Primary ²⁹Si hyperfine structure of E^′ centers in nm-sized silica: Probing the microscopic network structure

Point defects in fumed ∼7-nm-sized fumed silica nanoparticles have been studied by X-, K-, and Q-band electron spin resonance (ESR) following 10-eV irradiation. The E^′ defects are monitored as a function of post manufacture heat treatment with the sample brought into contact with “bulk” Si∕SiO₂ entities at elevated temperatures in vacuum (T_an=1005–1205 °C), i.e., the presence of an Si∕SiO₂ interface. This results in a drastic increase in E^′ defect density with increasing T_an, enabling us to resolve the primary ²⁹Si hyperfine (hf) structure of the E^′ centers located in the core region of the nanoparticles. Detailed analysis of the observed hf spectra reveals several items pointing to a modification of the specific network structure of the core region of the nanoparticles. An increased hf splitting of 438±2 G is observed compared to bulk silica (418±2 G) indicating that the core part E^′ centers exhibit a more pyramidal defect structure. Moreover, the increased primary hf splitting indicates that the core of the fumed silica particles is densified, possibly associated with the presence of more low-membered rings in the nm-sized silica network.