Charge Transport in Conjugated Aromatic Molecular Junctions:  Molecular Conjugation and Molecule−Electrode Coupling

Abstract

The conductance of a single molecule transport junction is investigated in the Landauer-Imry regime of coherent tunneling transport. Utilizing aromatic systems with thiol end groups, we have calculated using density functional theory the expected conductance of junctions containing molecules with different levels of conjugation and of different lengths. The calculated variations in transport junction conductance are explained in terms of the continuity of the conjugation path between leads. Molecular conjugation describes this continuity within the molecule, and the interfacial terms (spectral densities or imaginary parts of the self-energy) describe its continuity at the molecule/metal interface. We compare the results from junction conductance calculations with isolated molecule electronic structure calculations These density functional theory calculations suggest that for these dithiol molecules, transport occurs mostly through the occupied orbital manifold. The decay of the transport with length is found to be exponential for poly-Ph dithiol molecules. We compare the calculated conductance of conjugated aromatic molecules with their molecular orbital calculations and with the Green's function formulation and evaluate the relative significance of different factors (such as energetic alignment and spectral density) that control the conductance of molecules.

From fused aromatics to graphene-like nanoribbons: The effects of multiple terminal groups, length and symmetric pathways on charge transport

“…Conjugated aromatic molecules, such as polycyclic hydrocarbons, [1][2][3] and oligomers, such as phenylene-vinylene, 4 represent some of the most frequently investigated molecular wires. Despite the large body of conductivity studies found in the literature, a class of wires with transport properties suitable for molecular circuit usage has not yet been identified.…”

Tunneling across SAMs Containing Oligophenyl Groups

“…Although these SAMs are based on simple oligophenyls and share (we assume) a similar Ph-H//Ga 2 O 3 interface, they differ substantially in their physical and electronic interactions with the metal substrates that form the "bottom" electrodes. Our results indicate that SAMs of SPh n and C≡CPh n on gold, which are characterized by orbital overlap (conjugation) across the molecule, and between the molecule and the electrode, 17 have similar, low values of the attenuation factor β in the simplified Simmons equation 18 (eq. 1) and the injection current, J 0 (β = 0.28 ± 0.03 Å -1 and log|J 0 | = 2.7 ± 0.1 for Au/SPh n ; β = 0.30 ± 0.02 Å -1 and log|J 0 | = 3.0 ± 0.1 for Au/C≡CPh n ).…”

“…27,[38][39][40][41][42][43][44][45] These results presented here also agree with theoretical calculations by Ratner and coworkers; these authors predict (using density functional theory, DFT) a value of  = ~0.3 Å -1 for SAMs of polyphenyldithiolates by assuming a continuous conjugation of the molecules with the metal electrodes. 17 We compared the electrical properties of SAMs of SPh n to SAMs of C≡CPh n on Au TS (Figure 2b). Both molecular systems are conjugated; that is, the π-electrons are delocalized across the molecular backbone, but they differ by the chemical structure of the anchoring group (-S-vs. -C≡C-).…”

Electrical characteristics of conjugated self-assembled monolayers in large-area molecular junctions

“…Calculations using different formalisms resulted in ␤ = 0.17-0.51 Å −1 . [25][26][27][28][29][30][31][32][33] Kondo et al 27 and Liu et al 31 have studied the influence of the ring torsion angle between the phenyl rings and found ␤ as low as 0.17 and 0.24 Å −1 for planar para-phenylene systems, i.e., no torsion angle between adjacent phenyl rings. The experimentally determined coefficients in large-area molecular junctions match closely to these values.…”