Probing energy transfer in an ensemble of silicon nanocrystals

Time-resolved photoluminescence measurements of silicon nanocrystals formed by ion implantation of silicon into silicon dioxide reveal multi-exponential luminescence decays. Three discrete time components are apparent in the rise and decay data, which we associate with different classes of nanocrystals. The values of decay time are remarkably constant with emission energy, but the relative contributions of the three components vary strongly across the luminescence band. In keeping with the quantum confinement model for luminescence, we assign emission at high energies to small nanocrystals and that at low energies to large nanocrystals. By deconvolving the decay data over the full emission band, it is possible to study the migration of excitation from smaller (luminescence donor) to larger (luminescence acceptor) nanocrystals. We propose a model of diffusion of excitation between neighboring nanocrystals, with long lifetime emission being from the largest nanocrystal in the local neighborhood. Our data also allow us to study the saturation of acceptor nanocrystals, effectively switching off excitation transfer, and Auger recombination in non-interacting nanocrystals. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3622151

nanograined anatase titania based optochemical gas detection

Optochemical sensing properties of thick films of titanium dioxide (titania) in anatase phase have been studied and compared with tin dioxide cassiterite. Anatase titania exhibits a large photoluminescence response to nitrogen dioxide, which acts as a luminescence enhancer. Intrinsic surface phenomena rather than bulk defectivity are proposed to account for the behaviour and the experimental results are fitted with the Langmuir model. Good operational performances working at room temperature are achieved

Optical characterisation of silicon nanoclusters embedded in SiO2 and SiOxNy matrices

The original work presented in this thesis concentrates on the origin of the visible and near-IR luminescence from silicon nanocrystals (Si-NCs) embedded in two different matrices, namely SiO2 and SiOxNy, and prepared by three different chemical vapour deposition (CVD) techniques and ion implantation. The optical properties of these materials were studied by time-integrated photoluminescence, time correlated single photon counting (TCSPC) and time-resolved PL spectroscopy (TRPL) while the structural and compositional characterisation was carried out using ellipsometry, Fourier Transform Infrared spectroscopy (FTIR), Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS). As opposed to the near-IR emission, well investigated and commonly associated with phonon-assisted excitonic recombination, the visible emission from Si-NCs in SiO2 is still subject to debate, with the majority of studies pointing to a defect-related origin. Recent works suggested that it is possible to achieve no-phonon direct-like transitions in Si nanostructures and thus extract a more efficient emission in the visible range. These works have sparked new interest in the high-energy emission from Si-NCs and motivated that part of the present research that aims at exploring the possibility of radiative recombinations in a process that does not involve phonons. In this work, the analysis of the PL decay curves evidenced the coexistence of multiple lifetimes, with components mainly grouped around two regions in the microsecond and picosecond domain. The decrease in PL intensity with annealing is correlated with the shortening of PL lifetime, characteristic of a class of non-radiative defects. Although my time resolved PL study shows the presence of ultra-fast lifetime components that could play a role in the populations of energy levels at the point, there is no evidence in my samples for no-phonon recombinations via direct channels. Ion implanted samples of Si-NCs embedded in SiO2 were investigated by means of PL and TRPL at different photon fluxes. No significant dependence of the PL dynamics on emission energy was observed, indicating that decay lifetimes are not linked to different discrete sizes in the NCs size. I proposed a model of diffusion of excitation between neighbouring nanocrystals, with the saturation of acceptor nanocrystals providing a switching-off mechanism of the excitation transfer. The evolution of the PL emission with increasing silicon excess and annealing temperatures was found to be in agreement with a diffusion limited, Ostwald ripening process. For lower temperature treatments, a factor of 5 PL enhancement was observed and attributed to a thermal-activated carrier “recovery mechanism”, i.e. de-trapping of carriers from localized states within the band-gap to the states of Si-NCs. I propose that the PL contribution at around 2 eV observed in the same low temperature regime (10-100 K) arises from Si-NC-sensitized luminescent defects. At higher temperatures a monotonic quenching of the PL peak emission was observed and attributed to the enhancement of non-radiative recombination from defect states. Finally the Si-NC PL intensity in SiOxNy films was studied and found to be strongly dependent on the annealing conditions. RBS and XPS measurements showed that the composition of the thin films is significantly affected by oxygen contamination. Surprisingly, only the PL spectra of the as-deposited samples are well correlated with the evolving Si-NC size according to the quantum confinement (QC) model in which thin films containing larger clusters emit at lower energy. The formation of cracks after annealing the samples at temperatures from 400 °C to 1150 °C for 1 hour in forming gas, results in the suppression of the emission in the near-IR and in the arising of a defect-related emission peaking at higher energy

Time-correlated single-photon counting study of multiple photoluminescence lifetime components of silicon nanoclusters

We report time-resolved photoluminescence measurements of thin films of silica containing silicon nanoclusters (Si NCs), produced by PECVD and annealed at temperatures between 700 °C and 1150 °C. While the near infrared emission of Si NCs has long been studied, visible light emission has only recently attracted interest due to its very short decay times and its recently-reported redshift with decreasing NCs size. We analyse the PL decay dynamics in the range 450–700 nm with picosecond time resolution using Time Correlated Single Photon Counting. In the resultant multi-exponential decays two dominant components can clearly be distinguished: a very short component, in the range of hundreds of picoseconds, and a nanosecond component. In this wavelength range we do not detect the microsecond component generally associated with excitonic recombination. We associate the nanosecond component to defect relaxation: it decreases in intensity in the sample annealed at higher temperature, suggesting that the contribution from defects decreases with increasing temperature. The origin of the very fast PL component (ps time region) is also discussed. We show that it is consistent with the Auger recombination times of multiple excitons. Further work needs to be done in order to assess the contribution of the Auger-controlled recombinations to the defect-assisted mechanism of photoluminescence

Time-correlated single-photon counting study of multiple photoluminescence lifetime components of silicon nanoclusters

We report time-resolved photoluminescence measurements of thin films of silica containing silicon nanoclusters (Si NCs), produced by PECVD and annealed at temperatures between 700 °C and 1150 °C. While the near infrared emission of Si NCs has long been studied, visible light emission has only recently attracted interest due to its very short decay times and its recently-reported redshift with decreasing NCs size. We analyse the PL decay dynamics in the range 450–700 nm with picosecond time resolution using Time Correlated Single Photon Counting. In the resultant multi-exponential decays two dominant components can clearly be distinguished: a very short component, in the range of hundreds of picoseconds, and a nanosecond component. In this wavelength range we do not detect the microsecond component generally associated with excitonic recombination. We associate the nanosecond component to defect relaxation: it decreases in intensity in the sample annealed at higher temperature, suggesting that the contribution from defects decreases with increasing temperature. The origin of the very fast PL component (ps time region) is also discussed. We show that it is consistent with the Auger recombination times of multiple excitons. Further work needs to be done in order to assess the contribution of the Auger-controlled recombinations to the defect-assisted mechanism of photoluminescence