Multiscale Simulations of Quantum Structures (original) (raw)
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Atomic scale design of nanostructures
Molecular Physics, 2007
Recent advances in theoretical methods and high performance computing allow for reliable first-principles predictions of complex nanostructured materials and devices. This paper describes three examples: (i) non-equilibrium electron transport through molecular junctions, as a stepping stone for the design of molecular-scale devices and for integration of biomolecules with Si technology; (ii) polarization and piezoelectric properties of PVDF and related polymers; and (iii) the many-body optical spectrum of water. For the molecular junction, our results provide a qualitative picture and quantitative understanding of the mechanism leading to negative differential resistance for a large class of small molecules. For ferroelectric polymers, the calculations show that their polarization is described by cooperative, quantum-mechanical interactions between polymer chains. Nevertheless, the ab initio results lead to a simple parameterization of polarization as a function of copolymer concentration. Finally, our calculations explain the well-known redshift in the fundamental absorption of water as due to exciton delocalization upon aggregation.
The Journal of Chemical Physics, 1999
To study the electronic transport of molecular wire circuits, we present a time-independent scattering formalism which includes an ab initio description of the molecular electronic structure. This allows us to obtain the molecule-metal coupling description at the same level of theory. The conductance of junction ␣, ␣Ј xylyl dithiol and benzene-1,4-dithiol between gold electrodes is obtained and compared with available experimental data. The conductance depends dramatically on the relative position of the Fermi energy of the metal with respect to the molecular levels. We obtain an estimate for the injecting energy of the electron onto the molecule by varying the distance between the molecule and the attached gold clusters. Contrary to the standard assumption, we find that the injecting energy lies close to the molecular highest occupied molecular orbital, rather than in the middle of the gap; it is just the work function of the bulk metal. Finally, the adequacy of the widely used extended Hückel method for conductance calculations is discussed.
Physical Review B, 2006
Recent experimental studies demonstrated that self-assembled molecules sandwiched between metallic contacts can perform logic functions based on negative differential resistance ͑NDR͒. To understand the mechanism of NDR, the electronic structure and transport properties of one such junction, ferrocenyl alkanethiolate attached to a gold surface and probed with a scanning tunneling microscope tip, are investigated by large scale ab initio calculations. The I-V characteristics show strong NDR features at both positive and negative biases, in good agreement with the experimental data. The voltage-dependent transmission, potential drop profile, and molecular level alignment under bias suggest that the ferrocenyl group acts like a quantum dot and that the NDR features are due to resonant coupling between the highest occupied molecular orbital and the density of states of gold leads. The strength of the individual NDR peaks can be tuned by changing the tunneling distance or using suitable spacer layers.
The European Physical Journal E, 2005
Electric-field-assisted assembly has been used to place rod-shaped metal nanowires containing 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecules onto lithographically defined metal pads. These junctions exhibited negative differential resistance. The quantum chemical approach was used to compare the properties of Au-bonded 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecule and a molecule that does not exhibit the negative differential resistance, Au-bonded 4-[[4-(phenylethynyl) phenyl] ethynyl] benzenthiol. The influence of the static electric field and charge variation were modelled for both systems. PACS. 85.35.-p Nanoelectronic devices-73.63.-b Electronic transport in nanoscale materials and structures-31.15.-p Calculations and mathematical techniques in atomic and molecular physics (excluding electron correlation calculations)
Negative differential resistance – a decrease in current with increasing bias voltage – is a counterintuitive effect that is observed in various molecular junctions. Here, we present a novel mechanism that may be responsible for such an effect, based on strong Coulomb interaction between electrons in the molecule and electrons on the atoms closest to the molecule. The Coulomb interaction induces electron-hole binding across the molecule-electrode interface, resulting in a renormalized and enhanced molecule-electrode coupling. Using a self-consistent non-equilibrium Green’s function approach, we show that the effective coupling is non-monotonic in bias voltage, leading to negative differential resistance. The model is in accord with recent experimental observations that showed a correlation between the negative differential resistance and the coupling strength. We provide detailed suggestions for experimental tests which may help to shed light on the origin of the negative differential resistance. Finally, we demonstrate that the interface Coulomb interaction affects not only the I-V curves but also the thermoelectric properties of molecular junctions.
Physical Review Letters, 2008
A new mechanism is proposed to explain the origin of negative differential resistance (NDR) in a strongly coupled single molecule-metal junction. A first-principles quantum transport calculation in a Fe-terpyridine linker molecule sandwiched between a pair of gold electrodes is presented. Upon increasing applied bias, it is found that a new phase in the broken symmetry wavefunction of the molecule emerges from the mixing of occupied and unoccupied molecular orbital. As a consequence, a non-linear change in the coupling between molecule and lead is evolved resulting to NDR. This model can be used to explain NDR in other class of metal-molecule junction device.
Atomistic simulations of highly conductive molecular transport junctions under realistic conditions
Nanoscale, 2013
We report state-of-the-art atomistic simulations combined with high-fidelity conductance calculations to probe structureconductance relationships in Au-benzenedithiolate(BDT)-Au junctions under elongation. Our results demonstrate that large increases in conductance are associated with the formation of monatomic chains (MACs) of Au atoms directly connected to BDT. An analysis of the electronic structure of the simulated junctions reveals that enhancement in the s-like states in Au MACs causes the increases in conductance. Other structures also result in increased conductance but are too short-lived to be detected in experiment, while MACs remain stable for long simulation times. Examinations of thermally evolved junctions with and without MACs show negligible overlap between conductance histograms, indicating that the increase in conductance is related to this unique structural change and not thermal fluctuation. These results, which provide an excellent explanation for a recently observed anomalous experimental result [Bruot et al., Nature Nanotech., 2012, 7, 35-40], should aid in the development of mechanically responsive molecular electronic devices.
Computational chemistry for molecular electronics
Computational Materials Science, 2003
We present a synergetic effort of a group of theorists to characterize a molecular electronics device through a multiscale modeling approach. We combine electronic-structure calculations with molecular dynamics and Monte Carlo simulations to predict the structure of self-assembled molecular monolayers on a metal surface. We also develop a novel insight into molecular conductance, with a particular resolution of its fundamental channels, which stresses the importance of a complete molecular structure description of all components of the system, including the leads, the molecule, and their contacts. Both molecular dynamics and electron transport simulations imply that knowledge of detailed molecular structure and system geometry are critical for successful comparison with carefully performed experiments. We illustrate our findings with benzenedithiolate molecules in contact with gold.
Journal of Physics: Condensed Matter, 2014
The realization of metal–molecule junctions for future electronic devices relies on our ability to assemble these heterogeneous objects at a molecular level and understand their structure and the behavior of the electronic states at the interface. Delocalized interface states near the metal Fermi level are a key ingredient for tailoring charge injection, and such a delocalization depends on a large number of chemical, structural and morphological parameters, all influencing the spatial extension of the electron wavefunctions. Our large-scale dynamical simulations, combined with experiments, show that a double-decker organometallic compound (ferrocene) can be deposited on a Cu(111) surface, providing an ideal system to investigate the adsorption, the interface states and localized spin states at a metal–organometallic interface. Adsorbed ferrocene is shown to have a peculiar pattern and realizes a 2D-like interface state strongly resembling Shockley's surface state of Cu. By a subsequent deposition of single metal atoms on the adsorbed ferrocene, we analyze the sensitivity of the interface state to local modifications of the interface potential. This provides an insight into adsorption, spin configuration and charge redistribution processes, showing how to tune the electron behavior at a metal–molecule interface.