Size-dependent changes in the electronic structure of metal clusters as investigated by scanning tunneling spectroscopy

Pd, Ag, Cd and Au clusters of varying sizes have been investigated by scanning tunneling spectroscopy under ultra-high vacuum. The conductance of the clusters decreases markedly when the cluster diameter is ≤1 nm. A plot of the density of states at the Fermi level against the cluster volume varies nearly linearly up to a cluster volume of 4 nm3 (diameter ∼2 nm). Below a cluster diameter of 1 nm, an energy gap occurs, the value of which increases with the decrease in cluster size, reaching values up to 70 meV at small sizes. Clearly, the very small clusters tend to become non-metallic.