Non-bridging Oxygen Hole Center Defects Research Articles

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.

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.

Multifunctional Ag nanoparticle decorated Si nanowires for sensing, photocatalysis and light emission applications

We report on the fabrication of Ag nanoparticle (NP) decorated mesoporous Si nanowire (NW) heterostructure (HS) by a simple and low cost chemical process. The as-grown Si NWs are mesoporous in nature and the Ag NP decorated Si NWs (Ag@Si NWs) exhibit broadband light emission, ultralow reflectance, efficient photocatalytic degradation of organic dyes and excellent sensitivity for the detection of organic molecules over a wide range of concentration. The broadband white light photoluminescence emission from the bare Si NWs is explained on the basis of quantum confinement effect in Si NCs/NWs and the nonbridging oxygen hole center defects in the SiSiOx interface. High work function of the noble metal NPs facilitates the effective separation of the photoinduced electron-hole pairs in Si NWs, which enables the Ag@Si NWs to exhibit high photocatalytic efficiency for the degradation of organic dye. The Ag@Si NWs exhibited high potential and sensitivity for the selective and quantitative detection of different organic molecules at extremely low concentration down to 10−12 M by surface-enhanced Raman scattering and 10−11 M by fluorescence-based detection. These versatile properties of the Ag@Si NWs open up opportunities for a variety of energy and environmental applications, such as white light emission, solar cell, artificial photosynthesis, disposal of organic pollutant and bio-chemical sensors etc.

Cathodo- and photoluminescence emission of a natural Mg-Cr carbonate layered double hydroxide

Abstract Spectral emission of a natural layered double hydroxide (LDH) studied by cathodo- and photoluminescence (PL), displays complex glow curves with wavebands that could be attributable to (i) the paramagnetic Non-bridging Oxygen Hole Center defects (peaked at 330 nm), (ii) F o centers (oxygen vacancies trapping two electrons) associated with O vacancy-hole centers and Me–O bonding defects and dehydroxylation process (in the range of 360–500 nm), (iii) presence of Mn 2+ (0.15%) substituting Mg 2+ ions (530–640 nm) and (iv) a red-IR band mainly associated with Cr and Fe point defects. The presence of Cr 3+ ion activators induces characteristic PL emission peaks in the red-infrared region at 681, 688 and 696 nm linked to 2 E → 4 A 2 transitions. The sample (Mg 6 Cr 2 CO 3 (OH) 16 ·4H 2 O) previously characterized by environmental scanning electron microscope, X-ray diffraction, and X-ray fluorescence indicates that this natural hydrotalcite-like compound is associated with hydroxyl-rich silicates (nimite and clinochlore) and Mg-Fe carbonate and Ni-Fe carbonate LDH (pyroaurite and reevesite).

掺Yb3+石英玻璃中非桥氧空穴缺陷特性的研究

掺Yb³⁺石英玻璃中非桥氧空穴缺陷特性的研究

Properties of Non-Bridging Oxygen Hole Centers Defects in ${\rm Yb}^{3+}/{\rm Al}^{3+}$ Co-Doped Photonic Crystal Fiber by Using Powder Melting Technology

The Yb3+/Al3+ co-doped silica glass is prepared using the powder melting technology based on the high-frequency plasma furnace. The typical absorption and emission pattern of Yb3+ ions in the Yb3+/Al3+ co-doped silica glass and photonic crystal fiber (PCF) fabricated in our laboratory are demonstrated. The additional absorption peak at 260 nm and the 200 nm-excited emission peak at 630 nm indicate that the non-bridging oxygen hole centers (NBOHC) defects exist in the silica glass. Moreover, the luminescence peak at 735 nm is also related to the NBOHC defects. Spectral evolution of the NBOHC defects luminescence in relation to the excitation wavelength, excitation power, and PCF length are also analyzed. The laser slop efficiency of the PCF is 12.2%.

Role of hydrogen loading and glass composition on the defects generated by the femtosecond laser writing process of fiber Bragg gratings

The creation of fiber Bragg gratings (FBGs) in optical fibers by laser irradiation causes the formation of defects in the modified glass. We have used confocal fluorescence spectroscopy to identify the location and types of defects formed after writing FBGs with the femtosecond laser phase mask technique. Our results show that non-bridging oxygen hole centers (NBOHCs) and self-trapped excitons (Eδ’) are formed throughout all-silica core Sumitomo Z-fiber. Similar defects are observed for Ge-doped silica fiber, Corning SMF-28, but in this case the relative concentrations of NBOHC and Eδ’ vary from the core to the cladding. In both fibers, hydrogen loading prior to irradiation appears to passivate the defects except in the Ge-doped core where the NBOHC defects persist.

A spectroscopic comparison of femtosecond-laser-modified fused silica using kilohertz and megahertz laser systems

Waveguides were written in fused silica using both a femtosecond fiber laser with a 1MHz pulse repetition rate and a femtosecond amplified Ti:sapphire laser with a 1kHz repetition rate. Confocal Raman and fluorescence microscopies were used to study structural changes in the waveguides written with both systems. A broad fluorescence band, centered at 650nm, associated with nonbridging oxygen hole center (NBOHC) defects was observed after waveguide fabrication with the megahertz laser. With the kilohertz laser system these defects were only observed for pulse energies above 1μJ. Far fewer NBOHC defects were formed with the megahertz laser than with kilohertz writing, possibly due to thermal annealing driven by heat accumulation effects at 1MHz. When the kilohertz laser was used with pulse energies below 1μJ, the predominant fluorescence was centered at 550nm, a band assigned to the presence of silicon clusters (Eδ′). We also observed an increase in the intensity of the 605cm−1 Raman peak relative to the total Raman intensity, corresponding to an increase in the concentration of three-membered rings in the lines fabricated with both laser systems.

Enhanced and stable photoluminescence from partially oxidized porous Si coated with Si thin films

Photoluminescence (PL) spectra of partially oxidized porous Si (POPS) coated with Si thin films were examined using the 488 nm line of Ar+ laser. The obtained PL is stable, peaks at 1.763 eV with a blueshift of ∼60 meV, and its maximal intensity is seven times larger than that of the POPS. Spectral analysis and the experimental results from infrared spectroscopy and electron spin resonance suggest that the enhanced and stable PL arises from optical transitions in the nonbridging oxygen hole centers (NBOHCs). Si coating mainly leads to introduction of the NBOHCs defects and thus makes the PL intensity enhanced. The blueshift of ∼60 meV is a result of the local equilibrium of NBOHCs defects under high temperature.

Characterization of Si+ ion-implanted SiO2 films and silica glasses

We report here electron spin resonance (ESR), Raman scattering, photoluminescence (PL), and absorption studies of Si+ ions implanted into silica glasses and thermally grown SiO2 films on Si wafers. The aim is to understand the defect formation and luminescence mechanism as the films were annealed at several temperatures. In as-grown Si-implanted films, paramagnetic defects in the form of Si–E′ and nonbridging oxygen hole centers (NBOHC) were detected by ESR. A luminescence band, centered at 650 nm, was observed in these samples. The E′ and NBOHC defects were annealed out in samples annealed above 500 °C, but Si dangling bonds in the form of Pb centers were observed. In films annealed above 800 °C, Si nanocrystals of varying size between 1 and 5 nm were formed, as detected by transmission electron microscopy. A red PL band (>700 nm) and a Si-phonon band at 510 cm−1, gradually evolved as a function of anneal temperature. The observation of the Si-phonon mode with a 15 cm−1 shift in the peak position from the bulk affirms the presence of Si clusters in the annealed ion-implanted SiO2 films and silica glasses. We attribute this red PL band to excitonic recombination in Si nanocrystals. The peak position of this PL band was found to shift with excitation wavelength. The shift of the PL spectra as a function of excitation energy was interpreted as the emission from Si particles of certain sizes that are excited by the incident energy. An energy band gap distribution due to a size distribution was used to model the PL spectra at each excitation energy.

Non-linear γ-ray activation of defect spins in vitreous silica

Previously published data showing electron spin resonance spin count versus ionizing radiation dose in bulk vitreous SiO 2 are compared for the two case of ∼ 1.25 MeV γ-rays and ∼ 8 keV X-rays. For both Suprasil 1 and Suprasil W1 materials, the non-linear production of paramagnetic states with increasing dose for γ-rays at doses below ∼ 175 Mrad is shown to have nearly identical forms to those for X-rays at doses below ∼ 8 Mrad. It is then argued that the non-linearity of the γ-ray data has the same physical origin as previously ascribed to the non-linearity of the X-ray data: an exponentially saturating ‘activation’ of pre-existing defects, accompanied by a linearly increasing number of newly created defects. The data for E′ and non-bridging oxygen hole center defects show that ∼ 1.25 MeV γ-rays are at least an order of magnitude more efficient per unit of dose for activating pre-existing defects than are ∼ 8 keV X-rays.

Radiation damage and the role of structure in amorphous SiO 2

The influence of structure on the creation efficiency of oxygen-vacancy center and non-bridging oxygen-hole center defects in amorphous SiO 2 by ionizing radiation is examined. In undensified, amorphous SiO 2 the defect creation depends significantly upon the OH content of the network because of self-annealing effects due to the trapping of O 2 molecules (dry silica) or water-related molecules (wet silica). Oxygen vacancy defect creation is argued to be due to displacement of network O atoms — precursor transformation accounts for at most 10 14 cm −3. It is suggested that strained Si-O-Si bonds may act as precursors for both oxygen vacancy and nonbridging oxygen defects. Metastable densification of the SiO 2 structure results from a process of Si-O-Si intertetrahedral bridging bond angle reduction coupled with a reduction in the average number of members in a ring. Diffusion of molecules involved in defect annealing is found to be impeded by the densification process. Defect creation is found to be very much enhanced in densified, amorphous SiO 2 produced by hydrostatic compression or resulting directly from deposition in chemical vapour deposited films. This is interpreted as due to a process of defect creation due to strained bond cleavage. The enhancement is found to be up to several orders of magnitude for oxygen vacancy defects and of the order of 40 for nonbridging oxygen defects. The bond cleavage interpretation appears to be valid for both wet and dry forms of densified silicas.