Unraveling exciton dynamics in amorphous silicon dioxide: Interpretation of the optical features from 8 to 11 eV.

Physical review / B 83, 174201 (2011). doi:10.1103/PhysRevB.83.174201

Sturctural disorder and silanol groups content in amorphous SiO2

We present a study on the features of the Urbach edge in amorphous silicon dioxide a-SiO2 . The effects of temperature on the absorption edge in the range from 4 to 300 K were studied in both materials having negligible dry, 1017 cm−3 and significant wet, 1019 cm−3 silanol groups contents. Remarkable differences in the values and in the temperature dependence of the Urbach energy in the dry and wet samples were observed. These differences are interpreted as a consequence of a drastic reduction in the degree of disorder in wet materials, which turn out to be characterized by an electronic structure more similar to that of crystalline quartz. Furthermore, our results indicate that silanol gro…

Absorption edge in silica glass

Measurements of optical absorption in the v-UV range in a variety of silica glass are used to determine the width of the absorption edge (Urbach energy). Measured values range from 60 meV up to 180 meV. So high a variability over silica types is tentatively ascribed to the different disorder degree, which characterizes different materials.

Thermal evolution of the OH-related infrared absorption lineshape in synthetic wet silica

Temperature effects on the IR absorption bands of hydroxyl and deuteroxyl groups in silica glass

Abstract In this report we will show the changes of the band shape of the hydroxyl group (SiOH) infrared (IR) absorption band (∼3670 cm −1 ) in silica glass induced by lowering temperature in the range from 290 to 20 K. This band is considered as the overlap of several spectral components associated to the vibrational activity of hydroxyl groups in different bond configurations. By a suitable analysis of the experimental band profile in terms of different sub-bands, we studied the thermal evolution of each component and we reconsidered their assignations. For comparison we examined the SiOD absorption band (∼2710 cm −1 ) as a function of temperature as well. Our data can be interpreted as a…

Vacuum-ultraviolet absorption of amorphous SiO2: intrinsic disorder and role of silanol groups

We present a study on the vacuum-ultraviolet VUV absorption properties of amorphous SiO2 a-SiO2 with high concentrations of silanol groups Si-OH . We found that the absorption spectra are made up of a couple of exponential profiles. The first, in the range from 7.5 to 8.1 eV, was attributed to Si-OH group absorption, while the second, in the range from 8.1 to 8.25 eV, was ascribed to the intrinsic absorption. The VUV absorption cross section of Si-OH groups in a-SiO2 was determined as well. The intrinsic absorption was found to be affected by the Si-OH concentration: high silanol group contents allow lower values of the Urbach energy. This result is interpreted in terms of a reduction in th…

Irradiation effects on the absorption edge of silica glass

Abstract Vacuum ultraviolet absorption experiments were carried out on a variety of specimens of amorphous silica β-irradiated at different doses from ∼103 to 5 × 106 kGy. Changes in the width of the absorption (Urbach) edge were investigated. These changes strongly depend on the kind of silica considered: in particular the Urbach energy of silica of industrial manufacture increases in the irradiated samples, whereas in sol–gel silica it is poorly influenced by the irradiation. The fictive temperature of the different materials before and after irradiation was also monitored. The changes of the Urbach energy and of the fictive temperature are tentatively discussed considering the disorder d…

Structural organization of silanol and silicon hydride groups in the amorphous silicon dioxide network

We present a study on the effects of an isothermal annealing treatment on a-SiO 2 having a significant content of silanol hydride groups (Si-H). We examined the properties of the IR absorption bands of silanol (Si-OH) and silicon hydride groups as a function of the duration of the thermal treatment. We showed that the Si-OH and Si-H groups contents decrease in a linearly correlated way. The annealing dynamics suggest that the two species are close to each other in the amorphous network. We showed that the profile of the silanol groups absorption band is the same as that observed in other commercial a-SiO 2 materials, irrespectively of the concomitant presence of nearby Si-H groups, and, mor…

A two-component model for the 2260cm−1 infrared absorption band in electron irradiated amorphous SiO2

Abstract We report an experimental study by infrared absorption (IR) measurements focused on the effects of electron irradiation in the dose range from 1.2 × 10 3  kGy to 5 × 10 6  kGy on the intrinsic band peaked at 2260 cm − 1 in amorphous silicon dioxide (a-SiO 2 ) materials. This IR band is particularly relevant as it is assigned to an overtone of the strong asymmetric stretching vibration of Si–O–Si bridges and consequently it is intimately related to the Si–O–Si bond angle distribution. In a recent work we have shown that structural modifications induced by irradiation take place through the nucleation of confined high-defective and densified regions statistically dispersed into the w…

Inhomogeneity Effects On Point Defects Studied By Photoluminescence Time Decay In SiO2.

Evidence of delocalized excitons in amorphous solids

We studied the temperature dependence of the absorption coefficient of amorphous ${\mathrm{SiO}}_{2}$ in the range from 8 to 17.5 eV obtained by Kramers-Kronig dispersion analysis of reflectivity spectra. We demonstrate the main excitonic resonance at 10.4 eV to feature a close Lorentzian shape redshifting with increasing temperature. This provides a strong evidence of excitons being delocalized notwithstanding the structural disorder intrinsic to amorphous ${\mathrm{SiO}}_{2}$. Excitons turn out to be coupled to an average phonon mode of 83 meV energy.

Structural disorder and silanol groups content in amorphousSiO2

We present a study on the features of the Urbach edge in amorphous silicon dioxide $(a{\text{-SiO}}_{2})$. The effects of temperature on the absorption edge in the range from 4 to 300 K were studied in both materials having negligible (dry, $l{10}^{17}\text{ }{\text{cm}}^{\ensuremath{-}3}$) and significant (wet, $g{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$) silanol groups contents. Remarkable differences in the values and in the temperature dependence of the Urbach energy in the dry and wet samples were observed. These differences are interpreted as a consequence of a drastic reduction in the degree of disorder in wet materials, which turn out to be characterized by an electronic struc…

Temperature dependence of the absorption properties of silanol groups in amorphous SiO2: Are silanol groups organized in clusters?

Vacuum-ultraviolet absorption of amorphousSiO2: Intrinsic contribution and role of silanol groups

We present a study on the vacuum-ultraviolet (VUV) absorption properties of amorphous ${\text{SiO}}_{2}$ $(a{\text{-SiO}}_{2})$ with high concentrations of silanol groups (Si-OH). We found that the absorption spectra are made up of a couple of exponential profiles. The first, in the range from $\ensuremath{\sim}7.5$ to $\ensuremath{\sim}8.1\text{ }\text{eV}$, was attributed to Si-OH group absorption, while the second, in the range from $\ensuremath{\sim}8.1$ to $\ensuremath{\sim}8.25\text{ }\text{eV}$, was ascribed to the intrinsic absorption. The VUV absorption cross section of Si-OH groups in $a{\text{-SiO}}_{2}$ was determined as well. The intrinsic absorption was found to be affected by…

Temperature dependence of the absorption properties of silanol groups in amorphous : Are silanol groups organized in clusters?

Abstract We present a study on the vacuum-ultraviolet (VUV) and infrared (IR) absorption of silanol groups in amorphous silicon dioxide ( a - SiO 2 ) in the range of temperature from 4 to 300 K. The observed temperature induced modifications of IR and VUV absorption spectra are interpreted as due to a process of conversion of free into H-bonded silanol groups. The changes of the amplitude of the VUV absorption spectra are shown to be linearly correlated to the changes of the IR absorption of free Si–OH groups. This point together with the evidence that the shape of the Si–OH VUV absorption does not depend on temperature demonstrates that the different silanol group sub-species have differen…