Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.

Electron transport in molecular junctions.

The interaction of light with molecular conduction junctions is attracting growing interest as a challenging experimental and theoretical problem on one hand, and because of its potential application as a characterization and control tool on the other. It stands at the interface between two important fields, molecular electronics and molecular plasmonics and has attracted attention as a challenging scientific problem with potentially important technological consequences. Here we review the present state of the art of this field, focusing on several key phenomena and applications: using light as a switching device, using light to control junction transport in the adiabatic and non-adiabatic regimes, light generation in biased junctions and Raman scattering from such systems. This field has seen remarkable progress in the past decade, and the growing availability of scanning tip configurations that can combine optical and electrical probes suggests that further progress towards the goal of realizing molecular optoelectronics on the nanoscale is imminent.

/pdf/molecular-optoelectronics-the-interaction-of-molecular-2myr3yqomr.pdf

Molecular optoelectronics: the interaction of molecular conduction junctions with light

The electroluminescence of a polythiophene wire suspended between a metallic surface and the tip of a scanning tunneling microscope is reported. Under positive sample voltage, the spectral and voltage dependencies of the emitted light are consistent with the fluorescence of the wire junction mediated by localized plasmons. This emission is strongly attenuated for the opposite polarity. Both emission mechanism and polarity dependence are similar to what occurs in organic light emitting diodes (OLED) but at the level of a single molecular wire.

/pdf/electroluminescence-of-a-polythiophene-molecular-wire-1g0ye324ng.pdf

Electroluminescence of a polythiophene molecular wire suspended between a metallic surface and the tip of a scanning tunneling microscope.

Electroluminescence (EL) in scanning tunneling microscopy (STM), which enables spectroscopy with submolecular spatial resolution, is shown to be due to radiative ionization with vibronic shape resonances that carry Fano line profiles. Since Fano progressions retain phase information, the spectra can be transformed to the time domain to reconstruct the vibronic motion. In effect, measurements within a molecule are accessible with joint space–time resolution at the A–fs limit. We demonstrate this through EL-STM on the Jahn–Teller-active Zn-etioporphyrin radical anion and visualize the orbiting motion of scattered electrons upon sudden reduction and oxidation. We discuss the elements that enable spectroscopy with submolecular spatial resolution through EL-STM and the closely related STM-Raman process.

Vibronic Motion with Joint Angstrom–Femtosecond Resolution Observed through Fano Progressions Recorded within One Molecule

We propose a scheme for calculation of linear optical response of current-carrying molecular junctions for the case when electronic tunneling through the junction is much faster than characteristic time of external laser field. We discuss relationships between nonequilibrium Green’s function (NEGF) and time-dependent density functional theory (TDDFT) approaches and derive expressions for optical response and linear polarizability within NEGF-TDDFT scheme. Corresponding results for isolated molecule, derived within TDDFT approach previously, are reproduced when coupling to contacts is neglected.

/pdf/linear-optical-response-of-current-carrying-molecular-1lyz9nhass.pdf

Linear optical response of current-carrying molecular junction: a nonequilibrium Green's function-time-dependent density functional theory approach.

We explore theoretically the electroluminescence of single molecules. We adopt a local-electrode framework that is appropriate for scanning tunneling microscopy (STM) experiments where electroluminescence originates from individual molecules of moderate size on complex substrates: Couplings between the STM tip and molecule and between the molecule and multiple substrate sites are treated on the same footing as local electrodes contacting the molecule. Electron flow is modeled with the Lippmann-Schwinger Green's function scattering technique. The evolution of the electronic energy levels of the molecule under bias is modeled assuming the total charge of the molecule to be invariant, consistent with Coulomb blockade considerations, but the electronic occupations of the molecular highest occupied molecular orbital and lowest unoccupied molecular orbital levels vary with changing bias. The photon-emission rate is calculated using Fermi's golden rule. We apply this theoretical approach to the STM/Zn-etioporphyrin/${\text{Al}}_{2}{\text{O}}_{3}/\text{NiAl}(110)$ system and simulate various configurations of coupling strength between the molecule and substrate. We compare our results to the experimental observations of Qiu et al. [Science 299, 542 (2003)] for this system and find that our model provides a comprehensive explanation of a multitude of previously unexplained observations. These include the different types of current-voltage characteristics (CVCs) that are observed experimentally, the observed association of electroluminescence with some CVCs and not others, and key properties of the observed photon spectra. Theoretical predictions are presented for further single-molecule electroluminescence experiments.

Understanding the electroluminescence emitted by single molecules in scanning tunneling microscopy experiments

We explore theoretically the scanning tunneling microscopy (STM) of single molecules on substrates using a framework of two local probes. This framework is appropriate for studying electron flow in tip-molecule-substrate systems where a thin insulating layer between the molecule and a conducting substrate transmits electrons nonuniformly and thus confines electron transmission between the molecule and substrate laterally to a nanoscale region significantly smaller in size than the molecule. The tip-molecule coupling and molecule-substrate coupling are treated on the same footing, as local probes to the molecule, with electron flow modelled using the Lippmann-Schwinger Green-function scattering technique. STM images are simulated for various positions of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We find that these images have a strong dependence on the substrate probe position, indicating that electron flow can depend strongly on both tip position and the location of the dominant molecule-substrate coupling. Differences in the STM images are explained in terms of the molecular orbitals that mediate electron flow in each case. Recent experimental results, showing STM topographs of Zn(II)-etioporphyrin I on $\\text{alumina}∕\\mathrm{NiAl}(110)$ to be strongly dependent on which individual molecule on the substrate is being probed, are explained using this model. A further experimental test of the model is also proposed.

http://absimage.aps.org/image/MAR05/MWS_MAR05-2004-001122.pdf

Two-probe theory of scanning tunneling microscopy of single molecules

We explore theoretically the scanning tunneling microscopy (STM) of single molecules on substrates using a framework of two local probes. This framework is appropriate for studying electron flow in tip-molecule-substrate systems where a thin insulating layer between the molecule and a conducting substrate transmits electrons nonuniformly and thus confines electron transmission between the molecule and substrate laterally to a nanoscale region significantly smaller in size than the molecule. The tip-molecule coupling and molecule-substrate coupling are treated on the same footing, as local probes to the molecule, with electron flow modelled using the Lippmann-Schwinger Green-function scattering technique. STM images are simulated for various positions of the stationary (substrate) probe below a Zn(II)-etioporphyrin I molecule. We find that these images have a strong dependence on the substrate probe position, indicating that electron flow can depend strongly on both tip position and the location of the dominant molecule-substrate coupling. Differences in the STM images are explained in terms of the molecular orbitals that mediate electron flow in each case. Recent experimental results, showing STM topographs of Zn(II)-etioporphyrin I on $\text{alumina}∕\mathrm{NiAl}(110)$ to be strongly dependent on which individual molecule on the substrate is being probed, are explained using this model. A further experimental test of the model is also proposed.

Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina

We explore theoretically the principles that govern photon emission from single-molecule conductors carrying electric currents between metallic contacts. The molecule and contacts are represented by a generic tight-binding model. The electric current is calculated using the Landauer theory and the photon emission rate is obtained using Fermi's golden rule. The bias dependence of the electronic structure of the molecular wire is included in the theory in a simple way. Conditions under which significant photon emission should occur are identified and photon spectra are calculated. We predict the photon emission rate to be more sensitive than the electric current to coupling asymmetries between the molecule and contacts. This has important implications for the design and interpretation of scanning tunneling microscopy experiments searching for electroluminescence from individual molecules. We discuss how electroluminescence may be used to measure important characteristics of the electronic structure of molecular wires such as the HOMO-LUMO (highest occupied molecular orbital--lowest unoccupied molecular orbital) gap and the location of the Fermi level of the contacts relative to the HOMO and LUMO. The feasibility of observing photon emission from Au/benzene-dithiolate molecular wires is also discussed.

Theoretical study of photon emission from molecular wires

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Papers

Understanding the electroluminescence emitted by single molecules in scanning tunneling microscopy experiments

Two-probe theory of scanning tunneling microscopy of single molecules

Two-probe theory of scanning tunneling microscopy of single molecules: Zn(II)-etioporphyrin on alumina

Theoretical study of photon emission from molecular wires