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Probing Potential-Tuned Resonant Tunneling through Redox Molecules



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VOLUME 76 NUMBER 21 PHYSICAL REVIEW LETTERS 20 MAY 1996 Probing Potential Tuned Resonant Tunneling through Redox Molecules with Scanning Tunneling Microscopy N J Tao Department of Physics Florida International University Miami Florida 33199 Received 28 August 1995 We have studied electron transfer through redox molecules adsorbed on a conductive substrate with scanning tunneling microscopy STM in aqueous solution By adjusting the substrate potential the Fermi levels of the substrate and tip can be easily shifted relative to the energy levels of the molecules Aligning the Fermi levels to an energy level a nearly tenfold increase in the tunneling current that flows between the substrate and the STM tip via the molecules due to resonant tunneling is observed This experiment also shows that structurally similar molecules may be identified with STM based on their different redox properties S0031 9007 96 00248 7 PACS numbers 82 45 z 73 40 Gk 85 65 h 87 64 Dz The rapid progress in electronics over the last several decades relies critically on miniaturization which has been realized by carving smaller and smaller features on semiconductor chips Another approach towards ultimate miniaturization that has attracted much interest is to build a circuit with single molecules 1 These two approaches begin to overlap as semiconductor devices shrink towards the nanometer scale On such a small scale the discrete nature of charge and energy becomes important and devices such as quantum dots begin to exhibit atomic or molecular properties 2 An important task in developing molecular devices is to understand electron transfer between molecules and electrodes 1 Using a scanning tunneling microscope Joachim et al 3 have recently studied electron tunneling through single C 60 molecules in ultrahigh vacuum as a function of tip displacement In this work we attach redox molecules to a conductive substrate in aqueous solution and study the electron transfer through the molecules between the



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