/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

Molecular conductance junctions are structures in which single molecules or small groups of molecules conduct electrical current between two electrodes. In such junctions, the connection between the molecule and the electrodes greatly affects the current-voltage characteristics. Despite several experimental and theoretical advances, including the understanding of simple systems, there is still limited correspondence between experimental and theoretical studies of these systems.

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Electron transport in molecular wire junctions.

The conductance of a single molecule connected to two gold electrodes was determined by repeatedly forming thousands of gold-molecule-gold junctions. Conductance histograms revealed well-defined peaks at integer multiples of a fundamental conductance value, which was used to identify the conductance of a single molecule. The resistances near zero bias were 10.5 +/- 0.5, 51 +/- 5, 630 +/- 50, and 1.3 +/- 0.1 megohms for hexanedithiol, octanedithiol, decanedithiol, and 4,4' bipyridine, respectively. The tunneling decay constant (betaN) for N-alkanedithiols was 1.0 +/- 0.1 per carbon atom and was weakly dependent on the applied bias. The resistance and betaN values are consistent with first-principles calculations.

http://science.sciencemag.org/content/sci/301/5637/1221.full.pdf

Measurement of Single-Molecule Resistance by Repeated Formation of Molecular Junctions

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems1. Experiments to date have examined a multitude of molecules conducting in parallel2,3, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions4,5 or scanning probes6,7 as well as three-terminal single-molecule transistors made from carbon nanotubes8, C60 molecules9, and conjugated molecules diluted in a less-conducting molecular layer10. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.

Coulomb blockade and the Kondo effect in single-atom transistors

Since it was first suggested1 that a single molecule might function as an active electronic component, a number of techniques have been developed to measure the charge transport properties of single molecules2,3,4,5,6,7,8,9,10,11,12. Although scanning tunnelling microscopy observations under high vacuum conditions can allow stable measurements of electron transport, most measurements of a single molecule bonded in a metal–molecule–metal junction exhibit relatively large variations in conductance. As a result, even simple predictions about how molecules behave in such junctions have still not been rigorously tested. For instance, it is well known13,14 that the tunnelling current passing through a molecule depends on its conformation; but although some experiments have verified this effect15,16,17,18, a comprehensive mapping of how junction conductance changes with molecular conformation is not yet available. In the simple case of a biphenyl—a molecule with two phenyl rings linked by a single C–C bond—conductance is expected to change with the relative twist angle between the two rings, with the planar conformation having the highest conductance. Here we use amine link groups to form single-molecule junctions with more reproducible current–voltage characteristics19. This allows us to extract average conductance values from thousands of individual measurements on a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine-squared relation predicted for transport through π-conjugated biphenyl systems13.

/pdf/dependence-of-single-molecule-junction-conductance-on-3kczbcpq89.pdf

Dependence of single-molecule junction conductance on molecular conformation

A reliable method has been developed for making through-bond electrical contacts to molecules. Current-voltage curves are quantized as integer multiples of one fundamental curve, an observation used to identify single-molecule contacts. The resistance of a single octanedithiol molecule was 900 ± 50 megohms, based on measurements on more than 1000 single molecules. In contrast, nonbonded contacts to octanethiol monolayers were at least four orders of magnitude more resistive, less reproducible, and had a different voltage dependence, demonstrating that the measurement of intrinsic molecular properties requires chemically bonded contacts.

https://science.sciencemag.org/content/sci/294/5542/571.full.pdf

Reproducible Measurement of Single-Molecule Conductivity

We compile, compare, and discuss experimental results on low bias, room-temperature currents through orgnaic molecules obtained in different electrode-molecule-electrode test-beds. Currents are normalized to single-molecule values for comparison and are quoted at 0.2 and 0.5 V junction bias. Emphasis is on currents through saturated alkane chains where many comparable measurements have been reported, but comparison to conjugated molecules is also made. We discuss factors that affect the magnitude of the measured current, such as tunneling attenuation factor, molecular energy gap and conformation, molecule/electrode contacts, and electrode material.

Comparison of Electronic Transport Measurements on Organic Molecules

Electrical contacts between a metal probe and molecular monolayers have been characterized using conducting atomic force microscopy in an inert environment and in a voltage range that yields reversible current-voltage data. The current through alkanethiol monolayers depends on the contact force in a way that is accounted for by the change of chain-to-chain tunnelling with film thickness. The electronic decay constant, βN, was obtained from measurements as a function of chain length at constant force and bias, yielding βN = 0.8±0.2 per methylene over a ±3 V range. Current-voltage curves are difficult to reconcile with this almost constant value. Very different results are obtained when a gold tip contacts a 1,8-octanedithiol film. Notably, the current-voltage curves are often independent of contact force. Thus the contact may play a critical role both in the nature of charge transport and the shape of the current-voltage curve.

https://iopscience.iop.org/article/10.1088/0957-4484/13/1/302/pdf

Making electrical contacts to molecular monolayers

The complex band structure of a periodic system is the conventional band structure extended to complex Bloch k vectors. The k vectors with an imaginary part describe spatially decaying wave functions and arise in, for example, the analysis of impurity and surface states. They also represent the quantum tunneling states which are vehicles of electron transport through a barrier such as a thin oxide layer or a molecule. We present a method for obtaining the complex band structure of a molecule which is composed of repeating units. The complex band structures of some simple organic molecules (n-alkanes, alkenes, and linked benzene rings) will be determined and used to analyze electron conduction through molecules (e.g., octanedithiol) connected between gold electrodes. The form of the complex band structure clearly elucidates the molecule length dependence of the tunneling current and also suggests the energy for alignment of the Fermi level.

Complex band structure, decay lengths, and Fermi level alignment in simple molecular electronic systems

Molecular electronic devices require at least two electrical contacts to one (or more) molecule(s). Single molecules are reliably probed by bonding one end to a gold substrate and the other end to a gold nanocrystal. The circuit is completed with a gold-coated atomic force microscope probe. Measurements of the decay of electronic current with the length of n-alkanedithiol molecules in these single-molecule nanojunctions are reported as a function of the applied bias. The value of the decay constant near zero bias was obtained from measurements in the ohmic region of the current−voltage curves. The electron tunneling decay rate is significantly smaller (βN = 0.57 ± 0.03) than observed for molecules bonded at just one end (βN ≈ 1), and it falls to even smaller values as the applied bias is increased. Both these effects are quantitatively accounted for by a large shift in molecular levels caused by the attachment of wires at each end.

John K. Tomfohr

Papers

Reproducible Measurement of Single-Molecule Conductivity

Comparison of Electronic Transport Measurements on Organic Molecules

Making electrical contacts to molecular monolayers

Complex band structure, decay lengths, and Fermi level alignment in simple molecular electronic systems

Changes in the Electronic Properties of a Molecule When It Is Wired into a Circuit