掃描式穿隧顯微術於鎳金屬核紫質錯合物的分子自組裝及電子結構之研究

An STM/STS Study of Self-Assembly and Electronic Structure of Ni(II)-5,15-dioxaporphyrin at Liquid-Solid Interface

陳倉葦(Tsang-Wei Chen)

指導教授 ： 陳俊顯

國立臺灣大學/理學院/化學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100583

摘要

本論文使用掃描穿隧顯微術(scanning tunneling microscopy, STM)，研究鎳金屬核雙氧卟啉(Ni(II)-5,15-dioxaporphyrin, NiDOP)在石墨表面的分子自組裝現象與其電子結構(electronic structure)。
分子自組裝的基材是具二維晶型的高定向熱裂解石墨(highly oriented pyrolytic graphite, HOPG)，STM實驗是在含NiDOP的辛酸(octanoic acid)溶液中進行，亦即在液固界面觀察NiDOP。STM影像呈現NiDOP單一分子的中心為凹陷，環繞著的，似是共軛環上的四個吡咯(pyrrole)，顯示吡咯的電子穿隧機率高於中心的鎳金屬。NiDOP自組裝的單層分子構型堆疊可分為三種，為分子溶液濃度約為0.2 mM下觀察到的hexagonal-DOP、以及0.05 mM條件的vertical-DOP和parallel-DOP排列，三種排列的分子膜厚皆約為1.8 Å。較高濃度或施加大偏壓電場脈衝，可獲得膜厚約3.0 Å之分子雙層。
掃描穿隧能譜(scanning tunneling spectroscopy, STS)分析hexagonal-DOP排列之NiDOP單分子層，顯示分子在−0.6 eV及+1.7 eV處開始呈現較高的能態密度，兩處應分別為分子的HOMO及LUMO的邊緣(edge)。由於STM影像反映電子雲分布，−0.6 eV的影像與密度泛函理論(density functional theory, DFT)模擬的分子HOMO電子雲形狀相似。越往正偏壓之影像越接近模擬之LUMO電子雲分佈，然而正偏壓條件下的影像極不穩定，故僅能推測+1.7 eV起增大之能態密度為分子的LUMO。分子之HOMO-LUMO能隙為2.3 eV，與DFT模擬及循環伏安法(cyclic voltammetry)測得的結果相似。前述之NiDOP雙層的STS能譜HOMO-LUMO能隙約為1.9 eV，較分子單層窄，推測其為NiDOP分子間作用力造成分子能階產生Davydov splitting的結果。

關鍵字

掃描穿隧顯微術 ； 掃描穿隧能譜 ； 高定向熱解石墨 ； 紫質 ； 分子自組裝 ； 液固界面 ； 能態密度

並列摘要

In this study, we presents the molecular self-assembly and electronic structure of Ni(II)-5,15-dioxaporphyrin (NiDOP) on the surface of graphite by scanning tunneling microscopy (STM).
The substrate for molecular self-assembly is highly oriented pyrolytic graphite (HOPG), which possesses two dimensional crystal form. The STM experiment was conducted at a liquid-solid interface: in the solution of NiDOP-dissolved octanoic acid on HOPG. There are three forms of NiDOP self-assembly structures could be observed under different solution concentration, which are hexagonal-DOP form at 0.2 mM solution concentration, and vertical-DOP and parallel-DOP form at 0.05 mM. The thickness of these NiDOP self-ssembly molecular films were all around 1.8 Å. With higher solution concentration or larger bias voltage, NiDOP molecular double-layer film with the the thickness of 3 Å could be observed.
The dI/dV spectrum of hexagonal-DOP layer obtained by scanning tunneling spectroscopy (STS) indicated some higher density of states of NiDOP started at −0.6 eV and +1.7 eV. With the evidence of density functional theory (DFT), we identified −0.6 eV as NiDOP molecular HOMO and +1.7 eV as LUMO. Thus, the HOMO-LUMO enegy gap was 2.3 eV, which was similar to the results of DFT calculation and cyclic voltammetry experiment of NiDOP. The STS study of NiDOP double-layer film showed a smaller HOMO-LUMO gap (1.9 eV) than that of the single NiDOP layer flim, which could be due to the result of HOMO and LUMO energy level splitting (Davydov splitting) caused by the intermolecular force between two NiDOP layers.

並列關鍵字

scanning tunneling microscopy ； scanning tunneling spectroscopy ； highly oriented pyrolytic graphite ； porphyrin ； self-assembly ； solid-liquid interface ； density of state

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