Ytterbium femtosecond fiber laser ablation synthesis of silicon nanocrystals in water: Laser frequency and pulse energy dependence

Abstract

The utilization of femtosecond laser pulses in synthesizing spherical silicon nanocrystals (SiNCs) poses great potential in producing stable nanostructures with chemically pure surfaces and easy-to-manipulate optical properties. In this work, we present an approach that enhances the production and optical quality of surfactant-free spherical SiNCs synthesized in water by ablating silicon wafers with ytterbium fiber femtosecond laser operating at different pulse energies and repetition rates. By further post-synthesis laser treatment of SiNCs suspensions, fragmentation of larger aggregates is observed leading to absorption broadening and absorption-band blueshift. Furthermore, the increase in the repetition rate, from 1 to 500 kHz, of laser pulses having energies of 20 and 240 μJ leads to a decrease in the Si wafer ablation efficiency from 35 to 1.15 pg/pulse; and from 440 to 5.2 pg/pulse, respectively. Using laser pulses with energy of 240 μJ and a repetition rate of 50 kHz results in maximum SiNCs productivity of 33 mg/h. The dependence of the nanostructures's size and ablation efficiency on the pulse energy and repetition rate is attributed to thermal effects on the Si/water interface and within the water layer illuminated by the strong laser radiation.