Time-Resolved Imaging and Photoluminescence of Gas-Suspended Nanoparticles Synthesized by Laser Ablation: Dynamics, Transport, Collection, and Ex Situ Analysis

The dynamics of gas phase nanoparticle formation by pulsed laser ablation into background gases are revealed by imaging photoluminescence and Rayleigh-scattered light from gas-suspended SiOx nanoparticles following ablation of c-Si targets into 1-10 Torr He and Ar. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described. The first in situ time-resolved photoluminescence spectra from 1-10 nm diameter silicon particles were measured as the nanoparticles were formed and transported. Three spectral bands (1.8, 2.5 and 3.2 eV) similar to photoluminescence from oxidized porous silicon were measured, but with a pronounced vibronic structure. The size and composition of individual gas-condensed nanoparticles were determined by scanning transmission electron microscopy and correlated with the gas-phase photoluminescence. Weblike-aggregate nanoparticle films were collected at room temperature and 77K on c-Si substrates. After standard passivation anneals, the films exhibited strong room temperature photo-luminescence consisting of 3 spectral bands in agreement with the gas-phase measurements, however lacking the vibronic structure. These techniques demonstrate new ways to study and optimize the luminescence of novel optoelectronic nanomaterials during synthesis in the gas phase, prior to deposition.