Molecular wire

Molecular wires (or sometimes called molecular nanowires) 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. Molecular wires (or sometimes called molecular nanowires) 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. 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 (EMACs) which comprise strings of transition metal atoms directly bonded to each other. Molecular wires containing paramagnetic inorganic moieties can exhibit Kondo peaks. Unlike the more usual nanowires (which are very thin crystals), 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 microsope tips. 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 (Fermi liquid) behavior. Important concepts are those introduced by Tomonaga, Luttinger and Wigner. Effects caused by classical Coulomb repulsion (called Coulomb blockade), interactions with vibrational degrees of freedom (called phonons) and Quantum Decoherence have also been found to be important in determining the properties of molecular wires. Various methods have been developed for the synthesis of different types of molecular wires (e.g. organic molecular wires and inorganic 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 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(phenylene ethylnylene) type molecular wire (B) was synthesized starting from readily available 1-bromo-4-iodobenzene (A). 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 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.