In recent years, nickel nanoparticles (NPs) have increased scientific interest because of their extensive prospects in catalysts, information storage, large-scale batteries and biomedicine. Several works on Ni NPs generation by laser ablation have appeared in the literature in the last years, using different pulsed laser regimes and different media have been published recently. In this work we analyze the characteristics of species, structure (bare core or core-shell), configuration and size distribution of NPs generated by fs pulse laser ablation over a Ni solid target in n-heptane and water. We explore the presence of NiO-Ni core-shell and hollow Ni (or air-Ni) NPs in the colloids obtained. These were experimentally characterized using AFM and TEM microscopy, as well as Optical Extinction Spectroscopy (OES). Extinction spectra were modeled using Mie theory through an appropriate modification of the complex experimental dielectric function, taking into account a size-dependent corrective term for each free and bound electron contribution. Experimental UVvisible- NIR spectra were reproduced considering a size distribution of bare core, hollow and core-shell structures NPs. In both media, Ni NPs shape and size distribution agrees with that derived from TEM and AFM analysis.
The synthesis and study of optical properties of copper nanoparticles are of great interest since they are
applicable to different areas such as catalysis, lubrication, conductive thin films and nanofluids. Their optical properties
are governed by the characteristics of the dielectric function of the metal, its size and environment.
The study of the dielectric function with radius is carried out through the contribution of free and bound
electrons. The first one is corrected for size using the modification of the damping constant. The second one takes into
consideration the contribution of the interband transitions from the d-band to the conduction band, considering the larger spacing between electronic energy levels as the particle decreases in size below 2 nm.
Taking into account these specific modifications, it was possible to fit the bulk complex dielectric function, and
consequently, determine optical parameters and band energy values such as the coefficient for bound electron
contribution <i>Q</i><sub>bulk</sub> = 2 x 10<sup>24</sup>, gap energy <i>E</i><sub>g</sub> = 1.95 eV, Fermi energy <i>E</i><sub>F</sub> = 2.15 eV and damping constant for bound electrons γ<sub>b</sub> = 1.15 x 10<sup>14</sup> Hz.
The fit of the experimental extinction spectra of the colloidal suspensions obtained by 500 μJ ultrashort pulse
laser ablation of solid target in water and acetone, reveals that the nanometric and subnanometric particles have a Cu-
Cu<sub>2</sub>O structure due to an oxidation reaction during the fabrication. The results were compared with those obtained by AFM, observing a very good agreement between the two techniques, showing that Optical Extinction Spectroscopy (OES) is a good complementary technique to standard electron microscopy.