Dynamic structure in nanoclusters and its effect on size & shape modeling

As recently as 10-15 years ago, nanoparticles were described qualitatively: oblate and hemispherical, discs and rafts. Today we discriminate between different faceted nano-architectures with unprecedented accuracy using EXAFS. As an illustration (Fig. 1), we will describe a Pt particle in equilibrium with support and adsorbate. We will show how to resolve analytically metal-support, metal-adsorbate, and support-adsorbate and obtain the size and shape information. We will show new methods for circumventing the main limitation of EXAFS analysis. EXAFS works best for ordered systems while nanoparticles have strong, asymmetric bond length disorder. They also have nonbulklike dynamics: the surface bonds are soft, while the interior bonds are stiff, resulting in a bimodal distributions of bonds. This heterogeneity, if not accounted for, results in smaller coordination numbers and, hence, in smaller particle sizes compared to the actual ones [1]. Conventional EXAFS analysis, based on the solution of "inverse problem" where experimental spectra is an input, and the unknown cluster architecture is an output, is unstable for systems with strong and asymmetric disorder. We will present a new methodology for the analysis of nm-scale clusters based on solving "direct problem". The starting point is the model system investigated using DFT-MD simulations to produce theoretical EXAFS signals that could be directly compared to the experimental results. The information that is learned from theory can be compared with traditional EXAFS fitting results to identify and rationalize any errors in the experimental fit. Our study, using both supported [2] and unsupported [3] clusters, demonstrates that DFT-MD simulations accurately depict complex experimental systems, and shows the advantages of using a combined experimental/theoretical approach over standard EXAFS fitting methodologies for determining the structural and dynamic parameters of metallic nanoparticles.

Progress and challenges in the theory and interpretation of x-ray spectra

There has been dramatic progress in recent years both in calculations and the interpretation of various x-ray spectra, ranging from extended x-ray absorption fine structure (EXAFS) and diffraction-anomalous fine structure (DAFS) to near-edge structure (XANES) and inelastic x-ray scattering (IXS). Using synchrotron radiation x- ray sources, these spectroscopies have become powerful probes of complex materials ranging from catalysts and minerals to bio-structures and aqueous systems. Together with advances in analysis techniques, these methods permit an interpretation of spectra in terms of structural, electronic, magnetic and vibrational properties. We summarize these advances first with a heuristic description of the real-space approach used in the electronic structure and spectroscopy codes developed by our group [1]. Our approach is based on real-space multiple-scattering Green's function techniques, rather than wave-functions. This simplifies calculations of excited states and x-ray spectra, particularly the inclusion of key many-body effects and relativistic corrections. The approach is illustrated with applications to various x-ray spectra of complex materials. For example, DAFS takes advantage of the fine structure in the intensity of Bragg diffraction peaks near an absorption edge, and provides unique information that combines EXAFS and XRD experiment. We also discuss some recent theoretical developments leading to a new generation of codes including FEFF9 [2] and extensions for treating strongly correlated systems.