Effects of surface morphology and composition on saturable absorption and charge carrier dynamics in oxidized Si nanocrystal thin films

Abstract

Thin films of silicon semiconductor nanocrystals (Si NCs) have a wide range of applications, from serving as saturable absorbers in mode-locked fiber lasers to being used in photovoltaic materials. This study investigates the relationship between the size, structure, morphology, and composition of Si NCs synthesized by femtosecond laser ablation and the optical properties of their thin films deposited on glass substrates. Femtosecond transient absorption spectroscopy of two sets of Si NCs synthesized by laser pulses layered on glass showed that photoinduced excitons efficiently transfer from the crystalline core to trap states-rich shell within a few picoseconds. The ultrafast trapping is followed by excitons redistribution among localized states on a time scale of over tens of picoseconds. Further depopulation of the trap states into the valence band (VB) occurs on a nanosecond timescale, confirming the existence of long-lived trap states. Z-scan measurements revealed that an increase in the laser pulse intensity utilized for Si NCs synthesis reduces the saturation intensities of saturable absorption in thin films from 32.8 MW/cm² to 1.58 MW/cm². These findings underscore the significance of surface properties in optimizing Si NC-based optoelectronic devices and their potential impact on the future design of Si nanocrystal-based optical materials.