Singlet oxygen generation by nanoporous silicon: photoluminescence dynamics in magnetic field.

Singlet oxygen generation in porous silicon (PSi) was investigated by a magneto-optical experiment. Photoluminescence (PL) quenching due to an energy transfer (ET) process mediated by an exchange interaction was monitored in the spectral range 1.4-2.5 eV and in a magnetic field of 0-6 Tesla at different levels of oxygen concentration and excitation pump power. When a magnetic field was applied, both PL recovery and, for magnetic fields below 2 Tesla and high concentrations of oxygen, an unusual additional pump power dependent quenching of the PL was observed. A rate equation model describing the behavior of PL from PSi with oxygen adsorbed at cryogenic temperatures in magnetic field was developed. The model has been expanded to cover the ET process as a function of the nanoparticle size.

Observation of oxygen dimers via energy transfer from silicon nanoparticles.

Energy transfer from photo-excited excitons confined in silicon nanoparticles to oxygen dimers adsorbed on the nanoparticle surfaces is studied as a function of temperature and magnetic field. Quenching features in the nanoparticle photoluminescence spectrum arise from energy transfer to the oxygen dimers with and without the emission of Si TO(Δ) phonons and, also, with and without the vibrational excitation of the dimers. The dependence of the quenching on magnetic field shows that energy transfer is fast when a dimer is present, allowing an estimate of the proportion of the nanoparticles with adsorbed dimers.

Magnetic field dependence of singlet oxygen generation by nanoporous silicon.

Energy transfer from photoexcited excitons localized in silicon nanoparticles to adsorbed oxygen molecules excites them to the reactive singlet spin state. This process has been studied experimentally as a function of nanoparticle size and applied external magnetic field as a test of the accepted understanding of this process in terms of the exchange coupling between the nano-Si exciton and the adsorbed O2 molecules.

Porous silicon nanocrystals in a silica aerogel matrix.

Silicon nanoparticles of three types (oxide-terminated silicon nanospheres, micron-sized hydrogen-terminated porous silicon grains and micron-size oxide-terminated porous silicon grains) were incorporated into silica aerogels at the gel preparation stage. Samples with a wide range of concentrations were prepared, resulting in aerogels that were translucent (but weakly coloured) through to completely opaque for visible light over sample thicknesses of several millimetres. The photoluminescence of these composite materials and of silica aerogel without silicon inclusions was studied in vacuum and in the presence of molecular oxygen in order to determine whether there is any evidence for non-radiative energy transfer from the silicon triplet exciton state to molecular oxygen adsorbed at the silicon surface. No sensitivity to oxygen was observed from the nanoparticles which had partially H-terminated surfaces before incorporation, and so we conclude that the silicon surface has become substantially oxidised. Finally, the FTIR and Raman scattering spectra of the composites were studied in order to establish the presence of crystalline silicon; by taking the ratio of intensities of the silicon and aerogel Raman bands, we were able to obtain a quantitative measure of the silicon nanoparticle concentration independent of the degree of optical attenuation.