Polymer Nanowires Conjugated by Controlled Chain Polymerization
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The present-day silicon-based electronic devices have grown more powerful as their basic subunit, the transistor, has shrunk in size. However, the laws of quantum mechanics and the limitations of fabrication techniques may soon prevent further reduction. Thus we must develop a novel device concept, which can be applied at the nanometer scale. This exploration of a novel device concept beyond silicon-based transistors is analogous to the search for a replacement for the vacuum tube half a century ago, and its discovery will make a profound impact on society in this century. Many ideas have been proposed for the novel device concept, and one promising possibility is the construction of novel electronic devices using organic molecules. However, even if we could make individual molecules that function as single-molecule transistors or diodes, the next problem would be the absence of good methods by which to arrange them in the desired pattern and to interconnect them with one another. Regarding this problem, it is necessary to develop a method of fabricating conductive wires of nanometer width (nanowires) at designated positions.
We usually use metals for wires whose width is larger than the order of micrometers. However, in the case of nanowires, the use of metals becomes much more difficult because of their poor stability and difficulties in their fabrication. If we want to use metals for nanowires, we must devise some method of stabilizing them. Hence many materials other than metals have been considered as candidates for nanowires, including inorganic compounds, carbon nanotubes, DNA, and other conductive organic molecules or polymers, although we must conduct further basic studies before we can put them to practical use.
In this article, we will present a method of fabricating a conjugated polymer nanowire at designated positions by controlling linear chain polymerization using the probe tip of a scanning tunneling microscope (STM). A demonstration is presented for a self-ordered monomolecular layer of 10,12-pentacosadiynoic acid and 10,12-nonacosadiynoic acid, which are amphiphilic diacetylene compounds, adsorbed on a graphite substrate. We have succeeded in controlling the initiation and termination of chain polymerization with a spatial precision on the order of 1 nm. The obtained polydiacetylene is a conjugated polymer, so that we can expect that it will become conductive when excess electrons or holes are supplied to it from the surroundings.