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Condensed Matter & Materials Physics

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Research

The focus of this programme is on carbon-based (organic) molecules with semiconducting properties. Although we use to think of plastics as insulators, polymers can also conduct electricity if the C-C bonds are arranged in an alternating single-double pattern, in which case the molecules are said to be "conjugated". Interestingly, the physics of charge injection, storage, and recombination in organics is enriched by several features specific to this class of semiconductors. Such effects arise from the peculiar nature of electron-phonon and electron-electron interactions in these materials, and include spatial self-localisation of the excitations, strong non-linear effects, ultrafast thermalisation of optically excited states, and disorder mediated processes.

A particularly exciting aspect of research in this area is that films of conjugated semiconductors only a few tens of nm thick are suitable for fabrication of Light-Emitting Diodes, LEDs, over large areas and at low cost. This motivates increasing interest from several companies world-wide (see for example Cambridge Display Technology, Dupont Displays, Philips Research, Merck Oled Materials and references therein). Similarly, a significant effort is being devoted to developing "all plastic" electronic circuits, capable of logic functions for information treatment. Another area of applications is in the fabrication of solar cells. For these, conjugated molecules can take advantage again of cheap and easy fabrication over large areas, and also of relatively high absorption coefficients, deriving from high oscillator strengths for the optical transitions. Most importantly, all these devices can be used as sophisticated investigative tools in order to access properties of the materials which are otherwise difficult or impossible to probe. An interesting prospect for development of the field is related to the chemical analogy of conjugated molecules with some of biological origin, which virtually opens the way to bio-compatible structures and devices, and maybe to artificial organs with complex functions, such as retinas.

The impact that these materials are likely to have on everyday life is testified by the award of the 2000 Nobel Prize for Chemistry to A. J. Heeger, A. G. MacDiarmid and H. Shirakawa for the discovery and development of conductive polymers.

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