Electrical transport through molecules has been much studied since it was proposed¹ that individual molecules might behave like basic electronic devices, and intriguing single-molecule electronic effects have been demonstrated^{2, 3}. But because transport properties are sensitive to structural variations on the atomic scale^{4, 5, 6, 7}, further progress calls for detailed knowledge of how the functional properties of molecules depend on structural features. The characterization of two-terminal structures has become increasingly robust and reproducible^{8, 9, 10, 11, 12}, and for some systems detailed structural characterization of molecules on electrodes or insulators is available^{13, 14, 15, 16, 17}. Here we present scanning tunnelling microscopy observations and classical electrostatic and quantum mechanical modelling results that show that the electrostatic field emanating from a fixed point charge regulates the conductivity of nearby substrate-bound molecules. We find that the onset of molecular conduction is shifted by changing the charge state of a silicon surface atom, or by varying the spatial relationship between the molecule and that charged centre. Because the shifting results in conductivity changes of substantial magnitude, these effects are easily observed at room temperature.

1. Department of Physics, 534 Avadh Bhatia Physics Lab, University of Alberta, Edmonton, Alberta T6G 2J1, Canada
2. National Institute for Nanotechnology, National Research Council of Canada, W6-010 ECERF, 9107-116th Street, Edmonton, Alberta T6G 2V4, Canada
3. Surface Science Research Centre, University of Liverpool, Liverpool L69 3BX, UK

Correspondence to: Robert A. Wolkow^1,2 Correspondence and requests for materials should be addressed to R.A.W. (Email: rwolkow@ualberta.ca).

Received 25 January 2005; Accepted 17 March 2005

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