On the Mechanism of Charge Transport in Single-Molecule Junction: Energy Alignment and Electronic Coupling of Bipyridine and Alkylbipyridine on Au, Pd, and Pt
Date Issued
2011
Date
2011
Author(s)
Chuang, Su-Ting
Abstract
Understanding the factors that influence charge transport properties in MMM devices (metal-molecule-metal devices) is a critical issue. Here especially the influence of the metal work function and the metal-molecule interface are considered. We measure the single-molecule conductance of a series of pyridyl-terminated molecules, 4,4''-bipyridine (BPY) and Py–(CH2)n–Py (n = 2, 3, and 4; Py = 4-pyridyl), on Au, Pd, and Pt electrodes by the method of STM-BJ (scanning tunneling microscopy break junction). The experimental data show that the conductance of BPY decreases as the electrode work function increases. The results demonstrate electron transport takes place via LUMO of BPY. Moreover, the conductance of Py–(CH2)n–Py molecules on Au is inferior to those on Pd and Pt. Since the α,ω-alkane moiety has large HOMO－LUMO gap, the effect of different work function is not observed. The contact resistance of Py–(CH2)n–Py (n = 2, 3, and 4) is smallest on Pt. Finally, by analyzing the transport mechanism based on transition voltage spectroscopy and density functional theory calculation, we find out the major factor that influence charge transport properties through pyridyl-terminated molecules. For BPY, disparity in the conductance is attributed to the different value of barrier height. However, for Py–(CH2)n–Py, the major factor that influence charge transport is the degree of electronic coupling between the headgroup and the electrode.
Subjects
molecular electronics
single-molecule conductance
STM
break junction
pyridine-based anchor
Type
thesis
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