Overcoming Imaging Limitations: 4D-STEM Pair Distribution Function for Resolving Structure of Supported Pt55 and Pt200 Clusters.

Metallic clusters in the 2-200 atom range exhibit exceptional functional properties arising from their large fraction of under-coordinated surface sites and quantum size effects. However, structural characterization of these clusters is challenging due to the absence of well-defined periodicity and long-range crystal structure. In this size regime, surface and finite-size effects dominate, leading to highly relaxed and/or multiply twinned configurations minimizing total energy. Experimental approaches using x-rays and electrons require particular care, as irradiation can distort the intrinsic structure of these clusters. To accurately resolve their structure, probing techniques must operate at sufficiently low dose to avoid atomic displacement from equilibrium sites. Here, we investigate the structure of Pt55/Pt200 clusters supported on amorphous Carbon from room temperature to catalytically relevant conditions using 4D-STEM-based pair distribution function (PDF) analysis, demonstrating its power to resolve intrinsic atomic arrangements. The atomic structures of Pt55/Pt200 clusters differ significantly from bulk Pt, exhibiting mixed characteristics of ideal isomeric configurations combined with contributions from chemical bonding to the substrate. The temperature dependence of the Pt-Pt bond length, measured from room temperature to 500°C, is used to extract the linear thermal expansion coefficient (TEC), which exhibits a pronounced enhancement of TEC with size confinement.