| Summary: | 博士 === 國立臺灣師範大學 === 物理學系 === 98 === When the electronic structure beyond the vacuum level of a metal film is probed by scanning tunneling spectroscopy (STS), both transmission resonance (TR) and Gundlach oscillation can be revealed in the tunneling spectrum. It is shown that the spectral intensity of the Gundlach oscillation can vary with observed location on a Ag/Si(111)7×7 surface and a reconstructed Au(111) surface. The variation can naturally be attributed to the local change of electron transmissivity and can be use to reveal the interface or surface structures. A general phenomenon is also observed that the total intensity in the range of the Gundlach oscillation is conserved, although its intensity distribution is location dependent. By inspecting the Gundlach oscillation on the Ag(100) surface, it is demonstrated that the transmission background can be correlated to the projected bulk band structure. Meanwhile, the observation of TR in the silver thin film on metal substrates shows that the occurrence of TR may depend on the band-gap feature found in the electronic structure at the interface. On the other hand, by observing Gundlach oscillations on Ag/Au(111), Ag/Cu(111), and Co/Cu(111) systems, we demonstrate that the work function difference is not the energy shift of the lowest order but the ones of higher order where a constant energy shift exhibits. Higher order Gundlach oscillations can thus be applied to determine the work function of thin metal films precisely. Therefore, it can be applied to reveal the oscillatory work function with thickness for the Pb islands possessed of quantum well states. The undulation of the work function is consistent with the theoretical calculations for a thickness from 6 to 15 atomic layers. While the island is grown layer-by-layer, it is found that the work function decreases (increases) whenever one (two) subband is added to the occupied states of the island. Finally, by means of STS, the electronic structures of C60 thin film on metal substrates are analyzed from Fermi level to above vacuum level. The results show that in addition to LUMO+1 and LUMO+2, several subbands related to the superatom molecular orbitals (SAMOs) appear validly at the energy even higher than the vacuum levels. We suggest that the limitation of the molecular orbitals should be dominated by the ionization energy of C60 rather than the work function. Besides, an extra feature existed at the energy over the ionization energy is found and should refer to the electron interferometry by the C60 thin film.
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