Non-bridging Oxygen Hole Centers Research Articles

Unusual Luminescence of Quartz from La Sassa, Tuscany: Insights on the Crystal and Defect Nanostructure of Quartz Further Developments

Quartz luminescence finds applications on many fields, but much work still needs to be done to precisely characterize it. In this work, we made further developments on the study of luminescence of quartz from La Sassa, Tuscany: a sample with unique properties in this regard. Photoluminescence (PL) measurements allowed study of the excitation profile of the previously reported luminescence, showing an excitation maximum at around 4.3 eV, among other minor ones. This kind of luminescence has also been studied as a function of X-ray irradiation, showing that ionizing radiation desensitizes the photoluminescence emissions. New radioluminescence (RL) measurements have been done to study the effect of thermal annealing at 1000 °C, showing a more complex emission picture in the red region (1.8–2.0 eV), with multiple emissions. The data presented here allow more precise assumptions regarding the assignment of the centers responsible for each emission. The assignment has been confirmed by chemical profiles measured by laser-ablation inductively coupled plasma (ICP) mass spectrometry. The previously tentative assignment of non-bridging oxygen hole centers (NBOHCs) has been disproved for the PL and LIF emissions and confirmed for the RL ones.

Study of the Effect of Nitric Acid in Electrochemically Synthesized Silicon Nanocrystals: Tunability of Bright and Uniform Photoluminescence

In this work, we show a correlation between the composition and the microstructural and optical properties of bright and uniform luminescent porous silicon (PSi) films. PSi films were synthesized by electrochemical etching using nitric acid in an electrolyte solution. PSi samples synthesized with nitric acid emit stronger (up to six-fold greater) photoluminescence (PL) as compared to those obtained without it. The PL peak is shifted from 630 to 570 nm by changing the concentration ratio of the HF:HNO3:(EtOH-H2O) electrolyte solution, but also shifts with the excitation energy, indicating quantum confinement effects in the silicon nanocrystals (Si-NCs). X-ray photoelectron spectroscopy analysis shows a uniform silicon content in the PSi samples that emit the strongest PL. High-resolution transmission electron microscopy reveals that the Si-NCs in these PSi samples are about ~2.9 ± 0.76 nm in size and are embedded in a dense and stoichiometric SiO2 matrix, as indicated by the Fourier transform infrared analysis. On the other hand, the PSi films that show PL of low intensity present an abrupt change in the silicon content depth and the formation of non-bridging oxygen hole center defects.

Production mechanism of fluorescent species for calcined zeolite

The fluorescence properties of NH4-form Y-type zeolites calcined under different atmospheres and temperatures were studied. Various analyses were performed on the calcined zeolites to elucidate the origin of the fluorescence. The electron spin resonance spectrum showed two signals corresponding to the E′-center and non-bridging oxygen hole center (NOBHC), which were generated the by collapse of the SiO4 tetrahedron. In the FT-IR spectra, the absorptions assigned to the Si–O vibration peaks decreased in the fluorescent samples. The SiO4 tetrahedron generated fluorescence if it had oxygen vacancy leading SiO3 and/or SiO2 radicals.

100‐Layer Error‐Free 5D Optical Data Storage by Ultrafast Laser Nanostructuring in Glass

AbstractThe demand for energy efficient data storage technologies with high capacity and long life span is increasingly growing due to the explosion of digital information in modern society. Here, a 5D optical data storage with high capacity and ultralong lifetime is realized by femtosecond‐laser‐induced anisotropic nanopore structures (type X modification) in silica glass. The ultrahigh transmission of this birefringent modification, >99% in the visible range, allows recording and retrieving thousands of layers of multibit digital data practically. Type X formation is associated with moderate free carrier density produced close to the energy threshold of avalanche ionization. Higher retardance with increased repetition rate at low pulse energy is attributed to accumulation of defects (nonbridging oxygen hole centers), enabling rapid imprinting of voxels by megahertz‐rate pulses. Data recording of 7 bits per voxel, i.e., 25 azimuth angles and 22 retardance levels is experimentally demonstrated with readout error as small as 0.6%. Furthermore, “The Hitchhiker's Guide to the Galaxy” by Douglas Adams is optically recorded with a data writing speed of 8 kB s−1 in 100 layers of voxels and the proven data readout accuracy of 100%.

Multiphoton process investigation in silica by UV femtosecond laser

We investigated the interaction processes between high intensity femtosecond ultraviolet laser pulses and amorphous silica, leading to permanent refractive-index changes that are at the basis of advanced manufacturing for photonics devices. The experiment, carried out as a function of the laser power, improves our understanding on the strong-field ionization process by the monitoring of the 1.9 eV and 2.65 eV emissions, related to nonbridging oxygen hole centers and self-trapped exciton, respectively, induced in the exposed glass region. Our results clearly proved that the UV laser light band-to-band absorption is allowed in the multiphoton ionization limit, whose consecutive relaxation leads to the generation of these photoluminescence (PL) signatures. Furthermore, we coupled the online PL investigation with post mortem analysis of the irradiated volume through phase contrast microscopy, Raman and steady state PL, providing a complete view of the silica-femtosecond laser interaction.

Unusual Luminescence of Quartz from La Sassa, Tuscany: Insights on the Crystal and Defect Nanostructure of Quartz

Quartz from La Sassa (Tuscany, Italy) presents a unique luminescence related to intrinsic and extrinsic defects in the crystal lattice due to the growth mechanisms in hydrothermal conditions. The bright fluorescence under the UV lamp was apparent to collectors since the early 1970s, and it entered the literature as a reference case of yellow-luminescent quartz. Early reports present the history of the discovery, the geological context, and preliminary luminescence measurements of the quartz nodules, suggesting various activators as potentially responsible of the peculiar luminescence effects: uranyl groups (UO22+), rare earths (Tb3+, Eu3+, Dy3+, Sm3+, Ce3+) and polycyclic aromatic compounds (PAH). Here, we report a full investigation of the La Sassa material, by a multi-analytical approach encompassing cathodoluminescence optical microscopy (OM-CL), laser-induced fluorescence (LIF), wavelength resolved thermally stimulated luminescence (WR-TSL), trace elements analysis by mass spectrometry (ICP-MS) and Raman spectroscopy (RS). The results provide a significant step forward in the interpretation of the luminescence mechanisms: the main luminescent centres are identified as alkali-compensated (mainly Li+ and Na+, K+ and H+) aluminum [AlO4/M+]0 centres substituting for Si, where the recombination of a self-trapped exciton (STE) or an electron at a nonbridging oxygen hole centre (NBOHC) are active.

Structural and Optical Modifications in the BaO-ZnO-LiF-B2O3-Yb2O3 Glass System after γ-Irradiation.

A Yb3+-doped borate glass system was examined for the structural and optical modifications after -irradiation. Among the studied 10BaO-20ZnO-20LiF-(50-x)B2O3-xYb2O3 (x = 0.1, 0.5, 0.7, and 1.0 mol%) glasses, the 10BaO-20ZnO-20LiF-49.9B2O3-0.1Yb2O3 glass showed the highest thermoluminescence intensity, trap density, and trap depth. The glass was irradiated with the optimum -dose of 1 kGy towards the analysis of radiation-induced defects. The amorphous nature was preserved before and after irradiation. The glass density slightly increased after irradiation. The structural rearrangement was evident from the Fourier transform infrared spectroscopy by the appearance and disappearance of some bonds after -irradiation. The transformation of [BO4] units into [BO3] units and non-bridging oxygens was deduced. The color of the glass darkened after irradiation and the optical absorption intensity enhanced between 250 and 700 nm. The optical bandgap reduced and Urbach energy increased upon -dose exposure. The electron spin resonance of the irradiated glass exhibited two signals at g = 2.0167 and g = 1.9938, corresponding to the non-bridging oxygen hole center and Boron E’-center, respectively.

Radiation resistance property of barium gallo-germanate glass doped by Nb2O5

Effects of Nb2O5 dopant on the radiation response of barium gallo-germanate (BGG) glass are studied mainly by electron paramagnetic resonance and absorption spectroscopy. Owing to the Nb5+↔Nb3+ interconversion in doped samples, formations of Ge-related non-bridging oxygen hole center and Ge-related electron center defects after γ-ray irradiation are inhibited. Thereby, Nb2O5 dopant can enhance radiation resistance of BGG glass, and 1.0% Nb2O5 concentration is the best.

An examination of the radiation-induced defects and thermoluminescence characteristics of Sm2O3 doped BaO–ZnO–LiF–B2O3 glass system for γ-dosimetry application

10BaO–20ZnO–20LiF-(50- x )B 2 O 3 - x Sm 2 O 3 ( x = 0, 0.1, 0.3, 0.5, 0.7, and 1.0 mol %) glasses were synthesized with an objective to examine the γ-radiation-induced defects and Thermoluminescence (TL) characteristics towards potential TL dosimetry application. The addition of 0.1 mol % Sm 2 O 3 to the host glass led to a drastic enhancement (almost two thousand times) in the TL intensity and concentration quenching was seen thereafter. The Sm 2 O 3 concentration and γ-dose for the examination of defects and TL features were optimized to be 0.1 mol % and 3 kGy, respectively. The lack of distinct peaks in the X-Ray Diffraction pattern confirmed the glass amorphous nature. The structural modification after irradiation was conveyed in the optical absorption spectroscopy by the presence of color centers and in Fourier Transform Infrared spectroscopy by the increase in the Non-Bridging Oxygens. The color centers were identified as Non-Bridging Oxygen Hole Center and Boron E ’ -center from the Electron Spin Resonance characterization. The glass manifested a good linear TL response between 0.25 and 3 kGy, high sensitivity, a minimum detection dose of 2 ± 0.2 Gy, reusability and reproducibility after annealing at 523 K for 15 min, Z e f f of 9.6, and trap activation energies between 0.47 and 1.28 eV. The application of the titled glass system as a potential TL dosimeter is proposed for food irradiation, and γ-environment (up to 3 kGy). • Sm 2 O 3 doped BaO–ZnO–LiF–B 2 O 3 glass system investigated for γ-TLD characteristics. • Two thousand times enhancement in TL intensity after 0.1 mol % Sm 2 O 3 incorporation. • TL characteristics suitable for high-dose γ-dosimetry (0.25–3 kGy). • Color centers post-irradiation evaluated through optical and structural properties.

Changes in macroscopic and microscopic properties of borosilicate glass irradiated with ions

Borosilicate glass has potential application in the vitrification of high-level waste; therefore, its radiation durability attracts extensive attention. Borosilicate glass samples were irradiated with Xe and Au ions. The samples after irradiation were analyzed by nanoindenter, the Raman, photoluminescence (PL) and infrared spectroscopy. With increase of irradiation dose, the hardness of NBS2 glass samples dropped and then saturated. The changes in PL and infrared spectra of NBS2 glasses samples were deduced. The relation between changes in Raman spectra and those in PL spectra was discussed. The results from PL and infrared spectra revealed the formation of a non-bridging oxygen hole center and transformation of BO4 to BO3 in the NBS2 glass after the irradiation of ions. Relations between the changes in hardness and microstructures of borosilicate glass were discussed.

Transient absorption with a femtosecond tunable excitation pump reveals the emission kinetics of color centers in amorphous silica.

We report a set of femtosecond (fs) transient absorption (TA) measurements following the dynamics of the so-called nonbridging oxygen hole center in silica, a model color center in wide bandgap amorphous solids, characterized by a very large Stokes shift between the UV excitation and its associated red emission at 1.9 eV. The changes in the TA spectrum were probed in the UV-visible range at various delays after photoexcitation and analyzed as a function of the UV excitation energy, in single-photon absorption conditions. The combination of the experiments helps to clarify the defect photocycle, highlighting how TA measurements with tunable UV excitation could represent a powerful tool to investigate the dynamics of color centers embedded in transparent materials.

Radiation Effects on Pure-Silica Multimode Optical Fibers in the Visible and Near-Infrared Domains: Influence of OH Groups

Signal transmission over optical fibers in the ultraviolet to near-infrared domains remains very challenging due to their high intrinsic losses. In radiation-rich environments, this is made even more difficult due to the radiation-induced attenuation (RIA) phenomenon. We investigated here how the number of hydroxyl groups (OH) present in multi-mode (MM) pure-silica core (PSC) optical fibers influences the RIA levels and kinetics. For this, we tested three different fiber samples: one “wet”, one “dry” and one with an intermediate “medium” OH content. The RIA of the three samples was measured in the 400–900 nm (~3 eV to ~1.4 eV) spectral range during and after an X-ray irradiation at a dose rate of 6 Gy(SiO2) s−1 up to a total accumulated dose of 300 kGy(SiO2). Furthermore, we evaluated the H2-pre-loading efficiency in the medium OH sample to permanently improve both its intrinsic losses and radiation response in the visible domain. Finally, the spectral decomposition of the various RIA responses allows us to better understand the basic mechanisms related to the point defects causing the excess of optical losses. Particularly, it reveals the relationship between the initial OH groups content and the generation of non-bridging oxygen hole centers (NBOHCs). Moreover, the presence of hydroxyl groups also affects the contribution from other intrinsic defects such as the self-trapped holes (STHs) to the RIA in this spectral domain.

Alteration of the chemical composition of fused silica surfaces via combined hydrogenous plasma treatment and UV laser irradiation

In this contribution, we report on the influence of nitrogen-hydrogen plasma treatment on the chemical composition of fused silica surfaces with and without additional argon fluoride laser irradiation. Therefore, the compositions of pristine and plasma treated fused silica samples were investigated by secondary ion mass spectrometry before and after laser irradiation. Transmission data were moreover obtained via UV-VIS spectroscopy. The plasma treatment most likely generates optically active defects such as non-bridging oxygen hole centers (NBOHCs), oxygen-deficiency-related defects (ODCs), and E' centers, which serve as precursor sites for hydrogen saturated defects. The resulting enhanced laser coupling leads to a loss of oxygen from the fused silica network and a further generation of defect centers. Possible explanatory approaches for the observed plasma- and laser-induced changes in carbon, hydrogen, and oxygen content are discussed. This includes the formation of silicon monoxide, hydrogen saturated defects, and nanocrystalline domains in the matrix containing silicon-carbon bonds.

Multiphoton-Absorption-Excited Up-Conversion Luminescence in Optical Fibers

We experimentally demonstrate a previously unforeseen nonlinear effect in optical fibers: up-conversion luminescence generation excited by multiphoton absorption of femtosecond infrared pulses. We directly estimate the average number of photons involved in the up-conversion process, by varying the wavelength of the pump source. We highlight the role of nonbridging oxygen hole centers and oxygen-deficient center defects and directly compare the intensity of side-scattered luminescence with numerical simulations of pulse propagation.

Impact of substitution of B2O3 by Bi2O3 on the structural changes and associated oxygen defect centers in B2O3-TeO2 glass.

Bulk samples of compositions x(Bi2O3)-(0.5-x)(B2O3)-0.5(TeO2) with x = 0.05, 0.1, 0.2, 0.3 and 0.4 are prepared using melt quenching technique The IR absorbance revealed that at x = 0.05 the network is mainly constituted by [BiO3] and [BiO5], [BO3] trigonal pyramid and [BO4] tetrahedron, [TeO4] trigonal bipyramidal and [TeO3] trigonal pyramid structural units. Increasing Bi2O3 content over 0.05 transforms [BO4] to [BO3], [TeO3] to [TeO4] and [BiO3] into [BiO5]. The calculated ratio of BO4 in the structural network showed a decrease against the increase in Bi2O3which is explained according to the density of bridging and non-bridging oxygen defects. Temperature dependent capacitance-frequency measurements showed relaxation peaks which argued to the non-bridging Oxygen hole centers (NBOHC) trap defects. The emission rate of these NBOHC defects appeared that its energy states are located as deep levels with respect to the conduction band edge.

Luminescence of non-bridging oxygen hole centers as a marker of particle irradiation of α-quartz

The origin of the “red” emission bands in the 600 nm–700 nm region, observed in quartz crystals used for luminescence dating and environmental dosimetry, is still controversial. Their reported spectral and lifetime characteristics are often similar to those of oxygen dangling bonds (“non-bridging oxygen hole centers, NBOHCs”) in glassy silicon dioxide. The presence of these “surface radical type” centers in quartz crystal requires sites with highly disordered local structure forming nano-voids characteristic to the structure of glassy SiO2. Such sites are introduced in the tracks of nuclear particles (α-irradiation, neutrons, ions). In case of electrons they are created only at large doses (>5 GGy), approaching amorphization threshold. This study reports a comparison of NBOHC photoluminescence in synthetic quartz and silica glass irradiated by neutrons or 2.5 MeV electrons, and suggests that the red NBOHC photoluminescence band in quartz may serve as a selective marker of an exposure to particle irradiation. It can be distinguished from other red-region luminescence bands by lifetimes in 5–25 μs range, characteristic vibrational structures in the low-temperature spectra and presence of resonance excitation band at ~620 nm.

Silicon negative ion implantation induced vacancy defects in thermally grown SiO2 thin films

ABSTRACT Thermally grown SiO2 thin films on a silicon substrate implanted with 100 keV silicon negative ions with fluences varying from 1 × 1015 to 2 × 1017 ions cm−2 have been investigated using Electron spin resonance, Fourier transforms infrared and Photoluminescence techniques. ESR studies revealed the presence of non-bridging oxygen hole centers, E′-centers and Pb-centers at g-values 2.0087, 2.0052 and 2.0010, respectively. These vacancy defects were found to increase with respect to ion fluence. FTIR spectra showed rocking vibration mode, stretching mode, bending vibration mode, and asymmetrical stretching absorption bands at 460, 614, 800 and 1080 cm−1, respectively. The concentrations of Si–O and Si–Si bonds estimated from the absorption spectra were found to vary between 11.95 × 1021 cm−3 and 5.20 × 1021 cm−3 and between 5.90 × 1021 cm−3 and 3.90 × 1021 cm−3, respectively with an increase in the ion fluence. PL studies revealed the presence of vacancies related to non-bridging oxygen hole centers, which caused the light emission at a wavelength of 720 nm.

Anti‐Counterfeiting Microstructures Induced by Ultrashort Laser Pulses

Anti‐counterfeiting microstructures are induced by ultrashort laser pulses in four kinds of materials. 1) Computer‐generated holograms of “SIOM” are written pixel‐by‐pixel using focused laser pulses by evaporation of aluminum films on silica substrates. Pixels are well‐defined holes, which can be reconstructed by He–Ne laser. 2) On stainless steel, surface ripples are induced with its orientation perpendicular to laser polarization. With “S‐O” and “I‐M” written with orthogonal laser polarizations, selective display of “SIOM” is achieved. Spatially overlapped “E” and “F” are also selectively displayed. 3) Luminous point defect of non‐bridging oxygen hole center (NBOHC) is generated in hydroxyl‐fused silica by ultrashort laser pulses. Due to photoluminescence (PL) of NBOHC, a barcode fabricated in hydroxyl‐fused silica is invisible in sunlight, while can be read out by 254 nm UV light. 4) Long persistent luminescence (LPL) is a phenomenon that PL continues for an obvious length of time when excitation source is removed. In homemade Mn2+‐doped germanate glass, LPL is observed from characters of “SIOM” written by laser pulses. “SIOM” then act as luminous centers. An exposure of 365 nm UV light enables the reappearance of LPL in “SIOM.” These phenomena can enable anti‐counterfeiting applications, and the mechanisms are also discussed.

Tunable point defects in hydroxyl fused silica enabled by ultrashort laser pulses: photostimulated luminescence and functional module fabrication

Three kinds of point defects, SiE’ center (≡ Si·), unrelaxed oxygen deficiency center (ODC (II)) and non-bridging oxygen hole center (≡ Si−O·, NBOHC), have been generated in hydroxyl fused silica by ultrashort pulsed laser irradiation. Hydroxyl is proved to be a decisive component for defect producing: NBOHC originates directly from hydroxyl; Hydroxyl facilitates the generation of SiE’ in an indirect way; No obvious relevance could be built between ODC (II) and hydroxyl. By improving hydroxyl content to 1000 ppm, NBOHC becomes the dominant defect species and its red luminescence is hence discernible to naked eye. Intended for application, high hydroxyl fused silica is screened out as the desired candidate, and NBOHC becomes the final interested defect. NBOHC’s intrinsic features of lifetime and temperature stability and extrinsic properties of laser condition dependence are specifically and systematically investigated. Prospective use of defect manipulation and fabrication in one-chip exploited for anti-counterfeiting and lab-on-a-chip is also discussed.

Time dependence of silica optical properties during the implantation of fast hydrogen ions: Computer modeling

The paper deals with a computer simulation of the creation and chemical annealing behavior of non-bridging oxygen hole centers (NBOHC) in silica exposed to long-time hydrogen implantation. Calculated time dependence of the number of NBOHCs and the intensity of red band luminescence associated with these defects were in a good agreement with experimental data. It allowed estimating the average cross-section of the radiation production of NBOHCs, the radiation-enhanced hydrogen diffusion coefficient and the reaction rate constant for NBOHC’s passivation by implanted hydrogen. We also paid attention to the physical reasons for some discrepancy between simulated dependences and experimental data as well as between calculated parameters and our previous estimations based on some simplifying assumptions.