| Summary: | 碩士 === 國立臺灣大學 === 化學研究所 === 105 === Single-molecule conductance is an important index to show the electron transport efficiency through the Metal–Molecule–Metal junction. The closer the molecular frontier orbitals and Fermi levels are, namely higher degree of energy level alignment between Fermi levels and molecular frontier orbitals, the higher transmission probability would be, rendering higher conductance.
Our group simulated the transmission spectra of 1,3-propanedithiol and 1,4-bis(2-(4-(triphenylphosphinylauryl)ethynylphenyl)ethynyl)benzene (OPE3) under small bias, finding the HOMOs of these molecules are the major conducting orbitals. OPE3 has better headgroup-electrode coupling, and also has wider HOMO peak on the spectra. Hence, we can infer that diamine will show the narrowest HOMO peak because of its weakest headgroup-electrode coupling. We use Scanning tunneling microscopy break junction combined with homemade organic phase reference electrode, using electrochemical system to control the potential of the working electrode, Ewk(=EF), for tuning the extent of energy level alignment. Operate i-Ewk scanning on the molecules with fixed bias between electrodes to observe the change of conductance while tuning Ewk. The results suggest that when Ewk scanning toward HOMO, the extent of energy level alignment increase, and the conductance of three molecules increase accordingly. However, the increase is due to the extent of the coupling. And we also found that the shape of the electrode would influence the transmission spectra, causing two trends of i-Ewk curves.
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