Imaging and Spectroscopy of Defect Luminescence and Electron−Phonon Coupling in Single SiO₂ Nanoparticles

Abstract

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.

Photoluminescence of a single quantum emitter in a strongly inhomogeneous chemical environment

“…This effect can be also related to a redistribution of defect states within a particle, which has been recently observed as a reversible flipping of the transition dipole moment, while the particle orientation itself was fixed 7 . This can happen if another defect in a NP becomes energetically more favorable and then serves as the energy trap from which luminescence occurs.…”

“…A typical example of an emitter that is chemically bound to a matrix is a luminescent centre in a SiO 2 nanoparticle (NP) 7,8 . It has been shown that the PL from SiO 2 structures in the visible spectral range arises from non-bridging oxygen centres [9][10][11] , neutral oxygen vacancies 11,12 , and hydrogen-related species 13 .…”

“…It has been shown that the PL from SiO 2 structures in the visible spectral range arises from non-bridging oxygen centres [9][10][11] , neutral oxygen vacancies 11,12 , and hydrogen-related species 13 . Measurements of single SiO 2 nanoparticle (NP) photoluminescence revealed a strong coupling between the electronic transition and collective vibrations in the SiO 2 network due to a relaxation mechanism involving charge transfer 7,8 . In particular, the PL spectra of a single SiO 2 NP exhibit, besides a narrow zero-phonon line, one or even two satellite peaks at the lower-energy end, which are related to the excitation of one or two longitudinal optical phonons in the particle.…”

“…In particular, the PL spectra of a single SiO 2 NP exhibit, besides a narrow zero-phonon line, one or even two satellite peaks at the lower-energy end, which are related to the excitation of one or two longitudinal optical phonons in the particle. Furthermore, it has been shown that like single dye molecules, SiO 2 NPs possess both linear excitation and linear emission transition dipole moments 7 . In contrast to luminescent centres in nanodiamonds that possess a regular crystal structure, defect centres in SiO 2 NPs are embedded inside an amorphous matrix 14 .…”

Silica nanoparticles assisted photodegradation of acridine orange in aqueous suspensions

“…The presence of optically active defects on amorphous silica nanomaterials has been reported [54,55]. Upon light absorption, in the silica NPs, an electron is promoted from the valence band forming charge carrier species (electron-hole).…”