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Recent results on the photoluminescence properties of silicon nanocrystals embedded in silicon oxide are reviewed and discussed. The attention is focused on Si nanocrystals produced by high-temperature annealing of silicon rich oxide layers deposited by plasma-enhanced chemical vapor deposition. The influence of deposition parameters and layer thickness is analyzed in detail. The nanocrystal size can be roughly controlled by means of Si content and annealing temperature and time. Unfortunately, a technique for independently fine tuning the emission efficiency and the size is still lacking; thus, only middle size nanocrystals have high emission efficiency. Interestingly, the layer thickness affects the nucleation and growth kinetics so changing the luminescence efficiency.

The local environment of light emitting silicon nanocrystals (Sync) embedded in amorphous SiO2 has been studied by x-ray absorption spectroscopy (XAS) and by ab-initio total energy calculations. Si-nc have been formed by PECVD deposition of SiOx with different Si content (from 35 to 42 at.%) and thermal annealing at high temperature (1250 C). The comparison between total electron yield (TEY) and photoluminescence yield (PLY) spectra has allowed the identification of a modified region of SiO2 (about 1 nm thick) surrounding the Si-nc, which participates to the light emission of Si-nc. Total energy calculations, within the density functional theory, clearly show that Si-nc are surrounded by a cap-shell of stressed SiO2 with a thickness of about 1 run. The optoelectronic properties show the appearance of localized states not only in the Sinc core region but also in the modified SiO2 region.

EXAFS temperature dependent measurements have been carried out on the borate glass doped with silver iodide, (AgI)(0.55)(Ag2O.4B2O3)(0.45), at the K-edges of Ag and I, in the range 39-500 K Local negative thermal expansion has been measured in the explored temperature range; in the same range, crystalline AgI shows a positive trend. This behaviour, already found for another silver borate glass containing a higher content of silver iodide, was interpreted in terms of a relaxing of the AgI amorphous network within the silver borate host matrix. In the light of the results obtained for the glass (AgI)(0.55)(Ag2O.4B2O3)(0.45), having a much lower AgI concentration and showing an almost equal thermal expansion, a different interpretation is now necessary. The Ag-I first shell distance contraction is a specific characteristic of the AgI amorphous network within the silver borate host matrix: temperature increase favours shorter average distance of the mobile Ag+ cations from I ions. The host matrix influences, however the local thermal disorder around Ag and I species.

Light-emitting silicon nanocrystals embedded in SiO2 have been investigated by x-ray absorption measurements in total electron and photoluminescence yields, by energy filtered transmission electron microscopy and by ab initio total energy calculations. Both experimental and theoretical results show that the interface between the silicon nanocrystals and the surrounding SiO2 is not sharp: an intermediate region of amorphous nature and variable composition links the crystalline Si with the amorphous stoichiometric SiO2. This region plays an active role in the light-emission process.

CMOS circuitry dominates the current semiconductor market due to the astonishing power of silicon electronic integration technology. In contrast to the dominance of silicon in electronics, photonics utilises a diversity of materials for emitting, guiding, modulating and detecting light. In the last ten years a big research effort was aimed to render Si an optical active material so that it can be turned from an electronic material to a photonic material. For some the future of Si-based photonic lays in 'hybrid' solutions, for others the utilisation of more photonic functions by silicon itself. During the last two years many breakthroughs in the field have appeared. In this paper we will review what we believe the most important: optical gain in silicon nanostructures.

In recent decades, biocompatible and light emitting porous silicon (pSi) showed the possibility for use in biomedical applications. Unfortunately, until now the biological applications of light-emitting pSi have been severely limited due the immediate surface oxidation of this nanostructured material, followed by loss of optical properties and degradation in aqueous solutions. In view of drug delivery, the red-ox activity of pSi damages the carried drug. Herein, we have investigated the stabilization of pSi microparticles in aqueous solution by covalent attachment of polymers to a short-chain organic coating grafted onto inorganic silicon. The morphological, chemical and optical stability obtained using polyethylene glycol confirm this biocompatible polymer as a superior shell matrix nanostructure for use in biological environments.