A dramatic effect of water on single molecule conductance.
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1154-B04-02
A dramatic effect of water on single molecule conductance. Edmund Leary1, Horst Höbenreich1, Simon J. Higgins1, Harm van Zalinge1, Wolfgang Haiss1, Richard J. Nichols1, Christopher Finch,2 Iain Grace2 and Colin J. Lambert.2 1. Department of Chemistry, University of Liverpool, L69 7ZD, U.K. 2. Department of Physics, Lancaster University, Lancaster, LA1 4YB, U.K. ABSTRACT Simple alkanedithiols exhibit the same molecular conductance whether measured in air, under vacuum or under liquids of different polarity1. Here, we show that the presence of water ‘gates’ the conductance of a family of oligothiophene–containing molecular wires, and that the longer the oligothiophene, the larger is the effect; for the longest example studied, the molecular conductance is over two orders of magnitude larger in the presence of water, an unprecedented result suggesting that ambient water is a crucial factor to be taken into account when measuring single molecule conductances (SMC), or in the design of future molecular electronic devices. Theoretical investigation of electron transport through the molecules, using the ab initio nonequilibrium Green’s function (SMEAGOL) method2, shows that water molecules interact with the thiophene rings, shifting the transport resonances enough to increase greatly the SMC of the longer, more conjugated examples. INTRODUCTION We have examined the electrical properties of molecular junctions in which a πconjugated, often redox-active, unit is sandwiched between two thiahexyl ‘spacers’, with gold contacts3-5. Since the frontier orbitals of the alkyl groups are far from the Fermi energy of the contacts, while the frontier orbitals of the π–conjugated unit are closer to the Fermi energy, these molecules may be thought of as molecular analogs of double tunnelling barriers6. To examine the effect of varying the length (and hence degree of conjugation) of the π-conjugated unit, oligothiophenes were selected. Oligothiophenes, polythiophenes and their derivatives have been extensively investigated as semiconductors in organic thin film transistors and photovoltaic devices7, 8. We have found a remarkable effect of water on the SMC of longer oligothiophene molecules, and we report these results here. EXPERIMENT AND THEORY Low-coverage phases of 1–4 (as dithiolates) on clean, atomically–flat gold surfaces were prepared; X-Ray photoelectron spectra were consistent with the intact molecules ‘lying down’ and binding to Au through both thiol sulfurs. We employed the scanning tunnelling microscopy (STM)-based I(s) technique of Haiss et al3 to determine the single molecule conductances of 1– 4. Au STM tip is brought close to the surface (without making contact), the feedback loop is switched off and the tip is retracted while the tunneling current is monitored. In experiments where a molecule bridges between tip and substrate, the current at a given retraction distance is greater than in the absence of a molecule, and characteristic current plateaux are observed; see Figure 1B.
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