The electrostatic potential produced by a variety of self-assembled monolayers on Au(111) is measured using scanning probe techniques. The molecules chosen for this study contain thiol-terminated end groups and π-conjugated orbitals, making them suitable for molecular electronics applications. We have measured the surface potential of molecules having a symmetric structure and compared these results to those obtained from similar nonsymmetric molecules. The measured potential for nonsymmetric molecules scales with the dipole moment of the molecule comprising the monolayer. For symmetric molecules, the measured surface potential is essentially the same as the substrate. This result suggests that the dipole moment formed by the Au-S bond makes a small contribution to the measured surface potential. The dipole moment of a strong electron accepting molecule was intentionally modified by reaction with a strong electron acceptor. In this case, the surface potential produced by the self-assembled monolayer was found to change polarity after the formation of the charge-transfer complex.
This letter presents experimental evidence that the electrical conductance of a single molecule can be altered by a chemical binding event. Self-assembled monolayers of electron donor tetramethyl xylyl dithiol (TMXYL) have been synthesized and chemically switched to a conducting state by reaction with an electron acceptor tetracyanoethylene (TCNE). Low bias conductance measurements obtained by scanning tunneling spectroscopy under ultrahigh vacuum conditions show a change from insulating to ohmic behavior as a result of the electron donor/acceptor interaction.
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