Luminescence of modified nonbridging oxygen hole centers in silica and alkali silicate glasses

Abstract

This paper reports on the results of the investigation of the cathodoluminescence spectra of silica and alkali silicate glasses upon excitation with a pulsed electron beam (energy, 180 keV; current density, 700 A/cm 2 ; pulse duration, 2 ns). The luminescence band observed in the energy range 2.4-2.6 eV is assigned to modified structural defects of the ≡ Si-O·/ Me + type. These defects are revealed under high-density electronic excitation and, unlike the known L centers in alkali silicate glasses, are interpreted as a variety of nonbridging oxygen hole centers (defects of the dangling bond type) subjected to a disturbing action of the nearest neighbor alkali metal cations. The cathodoluminescence of similar centers is observed in neutron-irradiated silica glasses with lithium impurities; alkali silicate glasses with Li, Na, and K cations; and glasses in the two-alkali Na-K systems. It is established that the energy of the radiative transition of a modified nonbridging oxygen hole center, namely, ≡ Si-O·/ Me + , depends on the alkali cation type.

Structure and luminescence of intrinsic localized states in sodium silicate glasses

“…A more recent analysis of the photoluminescence suggested that alkali silicate glasses have at least two types of L centers, L 1 and L 2 , which are strongly dissimilar in their spectral and kinetic characteristics [13,16,17]. This was explained by the fact that the glass micro-structure contains fragments with different degrees of disorder in the environment of the L center.…”

Competition between green self-trapped-exciton and red non-bridging-oxygen emissions in SiO2 under interband excitation

“…2a we report the photoluminescence contour plots at 8 K of four silica variants (namely, p-spr, d-spr, p-cvd and s-spr) as a function of the bandto-band excitation energy, compared with the patterns excited in the sub-band-gap vacuum ultraviolet (VUV) range, which exemplify the variety of optical responses encountered in our study. Sub-band-gap VUV-excited spectra is dominated by defect-related emissions with excitation bands in the 6.5-8 eV region, ascribable to high excited states of oxygen-deficient centres (ODC) and NBO, with main emissions in the region 2.5-3.2 eV and at 1.9 eV 13,[27][28][29][30][31][32]34 . This sub-band-gap VUVexcited response is consistent with previous data, mainly showing ODC luminescence excited at about 5 eV, specifically in Sn-doped silica 29,42 , Ge-doped silica 29,42,43 , Si-rich silica 29,44 and silica from chemical vapour deposition (CVD) 45 , either treated and untreated.…”

“…e Schematics of energy levels with radiative (continuous arrows) and non-radiative (dashed arrows) transitions involved in STE decay and subsequent relaxation to α or β excited levels responsible for red and green emissions, respectively; Kinetic rates are indicated as in the text. f Representative experimental or theoretical photoluminescence spectra of NBO variants including: free NBO 30 , NBOs on crystal surfaces (cry-SiO 2 ) 36 , and alkali-perturbed NBOs 27 . g Photoluminescence spectra excited at 13.8 eV of pure synthetic quartz (p-q) at 8 K (blue curve) and 300 K (red curve).…”

“…In fact, X-ray-, electron-beam- 17,24,26 and two-photon-excited photoluminescence spectra 26 , typically show red and green-blue emissions in silica and green-blue luminescence in quartz. The greenblue component at 2.4-2.7 eV is commonly ascribed to STE and shares the spectral region with defect-related contributions of hydrogen-or alkali-decorated NBOs 26,27 and oxygen-deficient centres 28 . These defect-related emissions possess quite long lifetimes, similarly to the STE, and in some conditions with very similar values, from 8 to 12 μs at 300 K and few 10 1 μs at 90 K for hydrogen-perturbed NBOs 26 to about 10 ms, 112 μs and 10 μs for oxygen-deficient twofold coordinated Si, Ge and Sn sites in amorphous SiO 2 , respectively, at 300 K 29 .…”

“…The red photoluminescence at 1.9 eV is, instead, associated with NBOs, which are amongst the most investigated point defects in silica 30,31 , with main excitation channels at 2.0 eV, 4.8 eV and 6.8 eV and relatively long lifetime of about 10-20 μs [32][33][34] . Variants with slightly different spectral features due to interactions with nearby functional groups 26,27 or bridging 35 and non-bridging oxygens 36 are also commonly observed. Despite the fundamental role of elementary NBO point defects in bond-breaking events in SiO 2 , their red photoluminescence has never been observed in quartz, except for locally amorphized crystals by neutron irradiation 37,38 .…”