Core-shell structure in self-assembled lead/lead-oxide nanoclusters revealed by photoelectron spectroscopy

Nanoclusters containing metallic lead and lead oxide have been produced by self-assembly out of a primary mixture of lead atoms and oxygen in a reactive sputtering-based cluster source. Comparison of the valence and core-level responses in the photoelectron spectra shows that clusters have a core-shell structure with a lead-oxide core coated by an outer shell of metallic lead. This core-shell order is opposite to that typical for most microscopic and macroscopic materials. We explain this by the peculiarities of the cluster production kinetics and by the system's energy minimization striving due to what lead oxide is placed in the core of the mixed cluster.