Demonstration of enhanced surface mobility of adsorbate cluster species by surface acoustic wave excitation induced by a pulsed laser

Abstract

Fluorescence microscopy is used to explore the efficacy of surface acoustic waves (SAWs), as generated by a pulsed laser “tapping” a surface, to enhance the surface mobility of molecular adsorbates. The candidate adsorbate system under investigation comprises a series of gold clusters that are directly prepared on a silicon (111) surface by laser ablation of nanoparticles. Within the debris field the gold cluster Au8 is tracked because the fluorescence spectrum is known. The gold cluster is distinguished by using band-pass filters and tracked by fluorescence. The SAW source is a pulsed UV (355nm) laser operating at a repetition rate of 100Hz, where the laser fluence is set below the damage threshold of silicon. The experiment measures the location of the emitted light for a particular cluster through a high magnification (100X) imaging microscope that is integrated with a water-cooled (512x512 pixel) EMCCD camera. Image processing algorithms are used to track the light emission. Initial results show that the cluster Au8 moves approximately 0.5 Angstroms/s (when the excitation source is approximately 0.65 cm away). Diffusion and displacement data of adsorbed atoms and molecules on surfaces is sparse, though this value is similar for displacements of gold clusters on polymer films at 440 K, some metal-on-metal adatoms at room temperatures, and other nonmetal-metal interactions. The experiment also includes a laser heterodyne probe which measures the frequency distribution of the surface displacement induced by the pulsed SAWs. Results show that even at a source-to-probe distance of 1.8cm, frequency components up to 120MHz are present. These results suggest that “growing”/synthesizing thin films via surface aggregation of cluster compounds may be feasible.