Silver–silver oxide core–shell nanoparticles by femtosecond laser ablation: core and shell sizing by extinction spectroscopy

The generation of small silver metal nanoparticles (Nps) by ultrashort pulsed laser ablation has been an active area of research in recent years due to their interest in several fields of applied research such as biotechnology and material research, in particular those with sizes smaller than 10 nm. In general, laser ablation tends to produce environmentally clean metal Nps compared with wet chemical methods. However, since silver may be oxidized in the presence of water or ethanol, core–shell silver–silver oxide (Ag–Ag2O) Nps can be formed, whose size and thickness must be determined and characterized for functionalization related to future applications. This work analyses the size characteristics of core–shell Ag–Ag2O colloid nanostructures (smaller than 10 nm) obtained by femtosecond laser ablation of solid silver targets in different liquid media (water or ethanol) through the study of their optical extinction spectra. A fit of full experimental spectrum using Mie theory allows the determination of core size and shell thickness distributions as a function of fluence. The red-shift of the plasmon peak wavelength with respect to the bare-core peak wavelength at 400 nm, produced by the oxide shell, may be easily measured even for very small thicknesses. It was found that the dominant Ag2O effective thickness is inversely proportional to the fluence, reaching a maximum of 0.2 nm for a fluence of 60 J cm−2 and a minimum of 0.04 nm for a fluence of 1000 J cm−2.