Molecular conductance spectroscopy of conjugated, phenyl-based molecules on Au(111): the effect of end groups on molecular conduction

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Four families of conjugated molecules, containing between one and three phenyl rings and having both thiol (–SH) and isocyanide (–NC) end groups, have been synthesized and assembled as monolayers on flat Au(111) substrates. The conductance spectra G(V) for these molecular wires were systematically measured in UHV conditions using scanning tunneling microscope techniques. The measured conductance spectra for the molecules having thiol end groups are compared to a recent theory for molecular conduction. The favorable comparison indicates that the important properties influencing the conductance of short, conjugated molecular wires having thiol end groups and forming self-assembled monolayers on a Au(111) surface have been successfully identified. The isocyanide molecules reveal a shift in Fermi level of the molecule as a function of phenyl ring number that is opposite to that observed for the thiol-terminated molecules. The trends in molecular conductance determined from this systematic study are summarized and discussed and provide insight into the role played by bonding end groups in electronic conduction.