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Conductance measurement; Density functional theory calculations; Highest occupied molecular orbital; Homologous series; Length dependence; Odd-even effects; Transition voltage; Tunneling junctions
Abstract:
We compared the electrical properties of self-assembled monolayers (SAMs) formed on template-stripped Au from two homologous series of five different oligo(phenylene)s bearing alkane thiol tails. The terminal phenyl ring is substituted by a 4-pyridyl ring in one series, thus the two differ only by the substitution of C-H for N. We formed tunneling junctions using the liquid metal eutectic Ga-In (EGaIn) as a nondamaging, conformal top contact that is insensitive to functional groups and measured the current-density, J, tunneling decay constants, beta, and transition voltages, V-trans. Conductance measurements alone did not sufficiently differentiate the two series of molecules. The length dependences of the two series of SAMs produced values of beta of 0.44 and 0.42 angstrom(1) for pyridyl- and phenyl-terminated SAMs, respectively, which lie between the expected values for alkanethiolates and oligo(phenylene)s. The values of V-trans were 0.3 V larger for the phenyl-terminated SAMs than for the pyridyl-terminated SAMs. A comparison of the values of J to highest occupied molecular orbital (HOMO) levels determined by density functional theory (DFT) calculations revealed an odd-even effect for the phenyl-terminated SAMs but not the pyridyl-terminated SAMs. Plots of V-trans versus the measured shift in work function, measured with a Kelvin probe, reveal a roughly linear trend. Plots of the difference between HOMO and Fermi energies reveal a strong linear trend with two distinct series that clearly differentiate the two series of SAMs, even between SAMs with nearly identical HOMO levels, but only when the dipole-induced shift in vacuum level is considered. The influence of the electronic properties of the SAMs is clearly evident in the conductance data and highlights the importance of molecular dipoles in tunneling junctions comprising SAMs. Taken together, the data show that tunneling junctions incorporating EGaIn as a top contact are sensitive enough to differentiate SAMs that differ by the substitution of a single atom.
