Fractal-like nanostructures, size distribution and composition analysis of silver nanoparticles generated by femtosecond pulse laser ablation

JESICA M. J. SANTILLÁN; MARCELA B. FERNÁNDEZ VAN RAAP; DIEGO MURACA; DIEGO CORAL; PEDRO MENDOZA ZELIS; DANIEL C. SCHINCA ; LUCÍA B. SCAFFARDI

Trieste

Conferencia; Winter College on Optics: ?Fundamentals of Photonics - Theory, Devices and Applications?; 2014

ICTP

The study of optical properties of silver nanoparticles (Nps) is of great interest in the last years for your applications in several fields of science and technologies. We report on the formation of self-assembled fractal like Ag structures decorated with Ag nanoparticles (Nps). Fractals assemblies of 2 nm height and 5-15 mm large were observed on drops of diluted colloidal suspension over mica substrate dried in an oven when fast temperature rate is used. When the drops are dried at slow temperature rate, no fractals were observed, but only isolated single Nps. The original Nps were fabricated by ultrashort pulse laser ablation of a solid silver target in water, giving rise to colloidal suspensions. These suspensions were characterized with different techniques such as Atomic Force Microscopy (AFM) and Optical Microscopy (OM) for shape and size analysis of single Nps and fractal structures, Transmission Electron Microscopy (TEM) for morphological studies,electron diffraction for elemental composition analysis and Optical Extinction Spectroscopy (OES) and Small Angle X-ray Scattering (SAXS) to study size distribution, shape and core-shell structure of the obtained Nps. Fractal dimension was determined in colloidal suspensions as well as in 2D plane over mica substrate. With respect to the size composition of the colloidal suspensions the studies reveal that Nps are spherical in shape with typical size under 10 nm, presenting bare and core-shell structures. Modelling of the complex dielectric function, taking into account specific modifications with size on free and bound electron contributions, enabled to fit the bulk complex dielectric function, determining optical parameters and band energy values such as the coefficient for bound electron contribution = 2 ´1024 bulk Q , gap energy =1,91eV g E , Fermi energy = 4,12 eV F E , and damping constant for bound electrons =1,5´1014 Hz b g . Taking into account these modifications with size within the electrostatic approximation of Mie theory, we could determine the presence of Ag bare, Ag-Ag2O and air-Ag core-shell structures, as well as core-size and shell-thickness distribution through the fit experimental extinction spectra. The results from AFM and TEM analysis performed on the obtained samples agree with the sizing obtained by OES, showing that the latter is a good complementary technique to standard microscopy methods.