C60於鈷奈米島/Cu(111)的台階邊緣之掃描穿隧顯微術研究：雙金屬界面的C60電子結構

Abstract

本論文研究碳六十(Buckminsterfullerene, C60)在Cu(111)單晶表面的鈷奈米島台階邊緣與Cu(111)台階邊緣的吸附方向與其電子結構，利用超高真空-低溫-掃描穿隧顯微鏡(ultrahigh vacuum-low temperature-scanning tunneling microscope, UHV-LT-STM)進行兩種樣品製備與表面分析，在超高真空的環境下經由(1)熱阻式方式，蒸鍍碳六十到Cu(111)表面取得C60/Cu(111)樣品；與(2)先熱蒸鍍金屬鈷到Cu(111)表面，獲得兩個原子層厚的三角形鈷奈米島，再蒸鍍碳六十，以獲得C60-Co/Cu(111)樣品。STM進行影像掃描，觀察碳六十分子在兩種基材表面的吸附位向；在穿隧迴路連接鎖相放大器(lock-in amplifier)可獲得掃描穿隧能譜(scanning tunneling spectroscopy, STS)。碳六十在平整表面的吸附位向可分為六邊形(hexagon)中心朝上(h)、五邊形(pentagon)中心朝上(p)、六邊形-五邊形鍵朝上(h:p)、六邊形-六邊形鍵朝上(p:p)或頂端原子(apex atom)朝上(a)等等。STM影像顯示碳六十在Cu(111)台階邊緣的吸附方向為h，而在鈷奈米島台階邊緣則為h:p。STS能譜顯示碳六十分子在兩種台階邊緣的電子結構有明顯的差異，在Cu(111)台階邊緣的碳六十LUMO (lowest unoccupied molecular orbital)譜峰分裂，而在鈷奈米島邊緣的碳六十，除了LUMO穿隧譜峰分裂，連HOMO (highest occupied molecular orbital)的譜峰也分裂了，顯示分子與這兩種金屬原子有不同的軌域混成作用(p-d hybridization)。經Atomistix ToolKit (ATK)軟體模擬獲得的能態密度(Density of States)與掃描穿隧能譜進行比較，佐證經由台階的側向接觸能明顯改變碳六十分子的軌域分佈。

Adsorbate-substrate interactions are important and typical subjects in surface science because the interactions modulate the electronic structures of the adsorbate and thus offer a tactic to modify the interfacial properties. At step edges, in addition to the underneath substrate, lateral interactions from the vertical face of the step come into play. In this thesis work of C60, a bimetallic step edge was prepared by deposition of cobalt nanoislands on Cu(111). Hence, the effect on the electronic structures of C60 from the bimetallic substrate can be compared with that from the Cu(111) step edge. UHV-LT-STM/STS (ultrahigh vacuum-low temperature- scanning tunneling microscopy/spectroscopy) was employed to acquire, at the molecular level, STM images and STS spectra of C60 lodged at the edges of Cu steps and Co nanoislands on Cu(111). Topographic images for the former suggested an orientation of C60 with the hexagon ring facing Cu(111) and for the latter with a hexagon-pentagon bond resting on Cu(111). The discrepancy manifested the effect of aforementioned lateral interactions on the orientation of C60 adsorption. dI/dV spectra of C60 at Cu(111) step edge exhibited LUMO (lowest unoccupied molecular orbital) splitting, while at the edge of cobalt nanoislands additional splitting at the HOMO (highest occupied molecular orbital) level took place. The splitting involved the coupling of adsorbate states to those of the substrate. Different degrees of p-d hybridizations between C60 and metal atoms on the vertical face of the step are proposed for the disparities of dI/dV peaks.