Dynamic Structural Disorder in Supported Nanoparticles

Supported Pt based nanoclusters are of wide interest in nano-scale physics and have many industrial applications, yet an understanding of their structure is far from complete. Experimental probes such as x-ray absorption spectroscopy (XAS) only yield globally averaged properties, e.g., mean bond distances and mean-square radial disorder (MSRD), which can give a misleading characterization of such complex systems. To obtain a more detailed picture we have carried out finite temperature DFT/MD simulations\footnote{F. Vila \textit{et al.},\textit{UW preprint} (2012).} of Pt and PtSn nanoclusters up to 600 K (\textit{operando} conditions). These show that the nano-scale structure and charge distribution are inhomogeneous and dynamically fluctuating over several time-scales, ranging from fast (200-400 fs) bond vibrations to slow fluxional bond breaking ($>$10 ps). In particular the anomalous behavior of the MSRD is not static, but rather due to ``dynamic structural disorder'' (DSD) driven by stochastic motion of the center of mass over 1-4 ps time-scales. In addition the nanoclusters exhibit a semi-melted, Sn-rich surface. These findings show that, and how an improved XAS interpretation of supported nano-scale structure must take into account DSD and other structural inhomogeneities.