Electronic characterization of individual monolayer protected Au clusters by single electron tunneling force spectroscopy.

Gold monolayer protected clusters (MPCs) exhibit strong quantum confinement effects and size dependent electronic, optical and chemical properties. Chemical tuning of these properties can be achieved by established synthesis methods, providing an excellent system for the study of the relationship between chemical and electronic structure. In this paper, the first electronic spectra of individual Au MPCs (Au(25)) acquired by single electron tunneling force spectroscopy on non-conducting silicon dioxide surfaces are reported. A HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) energy gap is observed in the Au(25) spectra. Hysteretic charging of the particles is also observed while obtaining the energy spectra. The new single electron tunneling measurement methodology is described. A model explaining the measurements supports the existence of mid-HOMO-LUMO gap defect states.