IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 20 (2008) 374122 (11pp) doi:10.1088/0953-8984/20/37/374122 Electrochemical gate-controlled electron transport of redox-active single perylene bisimide molecular junctions C Li 1,2 , A Mishchenko 1 , Z Li 1 , I Pobelov 1,2 , Th Wandlowski 1,2 , X Q Li 3 ,FW¨ urthner 3 , A Bagrets 4,5 and F Evers 4,5 1 Department f¨ ur Chemie und Biochemie, Universit¨ at of Bern, CH-3012-Bern, Switzerland 2 Institute of Bio- and Nanosystems IBN 3 and Center of Nanoelectronic Systems for Information Technology, Research Center J¨ ulich, D-52425 J¨ ulich, Germany 3 Institut f¨ ur Organische Chemie, Universit¨ at W¨ urzburg, Am Hubland, 97074 W¨ urzburg, Germany 4 Institute of Nanotechnology, Research Center Karlsruhe, PO Box 3640, D-76021, Germany 5 Institut f¨ ur Theorie der Kondensierten Materie, Universit¨ at Karlsruhe, 76021 Karlsruhe, Germany E-mail: thomas.wandlowski@dcb.unibe.ch, wuerthner@chemie.uni-wuerzburg.de and ferdinand.evers@int.fzk.de Received 15 February 2008 Published 26 August 2008 Online at stacks.iop.org/JPhysCM/20/374122 Abstract We report a scanning tunneling microscopy (STM) experiment in an electrochemical environment which studies a prototype molecular switch. The target molecules were perylene tetracarboxylic acid bisimides modified with pyridine (P-PBI) and methylthiol (T-PBI) linker groups and with bulky tert-butyl-phenoxy substituents in the bay area. At a fixed bias voltage, we can control the transport current through a symmetric molecular wire Au|P-PBI(T-PBI)|Au by variation of the electrochemical ‘gate’ potential. The current increases by up to two orders of magnitude. The conductances of the P-PBI junctions are typically a factor 3 larger than those of T-PBI. A theoretical analysis explains this effect as a consequence of shifting the lowest unoccupied perylene level (LUMO) in or out of the bias window when tuning the electrochemical gate potential VG. The difference in on/off ratios reflects the variation of hybridization of the LUMO with the electrode states with the anchor groups. I T – E S(T) curves of asymmetric molecular junctions formed between a bare Au STM tip and a T-PBI (P-PBI) modified Au(111) electrode in an aqueous electrolyte exhibit a pronounced maximum in the tunneling current at −0.740, which is close to the formal potential of the surface-confined molecules. The experimental data were explained by a sequential two-step electron transfer process. (Some figures in this article are in colour only in the electronic version) 1. Introduction The ability to measure and control charge transport across nanometer-scale metal|molecule|metal junctions represents a key step toward the realization of molecular-based electron- ics [1–3]. Various experimental approaches have been em- ployed to study molecular junctions in two- and three-terminal configurations. These include the scanning probe microscopies (scanning tunneling microscopy (STM), scanning tunnel- ing spectroscopy (STS), conducting probe atomic force mi- croscopy (CP-AFM)) [4–15], crossed-wire junctions [16], me- chanical [17–21] and electromigration [22, 23] break junctions, nanopores [24] and mercury drop electrodes [25]. Approaches in condensed media, and in particular in an electrochemical en- vironment, offer unique opportunities to measure and to con- trol charge transport across single molecules [2]. The measured 0953-8984/08/374122+11$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1