Laser-induced conversion of noble metal-island films to dense monolayers of spherical nanoparticles

By postdeposition nanosecond laser irradiation at 532 nm with a relatively low laser fluence of typically similar to50 mJ/cm(2), Ag- and Au-island films sputter-deposited on mica could be converted to a dense monolayer of spherical nanoparticles 40-60 nm in average diameter without aggregation. The peak position and good sharpness of the corresponding surface plasmon (SP) resonance suggested no significant interparticle SP interactions for the dense two-dimensional arrays of spherical nanoparticles tens of nanometers in diameter. In higher laser fluence regime above 100 mJ/cm(2) or more, a complicated mode of film conversion resulting in strong deformation (flattening) of the SP band became dominant before the ablation mode finally set in. A crude estimate of the instantaneous temperature of the metal-island films as a function of laser fluence suggested that laser-induced melting is probably the major driving force for the conversion to good spherical nanoparticle films.