Electron spin resonance probing of E′-type defects in fumed silica nanoparticles

Results are presented of an electron spin resonance (ESR) characterization of E′ type defects observed in fumed ∼ 7-nm sized silica nanoparticles. This is accomplished through K- and Q-band ESR following 10-eV irradiation used to photo-dissociate H from passivated defects. The defect analysis provides access to the structure of the nanoscale on atomic level, herewith adding an extra dimension for analysis. To assess specific physico-chemical structural aspects of the particles, the E′ centers have been monitored as a function of thermal treatment in vacuum in the range 850–1115 °C, revealing the presence of two systems of E′ centers. The first one exhibits ESR parameters very similar to those of the familiar E′γ center encountered in bulk fused silica, while the second bath exhibits an altered zero crossing g value and line shape, attributed to variations in local structure. While the first E′ system most likely resides in the core regions of the nanoparticles, the second system may mainly be confined to the outer SiO2 layers exposing a structurally different, possibly more strained nature than macroscopic glassy SiO2. Probing of fundamental point defects in nanometer-sized silica particles thus reveals structural variations and non-uniformity, indicative of a structure different from bulk material.