Chemically Induced Metal-to-Insulator Transition inAu55Clusters: Effect of Stabilizing Ligands on the Electronic Properties of Nanoparticles

The cluster compound ${\mathrm{Au}}_{55}({\mathrm{PPh}}_3{)}_{12}{\mathrm{Cl}}_6$ has been reanalyzed by photoelectron spectroscopy giving direct evidence for a nonmetallic behavior of the individual Au clusters as long as their ligand shell remains intact. The exposure to x-rays during the measurements is found to partly decompose the shell by removal of the chlorine atoms, resulting in a metallic behavior of the clusters as demonstrated by a steplike intensity at the Fermi energy. These observations resolve a long-standing controversy about the metallic behavior of ligated Au clusters emphasizing, in addition, the influence of the local environment on the electronic properties of nanoscaled materials.