以電化學方法調控單分子電性：分子−電極接合點及分子主體電子傳遞效率之探討

影響單分子導電值的一項重要因素是分子與電極間能量匹配(energy-level alignment)的程度，即前緣分子軌域(frontier molecular orbitals)與電極費米能階(Fermi level)之間的能量差。本研究利用掃描穿隧顯微鏡(scanning tunneling microscope)產生奈米尺度電極微隙，並搭配電化學方法調控電極費米能階接近分子軌域能階的程度，量測飽和烷二胺(alkanediamines)以及苯乙炔寡聚物(oligo(phenyleneethynylene)s)系列之單分子導電值。結果顯示隨電極費米能階靠近分子HOMO時，此二系列分子導電值皆上升，即代表此二系列分子皆以HOMO作為主要傳遞電子之軌域。比較二系列在不同電位下的接觸導電值(contact conductance)與穿隧係數(tunneling decay constant)，發現分子導電值改變主要來自於接觸導電值，即代表電子通過電極−分子接合點效率對能量匹配的程度較為敏感。為了進一步探討實驗現象，吾人分別使用Simmons模型與Newns-Anderson模型導證接觸電阻值、穿隧係數與能量匹配程度的關係；於Simmons模型中，接觸導電值、穿隧係數與電極費米能階分別為二次方與平方根關係，而於Newns-Anderson模型則分別為四次方與對數關係，即代表能量匹配程度對接觸導電值的影響較為顯著，與實驗結論一致。

關鍵字

電化學掃描式穿隧顯微術；單分子電性；能量匹配

並列摘要

Transport efficiency plays an important role to single-molecule conductance as the electrons pass through an electrode−molecule−electrode junction. One of the important factors is the degree of energy level alignment between Fermi levels of electrodes and molecular frontier orbitals. Scanning tunneling microscope incorporated with electrochemical control was implemented to manipulate tip-substrate the gap suitable for single molecule conductance measurements and studies of energy level alignment. The conductance of alkanediamines and oligo(phenyleneethynylene)s increases as the electrochemical potential of the electrodes moves positively (i.e., the Fermi level of electrodes approaches the HOMO of molecules), suggesting the dominant transport pathway via HOMO for both molecule series. Via the comparison in the change of the contact conductance with that of the tunneling decay constant under different working potentials, it is concluded that the conductance change of molecules primarily comes from the contact conductance. Simmons model and Newns-Anderson model were both applied to derive the behaviors of contact conductance and tunneling decay constant associated with Fermi levels of electrodes. In Simmons model, the contact conductance and the tunneling decay constant are related with the Fermi level in the power of second order and the square root respectively. On the other hand, results by Newns-Anderson model show that the contact conductance and the tunneling decay constant are sensitive to Fermi level in the power of fourth order and in logarithm relation, respectively. Both models predict the contact conductance is more sensitive to the energy level alignment, consistent with the experimental results.

並列關鍵字

electrochemical scanning tunneling spectroscopy ； single-molecule conductance ； energy-level alignment

參考文獻

20. Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoshki, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L. Nature 2002, 417, 722.

10. Liu, I. P. C.; Benard, M.; Hasanov, H.; Chen, I-W. P.; Tseng, W. H.; Fu, M. D.; Rohmer, M. M.; Chen, C.-h. Lee, G.-H. Chem.-Eur. J. 2007, 13, 8667.

83. Choi, S. H.; Kim, B.; Frisbie, C. D. Science 2008, 320, 1482.

97. Choi, S. H.; Kim, B.; Frisbie, C. D. Science 2008, 320, 1482.

7. Lin, S.-Y.; Chen, I.-W. P.; Chen, C.-h.; Hsieh, M.-H.; Yeh, C.-Y.; Lin, T.-W.; Chen, Y.-H.; Peng, S.-M. J. Phys. Chem. B 2004, 108, 959.

國際替代計量

以電化學方法調控單分子電性：分子−電極接合點及分子主體電子傳遞效率之探討

全文下載

主題瀏覽