Reversal of the Direction of Rectification Induced by Fermi Level Pinning at Molecule–Electrode Interfaces in Redox-Active Tunneling Junctions

Control over the energy level alignment in molecular junctions is notoriously difficult, making it challenging to control basic electronic functions such as the direction of rectification. Therefore, alternative approaches to control electronic functions in molecular junctions are needed. This paper describes switching of the direction of rectification by changing the bottom electrode material M = Ag, Au, or Pt in M–S(CH2)11S–BTTF//EGaIn junctions based on self-assembled monolayers incorporating benzotetrathiafulvalene (BTTF) with EGaIn (eutectic alloy of Ga and In) as the top electrode. The stability of the junctions is determined by the choice of the bottom electrode, which, in turn, determines the maximum applied bias window, and the mechanism of rectification is dominated by the energy levels centered on the BTTF units. The energy level alignments of the three junctions are similar because of Fermi level pinning induced by charge transfer at the metal–thiolate interface and by a varying degree of additional charge transfer between BTTF and the metal. Density functional theory calculations show that the amount of electron transfer from M to the lowest unoccupied molecular orbital (LUMO) of BTTF follows the order Ag > Au > Pt. Junctions with Ag electrodes are the least stable and can only withstand an applied bias of ±1.0 V. As a result, no molecular orbitals can fall in the applied bias window, and the junctions do not rectify. The junction stability increases for M = Au, and the highest occupied molecular orbital (HOMO) dominates charge transport at a positive bias resulting in a positive rectification ratio of 83 at ±1.5 V. The junctions are very stable for M = Pt, but now the LUMO dominates charge transport at a negative bias resulting in a negative rectification ratio of 912 at ±2.5 V. Thus, the limitations of Fermi level pinning can be bypassed by a judicious choice of the bottom electrode material, making it possible to access selectively HOMO- or LUMO-based charge transport and, as shown here, associated reversal of rectification.The authors express thanks to the Ministry of Education (MOE) for supporting this research under award nos. MOE2018-T2-1-088 and R-143-000-B30-112. We also acknowledge the Prime Minister’s Office, Singapore, under its Medium Sized Centre program for supporting this research. This work was also funded by ITN iSwitch 642196, the DGI (Spain), projects FANCY (CTQ2016-80030-RA), GENESIS (PID2019-111682RB-I00) and MOTHER (MAT2016-80826- R), the Generalitat de Catalunya (2017-SGR-918), the Instituto de Salud Carlos III, through “Acciones CIBER”, and the Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” program for Centers of Excellence in R&D (FUNFUTURE; CEX2019-000917-S). The work in Mons was financially supported by the EC through the Marie Curie project ITN iSwitch (GA no. 642196). Computational resources were provided by the Consortium des É quipements de Calcul Intensif (CÉ CI) funded by the Belgian National Fund for Scientific Research (F.R.S.-FNRS) under grant 2.5020.11. J.C. is an FNRS research director.Peer reviewe