Molecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting "contact." It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, we show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to determine the impact of a variety of electronic factors that act to determine the tunnel barrier.energy alignment | molecular electronics | electronic coupling | charge transport | Fermi-level pinning T he conductance of electrical charge through and across molecular entities is the basis of molecular and organic electronics (1, 2). Understanding, controlling, and designing electronic circuits using organic molecules as components is a major goal of molecular electronics (3); however, it has been a challenge to identify all of the factors that govern the conductance of a molecular junction. Rather than being a simple property of the molecule itself, many circumstances contribute to the measured electronic properties of the junction. Some of the important features include the nature of the molecule-contact bonding (4), the properties of the contact materials (5, 6), the orientation of the molecules relative to the contacts (7), and the structure of the molecule (5,8,9). Although there is no general consensus on exactly how each of these features affects the conductance of the junction, it is generally agreed that the alignment of the molecular and contact energy levels is an important factor (10-13). The offset between the substrate Fermi energy (E f ) and the molecular orbital closest in energy to E f is often used to estimate charge transport barriers in the context of tunneling or charge injection models; however, it is increasingly clear that the situation is complex and that there is no simple meth...

Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein-electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode's perturbation of the Cu(II)-localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic features of the Cu(II) coordination sphere in transport characteristics that show directly the active role of the metal ion in resonance tunneling. Our results illustrate how quantum mechanical effects may dominate electron transport via protein-based junctions.

Making biomolecular electronics a reality will require control over charge transport across biomolecules.Here we show that chemical modulation of the coupling between one of the electronic contacts and the biomolecules in a solid-state junction allows controlling electron transport (ETp) across the junction. Employing the protein azurin (Az), we achieve such modulation as follows: Az is covalently bound by Au−S bonding to a lithographically prepared Au electrode (Au-Az). Au nanowires (AuNW) onto which linker molecules, with free carboxylic group, are bound via Au−S bonds serve as top electrode. Current−voltage plots of AuNW-linkerCOOH//Az-Au junctions have been shown earlier to exhibit step-like features, due to resonant tunneling through discrete Az energy levels. Forming an amide bond between the free carboxylic group of the AuNW-bound linker and Az yields AuNW-linkerCO-NH-Az-Au junctions. This Az-linker bond switches the ETp mechanism from resonant to off-resonant tunneling. By varying the extent of this amide bonding, the current−voltage dependence can be controlled between these two mechanisms, thus providing a platform for altering and controlling the ETp mechanism purely by chemical modification in a two-terminal device, i.e., without a gate electrode. Using results from conductance, including the energy barrier and electrodemolecule coupling parameters extracted from current−voltage fitting and normalized differential conductance analysis and from inelastic-electron-tunneling and photoelectron spectroscopies, we determine the Az frontier orbital energies, with respect to the Au Fermi level, for four junction configurations, differing only in electrode-protein coupling. Our approach and findings open the way to both qualitative and quantitative control of biomolecular electronic junctions.

Molecular electronics seeks to build circuitry using organic components with at least one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We measured the photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts. Where no molecular absorption takes place, the photocurrent is dominated by internal photoemission, which exhibits energy thresholds corresponding to interfacial transport barriers, enabling their direct measurement in a functioning junction.

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