Optical properties of femtosecond laser-synthesized silicon nanoparticles in deionized water

Silicon nanoparticles were prepared by ultrafast laser ablation of a silicon target in deionized water. The nanoparticles were characterized by using optical absorption, Raman spectroscopy, and transmission electron microscopy. The mean size is found to vary from 60 to 2.5 nm in the absence of any reducing chemical reagents, decreasing the pulse energy value. High-resolution transmission electron microscopy together with Raman spectroscopy confirms the crystalline structure of the generated silicon nanoparticles. The energy confinement of carriers which is evaluated from optical experiments varies from 90 to 550 meV when the mean nanoparticles size decreases from 60 to 2.5 nm. In particular, the evaluated nanoparticle sizes from optical analysis and the LCAO theoretical model are found in agreement with transmission electron microscopy and Raman measurements for the silicon nanoparticles with a size less than 6 nm. Finally, we present stability studies which show that the smallest nanoparticles aggregate over time.