Ag nanoparticles formed by femtosecond pulse laser ablation in water: self-assembled fractal structures

We report for the first time on the formation of self-assembled fractals of spherical Ag nanoparticles (Nps) fabricated by femtosecond pulse laser ablation of a solid silver target in water. Fractal structures grew both in two and three Euclidean dimensions (d). Ramified-fractal assemblies of 2 nm height and 5-14 mu m large, decorated with Ag Nps of 3 nm size, were obtained in a 2d geometry when highly diluted drops of colloidal suspension were dried at a fast heating rate over a mica substrate. When less-diluted drops were dried at slow heating rate, isolated single Nps or rosette-like structures were formed. Fractal aggregates about 31 nm size in 3d geometry were observed in the as-prepared colloidal suspension. Electron diffraction and optical extinction spectroscopy (OES) analyses performed on the samples confirmed the presence of Ag and Ag2O. The analysis of the optical extinction spectrum, using the electrostatic approximation of Mie theory for small spheres, showed the existence of Ag bare core, Ag-Ag2O and air-Ag core-shell Nps, Ag-Ag2O being the most frequent type [69 % relative abundance (r. a.)]. Core-size and shell-thickness distribution was derived from OES. In situ scatteringmeasurements of the Ag colloidal suspension, carried out by small-angle X-ray scattering, indicate a mass fractal composed of packaged < DSAXS- = (5 +/- 1) nm particles and fractal dimension df = 2.5. Ex situ atomic force microscopy imaging displayedwell-ramified structures, which, analyzed with box-counting method, yield a fractal dimension d(f) = 1.67. The growing behavior of these 2d and 3d self-assembled fractals is consistent with the diffusion- limited aggregation model.