Charge transport in molecular electronic junctions: compression of the molecular tunnel barrier in the strong coupling regime.

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.

Epitaxial growth of nanostructured gold films on germanium via galvanic displacement.

This work focuses on the synthesis and characterization of gold films grown via galvanic displacement on Ge(111) substrates. The synthetic approach uses galvanic displacement, a type of electroless deposition that takes place in an efficient manner under aqueous, room temperature conditions. Investigations involving X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were performed to study the crystallinity and orientation of the resulting gold-on-germanium films. A profound effect of HF(aq) concentration was noted, and although the SEM images did not show significant differences in the resulting gold films, a host of X-ray diffraction studies demonstrated that higher concentrations of HF(aq) led to epitaxial gold-on-germanium, whereas in the absence of HF(aq), lower degrees of order (fiber texture) resulted. Cross-sectional nanobeam diffraction analyses of the Au-Ge interface confirmed the epitaxial nature of the gold-on-germanium film. This epitaxial behavior can be attributed to the simultaneous etching of the germanium oxides, formed during the galvanic displacement process, in the presence of HF. High-resolution TEM analyses showed the coincident site lattice (CSL) interface of gold-on-germanium, which results in a small 3.8% lattice mismatch due to the coincidence of four gold lattices with three of germanium.

Heteroepitaxial growth of gold nanostructures on silicon by galvanic displacement.

This work focuses on the synthesis and interfacial characterization of gold nanostructures on silicon surfaces, including Si(111), Si(100), and Si nanowires. The synthetic approach uses galvanic displacement, a type of electroless deposition that takes place in an efficient manner under aqueous, room-temperature conditions. The case of gold-on-silicon has been widely studied and used for several applications and yet, a number of important, fundamental questions remain as to the nature of the interface. Some studies are suggestive of heteroepitaxial growth of gold on the silicon surface, whereas others point to the existence of a silicon-gold intermetallic sandwiched between the metallic gold and the underlying silicon substrate. Through detailed high resolution transmission electron microscopy (TEM), combined with selected area electron diffraction (SAED) and nanobeam diffraction (NBD), heteroepitaxial gold that is grown by galvanic displacement is confirmed on both Si(100) and Si(111), as well as silicon nanowires. The coincident site lattice (CSL) of gold-on-silicon results in a very small 0.2% lattice mismatch due to the coincidence of four gold lattices to three of silicon. The presence of gold-silicon intermetallics is suggested by the appearance of additional spots in the electron diffraction data. The gold-silicon interfaces appear heterogeneous with distinct areas of heteroepitaxial gold on silicon, and others, less well-defined, where intermetallics may reside. The high resolution cross-sectional TEM images reveal a roughened silicon interface under these aqueous galvanic displacement conditions, which most likely promotes nucleation of metallic gold islands that merge over time: a Volmer-Weber growth mechanism in the initial stages.