Paper SS+AS+NS-MoM1
Measuring Homogeneity of Nanocluster Size Distributions using Charge Resolved Low Energy Alkali Ion Scattering

Monday, October 28, 2013, 8:20 am, Room 201 A

Gold nanoclusters on silicon dioxide are interrogated by charge-state resolved Low Energy Ion Scattering (LEIS). The LEIS data are analyzed in a novel way that quantitatively reveals information about the homogeneity of the clusters’ size distribution. The method utilizes the fact that the neutralization probability and the probability of multiple scattering (MS) both depend on the cluster size, but in different ways. Smaller nanoclusters are more likely to neutralize scattered alkali ions [1], while larger nanoclusters lead to more MS. Thus, in a system with a bimodal distribution of cluster sizes, multiply scattered ions are less likely to be neutralized. This leads to a difference in the spectral shapes of the scattered ions and neutrals. In contrast, a system with a homogeneous cluster size distribution yields identical spectral shapes. LEIS has advantages over the use of STM for measuring cluster size distributions, as it is easier to deploy over a wide range of preparation conditions and STM cannot probe disordered materials.

We collected time-of-flight LEIS spectra for 2 keV Na⁺ ions scattered from Au nanoclusters formed by deposition onto SiO₂ at room temperature and after annealing to increasing temperatures. The inhomogeneity in the cluster size distribution is quantified with a metric obtained by analyzing spectra collected for different charge states of the scattered projectiles. It is found that the cluster sizes are fairly uniform after the initial deposition, but a heterogeneous distribution, presumably due to Ostwald ripening, develops after annealing above 700 K.

[1] G. F. Liu, Z. Sroubek, and J. A. Yarmoff, Phys. Rev. Lett. 92, 216801 (2004).