Trap-Driven Exciton Recombination Processes within Femtosecond Laser-Synthesized Strongly Oxidized Silicon Nanoparticles

Trap states are central to silicon nanoparticle (Si NP) photoluminescence and charge transport properties, significantly influencing their potential for real-life applications. Laser ablation, typically conducted in a controlled environment, is an efficient technique for Si NPs synthesis that inherently minimizes the incorporation of impurities. This work presents results from time-integrated and time-resolved spectroscopic and microscopic studies of Si NPs synthesized by femtosecond laser ablation. The stationary spectroscopic and microscopic results indicate that the amorphous Si phase and oxidized SiO_x mesoporous phase cover the crystalline Si core. The analysis of time-resolved transient absorption spectra and dynamics indicates that the exciton initially photoinduced in the crystalline Si core is transferred to the shell-related trap states within approximately 2 ps. The trapped charges are further redistributed among localized states on a time scale of a few picoseconds. Finally, the traps in various Si NPs samples are depopulated into the valence band (VB) within approximately 170-190 and 1600-2100 ps. Our results highlight the importance of understanding the role of surface composition and morphology in shaping the complex photoinduced processes in the excited states of laser-synthesized Si NPs. Improving surface performance will help to increase Si NPs size-tunable photoluminescence efficiency, paving the way for developing materials suitable for future photoemitting devices.