Researchers use metal crystal to organize organic materials, overcoming key stumbling block to organic nanoelectronics. By using molecular materials instead of silicon semiconductor, we could one day build transistors that are ten times smaller than what currently exists.
They demonstrated perfectly ordered arrays of one of the most important organic conducting polymer, PEDOT [polyethylenedioxythiophene]. The researchers attained five nanometer resolution for lines of PEDOT by copying the already perfect crystalline structure of thin-film copper, using a simple low-temperature deposition technique whereby “monomers” self-assembled into polymers atop the inorganic material.
The technique sounds deceptively simple. The team used an inorganic material – a crystal of copper – as a template. When molecules are dropped onto the crystal, the crystal provokes a chemical reaction and creates a conducting polymer. By using a scanning probe microscope that enabled them to see surfaces with atomic resolution, the researchers discovered that the polymers had imitated the order of the crystal surface. The team is currently only able to produce the reaction in one dimension, i.e. to make a string or line of molecules. The next step will be to add a second dimension in order to make continuous sheets (“organic graphite”) or electronic circuits.
One of the great challenges in surface chemistry is to assemble aromatic building blocks into ordered structures that are mechanically robust and electronically interlinked—i.e., are held together by covalent bonds. We demonstrate the surface-confined growth of ordered arrays of poly(3,4-ethylenedioxythiophene) (PEDOT) chains, by using the substrate (the 110 facet of copper) simultaneously as template and catalyst for polymerization. Copper acts as promoter for the Ullmann coupling reaction, whereas the inherent anisotropy of the fcc 110 facet confines growth to a single dimension. High resolution scanning tunneling microscopy performed under ultrahigh vacuum conditions allows us to simultaneously image PEDOT oligomers and the copper lattice with atomic resolution. Density functional theory calculations confirm an unexpected adsorption geometry of the PEDOT oligomers, which stand on the sulfur atom of the thiophene ring rather than lying flat. This polymerization approach can be extended to many other halogen-terminated molecules to produce epitaxially aligned conjugated polymers. Such systems might be of central importance to develop future electronic and optoelectronic devices with high quality active materials, besides representing model systems for basic science investigations.
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