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

对烧结气相二氧化硅的结构和PL性能进行了较详细的研究。红外和拉曼测量结果表明，完全烧结透明样品的网络结构与块体硅玻璃的网络结构平均相当。然而，烧结后的气相二氧化硅的FESEM图像表明，所得的透明样品仍然具有源自原始气相二氧化硅颗粒的边界和界面区域。时间分辨PL测量表明，烧结透明样品的PL光谱至少由位于-410、-360和-510 nm的三个发射带组成。在不同的时间尺度下，这三个PL波段分别用不同的衰减曲线来表征。我们认为光激发电子的阱控制扩散是导致缓慢拉伸指数PL分量的原因。另一方面，在经历上述色散扩散过程之前，快速纯指数PL分量是由电子-空穴对的辐射复合产生的。-360 nm PL波段在微秒级时间尺度上呈指数衰减。观察到的纯指数衰减表明，与~ 360 nm PL发射有关的电子态是局域化的，可以简单地用双态模型表示。虽然在氧化多孔硅和多孔二氧化硅中也有类似的紫外发光带，但目前的~ 360 nm发光带的衰减时间（~ μs）比以前报道的发光带的衰减时间（~ ns）要长得多，这表明这些发光带的来源不同。

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.