Surface plasmon resonance analysis of Ag nanoparticles generated by pulsed laser ablation

Usually the synthesis of such structures is performed using ion implantation techniques or chemical reaction methods or ablating metal targets inside liquid solutions, while here we propose pulsed laser ablation in vacuum for the generation of these particles without any catalytic environment and annealing procedures for their activation. Silver targets were ablated in a vacuum chamber at (10^-7 Torr) by Nd:YAG high power pulsed laser at room temperature. The consequent deposition on Si-substrates covered by a ~50 nm thick SiO₂ results in the formation of well separated nanometric spheroidal particles of Ag with a diameter of 5-10 nanometers depending on the deposition time. The generation of silver nanoparticles was confirmed by scanning electron microscopy analysis (SEM). The kinetic energy (2 keV) of Ag ions of the non-equilibrium plasma produced by the high power pulse was measured by the aid of Faraday's cup inserted in the interacting chamber. Monte Carlo simulations of ions tracks in solid targets (TRIM) reveal that silver ions are implanted in a region thinner than 10 nm just under the surface. Optical properties of the samples were studied by variable angle ellipsometric spectroscopy (VASE). The ellipsometric spectra were modeled with a 2-layer model. Layer-1 is modeled using the dielectric function of SiO₂ and a variable thickness, while Layer-2 is best modeled with a single Lorenztian-oscillator and a constant layer thickness of 7 nm. The imaginary part of the refractive index for layer-2 reveals an absorption band in the energy range characteristic for surface plasmon resonances (SPR) of Ag nanoparticles. The maximum of SPR shifts 372 nm to 414 nm for longer deposition time indicating an increase of the average particle size [1]. Interestingly, although silver nanoparticles are located within the SiO₂ layer, nanparticle formation occurs during PLD and needs no additional forming or annealing step.