A local density of electronic states and a local HOMO-LUMO gap of a monatomic-layered platinum cluster with a size of 30 (Pt-30 cluster disk) on a silicon(1 1 1) surface were measured by space-resolved tunneling spectroscopy. It has been found that (1) both occupied and unoccupied electronic orbitals are delocalized due to Pt-Pt metallic interaction in the cluster disk and (2) electrons in the occupied orbitals of the cluster disk are transferred to the adjacent silicon atoms so as to induce two-dimensional charge polarization of the cluster disk. The center of the cluster disk is charged positively, while the boundary between the cluster disk and the silicon surface is charged negatively. (C) 2010 Elsevier B.V. All rights reserved.