The Chameleonic Nature of Electron Transport through π-Stacked Systems (original) (raw)
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Journal of the American Chemical Society, 2011
Understanding electron transport across π-π-stacked systems will help to answer fundamental questions about biochemical redox processes and benefit the design of new materials and molecular devices. Herein we employed the STM break-junction technique to measure the single-molecule conductance of multiple π-π-stacked aromatic rings. We studied electron transport through up to four stacked benzene rings held together in an eclipsed fashion via a paracyclophane scaffold. We found that the strained hydrocarbons studied herein couple directly to gold electrodes during the measurements; hence, we did not require any heteroatom binding groups as electrical contacts. Density functional theorybased calculations suggest that the gold atoms of the electrodes bind to two neighboring carbon atoms of the outermost cyclophane benzene rings in η 2 fashion. Our measurements show an exponential decay of the conductance with an increasing number of stacked benzene rings, indicating a nonresonant tunneling mechanism. Furthermore, STM tip-substrate displacement data provide additional evidence that the electrodes bind to the outermost benzene rings of the π-π-stacked molecular wires.
Quantum Transport Through Heterocyclic Molecules
International Journal of Modern Physics B, 2009
We explore electron transport properties in molecular wires made of heterocyclic molecules (pyrrole, furan and thiophene) by using the Green's function technique. Parametric calculations are given based on the tight-binding model to describe the electron transport in these wires. It is observed that the transport properties are significantly influenced by (a) the heteroatoms in the heterocyclic molecules and (b) the molecule-to-electrodes coupling strength. Conductance (g) shows sharp resonance peaks associated with the molecular energy levels in the limit of weak molecular coupling, while they get broadened in the strong molecular coupling limit. These resonances get shifted with the change of the heteroatoms in these heterocyclic molecules. All the essential features of the electron transfer through these molecular wires become much more clearly visible from the study of our current-voltage (I-V) characteristics, and they provide several key information in the study of molecul...
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.
The Journal of Chemical Physics, 2008
The effect of metal-molecule coupling on electron transport is examined in the prototypical case of alkane chains sandwiched between gold contacts and bridged by either amine or thiol groups. The results show that end group functionalization plays a crucial role in controlling electron transport, and that the symmetries and spatial extent of orbitals near the Fermi level control the conductivity rather than the strength of the bonding. For amine/Au and thiol/Au junctions, a crossover in conductivity with increasing bias is predicted.
Electron transport in multiterminal molecular devices: A density functional theory study
Physical Review B, 2010
The electron transport properties of a four-terminal molecular device are computed within the framework of density functional theory and nonequilibrium Keldysh theory. The additional two terminals lead to new properties, including a pronounced negative differential resistance not present in a two-terminal setup, and a pseudogating effect. In general, quantum interference between the four terminals and the central molecule leads to a complex nonlinear behavior of the current, which depends on the alignment of individual molecular states under bias and their coupling to the leads.
First-Principles Calculation of Transport Properties of a Molecular Device
Physical Review Letters, 2000
We report first-principles calculations of the current-voltage (I-V ) characteristics of a molecular device and compare with experiment. We find that the shape of the I-V curve is largely determined by the electronic structure of the molecule, while the presence of single atoms at the molecule-electrode interface play a key role in determining the absolute value of the current. The results show that such simulations would be useful for the design of future microelectronic devices for which the Boltzmann-equation approach is no longer applicable. PACS numbers: 73.40.Jn, 73.40.Cg, 73.40.Gk, 85.65. + h Conventional Si-based microelectronics is likely to reach its limit of miniaturization in the next 10-15 years when feature lengths shrink below 100 nm. The main problem is the onset of quantum phenomena, e.g., tunneling, that would make scaled-down conventional devices inoperable. Successor technologies currently under development, such as tunneling field-effect transistors and single-electron transistors, are in fact based on quantum phenomena. For the ultimate miniaturization below nm, devices made from single molecules are currently attracting attention. Prototypes have already been fabricated. Reed et al. reported I-V characteristics of single benzene-1,4-dithiolate molecules. Alivisatos and coworkers [2] reported similar I-V characteristics of semiconductor and metal nanoclusters between gold electrodes. Dekker and co-workers [3] reported transistorlike behavior in carbon nanotubes. Similar devices have been demonstrated by Avouris and co-workers using single-walled and multiwalled carbon nanotubes .