimized PL from Er silicates sensitized by luminescent centers (LCs) such as neutral oxygen vacancy (NOV) or non-bridging oxygen hole center (NBOHC) have been achieved in the film annealed at 1000 °C for 30 min followed by 1100 °C for 30 min, which is composed of well-crystallized y-Er2Si2O7 embedded with a-Si clusters.

Structural and optical properties of γ-ray irradiated BaF2– Bi2O3–SiO2Glasses

The influence of γ-ray irradiation on the optical and structural properties of BaF2– Bi2O3–SiO2 glasses has been studied. The absorption spectra of respective glasses are found to shift to higher wavenumber side as dosage rate of gamma irradiation increases. This is because of the formation of non-bridging oxygen hole centers (NBOHC) and electrons due to ionizing power of BaF2. The optical band gap value decreases from 3.07 to 2.61eV with an increase in dosage rate as well as alkaline content. The Urbach energy of glasses increases due to gamma irradiation may be attributed to formation of non-bridging oxygen in the system. The electron paramagnetic resonance (EPR) spectra exhibit resonance signals at g = 4.17 and g = 2.17 due to triplet hole pairs localized on apical oxygens. There were no significant variations in Raman spectra and this may be due to high content of Bismuth that is known for shielding of gamma irradiation. Photoluminescence (PL) studies reveal that both pure and irradiated barium silicate glasses exhibit white light where red region is predominant than blue in irradiated glasses.

Photoluminescence of Yb3+/Ce3+ co-doped aluminosilicate glasses under ultraviolet irradiation

The photoluminescence and decay properties of Yb3+/Ce3+ co-doped aluminosilicate glasses were investigated under ultraviolet irradiation. A strong excitation band was ascribed to 4f → 5d transitions of Ce3+ ions. A broad ultraviolet-visible emission band belonged to 5d → 4f transitions of Ce3+ ions and oxygen-deficient centers (ODCs) in the host matrix, and the visible-near infrared emission band was attributed to non-bridging oxygen hole centers. Comparative tests of the core composition were performed. The results showed Yb2O3 and the ODCs played a crucial role on ultraviolet-visible emission band. ODCs rather than Ce3+ ions were responsible for the 1–35 μs lifetime decay of the ultraviolet-visible emission, but had no contribution to the longer decay lifetimes (77.58 μs) for Ce-free Yb3+-doped glasses. A complex schematic energy-level diagram including the electron transfer mechanism such as Ce3+-Yb3+ → Ce4+-Yb2+, the cooperative up- and down-conversion processes was proposed to describe different luminescence mechanisms UV-expose.

Photoluminescence comparison of SRO-LPCVD films deposited on quartz, polysilicon and silicon substrates

In the present work, photoluminescence spectra of silicon-rich oxide monolayers (SRO10 and SRO25) and bilayers (SRO25/10, SRO10/25) films deposited on quartz, polysilicon on quartz and silicon substrates are compared. Silicon-rich oxide films were deposited using Low Pressure Chemical Vapor Deposition (LPCVD) technique. The films were characterized by Ellipsometry, Fourier Transform Infrared Spectroscopy, Secondary Ion Mass Spectrometry, Optical Transmittance, Photoluminescence, Scanning Electron Microscope, and High-Resolution Transmission Electron Microscopy. The results show that the bilayer films, deposited on polysilicon film on quartz have the stronger photoluminescence, which is indicative that there are different types of defects, Silicon nanocrystals, Silicon nanoparticles, and amorphous silicon nanoparticles that improve the Photoluminescence response. Also, it was observed constructive interferences in the transmittance a spectrum of silicon-rich oxide films/polysilicon films on quartz, due to that their refractive indices are different and they are together. This, when a beam of light crosses the interface between materials with different refraction indexes, its direction of propagation is altered, and the bigger is the difference of its refractive indexes, the greater will be the beam refraction. In all materials known this phenomenon is called as positive refraction. The photoluminescence is related to silicon dioxide defects such as weak oxygen bonds, neutral oxygen vacancy, non-bridging oxygen hole centers, or positively charged oxygen vacancies. Moreover, it is a combined effect where the bandgap energy acquires some direct bandgap properties due to quantum confinement, both effects (defects and quantum confinement) are important to increase the photoluminescence intensity. The average diameters of Silicon nanocrystals were estimated from the band gap energy and also by High-Resolution Transmission Electron Microscopy obtaining an average diameter of 3.8 ± 0.08 nm. Comparative analysis of optical and structural properties was performed on all samples.

Investigations of interstitial Bi0 interacting with intrinsic defects in bismuth-doped silica optical fiber

The interaction mechanisms of interstitial Bi0 with intrinsic defects in bismuth-doped silica optical fiber are investigated by first-principles calculations. The results reveal that interstitial Bi0 easily interact with peroxygen defects and the reaction products are more stable. By analyzing the effective charges of the reaction products of interstitial Bi0 interacting with peroxy linkage (POL), non-bridging oxygen hole center with silicon dangling bond (NBOHC-Eʹ), and peroxy radicals with silicon dangling bond (POR-Eʹ), it finds that Bi atom is nearly divalent. For interstitial Bi0 interacting with oxygen deficiency center (ODC(I)), it forms a complex with two Si atoms in the defect. The method will be effective to understand interaction mechanism of interstitial Bi0 with intrinsic defects in bismuth-doped silica optical fiber.

Energy transfer channel between silicon nanocrystals and an optical center emitting above their bandgap

This paper reports on the observation of an electronic energy transfer channel between silicon nanocrystals and the SiO2 matrix they are embedded in. This energy transfer manifests itself by photoluminescence at 650 nm, i.e. above the indirect band gap, in superlattices formed by alternating layers of P- or B-doped as well as undoped silicon nanocrystals (NCs) and stoichiometric SiO2 or SiOxNy. This band is only observable under strong (above ~13 mJ/cm2) UV femtosecond excitation. A detailed investigation of the excitation intensity dependence together with luminescence dynamics enabled us to ascribe this band to a non-bridging oxygen hole center located in the SiO2 (or SiOxNy) matrix, to study the influence of NC treatment on its photoluminescence and to determine the energy transfer mechanism.

Cooperative defect-enriched SiO2 for oxygen activation and organic dehydrogenation

Tailoring oxide functionalities via defect engineering is an effective approach in many advanced materials. Here we report that introducing defect sites in SiO2 via sequential hydrogenation and UV light pre-treatment steps can invoke synergism for oxygen activation and organic dehydrogenation under ambient conditions in the absence of light. The hydrogenation step is proposed to break SiOSi bonds to give an E′δ center – silanol pair. UV light pre-treatment then serves to dehydroxylate the silanol group to give a non-bridging oxygen hole centers (NBOHC). The two defect sites work in harmony to activate oxygen and in turn oxidize formic acid. First-principles calculations indicated that UV light pre-treatment lowered the energy barrier for oxygen activation and formic acid decomposition on the defect sites. Re-illuminating the used SiO2 led to partial recovery of the defect sites and hence oxidation performance. The study demonstrates the capacity for generating synergistic defects in simple metal oxides as an effective way to enhance performance for oxidation processes.

Radiation effect on the UV-green thermally stimulated luminescence emission of a natural Na-rich aluminosilicate

This paper reports on the thermoluminescence (TL) emission (from 250 to 500 nm) of a 10 Gy irradiated natural Na-rich aluminosilicate from Minas Gerais (Brazil), previously characterized by Raman spectroscopy and differential thermal analysis techniques. 3D-TL spectrum of the analyzed sample displays a complex glow curves with five different emissions bands peaked at 290, 380, 420, 460 and 500 nm that could be associated with structural and point defects. The UV-blue emission bands have the following characteristics: (i) the 290 nm waveband could be due to defects-sites by the presence of Na ions; (ii) the 380 nm band, caused by intrinsic defects in the lattice (i.e. Non-Bridging Oxygen Hole Centers (NBOHCs), [AlO4]o centres, alkali planar defects and Oxygen Defect Centers (ODCs), among others); (iii) the 420 nm waveband, linked to the recombination on a centre formed from a hole-oxygen atom adjacent to two aluminum atoms (Al-O-Al) and Al-O-Si complexes; (iv) the 460 nm band, associated with the presence of Ti4+ in the plagiclase crystal lattice. The green waveband could be linked to de-excitation of Mn2+ ions. The estimation of the activation energy (Ea) from the 10 Gy-induced TL glow curves analyzed by means of the initial rise method display linear regressions with acceptable r coefficients (r > 0.977) that reveal three groups of components with traps at different depth in the crystal lattice of the albite with Ea values in the range of 0.5–0.64 eV.

A comparative study of the point defect accumulation in the Ar+−implanted and neutron-irradiated silicon dioxide

The ion-implantation of silica is widely used in optoelectronics and neighboring areas. Si + ions serve for formation of clusters/nanocrystals in components of microelectronic devices; Ge+, P+, and B+ are used for controlling refraction index in lightguides. All enumerated ions interact chemically with oxygen atoms in SiO2, thus shading the stoichiometric distortions caused by direct impact interactions of particles with a host. In this research, the optically-active defects in the silica network were induced by embedding of chemically inert Ar + ions and thermal neutrons. A set of bands belonging to neutral oxygen vacancies (≡Si-Si≡), non-bridging oxygen hole centers (NBOHC) and some other point defects were detected in the photoluminescence spectra of implanted/irradiated samples. The intensity variation of the bands at the Ar + fluence increase was found to be caused by a competition between defect nucleation and their annealing or/and concentration quenching at high fluence. The ionization of atoms by charged particles resulted in the multivariant spectral manifestation of the defects of the same type due to their arrangement in differently distorted host localities. The diversification of defects produced by neutrons was lesser, because the neutron impact on the silica host was limited by the interactions with nuclei.

Light-Induced Synergistic Multidefect Sites on TiO2/SiO2 Composites for Catalytic Dehydrogenation

Here, we demonstrate that structural defects can induce catalytic reactivity in simple metal oxides to deliver cost-effective alternatives to noble metal group catalysts. We detail a strategy for introducing multiple defect sites in a binary TiO2–SiO2 composite to invoke synergism for oxygen activation. Hydrogenation and UV light pretreatment were applied to generate two distinct and adjacent defect sites, Ti3+ and silica-based nonbridging oxygen hole centers (NBOHC)—which work in unison to activate oxygen and oxidize formic acid under ambient conditions without light. The hydrogenation step was found to be crucial for rupturing Ti–O–Si bonds while first-principles calculations indicated that Si-doped TiO2 lowered the energy barrier for oxygen activation and formic acid dehydrogenation on the defect sites. Activity lost during the reaction was recoverable by catalyst reillumination. Defective metal oxides represent an appealing prospect in the pursuit of simple and readily accessible catalyst materials.

Effects of Hydroxyl Group in AlOx Gate Insulator on the Negative Bias Illumination Instability of In‐Ga‐Zn‐O Thin Film Transistors

Effects of hydroxyl group (OH) in the gate insulator (GI) of AlOx formed by atomic layer deposition on the negative bias illumination stress (NBIS) stability of amorphous InGaZnO (a‐IGZO) thin film transistors (TFTs) by changing the deposition temperature (Tdep) of the AlOx GI are studied. There are no significant differences in the electrical properties and stabilities of each device, such as field effect mobility (μFE), subthreshold swing (S.S.), turn on voltage (Von), positive bias temperature stress (PBTS), and negative bias temperature stress (NBTS). Meanwhile, the NBIS stability is improved, resulting in Von shift from −4.34 V to −2.48 V as the Tdep of the GI increased. This suggests that as the amount of OH increases in GI, more hole/and or ionized Vo trapping sites are formed in the bulk GI. Based on the energy level in photoluminescence (PL) spectra of ALD AlOx, it is suggested OH related‐bulk trapping sites in ALD AlOx as the origin of non‐bridging oxygen hole center (NBOHC) in AlOx GI.

Impact of heat treatment on NBOHC luminescence of OH-containing and H2-impregnated fused silica for deep-ultraviolet applications

OH-containing and H2-impregnated fused silica is the optical materials of choice for deep-ultraviolet (DUV) applications. Intrinsic and UV laser irradiation induced non-bridging oxygen hole center (NBOHC) defects play a central role in the optical properties of fused silica in the DUV spectra range. Understanding the NBOHC generation mechanism is of great importance to the performance improvement. In this paper, NBOHC-related 650 nm photoluminescence (PL) band is investigated in details by heat-treating the fused silica at different temperature and exciting the PL band at different wavelength. From the 650 nm PL characteristics of un-treated and heat-treated fused silica samples excited at 193 nm, 266 nm, 355 nm, and 532 nm, it is found that the 650 nm PL band of un-treated fused silica is excited at 193 nm and 266 nm mainly via a two-photon process, and cannot be excited with 355 nm and 532 nm. The un-treated fused silica also presented a reversible time delay (annealing) in 650 nm PL intensity following an abrupt change of excitation fluence at 193 nm, due to the recombination of NBOHCs with impregnated H2. On the other hand, for the heat-treated fused silica, the 650 nm PL band is excited by a combination of one- and two-photon processes at 193 nm and 266 nm with the one-photon process dominating, and by the one-photon excitation process at 355 nm and 532 nm. The quantum yield of 650 nm PL band decreases with the decreasing excitation photon energy. From these experimental results it is confirmed with no doubt that in un-treated fused silica NBOHCs are produced by 193 nm and 266 nm laser irradiation induced photolysis of an Si‒OH group via a two-photon process, with a minimum photolysis energy between 7.0 eV and 7.9 eV. In heat-treated fused silica NBOHCs are produced by heat treatment induced breakage of Si‒OH bonds. Heat treatment also removed totally the impregnated H2 in fused silica, that makes the heat-treatment produced NBOHCs become stable and show one-photon excited 650 nm PL band when excited at 193 nm, 266 nm, 355 nm, and 532 nm. In contrary to the un-treated fused silica, the heat-treated fused silica presented no reversible time delay in 650 nm PL intensity following the abrupt change of excitation fluence at 193 nm, due to the total removal of H2 in fused silica and the absence of recombination of NBOHCs with H2.

Accelerated synthesis of zeolites via radicalized seeds

By means of milling and heating, the zeolite crystals can be radicalized forming surface non-bridging oxygen hole center (NBOHC, ≡Si–O·). The corresponding g values of 2.0043 and 2.0048 unambiguously confirm the existence of the ≡Si–O· radicals in the milled and heated crystals via electron paramagnetic resonance. The active ≡Si–O· radicals can react with water to form hydroxyl free radicals to accelerate the crystallization of zeolite along with seeds. Radicalized crystals are used as seeds that show obvious accelerating effect in the crystallization of Na-A and nanosized silicalite-1 than the non-radicalized seeds. At the same crystallization time, solid products with much better crystallinity and more yields could be obtained using radicalized seeds compared with non-radicalized seeds. A new seeding-synthesis-related strategy is developed to accelerate the crystallization of zeolites.

Creation of glass-characteristic point defects in crystalline SiO2 by 2.5 MeV electrons and by fast neutrons

Point defects in crystalline SiO2, created by 2.5 MeV electron irradiation at dose below the amorphization threshold or by fast neutrons, were compared by luminescence spectroscopy. Oxygen dangling bonds (“non-bridging oxygen hole centers”, NBOHCs), peculiar to amorphous state of SiO2, were detected for the first time in electron-irradiated non-amorphized α-quartz crystal. Their presence may signal the formation of nucleation centers in crystal structure as the first step to radiation-induced amorphization. Compared to crystal, irradiated by 1019 cm−2 fast neutrons, their concentration was over 100 times lower, and their inhomogeneous broadening was at least 2.5 times smaller. Divalent silicons (“silicon oxygen deficiency centers”, SiODC(II)), inherent to oxygen-deficient or irradiated SiO2 glass, were detected in neutron-irradiated (1019 n/cm2) α-quartz but were not found after the electron irradiation. Radiation-induced interstitial O2 molecules, characteristic to irradiated glassy SiO2 and other oxide glasses, are found in α-quartz only after neutron irradiation. The oxygen atoms, displaced by the 2.5 MeV e− irradiation of α-quartz for fluences up to 1019 e−/cm2 evidently stays entirely in the peroxy linkage (Si-O-O-Si bond) form.