Imaging and spectroscopy of defect luminescence and electron-phonon coupling in single SiO2 nanoparticles.

Silicon nanocrystals were synthesized by CO(2) laser pyrolysis of SiH(4). The fresh silicon nanopowder was oxidized in water to obtain SiO(2) nanoparticles (NPs) exhibiting strong red-orange photoluminescence. Samples of SiO(2) NPs embedded in low concentration in a thin polymer layer were prepared by spin-coating a dedicated solution on quartz cover slides. Using an argon ion laser at 488 nm with higher-order laser modes (azimuthally and radially polarized doughnut modes) for excitation, the three-dimensional orientation of the nanoparticles' transition dipole moment was investigated in a confocal microscope. The linear transition dipole moment was found to be rather stable and randomly oriented. However, dynamical effects such as fluorescence intermittency and transition dipole moment flipping could also be observed. The spectral analysis of single SiO(2) NPs revealed double-peak spectra consisting of a narrow zero-phonon line and a broader phonon band being associated with the excitation of longitudinal optical phonons in the SiO(2) NP.