運用昆拉赫振盪量測局域功函數及研究其與電子結構之相互作用

Abstract

利用掃描穿隧能譜術可以探測高於真空能階之後的電子結構，包括穿透共振與昆拉赫震盪都可以於能譜中顯現。在銀薄膜於矽(111)7×7表面與金(111)表面的量測中發現昆拉赫震盪的峰值強度會隨著區域的不同而有所差異，這個差異是源自於各區域對電子的穿透率不同所產生，因而可以被運用來解析介面及表面的結構。另一個具有一般性的現象也在實驗中被觀察到，那就是即便昆拉赫震盪強度是具有局域性的差異，其總強度是守恒的，此外，藉由量測銀(100)表面的昆拉赫震盪，證明了塊材的能帶結構在某個表面上的投影是與穿透背景相關的。同時，藉由觀察銀薄膜在金屬基材上的穿透共振可以了解穿透共振會受到介面電子結構的影響，其發生與介面電子結構是否具有「類能隙特徵」有很大的關連性。另一方面，觀察銀/金(111)、銀/銅(111)與鈷/銅(111)三個系統的昆拉赫震盪，我們證實了薄膜與基底間的功函差異並非最低階峰值的能量平移，反而應該是存在於高階中的固定能量平移，因此高階的昆拉赫震盪可運用於精確地量測薄膜的功函數，故可用來量測具有量子井態的鉛島之震盪的功函數，從六層到十五層的實驗結果中發現其量測到的震盪形式與先前理論計算的結果相當吻合，由於鉛是一層一層地成長，我們發現功函數的增減與鉛島中已佔據的量子井態增減相關。最後，同樣是藉由掃描穿隧能譜術，碳六時薄膜在金屬基材上從費米能階到超過真空能階的電子結構被清楚地解析出來，結果顯示除了一般熟知的最低未填滿分子軌域加一(LUMO+1)及加二(LUMO+2)之外，一些額外與「超級原子分子軌域」相關的次能帶也顯現出來，甚至是在超過真空能階之後，我們認為碳六十分子薄膜的電子軌域之極限應該不是真空能階，而是電子游離能。此外，我們也發現能譜中在超過游離能之後的額外特徵，此特徵應該是屬於電子受到分子薄膜干涉的結果。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.