Naked nanowires cover up

If an object is too small to even be seen by the unaided eye, does it matter if it is, in fact, naked? The answer, according to a team of scientists writing in the January 2, 2008, issue of the American Chemical Society Nano, is a resounding ‘yes’ – at least in the rapidly emerging world of nanotechnology. The researchers, from RIKEN and other Japanese research institutes, have created insulated wires for use in nanotechnology.

Nanotechnology is an evolving interdisciplinary branch of applied science, engineering and technology that essentially deals with the manipulation and utilization of devices at the level of atoms and molecules. The term ‘nano’ – derived from the Greek word for dwarf, ‘nanos’ – refers to nanometer, or one billionth of a meter, and describes infinitesimal particles that are only about a millionth the size of the period at the end of this sentence.

Such tiny scales have mind-boggling connotations. Nanotechnology is often described only in terms of how much smaller nanotech materials are than those used in conventional technologies, but this can be misleading, as the real excitement of nanotechnology – it promises to revolutionize all major fields – is ultimately in how it allows control of the physical, chemical and other properties of matter, which are fundamentally different when atoms and molecules are rearranged. At the nanoscale, materials’ electronic properties change, and they undergo radical increases in surface-area-to-volume ratios that dramatically alter their mechanical, thermal and catalytic characteristics.

For some years now, researchers have been able to arrange conductor or semiconductor atoms into miniscule rows only a few atoms wide, to fashion nanowires, a key step for nanoscale electronics. Intense research activity is being directed toward nanowire growth methods utilizing a ‘bottom-up’ approach involving electrochemical growth processes to self-assemble the nanowires atom by atom. Such self-assembly is a common feature within biological systems and works on the principles of ‘supramolecular chemistry,’ which is distinct from traditional chemistry in focusing on non-covalent forces – relatively weak, reversible interactions between molecules.

Most techniques produce naked wires that are so close together they short-circuit, so the RIKEN team focused on both the conducting and the insulating properties of their subject. Building on their previous work, the researchers synthesized distinct derivatives of tetrathiafulvalene (TTF, a common organic compound) containing various combinations of halogen (electronegative elements like chlorine, bromine and iodine) compounds. Starting from these synthesized materials, the authors used bottom-up techniques to create six different types of crystals, each exhibiting a key feature – the formation of an insulating network.

In these crystals, the positively charged TTF molecules are good conductors that stack together to form the actual nanowires, which are physically and electrically separated from each other by sheaths of insulating networks created by different types of halogen molecules. Integral to this sheath network are the special ‘halogen bonds’ that form between neutral iodine-containing molecules and negatively charged halides (salt versions of halogens). The researchers’ analysis showed that some of these sheaths provided good insulation that should limit the electrons from ‘hopping’ from wire to wire, which normally results in inefficient circuits.

The researchers believe that they will soon be able to fabricate more practical and functional two-dimensional and three-dimensional structures simply by varying the chemistry of the halogen compounds used. To tackle the difficulties of aligning and connecting nanowires into functional circuits, the researchers have used their findings to propose a roadmap that addresses all the major issues required for three-dimensional nanowiring to create high-density memory or logic circuits. Time will tell just where their road map leads.