We propose a model of the blue-light-emitting center in silicon and silica-based materials on the basis of the density functional theory calculations. It has been shown that the dehydroxylation reaction involved in the annealing process of surface hydroxyl groups results in the formation of a defect pair consisting of a dioxasilirane, vSi͑O 2 ͒, and a silylene, vSi:, center. The resulting defect pair has the allowed electronic transition energies of ϳ3.5 and ϳ5 eV, in agreement with the peak positions of the photoluminescence excitation spectra of the blue-light-emitting materials.Visible light emission from Si-based nanostructures, including porous and nanocrystalline silicon, has generated tremendous interest in view of the intriguing emission features and the possible application of optoelectronics. 1 The visible photoluminescence ͑PL͒ bands from Si-related nanostructured materials are commonly classified into two groups depending on their characteristics decay times. One is the socalled "slow band ͑S band͒," which is characterized by the decay times of the order of several microseconds at room temperature. It has been demonstrated that the position of the S band is affected by the dimensions of Si nanocrystals, extending from about 1.3 eV ͑950 nm͒ to 2.1 eV ͑590 nm͒. This provides the possible experimental evidence that quantum confinement plays a vital role in showing the S band. 1,2 The other widely-studied PL from Si-based materials is a blue-green band with decay times in the nanosecond regime and is hence termed the fast band or "F band." In contrast to the S band, there has been no clear experimental evidence that links the F band to any Si nanostructures; 3 rather, defective oxide structures are likely to be responsible for the F band because this type of PL is observed exclusively from oxidized porous silicon and the relevant silicon oxide-based materials. 1 Some researchers suggest that carbonaceous contaminations, e.g., carbonyl-related chromophores, are responsible for the F band. 4 However, these organic chromophores are not necessarily needed to show the F bandlike output; for example, as will be shown below, high-purity nanometer-sized amorphous silica ͑SiO 2 ͒ particles called fumed silica, which has been shown to have unique structural characteristics as compared to bulk amorphous silica, 5 also exhibit a similar blue PL band in the nanosecond time domain. Thus far, the complexity of microscopic details has not allowed us to perform reasonable theoretical investigations of the F band, and the physical origin of the blue emission is still unclear and a matter of controversy.Although there has been no accepted model of the F band, the experimental evidence is provided for a surface-related defect model. 6 One possible candidate of the surface emission center is the silanol groups ͑Si-OH͒ adsorbed on the surface of silicon and/or silicon-oxide structures. 7 Since the blue PL can be generally stimulated by immersion of the oxidized porous silicon into water or by a long-term aging in air, 7,8 it i...

We have studied the structure of nanometer-sized silica particles called fumed silica, which is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame, using infrared and Raman spectroscopies and a high-energy x-ray diffraction method. It has been demonstrated that the structure of fumed silica is not identical to that of the normal bulk silica glass in terms especially of the distribution of the size of silica rings. Three-and four-membered rings are more frequent in fumed silica than in the bulk silica glass. It has also been shown that the network structure of fumed silica is more flexible than that of the bulk one, probably explaining the reason why fumed silica can accommodate a large number of three-and four-membered rings in the structure.

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