Conductive Molecular Junctions Research Articles

Hybrid Molecular Junctions Using Au–S and Au−π Bindings

Electrically conductive molecular junctions are highly desirable components for applications in future molecular-scale electronic circuits. Here, we demonstrate an electrically conductive hybrid mo...

Investigation of the geometrical arrangement and single molecule charge transport in self-assembled monolayers of molecular towers based on tetraphenylmethane tripod

Multipodal molecular platforms were designed recently to establish a well-defined contact between molecular electronic components and metallic electrodes to manufacture working devices based on molecular electronics. In this work, we use electrochemical techniques, scanning tunneling microscopy break junction technique and theoretical approaches combining density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism to investigate the geometrical arrangement and single molecule charge transport in self-assembled monolayers (SAMs) of molecular towers anchored by tetraphenylmethane tripod on Au (111) surface. The effect of the molecular length as well as the role of the position of anchoring groups was addressed. Electrochemical double-layer capacitance measurements and reductive desorption studies combined with theoretical modeling clearly demonstrated that the molecular towers form densely packed SAMs, in which the individual molecules are attached to the Au (111) surface by the tripodal base and the principal molecular axis is directed away from the electrode surface, providing thus desired orientation. Temperature resolved single molecule conductance measurements combined with DFT/NEGF calculations showed that the electric charge is transported by tunneling via highly conductive molecular junctions formed by the tripodal base. Our combined experimental and theoretical work demonstrates that tetraphenylmethane tripods are suitable platforms to bear functional groups serving as molecular electronic components.

Supramolecular Systems and Chemical Reactions in Single-Molecule Break Junctions.

The major challenges of molecular electronics are the understanding and manipulation of the electron transport through the single-molecule junction. With the single-molecule break junction techniques, including scanning tunneling microscope break junction technique and mechanically controllable break junction technique, the charge transport through various single-molecule and supramolecular junctions has been studied during the dynamic fabrication and continuous characterization of molecular junctions. This review starts from the charge transport characterization of supramolecular junctions through a variety of noncovalent interactions, such as hydrogen bond, π-π interaction, and electrostatic force. We further review the recent progress in constructing highly conductive molecular junctions via chemical reactions, the response of molecular junctions to external stimuli, as well as the application of break junction techniques in controlling and monitoring chemical reactions in situ. We suggest that beyond the measurement of single molecular conductance, the single-molecule break junction techniques provide a promising access to study molecular assembly and chemical reactions at the single-molecule scale.

Orientation-Controlled Single Molecule-Electrode Junctions

Molecular scale electronic devices usually are designed by wiring a single molecule between two metal electrodes via anchoring groups. Conventional single molecule conductance (SMC) studies focus on molecules with functional groups that provide efficient electronic coupling and bind the organic molecular backbone to the metal electrodes. However, conductance is sensitive to the atomic level details of the molecule-electrode contact so that the anchoring groups end up being resistive spacers between the molecule and the metal, decreasing the single molecule junction conductivity. Thus creating well-defined, highly conductive molecular junctions to minimize resistance introduced by chemical linkers is a challenging experimental problem especially under ambient conditions.The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature using a combination of molecular assembly, Scanning Tunneling Microscopy (STM) imaging and STM break junction (STM-BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat-oriented mesitylene on Au(111) to enable direct tip/π contacts, a result that is not possible via conventional methods. The measured conductance of sandwiched Au/π/Au junction is ~0.1Go, two orders of magnitude higher than the conductance of phenyl rings connected via standard anchoring groups. A comparison with other benzene derivatives, that do not display long-range ordered structure, suggests that such structures, which hold the aromatic ring in place and parallel to the surface, are essential to the increased probability of the formation of orientation-controlled “sandwiched” molecular junctions.

Carbon tips for all-carbon single-molecule electronics.

We present here an exhaustive ab initio study of the use of carbon-based tips as electrodes in single-molecule junctions. Motivated by recent experiments, we show that carbon tips can be combined with other carbon nanostructures, such as graphene, to form all-carbon molecular junctions with molecules like benzene or C60. Our results show that the use of carbon tips can lead to relatively conductive molecular junctions. However, contrary to junctions formed with standard metals, the conductance traces recorded during the formation of the all-carbon single-molecule junctions do not exhibit clear conductance plateaus, which can be attributed to the inability of the hydrogenated carbon tips to form chemical bonds with the organic molecules. Additionally, we explore here the use of carbon tips for scanning tunneling microscopy and show that they are well suited for obtaining sample images with atomic resolution.

Erratum: Highly Conductive Molecular Junctions Based on Direct Binding of Benzene to Platinum Electrodes [Phys. Rev. Lett.PRLTAO0031-9007101, 046801 (2008)

Received 2 August 2012DOI:https://doi.org/10.1103/PhysRevLett.109.079903© 2012 American Physical Society

Transmission eigenvalue distributions in highly conductive molecular junctions.

Background: The transport through a quantum-scale device may be uniquely characterized by its transmission eigenvalues τn. Recently, highly conductive single-molecule junctions (SMJ) with multiple transport channels (i.e., several τn > 0) have been formed from benzene molecules between Pt electrodes. Transport through these multichannel SMJs is a probe of both the bonding properties at the lead–molecule interface and of the molecular symmetry.Results: We use a many-body theory that properly describes the complementary wave–particle nature of the electron to investigate transport in an ensemble of Pt–benzene–Pt junctions. We utilize an effective-field theory of interacting π-electrons to accurately model the electrostatic influence of the leads, and we develop an ab initio tunneling model to describe the details of the lead–molecule bonding over an ensemble of junction geometries. We also develop a simple decomposition of transmission eigenchannels into molecular resonances based on the isolated resonance approximation, which helps to illustrate the workings of our many-body theory, and facilitates unambiguous interpretation of transmission spectra.Conclusion: We confirm that Pt–benzene–Pt junctions have two dominant transmission channels, with only a small contribution from a third channel with τn << 1. In addition, we demonstrate that the isolated resonance approximation is extremely accurate and determine that transport occurs predominantly via the HOMO orbital in Pt–benzene–Pt junctions. Finally, we show that the transport occurs in a lead–molecule coupling regime where the charge carriers are both particle-like and wave-like simultaneously, requiring a many-body description.

Highly Conductive Molecular Junctions Based on Direct Binding of Benzene to Platinum Electrodes

Highly conductive molecular junctions were formed by direct binding of benzene molecules between two Pt electrodes. Measurements of conductance, isotopic shift in inelastic spectroscopy, and shot noise compared with calculations provide indications for a stable molecular junction where the benzene molecule is preserved intact and bonded to the Pt leads via carbon atoms. The junction has a conductance comparable to that for metallic atomic junctions (around 0.1-1G0), where the conductance and the number of transmission channels are controlled by the molecule's orientation at different interelectrode distances.