Infrared, Raman and Magnetic Resonance Spectroscopic Study of SiO2:C Nanopowders

Abstract

Optical and magnetic properties of SiO2:C nanopowders obtained by chemical and thermal modification of fumed silica were studied by Fourier transform infrared spectroscopy, Raman, continuous wave (CW) electron paramagnetic resonance (EPR), echo-detected EPR and pulsed electron nuclear double resonance (ENDOR) spectroscopy. Two overlapping signals of Lorentzian lineshape were detected in CW EPR spectra of the initial SiO2:C. The EPR signal at g = 2.0055(3) is due to the silicon dangling bonds, which vanishes after thermal annealing, and the second EPR signal at g = 2.0033(3) was attributed to the carbon-related defect (CRD). The annealing of the SiO2:C samples gives rise to the increase of the CRD spin density and shift to the higher g-values due to the appearance of the oxygen in the vicinity of the CRD. Based on the temperature-dependent behavior of the CRD EPR signal intensity, linewidth and resonance field position we have attributed it to the spin system with non-localized electrons hopping between neighboring carbon dangling bonds, which undergo a strong exchange interaction with a localized spin system of carbon nanodots. The observed motional narrowing of the CRD EPR signal in the temperature interval from 4 to 20 K indicates that electrons are mobile at 4 K which can be explained by a quantum character of the conductivity in the vicinity of the carbon layer. The electrons trapped in quantum wells move from one carbon nanodot to another by hopping process through the energy barrier. The fact that echo-detected EPR signal at g = 2.0035(3) was observed in SiO2:C sample annealed at T ann ≥ 700 °C serves as evidence that non-localized electrons coexist with localized electrons that have the superhyperfine interaction with surrounding 13C and 29Si nuclei located at the SiO2:C interface. The presence of the superhyperfine interaction of CRD with 1H nuclei indicates the existence of hydrogenated regions in SiO2:C sample.

Multiband light emission and nanoscale chemical analyses of carbonized fumed silica

“…Our attempts to identify pyrolytic carbon by Raman scattering using laser excitations at either 488 nm or 514 nm were not successful due to the strong background of the PL excited by the laser probe. As it has been demonstrated previously, carbon related Raman scattering can be detected in phenylsilica after annealing at temperature as high as 800 C. 27 These samples are black in colour, not luminescent under UV/VIS excitation, and well-defined Raman scattering spectra composed of typical D-and G-bands can be detected.…”

“…At higher treatment temperature, we observe a red shift and a decrease in the PL intensity: after 600 C, the emission consists of a single band centered around 450 nm (2.75 eV) with a FWHM close to 0.7 eV, while after 650 C the PL is centered around 490 nm (2.5 eV) with a FWHM close to 0.8 eV. We note that the PL is no longer observed in these experiments after thermal treatments at 700 C and higher when using UV excitation although it was demonstrated previously 27 that noticeable emission can be detected in the samples after annealing at 700 C when excited by visible and/or infrared laser.…”

Effect of Hydration Procedure of Fumed Silica Precursor on the Formation of Luminescent Carbon Centers in SiO2:C Nanocomposites

“…Any of the above mentioned mechanisms reduces the energy of emitted photons and broadens energy distribution of emission transitions. Further increase of the annealing temperature results in further red shift and quenching of PL that correlates with “graphitization” of carbon precipitates …”

Molecular characteristics of Salix cheilophila chemical components

“…The FT-IR is a rapid detection technology, with a high sensitivity, which is applied in the identification of chemical bond and functional groups of various compounds [7][8][9]. In this experiment, the changes of the Salix cheilophila and extracted residues' chemical groups were shown in fig.…”