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 AuTS-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, AuTS-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 (AuTS). When the SAM is exposed to cobalt(II) chloride, SAMs of the form AuTS-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) AuTS-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) AuTS-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.
Bibliographical noteFunding Information:
This work was supported by the National Science Foundation (NSF, CHE-18083681). We acknowledge the Materials Research and Engineering Center (MRSEC, DMR-1420570) at Harvard University for supporting XPS and UPS measurements, and providing access to the clean-room facilities. Sample characterization was performed in part at the Center for Nanoscale Systems (CNS) at Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the National Science Foundation (ECS-0335765). J.P. acknowledges fellowship support from the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education of Korea (2018R1A6A3A03013079). HJY acknowledges the support from the NRF of Korea (NRF-2019R1A2C2011003, NRF-2019R1A6A1A11044070) and the Future Research Grant (FRG) of Korea University. We thank Christian A. Nijhuis (Department of Chemistry, National University of Singapore), Victoria E. Campbell, and Philipp Rothemund for useful discussion and input. J.P. and G.M.W. conceived the research and designed the experiments. J.P., L.B., L.Y., M.P.S.M., S.E.R. and H.J.Y. performed the experiments and analyzed the data. L.B. performed the computational simulations. J.P., L.B., S.E.R., and G.M.W. wrote the manuscript.
© 2021 American Chemical Society.
ASJC Scopus subject areas
- Colloid and Surface Chemistry