Cluster synthesis and assembling in laser-generated plasmas

Abstract

There is an increasing interest in the production and control of nanoparticles and cluster-assembled nanometer-sized films. Pulsed laser deposition is a suitable technique to achieve this objective both with nanosecond (ns) and with femtosecond (fs) pulses. In the former case atomic, or molecular clusters grow during the propagation through an ambient gas of the plasma plume generated upon target irradiation. Deposition parameters including laser wavelength and energy density, target to substrate distance, nature and pressure of the ambient gas influence the plasma expansion and thus cluster size and kinetic energy, together with the related distributions. The size of the clusters at landing, their energy and their mobility on the substrate affect film growth and morphology, resulting in broad ranges of values of the physico-chemical properties of the deposited films. Controlling the onset of different stages of film growth is vital to obtain films with ad hoc tailored properties. After summarising the most popular models for the propagation of an ablation plume through an ambient gas at high pressure, a phenomenological model of the growth of clusters nucleated in the plume is introduced; the average asymptotic cluster size is deduced and compared with available data from selected experiments. Irradiation under vacuum conditions with ultra-short fs laser pulses is a dry physical method to synthesize nanoparticles; indeed in the majority of materials films made of a random stacking of nanoparticles whose sizes range between 10 and 100nm are obtained. Starting from experimental data on nanoparticle deposition, currently proposed alternative mechanisms for cluster synthesis and the essential parameters governing them are presented. The performance of standard substrates covered with noble-metal nanocluster arrays in Surface Enhanced Raman Scattering is discussed to illustrate the technological potentiality of such artificially built nanostructures.