  The electronic structure of Si(111)-(6x6)Au surface covered with submonolayer
amount of Pb is investigated using scanning tunneling spectroscopy. Already in
small islands of Pb with thickness of 1 ML Pb$_{(111)}$ and with the diameter
of only about 2 nm we detected the quantized electronic state with energy 0.55
eV below the Fermi level. Similarly, the I(V) characteristics made for the
Si(111)-(6x6)Au surface reveal a localized energy state 0.3 eV below the Fermi
level. These energies result from fitting of the theoretical curves to the
experimental data. The calculations are based on tight binding Hubbard model.
The theoretical calculations clearly show prominent modification of the I(V)
curve due to variation of electronic and topographic properties of the STM tip
apex.
