Reduced metallic properties of ligand-stabilized small metal clusters

Low-temperature ultrahigh-vacuum scanning tunneling microscopy was employed to analyze the electronic behavior of Au-55 clusters stabilized by [P(C6H5)(3)](12)Cl-6 ligands. At 7 K, the actual arrangement of the C6H5 rings of the ligand molecules could be imaged. Spectroscopic data reveal discrete energy levels with an average spacing of 170 meV that can be attributed to the Au-55 core. Additionally, characteristic charge-quantization phenomena were observed. Energy and charge quantization both support the view that the clusters consist of a metallic core extending slightly beyond the first close-packed shell of Au atoms.