The electronic structure of ultra-small Au clusters on thin MgO/Ag(001) films has been analyzed by scanning tunneling spectroscopy and density functional theory. The clusters exhibit two-dimensional (2D) quantum well states, whose shapes resemble the eigen-states of a 2D electron gas confined in a parabolic potential. From the symmetries of the HOMO and LUMO of a particular cluster, its electron filling and charge state is determined. In accordance to a DFT Bader-charge analysis, aggregates containing up to twenty atoms accumulate one to four extra electrons due to a charge transfer from the MgO/Ag interface. The HOMO - LUMO gap is found to close for clusters containing between 70 and 100 atoms.
@article{arxiv.0811.3812,
title = {Quantum well states in two-dimensional gold clusters on MgO thin films},
author = {X. Lin and N. Nilius and H. -J. Freund and M. Walter and P. Frondelius and K. Honkala and H. H/"akkinen},
journal= {arXiv preprint arXiv:0811.3812},
year = {2015}
}