Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States.
Department of Chemistry, Korea University, Seoul 02841, Korea.
J Am Chem Soc. 2021 Feb 3;143(4):2156-2163. doi: 10.1021/jacs.0c12641. Epub 2021 Jan 22.
This paper addresses the mechanism for rectification in molecular tunneling junctions based on alkanethiolates terminated by a bipyridine group complexed with a metal ion, that is, having the structure Au-S(CH)BIPY-MCl (where M = Co or Cu) with a eutectic indium-gallium alloy top contact (EGaIn, 75.5% Ga 24.5% In). Here, Au-S(CH)BIPY is a self-assembled monolayer (SAM) of an alkanethiolate with 4-methyl-2,2'-bipyridine (BIPY) head groups, on template-stripped gold (Au). When the SAM is exposed to cobalt(II) chloride, SAMs of the form Au-S(CH)BIPY-CoCl rectify current with a rectification ratio of = 82.0 at ±1.0 V. The rectification, however, disappears ( = 1.0) when the SAM is exposed to copper(II) chloride instead of cobalt. We draw the following conclusions from our experimental results: (i) Au-S(CH)BIPY-CoCl junctions rectify current because only at positive bias (+1.0 V) is there an accessible molecular orbital (the LUMO) on the BIPY-CoCl moiety, while at negative bias (-1.0 V), neither the energy level of the HOMO or the LUMO lies between the Fermi levels of the electrodes. (ii) Au-S(CH)BIPY-CuCl junctions do not rectify current because there is an accessible molecular orbital on the BIPY-CuCl moiety at both negative and positive bias (the HOMO is accessible at negative bias, and the LUMO is accessible at positive bias). The difference in accessibility of the HOMO levels at -1.0 V causes charge transfer-at negative bias-to take place via Fowler-Nordheim tunneling in BIPY-CoCl junctions, and via direct tunneling in BIPY-CuCl junctions. This difference in tunneling mechanism at negative bias is the origin of the difference in rectification ratio between BIPY-CoCl and BIPY-CuCl junctions.
本文探讨了基于金硫醇末端联吡啶基团与金属离子配位的分子隧道结的整流机制，即具有结构 Au-S(CH)BIPY-MCl（其中 M = Co 或 Cu）的金硫醇自组装单层（SAM）与共晶铟镓合金顶接触（EGaIn，75.5%Ga 24.5%In）。在这里，Au-S(CH)BIPY 是具有 4-甲基-2,2'-联吡啶（BIPY）头基的烷硫醇的自组装单层（SAM），位于模板剥离的金（Au）上。当 SAM 暴露于氯化钴(II)时，SAM 以 Au-S(CH)BIPY-CoCl 的形式整流电流，整流比为 = 82.0 在 ±1.0 V。然而，当 SAM 暴露于氯化铜(II)而不是氯化钴时，整流作用消失（= 1.0）。我们从实验结果得出以下结论：（i）Au-S(CH)BIPY-CoCl 结整流电流，因为只有在正偏压（+1.0 V）下，BIPY-CoCl 部分才有可访问的分子轨道（LUMO），而在负偏压（-1.0 V）下，HOMO 或 LUMO 的能级都不在电极的费米能级之间。（ii）Au-S(CH)BIPY-CuCl 结不整流电流，因为在负偏压和正偏压下，BIPY-CuCl 部分都有可访问的分子轨道（HOMO 可在负偏压下访问，LUMO 可在正偏压下访问）。-1.0 V 时 HOMO 能级的可访问性差异导致在 BIPY-CoCl 结中通过福勒-诺德海姆隧道在 BIPY-CoCl 结中发生电荷转移，而在 BIPY-CuCl 结中直接隧道。这种负偏压下隧道机制的差异是 BIPY-CoCl 和 BIPY-CuCl 结之间整流比差异的原因。
