Electron Transport through Conjugated Molecules: When the π System Only Tells Part of the Story (original) (raw)
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The Journal of Physical Chemistry C, 2008
We report on a class of molecules that exhibit nonlinear current/voltage behavior in the low bias tunneling regime. This interesting behavior is attributed to quantum interference. Using site models, we show that interference features, while common, do not necessarily occur at experimentally relevant energies, hindering realization in transport measurements. Calculations made using a nonequilibrium Green's function code show that quantum interference can be experimentally relevant in cross-conjugated molecules. A detailed bond length analysis of cross-conjugated molecules gives insight into why these molecules have interference at energetically accessible regions. The interference features are shown to be stable to both an electronic dephasing analysis and geometric fluctuations provided by molecular dynamics.
Beilstein Journal of Nanotechnology, 2011
Quantum interference effects offer opportunities to tune the electronic and thermoelectric response of a quantum-scale device over orders of magnitude. Here we focus on single-molecule devices, in which interference features may be strongly affected by both chemical and electronic modifications to the system. Although not always desirable, such a susceptibility offers insight into the importance of "small" terms, such as through-space coupling and many-body charge-charge correlations. Here we investigate the effect of these small terms using different Hamiltonian models with Hückel, gDFTB and many-body theory to calculate the transport through several single-molecule junctions, finding that terms that are generally thought to only slightly perturb the transport instead produce significant qualitative changes in the transport properties. In particular, we show that coupling of multiple interference features in cross-conjugated molecules by through-space coupling will lead to splitting of the features, as can correlation effects. The degeneracy of multiple interference features in cross-conjugated molecules appears to be significantly more sensitive to perturbations than those observed in equivalent cyclic systems and this needs to be considered if such supernodes are required for molecular thermoelectric devices.
Quantum interference in off-resonant transport through single molecules
Physical Review B, 2014
We provide a simple set of rules for predicting interference effects in off-resonant transport through single-molecule junctions. These effects fall in two classes, showing respectively an odd or an even number of nodes in the linear conductance within a given molecular charge state, and we demonstrate how to decide the interference class directly from the contacting geometry. For neutral alternant hydrocarbons, we employ the Coulson-Rushbrooke-McLachlan pairing theorem to show that the interference class is decided simply by tunneling on and off the molecule from same, or different sublattices. More generally, we investigate a range of smaller molecules by means of exact diagonalization combined with a perturbative treatment of the molecule-lead tunnel coupling. While these results generally agree well with GW calculations, they are shown to be at odds with simpler mean-field treatments. For molecules with spin-degenerate ground states, we show that for most junctions, interference causes no transmission nodes, but argue that it may lead to a non-standard gate-dependence of the zero-bias Kondo resonance.
Chemical Physics, 2009
Quantum coherent transport through a multiply-connected network is investigated by the free-electron network model (FENM). Within this model, we study p-conjugated molecules such as benzenedithiol (BDT) in order to understand the influence of nontrivial topological structures on the transport behavior. The analytical solutions for transmission functions and I-V characteristics of the simplest networked conjugated molecules are derived. Moreover, quantum effects such as resonance and interference are clearly revealed in this approach. We have also compared our FENM approach with the non-equilibrium Green's function (NEGF) method within tight-binding calculation.
The Chameleonic Nature of Electron Transport through π-Stacked Systems
Journal of the American Chemical Society, 2010
This paper shows that the symmetry breaking effects involved in chemisorbing π-stacked benzene rings to metallic electrodes can have a significant impact on the transport properties. Fully eclipsed stacks may not be optimal; rather stacks designed to favor interactions between particular sites lead to increased transport. Conversely, in the case of transport through an infinite stack of benzene rings, where the full 6-fold symmetry of the system is preserved, maximal overlap of the rings in the fully eclipsed structures maximizes transport.
Basic concepts of quantum interference and electron transport in single-molecule electronics
This tutorial outlines the basic theoretical concepts and tools which underpin the fundamentals of phase-coherent electron transport through single molecules. The key quantity of interest is the transmission coefficient T(E), which yields the electrical conductance, current–voltage relations, the thermopower S and the thermoelectric figure of merit ZT of single-molecule devices. Since T(E) is strongly affected by quantum interference (QI), three manifestations of QI in single-molecules are discussed, namely Mach–Zehnder interferometry, Breit–Wigner resonances and Fano resonances. A simple MATLAB code is provided, which allows the novice reader to explore QI in multi-branched structures described by a tight-binding (Hu¨ckel) Hamiltonian. More generally, the strengths and limitations of materials-specific transport modelling based on density functional theory are discussed.
Controlling Quantum Transport through a Single Molecule
Nano Letters, 2006
We investigate multiterminal quantum transport through single monocyclic aromatic annulene molecules, and their derivatives, using the nonequilibrium Green function approach within the self-consistent Hartree−Fock approximation. We propose a new device concept, the quantum interference effect transistor, that exploits perfect destructive interference stemming from molecular symmetry and controls current flow by introducing decoherence and/or elastic scattering that break the symmetry. This approach overcomes the fundamental problems of power dissipation and environmental sensitivity that beset nanoscale device proposals.
The Relation between Structure and Quantum Interference in Single Molecule Junctions
Nano Letters, 2010
Quantum interference (QI) of electron pathways has recently attracted increased interest as an enabling tool for singlemolecule electronic devices. Although various molecular systems have been shown to exhibit QI effects and a number of methods have been proposed for its analysis, simple guidelines linking the molecular structure to QI effects in the phase-coherent transport regime have until now been lacking. In the present work we demonstrate that QI in aromatic molecules is intimately related to the topology of the molecule's π system and establish a simple graphical scheme to predict the existence of QI-induced transmission antiresonances. The generality of the scheme, which is exact for a certain class of tight-binding models, is proved by a comparison to first-principles transport calculations for 10 different configurations of anthraquinone as well as a set of cross-conjugated molecular wires.