Nanoscale Dielectric Function of Fe, Pt, Ti, Ta, Al, and V: Application to Characterization of Al Nanoparticles Synthesized by Fs Laser Ablation

Abstract

Development and applications of new nanomaterials and nanocomposites that include metal nanoparticles have received much attention in the last years. However, there are relatively few studies concerning basic physical characteristics of the dielectric function at the nanoscale, which is needed for predicting their optical and plasmonic response. The size-dependent complex dielectric function of metal Fe, Pt, Ti, Ta, Al, and V nanoparticles (NPs) is calculated for the first time for an extended wavelength range from UV to FIR, based on experimental bulk complex refractive index measurements in the mentioned range at room temperature. Calculation is based on a “top-down” approach, based on a stepwise modification of the Drude model. Bulk plasma frequency (ω p) and damping constant (γ free) in this model are determined using a method that improves the relative uncertainties in their values and provide an insight about the wavelength range over which the metal may be considered Drude like. Validation of ω p and γ free values is demonstrated by the improved accuracy with which the experimental bulk dielectric function is reproduced. For nanometric and subnanometric scales, dielectric function is made size dependent considering size-corrective terms for free and bound electron contributions to the bulk dielectric function. These results are applied to analyze the synthesis of Al NP suspensions using a 120-fs pulse laser to ablate an Al solid target in n-heptane and water. The presence of Al, Al-Al2O3, and air-Al core-shell structures is also reported for the first time in these type of colloids. Analysis of the structure, configuration, sizing, and relative abundance was carried out using optical extinction spectroscopy (OES). Sizing results are compared with those provided by atomic force microscopy (AFM) studies.