Structure and formation mechanism of blue-light-emitting centers in silicon and silica-based nanostructured materials

Abstract

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...

Facile Synthesis and Luminescence Properties of Silica Submicron Rods

“…This theory has shown that the photoluminescence excitation wavelength of this defect pair is in the range of 240-364nm, which is in agreement with the photoluminescence excitation wavelength of silica gel at 350nm. Based on this we measure the photoluminescence emission wavelength of silica submicron rods and from Fig.3 we can easily see that the silica submicron rods exhibited blue light [18], which is in agreement with the result of the literature. And we can conclude that the blue emission of the obtained silica submicron rods is caused by the surface-associated defects.…”

Oxides of third period elements revisited: synthesis, structure and photoluminescence properties of silica, magnesia, and alumina

“…1), 44) similar to the case of oxidized porous silicon. Thus, the blue PL characteristics that were originally found in oxidized porous silicon can also be seen in silica-based nanostructured materials.…”

Thermal quenching properties of ultraviolet emitting centers in mesoporous silica

“…1,2 On the other hand, blue-green emission is ascribed to surface-related defects, including both C or OH groups adsorbed at the surface of the porous material. [1][2][3][4][5][6] As regards silica-based nanostructered materials, red, green, and blue emissions from silica nanoparticles, mesoporous silica, and silica nanotubes were reported with strong similarities with the spectroscopic features observed in nanoscaled silicon. [7][8][9][10] Since the thickness of the bulk silica walls in nanostructured silica materials ͓ϳ1 nm ͑Ref.…”

“…1 Beside the technological interest, the topic is also relevant from a basic research point of view, since the attribution of these optical features to quantum confinement effects or to defect-related mechanisms is still debated. [1][2][3][4][5][6] Concerning porous and nanocrystalline silicon, the emissions in the red to near-infrared region ͑2.1-1.3 eV͒ depend on the dimensions of the Si nanocrystals, providing experimental evidence of the role of the quantum confinement effects. 1,2 On the other hand, blue-green emission is ascribed to surface-related defects, including both C or OH groups adsorbed at the surface of the porous material.…”

“…It was recently reported that the blue emission in silicon nanostructure is due to a defect pair consisting of a dioxasilirane center and a silylene center formed by the de- hydroxylation of surface silanol groups. 6 The dependence of water content or aging condition of the blue band in silicon nanostructure strongly recalls that of the UV bands in porous silica, suggesting a dehydroxylationlike model also for these centers, where the interacting silanol groups are regarded as indirectly responsible for the UV emissions. Since the contribution of different types of interacting silanols depends on the porosity of the samples and the two centers showed very similar spectroscopic properties, it is interesting to investigate possible energy transfer or conversion mechanisms between the two species.…”