We report on the spectral and dynamical properties of photoluminescence under ultrashort (femtosecond and picosecond) laser pulse excitation in Si nanocrystals prepared by Si+-ion implantation into silica matrix. We concentrate on the luminescence properties that make it possible to distinguish several luminescence bands within a broad luminescence spectrum. In time-integrated luminescence saturation-related changes suggest a presence of two different recombination channels connected to the nanocrystal volume states and nanocrystal-SiO2 interface. The saturation of the luminescence can be explained in terms of Auger recombination. We investigated in detail fast sub-nanosecond luminescence, where significant differences were observed under UV excitation (linear pump-intensity dependence, PL maximum at 450 nm) and for the excitation wavelengths in the visible part of spectrum (superlinear pump-intensity dependence, broad luminescence band including wavelengths as short as 300 nm). We propose a rate-equations model based on the Auger and radiative/nonradiative recombination to interpret the experimental data.

We present a study of ultrafast carrier transfer from highly luminescent states inside the core of silicon nanocrystal (due to quasidirect transitions) to states on the nanocrystal-matrix interface. This transfer leads to a sub-picosecond luminescence decay, which is followed by a slower decay component induced by carrier relaxation to lower interface states. We investigate the luminescence dynamics for two different surface passivation types and we propose a general model describing spectral dependence of ultrafast carrier dynamics. Our results stress the crucial role of the energy distribution of the interface states on surface-related quenching of quasidirect luminescence in silicon nanocrystals. We discuss how to avoid this quenching in order to bring the attractive properties of the quasidirect recombination closer to exploitation.

We report on ultrafast photoluminescence (PL) spectroscopy of blue and red-emitting silicon nanostructures. We prepared samples of porous Si with different dominating
PL bands in time-integrated PL spectrum. These samples show also striking differences in ultrafast PL dynamics. For the red-emitting samples we observe rapid PL onset followed by a two-exponential decay, which is spectrally dependent. For the blue-emitting porous Si the PL builds up several picoseconds and decays monoexponentially with spectrally independent PL rise and decay times.
Our results indicate that a dominant part of the blue and red emission comes from two distinct species with
different energy states structures. This is furthermore supported by two-photon absorption induced PL measurements. The two-photon and one-photon absorption induced PL spectra, which are identical for the red-emitting samples, differ substantially for the blueemitting ones. However, polarization properties of PL suggest that the blue PL band originates from several overlapping PL bands with sample dependent importance.

Silicon nanocrystals are an extensively studied light-emitting material due to their inherent biocompatibility and compatibility with silicon-based technology. Although they might seem to fall behind their rival, namely, direct band gap based semiconductor nanocrystals, when it comes to the emission of light, room for
improvement still lies in the exploitation of various surface passivations. In this paper, we report on an original way, taking place at room temperature and ambient pressure, to replace the silicon oxide shell of luminescent Si nanocrystals with capping involving organic residues. The modification of surface passivation is evidenced by both Fourier transform infrared spectroscopy and nuclear magnetic resonance measurements. In addition, single nanocrystal spectroscopy reveals the occurrence of a systematic fine structure in the emission single spectra, which is connected with an intrinsic property of small nanocrystals since a very similar structure has recently been observed in specially passivated semiconductor CdZnSe nanoparticles. The organic capping also dramatically changes optical properties of Si nanocrystals (resulting ensemble photoluminescence quantum efficiency 20%, does not deteriorate, radiative lifetime 10 ns at 550 nm at room temperature). Optically clear colloidal dispersion of these nanocrystals thus exhibits properties fully comparable with direct band gap semiconductor nanoparticles.

článek vyjde do konce roku 2008