The aim of this study attempts to explore the surface effect imposed on the self-organized overlayer trans-stilbene (C14H12) on the silver-covered semiconductor surface, Ag/Ge(111), at ~100K under the ultra-high vacuum environment. Serving as a prototype molecule for investigating the trans-cis isomerization, trans-stilbene lying flat on the surface was found to be isomerized to the cis-form via a pair-wise fashion under UV-photon irradiation. Intense interest has arisen regarding how adsorbed trans-stilbene on the surface prepares for undergoing the photoisomerization, which is expected to differ from the one in gas and liquid phases. To thoroughly shed light upon this issue, this research adopted a joint experimental and theoretical approach in examining the surface and adsorbate characteristics with scanning tunneling microscopy (STM), surface Raman scattering, density functional theory (DFT) calculation and so on.
Results of these investigations let us to re-characterize the silver-germanium coexisting layer whose the most stable structure appears to be the hexagonal pattern in the STM image, instead of the well-known honeycomb pattern. To understand the STM images, the simulated electronic density distribution based on the periodic DFT calculation indicates that alternating bright and dark protrusions in the r3xr3 unit cell, referring to the hexagonal pattern, represent two silver trimers with different sizes. Furthermore, with trans-stilbene overlayer on the surface, large silver trimers are calculated to be an energetically favored adsorption sites for anchoring the benzoic moiety. A considerable interaction between the trans-stilbene and the silver trimers can be expected and have been examined by Raman spectroscopy. Self-organized submonolayer of trans-stilbene on Ag/Ge(111) yields a significant red shift in the C=C stretching of the olefinic group at 1625 cm-1 and enhanced Raman activity of the peak at 1568 cm-1 for phenyl ring stretching as compared to the corresponding one of a multilayer trans-stilbene. Electronic interaction between trans-stilbene and the underneath surface together with Raman spectra were calculated using the density functional theory. The calculated Raman spectra are in good agreement with the experimental results, supporting the calculated molecular geometry of the adsorbed trans-stilbene which exhibits a predominant bond elongation in the olefinic group and an molecular planarity. The analysis of the partial density of states shows that the lowest unoccupied molecular orbital is broadened and lowered to cross the surface Fermi level by the interaction with the surface, facilitating surface charge transfer and thus destabilizing the C=C double bond. In addition, self-organized overlayer of trans, trans-distyrylbenzene (C22H18, with one additional repeat unit of phenylenevinylene to stilbene) and trans-azobenzene (C12H10N2) on the surface have also been investigated. We found the similar adsorption behaviors for these two molecules as for trans-stilbene in interacting with the surface.

本研究主要的目地在於了解超高真空環境且在-173°C下自我組裝反式-二苯乙烯分子層(self-organized trans-stilbene overlayer)吸附在單層銀覆蓋鍺半導體(Ag/Ge(111))所具有的表面效應(surface effect)。二苯乙烯為異構化反應的簡單原型，已被廣泛的研究超過五十年。近年來我們將反式-二苯乙烯氣態分子單層吸附於金屬半導體表面上，發現它不但能夠整齊排列在表面上並且可以被光激發形成雙激子(bi-exciton)進行光異構化反應而產生順式-二苯乙烯。此發現引起我們對於在表面效應影響下的反式-二苯乙烯是如何準備異構化反應產生更進一步的興趣，並認為其性質可能不同於液相或氣相。為深入了解反式-二苯乙烯吸附行為，本研究結合掃描式穿隧顯微鏡和拉曼光譜儀的實驗觀測和第一原理模擬的計算方法，來探討吸附物和表面的性質以及之間的作用。
在表面形態的實驗觀測和理論模擬上，我們發現一最穩定銀-鍺表面結構存在週期排列的六面體圖案(hexagonal pattern)而不是過去所認為的蜂窩狀組織(honeycomb pattern)。經計算所得的最適化表面結構指出明暗影像(六面體圖案)在表面晶胞(surface unit cell)中是由二種不同大小的三聚銀(silver trimer)所構成。進一步計算吸附物在表面上的研究指出大的三聚銀是反式-二苯乙烯中苯官能基的吸附位置。此結果可以想見銀原子為表面扮演著重要的角色影響反式-二苯乙烯分子。在吸附分子—反式-二苯乙烯分子的拉曼振動實驗，我們觀察比較大約單層和多層的反式-二苯乙烯分子所具有不同的振動光譜特徵。我們發現與表面直接作用的反式-二苯乙烯分子其雙鍵拉伸振動產生一明顯紅移約在頻譜位置1625 cm-1 ，另外苯環拉伸振動的頻譜位置1568 cm-1強度也被增強。經由理論模擬表面效應及其吸附物的拉曼光譜計算，我們發現與實驗一致的結果並確認其光譜變化源自吸附的反式-二苯乙烯分子受到表面效應而改變其電子組態進而影響分子的結構。相較於未吸附的反式-二苯乙烯，結構最佳化的反式-二苯乙烯吸附在表面上呈現較長的碳碳雙鍵 (olefinic moiety) 和較扭曲的分子平面。根據吸附系統的電子組態分析，得知最低未填滿分子能階分佈變寬且朝向低能量位移並跨過表面費米能階(Surface Fermi level)而呈現部分填滿的狀態. 故我們認為吸附反式-二苯乙烯結構和對應的拉曼光譜變化起因於表面電子轉移到反式-二苯乙烯的最低未填滿分子能階 (lowest unoccupied molecular orbital, LUMO)。最後，我們也使用相同的實驗觀測和理論模擬探討另二種可自我組裝分子的吸附行為：反，反式-對苯二乙烯和反式-偶氮苯，發現均有類似反式-二苯乙烯的吸附行為和拉曼頻譜位移的現象。

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