Rectification in Molecular Tunneling Junctions Based on Alkanethiolates with Bipyridine-Metal Complexes

Abstract

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^TS-S(CH₂)₁₁BIPY-MCl₂ (where M = Co or Cu) with a eutectic indium-gallium alloy top contact (EGaIn, 75.5% Ga 24.5% In). Here, Au^TS-S(CH₂)₁₁BIPY is a self-assembled monolayer (SAM) of an alkanethiolate with 4-methyl-2,2'-bipyridine (BIPY) head groups, on template-stripped gold (Au^TS). When the SAM is exposed to cobalt(II) chloride, SAMs of the form Au^TS-S(CH₂)₁₁BIPY-CoCl₂ rectify current with a rectification ratio of r⁺ = 82.0 at ±1.0 V. The rectification, however, disappears (r⁺ = 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^TS-S(CH₂)₁₁BIPY-CoCl₂ junctions rectify current because only at positive bias (+1.0 V) is there an accessible molecular orbital (the LUMO) on the BIPY-CoCl₂ 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^TS-S(CH₂)₁₁BIPY-CuCl₂ junctions do not rectify current because there is an accessible molecular orbital on the BIPY-CuCl₂ 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-CoCl₂ junctions, and via direct tunneling in BIPY-CuCl₂ junctions. This difference in tunneling mechanism at negative bias is the origin of the difference in rectification ratio between BIPY-CoCl₂ and BIPY-CuCl₂ junctions.