Paper SS-TuP16
Ion Beam-Induced Formation and Interrogation of Metal Nanoclusters

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Metal nanoclusters are important systems that display quantum size effects and have remarkable catalytic activity. We utilize low energy (0.5-5 keV) ion beams as a simple, but efficient means for both the controlled production and the characterization of nanoclusters. Low energy Ar⁺ beams enable control of the size and shape of nanoclusters because of the curvature dependence of ion sputtering. The neutralization of scattered low energy alkali ions provides a sensitive measure of the localized electronic states. It was previously shown that scattered alkalis couple to atomic-like states specific to Au nanoclusters grown by deposition, and the neutralization probability is a function of the cluster size.¹ We further showed that sputtering a thin Au film on TiO₂(110) with Ar⁺ forms self-organized nanoclusters that display quantum size effects.² The scattered alkali neutral fractions increase as the cluster dimensions decrease, indicative of the quantum size effects, and the data shows that the electronic structure of clusters grown by deposition and by sputtering are similar. In this work, Li⁺, Na⁺ and K⁺ ions are employed to probe metal nanoclusters formed by both deposition and sputtering, and the dependence of neutralization on cluster size is compared. Au or Ag is first deposited on an insulating or semiconducting substrate, and self-organized clusters are produced either during the deposition, or by Ar⁺ sputtering a thin film. The ion scattering NF’s have different dependences with respect to cluster size for each alkali projectile. The microscopic mechanisms responsible for the formation of the nanoclusters, and for the charge exchange between scattered alkali ions and the nanoclusters, will be discussed.

¹G. F. Liu, Z. Sroubek, and J. A. Yarmoff, Phys. Rev. Lett. 92, 216801 (2004).
²P. Karmakar, G. F. Liu, Z. Sroubek, and J. A. Yarmoff, Phys. Rev. Lett. 98, 215502 (2007).