Inorganic molecular wires: Physical and functional properties of transition metal chalco-halide polymers

The rapid development of nanotechnology has lead to demands on new one-dimensional materials with new functional properties. Carbon nanotubes have received most attention, followed by nanowires of very different kinds. Most recently inorganic molecular wires - particularly molybdenum halide or chalcogenide cluster polymers - have emerged as a new type of one-dimensional materials with remarkable molecular-scale functionality. These transition metal chalco-halide molecular wires are unique in terms of structure and molecular properties, setting them apart from the plethora of nanowires and nanotubes discovered in recent years. Their one-dimensional polymer structure gives rise to some very unusual physical properties. Anionic bridges which bind Mo clusters together into one-dimensional chains are extraordinarily strong, yet highly deformable, giving rise to exceptionally high Young's moduli and nonlinear mechanical properties respectively. The very weak interaction between individual polymer chains within crystalline bundles leads to observation of extreme one-dimensional electronic and magnetic character on one hand, and also to easy dispersion in common polar solvents and ultralow shear moduli on the other. The sulfur atoms within the structure facilitate diverse functionalization chemistry to thiol-containing molecules, such as proteins. The connectivity of the molecular wire ends to gold nanoparticles and surfaces with covalent bonds and good electronic coupling enable self-assembled molecular-scale connections to be made between individual molecules. Experimental and theoretical progress in the field has been extremely rapid since 2004 and in this review we try and summarize the most important structural, mechanical, electronic and magnetic properties of molybdenum chalco-halide molecular wires. We also discuss their functional properties which make these new materials of great interest for a wide variety of diverse applications including conducting composites, nonlinear optics, field emission, tribology, spin-modulators, molecular and gas sensing and potentially revolutionary applications in molecular electronics. (c) 2008 Published by Elsevier Ltd.