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標題: | 電極電子結構對於跳躍積分及電子傳輸效率的影響:以單層鉛原子修飾的金電極為架構 Surface Electronic Structures of Pb-monolayer-modified Au on Interfacial Hopping Integrals and Electron Transport |
作者: | 江彥璋 Yen-Chang Chiang |
指導教授: | 陳俊顯 Chun-hsien Chen |
關鍵字: | 分子電子學,低電位沉積法,雙金屬電極, molecular electronics,underpotential deposition,bimetallic electrodes, |
出版年 : | 2023 |
學位: | 碩士 |
摘要: | 分子電子學領域常以電極-分子-電極的基本架構,從分子層級討論電荷傳輸的現象。電荷傳輸效率與電極-分子界面間的耦合(coupling)程度相關,顯見傳輸效率應深受電極的物理化學性質所影響。然而受限於金屬材料的高氧化活性,僅有零星的文獻例子以鈀、鉑、銀、鎳為電極進行單分子電性的量測,故而限縮此領域的電極材料於金,侷限了我們對電極-分子耦合現象的探索與應用。
本研究室曾運用電化學的低電位沉積(underpotential deposition, UPD)現象,於金電極表面修飾單層銅原子,製備不易形成氧化銅表面的雙金屬電極。本研究則是以UPD在金電極表面製備了單層鉛的電極(簡稱為PbUPD電極)。和金及過渡金屬相比,鉛金屬的費米能階(Fermi level)處擁有較豐富的p軌域的表面能態密度(surface density of states, SDOS)。依費米黃金定則(Fermi’s golden rule),電極-分子的耦合強度正比於表面能態密度、跳躍積分(hopping integral)的平方,因此預期在PbUPD電極架構下的電荷傳輸表現將優於金電極。 本研究選用以飽和烷為主幹的分子,以便聚焦討論電極-分子的耦合與傳輸效率的關係;分子頭基為羧酸基(-CO2H)和腈基(-CN)。相較於金電極,以PbUPD為電極時測得的單分子導電值提高了0.5~2個數量級。我們發現導電值與分子主幹及電極表面法線的夾角相關,推論是分子頭基電子雲與電極表面電子雲的相對位向,大幅影響跳躍積分值,而PbUPD電極表面的pz電子密度凸顯了上述特性。 本研究製備含主族元素的雙金屬電極,為分子電子學增加了電極材料的維度。該電極使單分子導電度提升接近2個數量級,顯示電極表面能態密度、跳躍積分之於分子級電荷傳輸的重要性。 Molecular electronics focuses on the charge transport at the molecular level with typically the rudimentary electrode-molecule-electrode configuration. The electrode-molecule coupling strength is a crucial factor for transport efficiency and thus the role that the electrodes play is equally important to that of the bridged molecule. However, the electrode materials are mainly gold and sporadically reported palladium, platinum, silver, and nickel. The exploration of other metals is limited due to their oxidative activity. In this study underpotential deposition (UPD), an electrochemical phenomenon, was used to modify a monolayer of lead on the surface of a gold electrode (abbreviated as PbUPD), creating a bimetallic electrode less prone to the formation of surface oxides. The molecules used in this study were saturated alkanes terminated with carboxylic acid and cyanide groups. The measurements of single-molecule junction conductance were carried out by a modified STM I(s) mode to preserve the Pb adlayer. For the model compounds, junction conductance of PbUPD was found 0.5 to 2 orders of magnitude more conductive than those with gold electrodes. The electrode-molecule coupling strength, described by Fermi's golden rule, explains the differences between junctions of PbUPD and gold electrodes. The coupling is proportional to the surface density of states (SDOS) and the square of the hopping integral. Density functional theory (DFT) calculations showed that Pb has a richer p-band SDOS near the Fermi energy than gold and other transition metals. Furthermore, the obtained conductance values are sensitive to the orientation of the molecule backbone relative to the surface normal, consistent with the overlapped degrees between the electron clouds of the molecular head group and the electrode surface. Hence, hopping integrals with the pz electron density on PbUPD surface are proposed to explain the superior conductance. In this study, the preparation of bimetallic electrodes containing main group elements adds a new dimension to molecular electronics. This electrode significantly improves the conductance of single-molecule junctions by two orders of magnitude, demonstrating the importance of electrode SDOS and hopping integrals in molecular-scale charge transport. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/91272 |
DOI: | 10.6342/NTU202304207 |
全文授權: | 同意授權(全球公開) |
顯示於系所單位: | 化學系 |
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