Laser pulse number dependent nanostructure evolution by illuminating self-assembled microsphere array

Abstract

Pulse number dependent evolution from nanodents to nanobumps has been studied on a bearing steel substrate, which was coated with a self-assembled monolayer of silica microspheres and repeatedly irradiated by an 800 nm femtosecond laser. Scanning electron microscope and atomic force microscope were employed to characterize nanopatterns, the dimensions of which were related to the laser pulse number and pulse fluences. The transformation depending on the number of laser pulses could be attributed to the changes of electric field distribution and material property after the impacts of multiple laser pulses, the process of which could be divided into three steps. First, the bottoms of silica microspheres were ablated because of the incubation effects from repeated irradiation. Second, strong plasmonic localization at the edges of the deep nanodents resulted in plasma-chemical reactions between ablated materials, which was confirmed by electromagnetic simulations. Third, recrystallized solid matter from ablated materials deposited in nanodents and then formed nanobumps, which was confirmed by transmission electron microscope and energy dispersive X-ray spectrometer analyses on their longitudinal sections.