Molecular wires

Molecular wires (or sometimes called molecular nanowires) are molecular-scale objects which conduct electrical current. They are the fundamental building blocks for molecular electronic devices. Their typical diameters are less than 3 nanometers, while their bulk lengths may be macroscopic, exteding to centimeters or more.

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Materials

Most work thus far has consisted of organic molecules. Higher conductivities originate from highly conjugated systems, while alkane chains are important in understanding basic charge transport and tunneling. A molecular wire in nature is DNA. Prominent inorganic examples are Li₂Mo₆Se₆ and Mo₆S_9-xI_x. Molecular wires containing paramagnetic inorganic moieties are interesting, in particular, because they can lead to observations of Kondo peaks.

Structure

  Unlike the more usual nanowires (which are very long crystals), molecular nanowires are composed of repeating molecular units, which may be organic (e.g. DNA) or inorganic (e.g. Mo₆S_9-xI_x). 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 Mo₆S_9-xI_x, the repeat units are Mo₆S_9-xI_x clusters, which are joined together by sulphur or iodine bridges. Molecular nanowires often aggregate in solution into swathches 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.

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 (Fermi liquid) behaviour. Important concepts are those introduced by Tomonaga, Luttinger and Wigner. Effects caused by classical Coulomb repulsion (called Coulomb blockade) have also been found to be important in determining the properties of MWs.

Use of Nanowires in Molecular Electronics

To be of use for connecting molecules together, MWs need to display some very important characteristics. The connectors between elements need to be able 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, it is essential that the connectors which join them have recognitive ability. They should be connectable to diverse entitites, such as gold metal surfaces (for connections to outside world), biomolecules (for nanosensors, nanoelectrodes, molecular switches) 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 very effectively in molecular scaffold fabrication. Very complex shapes have recently been demonstrated, but unfortunately metal coated DNA which is electrically conducting is much too thick to connect to individual molecules. Thinner coated DNA lacks electronic connectivity, and are not suitable for connecting molecular electronics elements. Some varieties of carbon nanotubes (CNTs) 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 functionalised 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. Recently possible routes for the construction of larger functional circuits using Mo₆S_9-xI_x 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.