Plasmonic properties and sizing of core-shell Cu-Cu2O nanoparticles fabricated by femtosecond laser ablation in liquids

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 Q(bulk) = 2 x 10(24), gap energy E-g = 1.95 eV, Fermi energy E-F = 2.15 eV and damping constant for bound electrons g gamma(b) = 1.15 x 10(14) Hz. The fit of the experimental extinction spectra of the colloidal suspensions obtained by 500 mu J ultrashort pulse laser ablation of solid target in water and acetone, reveals that the nanometric and subnanometric particles have a Cu-Cu2O 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.