Silicon Nanoparticles Produced by Femtosecond Laser Ablation in Ethanol: Size Control, Structural Characterization, and Optical Properties

Spherical silicon nanoparticles were prepared by laser ablation of a single crystal Si wafer immersed in 95% ethanol with a pulse duration shorter than the time of electron-phonon relaxation (from 35 to 900 fs). The size distribution depends on the pulse duration as well as the width of the size distribution, which increases with the increase of the laser pulse duration. High resolution transmission electron microscopy performed on 20-40 nm particles showed polycrystalline particles made up mainly of silicon (alpha-Si) crystallites with the diamond structure and in some cases cubic silicon carbide (SiC) inclusions. Electron energy loss spectroscopy data on the large particles are very similar to bulk Si. Raman analysis extended to small frequencies showed a downshift and an asymmetrical broadening of the first-order Si optical peak with respect to bulk Si in good agreement with a spatial confinement in 5-10 nm crystallites. The photoluminescence spectra present a maximum of emission band at about 640 nm.