Structure and optical properties of porous silica modified by electronic excitation process

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We have studied the structure and PL properties of sintered fumed silica in detail. From the results of infrared and Raman measurements, it has been shown that the network structure of the fully sintered transparent sample is, on average, comparable to that of the bulk silica glass. However, the FESEM image of the sintered fumed silica demonstrates that the resulting transparent sample still has boundary and interface regions derived from the original fumed silica particles. Time-resolved PL measurements have revealed that the PL spectrum of the sintered transparent sample consists at least of three emission bands located at -410, -360, and -510 nm. These three PL bands are characterized respectively by different decay curves in different times scales. We suggest that a trap-controlled diffusion of the photoexcited electrons is responsible for the slow stretched exponential PL component. On the other hand, the fast pure exponential PL component results from radiative recombinations of electron-hole pairs before experiencing the above dispersive diffusion process. The -360-nm PL band shows a pure exponential decay in the time scale of microseconds. The observed pure exponential decay implies that the electronic states related to the 〜360-nm PL emission are localized and can be represented simply by a two state model. Although similar UV PL bands have been reported in oxidized porous silicon and porous silica, the decay time (〜μs) of present 〜360-nm PL is much longer than that of the previously reported ones (〜ns), suggesting that these PL bands have different origins.