Abstract
Molecular junctions consisting of 2–20 nm thick layers of organic oligomers oriented between a conducting carbon substrate and a carbon/gold top contact have proven to be reproducible and reliable, and will soon enter commercial production in audio processing circuits. The covalent, conjugated bond between one or both sp2-hybridized carbon contacts and an aromatic molecular layer is distinct from the more common metal/molecule or silicon/molecule structures in many reported molecular junctions. Theoretical observations based on density functional theory are presented here, which model carbon-based molecular junctions as single molecules and oligomers between fragments of graphene. Electronic coupling between the molecules and the contacts is demonstrated by the formation of hybrid orbitals in the model structure, which have significant electron density on both the graphene and the molecule. The energies of such hybrid orbitals correlate with tunneling barriers determined experimentally, and electronic coupling between the two graphene fragments in the model correlates with experimentally observed attenuation of transport with molecular layer thickness. Electronic coupling is affected significantly by the dihedral angle between the planes of the graphene and the molecular π-systems, but is absent only when the two planes are orthogonal. Coupling also results in partial charge transfer between the graphene contacts and the molecular layer, which results in a shift in electrostatic potential which affects the observed tunneling barrier. Although the degree of partial charge transfer is difficult to calculate accurately, it does provide a basis for the “vacuum level shift” observed in many experiments, including transport and ultraviolet photoelectron spectroscopy of molecular layers on conductors.
Highlights
The field of Molecular Electronics investigates the behavior of molecules as elements in electronic circuits, with the intent of exploiting variations of molecular structure to realize unusual electronic functions [1,2,3,4]
The purpose of the current paper is to identify some of the “guides” that apply to how molecular structure affects electron transport in carbon-based molecular electronic devices
Electronic coupling is an important factor in understanding transport, both coupling between the molecule and the contacts, and between subunits within the molecular layer
Summary
The field of Molecular Electronics investigates the behavior of molecules as elements in electronic circuits, with the intent of exploiting variations of molecular structure to realize unusual electronic functions [1,2,3,4]. Charge transport through single molecules and through ensembles of molecules in nanoscale (1–20 nm) films has been studied experimentally since the late. The experimental paradigms are numerous [1,5], but they all share a common phenomenon of electrical communication between molecules and “contacts” made from conventional conductors or semiconductors. A basic element of Molecular Electronics is the “molecular junction (MJ)” consisting of molecules oriented between two conductors, with charge transport through the molecular layer. The vast majority of existing junction structures are based on metal/molecule bonding such as the Au/thiol self-assembled monolayers [1321], Langmuir–Blodgett films on metals [22,23], or molecules bonded to silicon [24,25,26,27]
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