PrimarySi29hyperfine structure ofE′centers in nm-sized silica: Probing the microscopic network structure

Point defects in fumed $\ensuremath{\sim}7\text{\ensuremath{-}}\mathrm{nm}$-sized fumed silica nanoparticles have been studied by $X$-, $K$-, and $Q$-band electron spin resonance (ESR) following $10\text{\ensuremath{-}}\mathrm{eV}$ irradiation. The ${E}^{\ensuremath{'}}$ defects are monitored as a function of post manufacture heat treatment with the sample brought into contact with ``bulk'' $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_{2}$ entities at elevated temperatures in vacuum $({T}_{\mathrm{an}}=1005\char21{}1205\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, i.e., the presence of an $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_{2}$ interface. This results in a drastic increase in ${E}^{\ensuremath{'}}$ defect density with increasing ${T}_{\mathrm{an}}$, enabling us to resolve the primary $^{29}\mathrm{Si}$ hyperfine (hf) structure of the ${E}^{\ensuremath{'}}$ 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 $438\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{G}$ is observed compared to bulk silica $(418\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{G})$ indicating that the core part ${E}^{\ensuremath{'}}$ 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.