Formation of nanostructures upon laser ablation of silver in liquids

The formation of a dense array of nanospikes is studied upon ablation of a silver target immersed into water or ethanol by 350-ps pulses from a neodymium laser. The average height of nanospikes is 50 nm and their density amounts to 10¹⁰cm^-2. The irradiated surface is studied by using reflection spectroscopy in the 200–600-nm range and an atomic force microscope. Small lateral dimensions of nanospikes confine plasmon oscillations of electrons, which is manifested in the colouring of the irradiated surface. The plasmon resonance of nanospikes is observed at 380 nm and shifts to the visible region upon oxidation in air. The initial spectrum of nanospikes is recovered after processing of their oxidised surface in an aqueous solution of ammonium. The use of nanospikes for observing surface enhanced Raman scattering is discussed.