Pentagonal tiles pave way towards organic electronics?

The findings are from scientists at the University of Cambridge and Rutgers University, New Jersey, who are developing new classes of organic thin films on surfaces. By studying the fundamental forces at play in self assembling thin films, the team say they are developing the knowledge that will allow them to tailor these films into molecular scale organic electronic devices, creating smaller components than would ever be possible with conventional fabrication techniques. According to Dr Holly Hedgeland of the Department of Physics at the University of Cambridge and one of the co authors of the paper, ring like molecules with unusual five fold (pentagonal) symmetry bind strongly to a copper surface due to a substantial transfer of charge. However, the molecules experience very little difficulty in sideways diffusion and exhibit surprisingly little interaction between neighbouring molecules. This unprecedented combination of features is ideal for the spontaneous creation of high density stable thin films, comprising a pavement of the organic pentagonal tiles, with potential applications in computing, display technologies and solar power. "With the semiconductor industry currently worth an estimated $249billion per year there is a clear motivation towards a molecular scale understanding of innovative technologies that could come to replace those we use today," said Dr Hedgeland. The interdisciplinary team have reported the first dynamical measurements for a new class of organic thin film where cyclopentadienyl molecules (C5H5) receive significant electronic charge from the surface, yet diffuse easily across the surface and show interactions with each other that are much weaker than would typically be expected for the amount of charge transferred. Hedgeland noted: "By coupling the experimental helium spin echo technique with advanced first principles calculations, we were able to study the dynamic behaviour of a cyclopentendienyl layer on a copper surface, and to deduce that the charge transfer between the metal and the organic molecule was occurring in a counter intuitive sense." Dr Marco Sacchi, of the Department of Chemistry at the University of Cambridge, who carried out the calculations that helped explain the new experimental results, added: "The key to the unique behaviour of cyclopentadienyl lies in its pentagonal symmetry, which prevents it latching onto any one site within the triangular (three fold) symmetry of the copper surface through directional covalent bonds, leaving it free to move easily from site to site; at the same time, its internal electronic structure is just one electron short of an extremely stable `aromatic' configuration, encouraging a high degree of charge transfer from the surface and creating a strong non directional ionic bond." Hedgeland concluded: "The unusual character of the charge transfer in this case prevents the large repulsive interactions between adjacent molecules that would otherwise have been expected, and hence should enable the formation of unusually high density films. At the same time, the molecules remain highly mobile and yet strongly bound to the surface, with a large degree of thermal stability. In all, this is a combination of physical properties that offers huge potential benefit to the development of new classes of self assembled organic films relevant for technological applications." The researchers' findings are reported in Physical Review Letters.