Molecule Nanoelectronics

We measure the conductance of over 30 amine terminated molecules by breaking Au point-contacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule-Au junctions is much less than the variability for diisonitrile and dithiol-Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2-8 carbon atoms in the hydrocarbon chain and for polyphenyl diamine with 1-3 benzene rings, our results show that conductance decreases exponentially with length. For biphenyl diamines with different substituents that alter the phenyl-phenyl dihedral angle, we find that the conductance decreases with increasing twist angle, following a cosine squared dependence. Finally, for a series of substituted benzene diamines, we find that electron donating substituents on the ring, which drive the occupied molecular orbitals up, increase the junction conductance while electron withdrawing substituents have the opposite effect. Thus for this measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.