單分子橋接系統電荷傳遞機制探究：雙苯環多炔分子主體和金、鈀及鉑電極接面之能障偶合

Abstract

The multibarrier model of metal-molecule-metal (MMM) junction was applied to detailed understanding of charge transport through single-molecule junctions, one of the critical issues for molecular electronics. This model consists of the contact barriers and bridge barrier, in which contact barriers are proportional to the size of the coupling of the headgroup–electrode and the bridge barrier is correlated to EF – EFMOs (the energy differences between Fermi level and frontier molecular orbitals). To systematically study the multibarrier model, we employ the conjugative molecules to lower the EFMOs and change electrode materials to tune EF and coupling strength. Carefully measured herein are the conductances of a series of methylsulfide-terminated biphenyl-oligoyne molecules on Au, Pd, and Pt electrodes by STM BJ (scanning tunneling microscopy break-junction). The results show that the contact resistance (Rc) decreased strongly as simulated bond length of CH3S–electrode decreased, ascribed to the bond strength increased. The characteristic transition voltage (Vtrans), extracted by the I-V curves, is an inflection point on a plot of ln(I/V2) vs. 1/V, consistent with a change in transport mechanism from direct tunneling to field emission and corresponding to total barrier height. The Vtrans values of model compounds decreased with increased conjugation (decreased HOMO–LUMO gap) and increased metal work function (the energy needed to move an electron from the Fermi level). Referring to the multibarrier model, Rc is determined by the contact barriers of molecule-metal interface, and Vtrans is attributed to the contact barrier and the bridge barrier, which was correlated with molecular conjugation. Putting together the aforementioned findings, we provide a comprehensive relation among molecular length, barrier height through MMM, and the measured conductance. In the measurement of I-V characteristics, NDR behavior appears when measuring 1,4-bis(4-(methylthio)phenyl)-buta-1,3-diyne (CH3S-Ph-(C≡C)2-Ph-SCH3) on Au, whereas such phenomena are not found on Pd and Pt electrodes. A plausible explanation for the mechanism of NDR in our research is that the energy of molecule orbital matches/mismatches the spatially discrete energy distribution of the electrode, resulting in diverse energy alignment with oligoynes.