光致熱電子影響穿隧效率之研究：以掃描穿隧顯微術之斷裂點接合法量測穿隧衰減常數及單分子電性

分子電子學旨在探討電極–分子–電極(electrode-molecule-electrode, EME)系統的電子傳遞，此傳遞效率和電子穿隧能障高度(barrier height)有關，而EME系統的傳遞效率與能障高度可以透過穿隧衰減常數值(tunneling decay constant)定性地評比。本論文研究電流受電磁場(光)的影響：電磁場與金屬奈米電極耦合產生的表面電漿共振(surface plasmon resonance)能量衰減，或者電磁場激發金屬電子，皆能使金屬裡的電子得到能量，產生熱電子，提升電子傳遞效率。我們以掃描穿隧顯微術破裂點接合法(scanning tunneling microscopy break junction, STM BJ)測量"沒有分子架接於電極的空間穿隧電流"以及"分子架接於電極的分子導電值"，並討論入射雷射光對電流的提升效果。前者實驗得到穿隧衰減常數，釐清電極、溶劑與入射電磁場三種變因的關係。我們量測電極分別為金和銀、溶劑分別為辛基苯(octylbenzene)和碳酸丙烯酯(propylene carbonate)及不入射/入射雷射光(532 nm)之穿隧衰減常數，了解金屬電極中的電子穿隧經溶劑到另一電極的難易程度。電極方面，銀電極系統的穿隧衰減常數小於金電極；當溶劑為碳酸丙烯酯時，其穿隧衰減常數較辛基苯小；照光時，穿隧衰減常數下降，且轉換電壓能譜(transition voltage spectroscopy, TVS)的最低點對應到的偏壓較小，意味著熱電子的產生將使穿隧能障下降。以上的現象與功函數、極性以及熱電子產生有關。將熱電子的概念延伸至"分子架接於電極"的實驗中，我們量測2,7–二胺基芴及4,4'–聯吡啶兩種分子在照光時導電值變化，並討論為何前者的導電值提升量較大，而後者提升量較小。

關鍵字

分子電子學；分子導電值；分子接合點；掃描穿隧顯微術；熱電子；表面電漿共振

並列摘要

The fundamental of molecular electronics involves electron-transport through electrode-molecule-electrode junctions. The transporting efficiency can be correlated to barrier height by tunneling decay constant. The interaction of molecular electronics with plasmons, collective oscillations of free electrons coupling to electromagnetic fields, has drawn lots of attentions. The formation of hot electrons from surface plasmon decay results in photocurrent. In addition, hot electron can also be generated from photoexcitation. We measure the tunneling phenomenon without molecules bridging between electrodes and the responses of gold and silver electrodes, solvent including octylbenzene and propylene carbonate, and electromagnetic field to tunneling decay constant by scanning tunneling microscopy break junction (STM BJ). The tunneling decay constant is reduced for silver electrodes in propylene carbonate when irradiated, that would be realized by comparing the work function of metal, polarity of solvent and by the presence of hot electrons. Following the concept of hot electrons, we investigate the increased conductance of molecules that binding to Au electrodes via head group under illumination and explain the differences in the conductance enhancement of 2,7-diaminofluorene and 4,4′-bipyridine.

並列關鍵字

molecular electronics ； molecular conductance ； molecular junction ； scanning tunneling microscopy ； hot electron ； surface plasmon resonance

參考文獻

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(3) Park, H.; Lim, A. K. L.; Alivisatos, A. P.; Park, J.; McEuen, P. L. Fabrication of metallic electrodes with nanometer separation by electromigration. Appl. Phys. Lett. 1999, 75, 301-303.

(4) Strigl, F.; Espy, C.; Bückle, M.; Scheer, E.; Pietsch, T. Emerging magnetic order in platinum atomic contacts and chains. Nat. Commun. 2015, 6.

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光致熱電子影響穿隧效率之研究：以掃描穿隧顯微術之斷裂點接合法量測穿隧衰減常數及單分子電性

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