Energy spectra of gold monolayer protected clusters measured by single electron tunneling force microscopy

An introduction to scanning probe microscopy techniques is presented briefly. A three-dimensional model is presented to calculate the tunneling rate of electron tunneling between a probe and localized states in a dielectric, and theoretical results are compared with experimental data. On the basis of force detection atomic force microscopy, a new technique, single electron tunneling force microscopy (SETFM), and its application are demonstrated and discussed in detail. SETFM can provide nanoscale spatial resolution imaging of electrically localized states in/on nonconducting surfaces. Electrons can be manipulated by shuttling between the probe and defects in a dielectric. Electronic spectra of individual nanostructures on a nonconducting surface can be measured by SETFM. As a promising component of nanodevices, monolayer protected clusters (MPCs) exhibit strong quantum confinement effects and size dependent electronic, optical and chemical properties. The energy levels of individual gold MPCs (Au25 & Au140) are directly measured by SETFM at room temperature in UHV. A clear electronic spectrum is obtained, showing a highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap for Au25 but not for Au 140. For both MPCs the single electron charging energy is measured. Spectral differences from particle to particle and energy relaxation are observed. The energy spectra obtained by this method are directly compared with existing electrochemical data. Finally, energy relaxation in spectra for Au25 is discussed.