Non-bridging Oxygen Hole Centers Research Articles

Cathodoluminescence response of natural and synthetic lanthanide-rich phosphates (Ln3+: Ce, Nd)

This paper reports on the cathodoluminescence (CL) emission of both natural and synthetic lanthanide-rich phosphates (Ln3+: Ce, Nd) previously characterized by X-ray Diffraction (XRD), Environmental Scanning Electronic Microscopy (ESEM) and Energy Dispersive Spectroscopy. The thermal treatment at 700°C performed on the synthetic sample obtained by chemical precipitation, promotes increasing of the crystallinity degree giving rise to a phase transition from the hexagonal (comprising monazite and rabdophane) into the monoclinic (cerianite and monazite) structures detected by XRD. Despite the size and the morphology of the grains are similar under ESEM, it could be appreciated significant differences among CL signals attending to the shape (with well-defined peaks for the annealed sample) and intensity (with lower emission for the non-thermally pretreated synthetic phosphate). The main wavebands centered at (i) 360, 380 and 490nm are associated respectively with 5D3/2 → 2F5/2 and 5D3/2 → 2F7/2 transitions as well as a redox reaction assigned to the presence of Ce3+, (ii) 276, 424, 516 and 531nm are linked respectively to 2G9/2→4I9/2, 2P1/2→4I9/2, 4G9/2→4I9/2 and 4G7/2→4I9/2 Nd3+ transitions and (iii) 400–490nm is due to non-bridging oxygen hole centers related to the tetrahedral PO43- groups or structural defects for the heated synthetic samples. The natural sample from Madagascar, with a very complex CL spectrum, displays a characteristic band emission in the green-yellow and red regions corresponding to [UO2]2+ groups and Sm3+ respectively.

Using NBOHC fluorescence to predict multi-pulse laser-induced damage in fused silica

We investigate the evolution of the fluorescence signal as function of repeated nanosecond laser pulses up to laser-induced damage in fused silica at 266 nm. We observed a maximum in the fluorescence spectrum at 664 nm linked to NBOHCs (Non-Bridging Oxygen Hole Centers). This peak increases with shot number for a fixed fluence until damage occurs. Thanks to the data from the in situ fluorescence measurements, we propose a new method to predict the occurrence of laser damage in high quality fused silica samples like Herasil©and Suprasil 1©. This new method allows to reduce drastically the time of test and the area under study that are required to record the reference data for damage occurrence. Furthermore we measured that the lifetime of the laser-induced modifications is about 2 h in Herasil.

Systematic control of optical features in aluminosilicate glass waveguides using direct femtosecond laser writing

Low loss optical waveguides inside aluminosilicate glasses have been successfully fabricated using direct femtosecond laser writing. To establish the influence of pulse energy and host variations on the optical waveguides have been tentatively explored and systematically studied with the help of different spectroscopic techniques. Isochronal annealing treatment effectively reduces the insertion losses to 1.01 ± 0.28 dB at 632.8 nm. A red shift of the Raman band has been observed with increasing Al2O3 content due to the bond angle variations. The point defects such as non-bridging oxygen hole centers have been corroborated by the photoluminescence studies and significant red-shift has also been documented with increasing Al2O3 content. In addition, there is no NBOHC defects perceived after isochronal annealing treatment inside the glass waveguides. Our results envisage that the present glass waveguides should be promising and potential for applications in passive waveguides and integrated photonic devices.

Optical, FTIR and ESR Spectral Investigations of Tungsten Ions in Barium Phosphate Host Glass and Effects of Gamma Irradiation

Tungsten oxide (0.25 – 3% WO3) doped barium phosphate glasses were prepared by the conventional melting annealing procedure. Spectroscopic FTIR infrared and ESR measurements were applied at 0 and 10 Mrad doses of gamma irradiations. The UV-Visible spectrum of the base glass consists of distinct ultraviolet peaks that can be correlated with trace ferric (Fe3+ ions) impurities. UV-Visible absorption spectra of WO3 – doped glasses show similar ultraviolet absorption as the base glass and a very broad visible band settled at about 720 nm and with the high content sample (3% WO3) two peaks are identified at 602 and 790 nm on the very broad visible absorption band. These additional broad band accompanied by the bluish color to the samples support the assumption of the formation or presence of pentavalent tungsten ions because of the reducing nature of the phosphate host glass. Gamma irradiation produces some extension of the UV absorption of the undoped glass spectrum accompanied by the generation of an induced visible broad band centered at 520 nm. These extra induced changes are related to some photochemical reactions leading to the partial oxidation of some ferrous ions to ferric ions through the radiation effect. The induced broad visible band is assumed to be related to the formation of phosphorus oxygen hole centers (POHC) or nonbridging oxygen hole centers (NBOHC) from interaction of gamma rays on the phosphate network. Gamma irradiation of WO3 doped glasses reveals the increase of the intensity of the very broad band and in the high content sample (3% WO3), a new induced band is generated at about 411 nm. These changes are assumed to be related to some photochemical reactions involving the conversion of some W6+ ions to W5+ ions by capturing electrons or by photo-reduction of W6+ ions to W4+ ions and thus imparting the additional peak at 411 nm. FTIR results of the studied glasses reveal the appearance of vibrational modes due to phosphate network groups mainly of metaphosphate units or P–O–P linkages as the response of the chemical composition 50% BaO – 50% P2O5. The infrared results of the radiative samples have no notable changes due to compactness of the phosphate with noticeable amount of heavy Ba2+ ions which seem to retard the passage of liberated electrons or positive holes during the irradiation process and thus the network structure remains unaffected. ESR spectra of WO3 – doped glasses reveal distinct signals due to the unpaired pentavalent ions which are stable towards irradiation due to the existence of a high content of the heavy Ba2+ ions which block the passage of negative and positive holes during irradiation.

Coupled irradiation-temperature effects on induced point defects in germanosilicate optical fibers

We investigated the combined effects of temperature and X-rays exposures on the nature of point defects generated in Ge-doped multimode optical fibers. Electron paramagnetic resonance (EPR) results on samples X-ray irradiated at 5 kGy(SiO2), employing different temperatures and dose rates, are reported and discussed. The data highlight the generation of the Ge(1), Ge(2), E′Ge and E′Si defects. For the Ge(1) and Ge(2), we observed a decrease in the induced defect concentrations for irradiation temperatures higher than ~450 K, whereas the E′ defects feature an opposite tendency. The comparison with previous post-irradiation thermal treatments reveals peculiar effects of the temperature increase during the irradiation. Such difference, confirmed also by online radiation-induced attenuation measurements, has to be considered for practical use of these fibers in a mixed environment. Importantly, even if post-irradiation fading should be considered, the Ge(1) and Ge(2) concentrations measured by postmortem EPR experiments in room-temperature-irradiated samples are quite representative of the concentrations induced in the temperature range 230–450 K regardless of the investigated dose rate. The enhancement of the E′ content can be related to the simultaneous generation of this defect with non-bridging oxygen hole center from strained bonds implying a relevant modification of the defects generation/formation processes in the host glass matrix.

Cathodoluminescence of moldavites

AbstractA systematic study of a large set of moldavites and the application of cathodoluminescence (CL)‐spectroscopy with a detailed discussion of spectral features is presented. Optical CL microscopy and spectroscopy (OM‐CL) were performed on 57 moldavite samples from different substrewn‐fields in Germany and the Czech Republic. The extracted CL data were supported by SEM‐EDX analysis. In general, two different kinds of CL colors can be distinguished: different shades of green in the matrix of the tektite glasses and a variation of blue color for lechatelierite inclusions (a pure silica‐glass phase). Spectral analysis of these colors shows three CL emission bands for green and five bands for blue c. Most CL activators are structural defects of the local glass network, influenced by the crystal field. The visible green CL emission is caused by defects related to strong local disorder as well as Al‐O−‐Al defects. The blue CL emission is activated by different types of lattice defects such as nonbridging oxygen‐hole center (NBOHC), self‐trapped excitons (STE), and oxygen deficiency centers (ODC). Intensity variations of the CL emissions were observed for samples from the different localities, but there is no direct correlation between substrewn‐fields and CL characteristics. Nevertheless, CL microscopy is a powerful tool for the high‐contrast visualization of internal textures such as streaks and lechatelierite in the tektite matrix due to the luminescence properties of the defect structures in the glassy network.

Study on the phase transition of fracture region of laser induced damage in fused glass by focused nanosecond pulse

As a key optical material in high-energy laser system, the laser induced damage of fused silica glass has been one of the bottlenecks to limit output of the laser influence. In this paper, the morphology of fractures on fused silica glass surface as well as phase transition caused by laser pulse shock wave have been investigated in theory and experiment, and we found that the whole fracture region can be divided into hackle zone, mist zone and mirror zone, which were produced by the tip of hoop stress formed in glass under the action of laser plasma shock wave. During cracks initiating and propagating of fused silica glass, the existing of cracks can reduce the relative content of three- and four-number rings structures corresponding to the stishovite and the coesite which caused by the absence oxygen free, this kind of change may destroy the mesh topology severely, and induce phase transitions. From the micro level, the non-bridging oxygen hole center and E' center lead to the decrease of the optical band gaps as well as transmittance which seriously affect the performance of glass.

Irradiation temperature effects on the induced point defects in Ge-doped optical fibers.

We present an experimental investigation on the combined effects of temperature and irradiation on Ge-doped optical fibers. Our samples were X-ray (10 keV) irradiated up to 5 kGy with a dose rate of 50 Gy(SiO2)/s changing the irradiation temperature in the range 233-573 K. After irradiation we performed electron paramagnetic resonance (EPR) and confocal microscopy luminescence (CML) measurements. The recorded data prove the generation of different Ge related paramagnetic point defects and of a red emission, different from that of the Ge/Si Non-Bridging Oxygen Hole center. Furthermore, by comparing the behaviour of the EPR signal of the Ge(1) as a function of the irradiation temperature with the one of the red emission we can exclude that this emission is originated by the Ge(1).

Study of Radiation Effects on Er3+-Doped Nanoparticles Germano-Silica Fibers

The main goal of the present work is to study the impact of Er–SiO2 and Er–Al2O 3 nanoparticles on the radiation induced defects in germano-silica optical fibers with Al/Ge ranges over 0.1–150. These fibers are prepared by a modified chemical vapor deposition technique and are characterized by optical absorption and electron paramagnetic resonance spectroscopy. High amount of Ge, Er–SiO2 nanoparticles-derived optical fibers exhibit lower radiation induced attenuation than those with a low Ge content undoped or doped with Al2O3 nanoparticles. Furthermore, the behavior of point defects namely GeE’, Ge(1), SiE’, nonbridging oxygen hole centers, and H(1) is reported according to the Al/Ge ratio. In contrast to the optical fiber preforms, no Al-related defects are found in the optical fibers. Therefore, the results evidence the strong radiation tolerance by the virtue of the nanoparticle doping, which is enabling technology for the development of the photonic and space applications in the radiation fields.

In Situ Luminescence Measurement from Silica Glasses Irradiated with 20 keV H− Ions

A new low-energy negative-ion induced luminescence setup was recently developed at the injector of the GIC4117 2 × 1.7 MV Tandem accelerator in Beijing Normal University. In situ luminescence measurements are performed on silica glass by using 20keV H− ions at room temperature. Gauss fitting of the spectra revealed six overlapping components at about 2.7 eV, 2.4eV, 1.9eV, 1.8eV, 4.2eV, and 3.6eV, which except for the new observed emission band at 3.6 eV are assigned to the creation of type II oxygen-deficient centers, E' centers, non-bridging oxygen hole centers with different precursor states, and type-I oxygen-deficient centers. The fitted results of the saturation concentration show that self-trapped exciton recombination at type-II oxygen-deficient centers is the main luminescence emission process. The evolution of the luminescence intensity and full width at half maximums as a function of ion fluence is also discussed. It is found that the number of recombination centers reaches its maximum at lower fluence, and the area ratio between blue bands and red bands is much lower than that under high energy H+ ion irradiation.

Optical character enrichment of NdF3 – doped lithium fluoroborate glasses

Optical spectra in combination with Fourier transform infrared FTIR spectra of base undoped lithium fluoroborate glasses and samples that doped with NdF3 were measured before and after subjecting to (8Mrad) gamma irradiation. Base glass reveals strong absorption in the UV region attributed to unavoidable iron impurity (Fe3+ ions) while gamma irradiated sample shows extended additional peaks related to the photochemical reactions of some Fe2+ impurities transformed to Fe3+ ions by acquiescent positive holes while the induced visible band is related to (BOHC) boron oxygen hole center or (NBOHC) nonbridging oxygen hole center. The NdF3-doped samples display the same strong UV spectra as the undoped sample beside additional characteristic visible absorption peaks due to the presence of Nd3+ ions. UV–Vis. absorption spectra were applied to assess their optical properties, comprising indirect and direct optical energy band gaps taking into account the effect of gamma irradiation. Infrared absorption spectra of the undoped lithium fluoroborate glass reveal characteristic IR bands due to both BO3 and BO4 borate groups as usually observed in various alkali–oxide B2O3 glasses. Suggested BO3F tetrahedral units are advanced to interpret the close similarity of the IR spectra obtained from fluoroborate glass with alkali or alkaline earth oxide borate glasses. The increase of NdF3 content causes minor changes in the IR spectra that can be attributed to the non-conventional loose OH, H2O or similar vibrations while the structural building borate units remain unchanged as evidence from the deconvoluted FTIR data before and after the irradiation process.

Multicolor photoluminescence in ITQ-16 zeolite film

Exploring the native defects of zeolites is highly important for understanding the properties of zeolites, such as catalysis and optics. Here, ITQ-16 films were prepared via the secondary growth method in the presence of Ge atoms. Various intrinsic defects of ITQ-16 films were fully studied through photoluminescence and FTIR characterizations. It was found that both the as-synthesized and calcined ITQ-16 films displayed multicolor photoluminescence including ultraviolet, blue, green and red emissions by exciting upon appropriate wavelengths. The results indicate that Si―OH and non-bridging oxygen hole centers(NBOHCs) are responsible for the origin of green and red emissions at 540―800 nm, while according to a variety of emission bands of calcined ITQ-16 film, blue emission bands at around 446 and 462 nm are attributed to peroxy free radicals(≡SiO2), ultraviolet emissions ranging from 250 nm to 450 nm are suggested originating from a singlet-to-triplet transition of two-fold-coordinated Si and Ge, respectively.

Effect of substrate temperature on structural and optical properties of ZnO nanostructures grown by thermal evaporation method

In this study, ZnO nanostructures were synthesized on SiO2/Si substrate by thermal evaporation method. The dependence of the crystalline phase, morphologies and chemical composition of the samples grown at different substrate temperatures were systematically studied. The XRD, Raman spectra, FTIR spectra and XPS results reveal the existence of Zn2SiO4 phase, beside the ZnO phase. The Zn2SiO4 phase dominates at high substrate temperature and vise versa. Under UV excitation at room temperature, the samples show three distinct emission bands namely UV (~380nm), green (~525nm), and NIR (730nm). The increase of the PL intensity of the NIR emission with increasing substrate temperature, indicates the relation between this emission and the higher Zn2SiO4 phase content in the samples. Origin of the rarely observed NIR emission band is attributed to the energy transition from non-bridging oxygen hole centers of SiO2 to the Zni and Vo states of Zn2SiO4.

Resonance Raman of oxygen dangling bonds in amorphous silicon dioxide

We investigate the origin of a resonance Raman band induced by ionizing radiation in amorphous silicon dioxide (silica glass), which can be detected under ultraviolet laser excitation. A silica sample, rich of oxygen-excess related defects, was prepared by treating some length of a pure-silica-core multimode fiber in an O2 atmosphere (at high temperature and pressure) and by irradiating it with X-rays at 10 MGy(SiO2) dose. A micro-Raman study revealed a gaussian band peaking at 896 cm−1 with a full width at half maximum of 32 cm−1, which could be detected by exciting the sample with the 325-nm line of a HeCd laser. This spectral feature is absent in the Raman spectra performed with the 442-nm line of the same laser. On the basis of several experimental evidences and some complementary literature data, the 896 cm−1 band is assigned to a resonance Raman scattering of oxygen dangling bonds, often called non-bridging oxygen hole centers. Copyright © 2016 John Wiley & Sons, Ltd.

Influence of Gold Nanoparticles on Luminescence of Eu3+ Ions Sensitized by Structural Defects in Germanate Films

This investigation shows new possibilities of gold nanoparticles using for rise of Ln3+ ions luminescence in oxide matrixes at ultraviolet synhrotron’s excitation on example of germanate films doped with Eu and Au. It has been established that formation of gold nanoparticles (AuNPs) in such films leads to the enhancement of the Eu3+ ions luminescence intensity by many times at excitation in broad absorption bands having λmax about 120 and 180 nm. The discovered effect is explained by joint influence of the following factors: (1) rise in the nonbridging oxygen hole centers (NBOHCs) concentration as a result of breakage of the O–Eu bonds in the ≡Ge–O–Eu= bridges by the forming AuNPs; (2) nonradiative transfer of excitations from excitons to these centers; (3) effective sensitization of the Eu3+ ions luminescence by the centers via the charge transfer (CT) state of the O2– + Eu3+ complexes; (4) increase in the optical path of the exciting radiation due to the intensification of light scattering by the AuNPs,...

Stable structure and optical properties of fused silica with NBOHC-E′ defect**Project supported by the National Basic Research Program of China (Grant No. 2014CB643900), the Open Fund of IPOC (BUPT), the Open Program of State Key Laboratory of Functional Materials for Informatics, the National Natural Science Foundation for Theoretical Physics Special Fund “Cooperation Program” (Grant No. 11547039), and Shaanxi Provincial Institute

First-principles method is used to simulate the stable structure and optical properties of a 96-atom fused silica. The preferable structure of NBOHC-E′ (non-bridging oxygen hole center (NBOHC) and E′ center) pair defect is predicted to be located at 2.4 Å for the Si–O bond length. The quasi-particle G0W0 calculations are performed and an accurate band gap is obtained in order to calculate the optical absorption properties. With the stretching of the Si1–O1 bond, an obvious redshift can be observed in the absorption spectrum. In the case of NBOHC-E′ pair, the p-orbital DOS of Si1 atom will shift to the conduction band. Two obvious absorption peaks can be observed in the absorption spectrum. The calculation reproduced the peak positions of the well-known optical absorption bands.

SU‐F‐T‐166: On the Nature of the Background Visible Light Observed in Fiber Optic Dosimetry of Proton Beams

Purpose:The nature of the background visible light observed during fiber optic dosimetry of proton beams, whether it is due to Cherenkov radiation or not, has been debated in the literature recently. In this work, experimentally and by means of Monte Carlo simulations, we shed light on this problem and investigated the nature of the background visible light observed in fiber optics irradiated with proton beams.Methods:A bare silica fiber optics was embedded in tissue‐mimicking phantoms and irradiated with clinical proton beams with energies of 100–225 MeV at Roberts Proton Therapy Center. Luminescence spectroscopy was performed by a CCD‐coupled spectrograph to analyze in detail the emission spectrum of the fiber tip across the visible range of 400–700 nm. Monte Carlo simulation was performed by using FLUKA Monte Carlo code to simulate Cherenkov light and ionizing radiation dose deposition in the fiber.Results:The experimental spectra of the irradiated silica fiber shows two distinct peaks at 450 and 650 nm, whose spectral shape is different from that of Cherenkov radiation. We believe that the nature of these peaks are connected to the point defects of silica including oxygen‐deficiency center (ODC) and non‐bridging oxygen hole center (NBOHC). Monte Carlo simulations confirmed the experimental observations that Cherenkov radiation cannot be solely responsible for such a signal.Conclusion:We showed that Cherenkov radiation is not the dominant visible signal observed in bare fiber optics irradiated with proton beams. We observed two distinct peaks at 450 and 650 nm whose nature is connected with the point defects of silica fiber including oxygen‐deficiency center and non‐bridging oxygen hole center.

Study of luminescence of Mn-doped CaB4O7 prepared by wet chemical method

In this study, manganese (Mn) doped Calcium Tetraborate (CaB4O7) samples were prepared by the wet chemical method. Under beta irradiated, CaB4O7:Mn showed thermally stimulated luminescence (TSL) glow peaks at approximately 85 °C and 220 °C with a heating rate of 5 °C/s. Peak shape (PS) and various heating rates (VHR) methods were applied for determining the trap parameters such as order of kinetic (b), activation energy (E) and frequency factor (s). The results indicate that the main dosimetric peak of CaB4O7:Mn follows the second-order kinetic model. The thermal fading ratio of the material is around 15% at the end of one month period of storage. The results of the TL studies carried out on the CaB4O7:Mn dosimeter revealed that it has high sensitivity, which makes it very valuable in various dosimetric applications like space, medical, personal dosimetry and dating. Radioluminescence (RL) of CaB4O7:Mn was also studied. It is shown that the RL spectrum contains a wide band centered at 530 nm assigned to Mn2+ ion emission. A broad band emission feature peaked at 350 nm assigned to the non-bridging oxygen hole center (NBOHC) is also discussed and a model is proposed to explain this broad band feature.

Effect of gamma rays on the structural and optical properties of a new hybrid monolith silica sucrose xerogel

A new hybrid monolithic silica xerogel containing sucrose has been synthesized via the sol–gel method. The resulting sucrose silica xerogels were exposed to gamma radiation with a dose in the range of 10–100kGy. The effect of gamma rays on the physical and structural properties of sucrose silica xerogels was investigated by Fourier transform infrared spectroscopy (FTIR) and UV visible spectroscopy. The results of FTIR spectroscopy show that the irradiation process causes changes in the network's structure by inducing defects. The UV visible spectra present three main band defects; E′ center, Non-Bridging Oxygen Hole center (NBOHC) and carbonyl radicals in irradiated sucrose silica xerogel samples. Additionally, the results show that the values of optical band gap energy depend on irradiation. Therefore, generated defects suggest that the behavior of sucrose silica xerogels was modified from an insulator (Eg=5.82eV) to a semiconductor (Eg=3.16eV) under gamma irradiation.

Formation of silver nanoparticles in silicate glass using excimer laser radiation: Structural characterization by HRTEM, XRD, EXAFS and optical absorption spectra

Plasmonic silver nanostructures in surfaces of soda-lime silicate glasses were generated using Ag+ ↔ Na+ ion exchange and UV laser irradiation (ArF laser, 193 nm) with different number of ns laser pulses (from 2 to 5000). To identify the correlations between the optical properties (surface plasmon resonance (SPR) parameters) and atomic structure of silver nanoparticles and their agglomerations, characterization of the samples was performed by HRTEM, XRD, optical absorption in visible range and Ag K-edge EXAFS spectra. Analysis of the optical spectra was performed using a Mie theory approach, accounting for the most plausible defect centers in silicate glass like hole trap centers and non-bridging oxygen hole centers. Processing of Ag K-edge EXAFS yielded values of Ag-Ag and Ag-O interactions averaged over ionic and neutral states of silver. The consistent treatment of HRTEM and XRD data, the behavior of features in optical spectra and the obtained dependence of Ag-Ag and Ag-O structural parameters upon the number of laser pulses enabled to suggest a mechanism of plasmonic Ag nanoparticles formation in silicate glass under UV laser irradiation.