Quantization of electronic states in individual oxide-supported silver particles

The electronic properties of Ag particles on Al2O3/NiAl(1 1 0) have been investigated by low-temperature scanning tunnelling microscopy and spectroscopy. Conductance measurements on single clusters revealed a series of equidistant resonances around the Fermi level. The energy separation between these peaks decreases with increasing cluster size. The resonances were interpreted as a discretization of the electronic states along the cluster height. In conductance images of the Ag particles, resonance levels appear as concentric circles of enhanced conductance with a diameter depending on the sample bias. This behaviour reflects the influence of the tip electric field on level positions in the clusters.