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Strong and broad near infrared (NIR) emission peaked at ∼855 nm upon optimal excitation at 342 nm has been observed from molecule-like silver clusters (MLSCs) confined in zeolite A assisted by thermal activation. To the best of our knowledge, this is the first observation of NIR emission peaked at longer than 800 nm from MLSCs confined in solid matrices. The decay time of the NIR emission is over 10 ls, which indicates that it is a spin-forbidden transition. The ∼855 nm NIR emission shows strong dependence on the silver loading concentration and the thermal activation temperature. [ABSTRACT FROM AUTHOR]

Electrochemical etching of a (110) oriented Si wafer results in a porous silicon (PSi) layer which exhibits a strong in-plane optical birefringence. We study the refractive index ellipsoid of (110) PSi by angle-resolved optical transmittance measurements and reveal that it is a biaxial crystal. The angle-resolved transmission electron microscope observations demonstrate that pores grow along the directions in between the <100> crystal axes and the etching current flow and these directions depend on the etching current density. The etching current density dependence of the pore direction indicates that the shape of the index ellipsoid can be controlled by the etching condition. [ABSTRACT FROM AUTHOR]

Boron (B) doped Si nanocrystals (Si-ncs) dispersed in hydrofluoric (HF) acid solution are prepared by dissolving borosilicate films containing B-doped Si-ncs in HF solution. We find that the etching rate of B-doped Si-ncs is much smaller than that of undoped Si-ncs. The difference of the etching rate allows us to extract only doped Si-ncs in the mixture of doped and undoped Si-ncs and observe the photoluminescence (PL) due to the transition from the conduction band to the acceptor state. The PL was very broad with the maximum around 1.15 eV. From the analysis of the PL data obtained for the samples prepared under different conditions and different etching time, preferential doping sites of B atoms are estimated. The data suggests that B-doped Si-ncs consists of intrinsic cores and heavily B-doped shells. [ABSTRACT FROM AUTHOR]

Comprehensive studies on nonlinear refractive indices (n2) of SiO2 films containing Si nanocrystals and/or nanoclusters (SiO2:Si-ncs) are performed. The comparison of the nonlinear refractive indices with the electron spin resonance signals reveals that defect states play a major role in the large n2 when the annealing temperature is low, i.e., when Si nanoclusters exist in films. On the other hand, when Si nanocrystals are grown by high-temperature annealing, the contribution of defect states becomes small and that of the quantized electronic states of Si nanocrystals becomes large. The present results demonstrate that both the defect states and the quantized electronic states should be taken into account to explain the origin of large n2 of SiO2:Si-ncs and to optimize the structure to maximize n2. [ABSTRACT FROM AUTHOR]

Nonlinear optical properties of Si nanocrystals (Si-ncs) doped SiO2 prepared by a cosputtering method were studied by z-scan technique in a femtosecond regime at around 1.6 eV. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β) were strongly enhanced compared to those of bulk Si and found to be about ∼2×10-13 cm2/W and ∼0.8 cm/GW, respectively. In the photon energy region from 1.48 to 1.65 eV, the n2 and β spectra followed the absorption spectra and no enhancement was observed in the band-edge photoluminescence region. In the diameter range of 2.7–5.4 nm, the size dependence of n2 coincided well with that calculated by a pseudopotential approach, suggesting that the discrete energy states of Si-ncs are responsible for the observed enhanced optical nonlinearity. [ABSTRACT FROM AUTHOR]

The mechanism of energy transfer from silicon nanocrystals (nc-Si) to erbium ions (Er[sup 3+]) in SiO[sub 2] films containing nc-Si and Er was studied by analyzing delayed infrared luminescence from Er[sup 3+]. It was found that, to theoretically reproduce the rising part of the time-dependent luminescence intensity, two different energy transfer processes, i.e., fast and slow processes, should be considered. From the fitting of the delayed luminescence to a model, the ratio of the two energy transfer processes and the energy transfer rate of the slow process were estimated. The ratio exhibited a clear dependence on the luminescence peak energy of Si nanocrystals, which act as photosensitizers for Er[sup 3+], indicating that the ratio depends on the size of nc-Si. The ratio of slow to fast processes increased with the decrease in size; this observation is a strong indication that the fast process is the direct inheritance of the process in bulk Si:Er systems, and the slow process is a characteristic process occurring only in nc-Si:Er systems. The energy transfer rate of the slow process was found to depend on the recombination rate of excitons in nc-Si. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The article presents a complete comparative analysis of the different optical properties exhibited by the different films containing Si nanocrystals or nanoclusters. The nonlinear refractive indices of these compounds is shown to be highly dependent on the defects present in the films.

Nonlinear optical properties of Si nanocrystals (Si-ncs) doped Si[O.sub.2] prepared by a cosputtering method are analyzed by z-scan method in a femtosecond regime at around 1.6 eV. The nonlinear refractive index and nonlinear absorption coefficient are enhanced when compared to those of bulk Si.

The mechanism of energy transfer form silicon nanocrystals (nc-Si) to erbium ions [Er(super 3+)] in SiO2 films containing nc-Si and Er is studied by analyzing delayed infrared luminescence from Er(super 3+). Findings show that the ratio of slow to fast processes increased with the decrease in size, which is a strong indication that the fast process is the direct inheritance of the process in bulk Si: Er systems, and the slow process is a characteristic process occurring only in nc-Si:Er systems.