Morphology and electron confinement properties of Co clusters deposited on Au(111)

We present the first measurements of both the morphology and electronic structure of preformed Co nanoclusters in the size range of a few up to several hundreds of atoms by means of low- temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Co clusters are produced in the gas phase and deposited under controlled ultra-high vacuum conditions onto clean Au(111) with low density, well below complete coverage of the Au(111) substrate. We find that smaller clusters, which typically contain less than 20 atoms, exhibit a significant surface mobility with subsequent aggregation, whereas larger clusters turn out to be immobile. From a systematic analysis of the cluster height distribution, we infer that the approximately spherical clusters experience only a restricted flattening and have a multilayered structure, which sometimes is observed to exhibit hexagonal facets, pointing to a truncated octahedral shape. Furthermore, detailed STS measurements on individual Co clusters reveal the presence of various size- and shape-dependent maxima with large energy spacings of the order of 100 meV, as well as an occupied state around -200 meV that originates from the Co 3d band. Our findings provide direct evidence for the existence of strong electron confinement effects in the Co clusters stemming from delocalized Co valence electrons. Our results provide an original contribution to the understanding of cluster physics and in particular of the specific electronic structure.