Molecular wires are molecular chains that conduct electric current. They are the proposed building blocks for molecular electronic devices. Their typical diameters are less than three nanometers, while their lengths may be macroscopic, extending to centimeters or more.
Examples
Most types of molecular wires are derived from organic molecules. One naturally occurring molecular wire is DNA. Prominent inorganic examples include polymeric materials such as Li2Mo6Se6 and Mo6S9−xIx, , and single-molecule extended metal atom chains which comprise strings of transition metal atoms directly bonded to each other. Molecular wires containing paramagnetic inorganic moieties can exhibit Kondo peaks.
Structure
Unlike the more usual nanowires, molecular nanowires are composed of repeating monomers. In the case of DNA, the repeat units are the nucleotides with a backbone made of sugars and phosphate groups joined by ester bonds. Attached to each sugar is one of four types of bases. In case of Mo6S9−xIx, the repeat units are Mo6S9−xIx clusters, which are joined together by flexible sulfur or iodine bridges. Molecular nanowires can be manipulated and investigated as single molecules, but they often aggregate in solution into swatches or bundles. In the case of the Mo chalcogenide-halides, they grow in the form of ordered strands, in which the individual strands are linked by very weak Van der Waals forces. Individual molecules can be manipulated, ordered and their length can be controlled with atomic force microscope tips.
Conduction of electrons
Molecular wires conduct electricity. They typically have non-linear current-voltage characteristics, and do not behave as simple ohmic conductors. The conductance follows typical power law behavior as a function of temperature or electric field, whichever is the greater, arising from their strong one-dimensional character. Numerous theoretical ideas have been used in an attempt to understand the conductivity of one-dimensional systems, where strong interactions between electrons lead to departures from normal metallic behavior. Important concepts are those introduced by Tomonaga, Luttinger and Wigner. Effects caused by classical Coulomb repulsion, interactions with vibrational degrees of freedom and Quantum Decoherence have also been found to be important in determining the properties of molecular wires.
Synthesis
Various methods have been developed for the synthesis of different types of molecular wires. The basic principle is to build repetitive molecular structures that are connected together to conduct electricity. Organic molecular wires are usually synthesized via transition metal-mediated cross-coupling reactions. Inorganic molecular wires are grown at conditions that allow formation of one-dimensional inorganic structures.
Organic Molecular Wires
Organic molecular wires usually consist aromatic rings connected by ethylene group or acetylene groups. Transition metal-mediated cross-coupling reactions are used to connect simple building blocks together in a convergent fashion to build organic molecular wires. For example, a simple oligo type molecular wire was synthesized starting from readily available 1-bromo-4-iodobenzene. The final product was obtained through several steps of Sonogashira coupling reactions. Other organic molecular wires include carbon nanotubes and DNA. Carbon nanotubes can be synthesized via various nano-technological approaches. DNA can be prepared by either step-wise DNA synthesis on solid-phase or by DNA-polymerase-catalyzed replication inside cells.
Inorganic Molecular Wires
Inorganic molecular wires consist of one-dimensional cluster structures that are usually held together by ionic bonds. These materials can be synthesized at high temperature. For example, a typical synthesis of Mo6S9-xIx was performed in sealed and vacuumed quartz ampoule. The commercially available molybdenum sheet metal, sulfur powder, and iodine were mixed at the desired ratio. The ampoule was slowly heated to a temperature of 1343 K, the sample was kept at that temperature for 72 hours and was then slowly cooled down to obtain the molecular wires.
To be of use for connecting molecules, MWs need to self-assemble following well-defined routes and form reliable electrical contacts between them. To reproducibly self-assemble a complex circuit based on single molecules. Ideally, they would connect to diverse materials, such as gold metal surfaces, biomolecules and most importantly, they must allow branching. The connectors should also be available of pre-determined diameter and length. They should also have covalent bonding to ensure reproducible transport and contact properties. DNA-like molecules have specific molecular-scale recognition and can be used in molecular scaffold fabrication. Complex shapes have been demonstrated, but unfortunately metal coated DNA which is electrically conducting is too thick to connect to individual molecules. Thinner coated DNA lacks electronic connectivity and is unsuited for connecting molecular electronics components. Some varieties of carbon nanotubes are conducting, and connectivity at their ends can be achieved by attachment of connecting groups. Unfortunately manufacturing CNTs with pre-determined properties is impossible at present, and the functionalized ends are typically not conducting, limiting their usefulness as molecular connectors. Individual CNTs can be soldered in an electron microscope, but the contact is not covalent and cannot be self-assembled. Possible routes for the construction of larger functional circuits using Mo6S9−xIx MWs have been demonstrated, either via gold nanoparticles as linkers, or by direct connection to thiolated molecules. The two approaches may lead to different possible applications. The use of GNPs offers the possibility of branching and construction of larger circuits.
Other research
Molecular wires can be incorporated into polymers, enhancing their mechanical and/or conducting properties. The enhancement of these properties relies on uniform dispersion of the wires into the hostpolymer. MoSI wires have been made in such composites, relying on their superior solubility within the polymer host compared to other nanowires or nanotubes. Bundles of wires can be used to enhance tribological properties of polymers, with applications in actuators and potentiometers. It has been recently proposed that twisted nanowires could work as electromechanical nanodevices to measure forces and torques at nanoscale with great precision.
