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

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(CH 2 ) 11 BIPY-MCl 2 (where M = Co or Cu) with a eutectic indium–gallium alloy top contact (EGaIn, 75.5% Ga 24.5% In). Here, Au TS -S(CH 2 ) 11 BIPY 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(CH 2 ) 11 BIPY-CoCl 2 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(CH 2 ) 11 BIPY-CoCl 2 junctions rectify current because only at positive bias (+1.0 V) is there an accessible molecular orbital (the LUMO) on the BIPY-CoCl 2 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(CH 2 ) 11 BIPY-CuCl 2 junctions do not rectify current because there is an accessible molecular orbital on the BIPY-CuCl 2 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 2 junctions, and via direct tunneling in BIPY-CuCl 2 junctions. This difference in tunneling mechanism at negative bias is the origin of the difference in rectification ratio between BIPY-CoCl 2 and BIPY-CuCl 2 junctions.