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27 August 2003 In-situ Raman spectroscopy of electrically generated species in fullerene thin films
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Organic materials have, in recent decades, been shown to be insulators, semiconductors, or even metallic when doped and the prospect of cheap, easily fabricated devices has attracted much interest. Primitive devices have been demonstrated and yet potentially competitive performance has been limited to polymer light emitting diodes. The recent report that lattice expanded C60 single crystals can be made superconducting, with a transition temperature of 117K, by the injection of charge via a FET type geometry has once again highlighted the potential of C60 in the development of molecular electronic devices. In light of the aforementioned report it is essential that a true understanding of the inter- and intramolecular processes in terms of their contribution to the electronic transport be obtained. In this study the current voltage characteristics of C60 thin film sandwich structures fabricated by vacuum deposition on indium tin oxide (ITO) with an aluminium top electrode are presented and discussed. A strongly non-linear behavior and a sharp increase in the device conductivity was observed at relatively low voltages (~2V), at both room and low temperatures (20K). At room temperature the system is seen to collapse, and in situ Raman measurements indicate a solid state reduction of the fullerene thin film to form a polymeric state. The high conductivity state was seen to be stable at elevated voltages and low temperatures. This state is seen to be reversible with the application of high voltages. At these high voltages the C60 film was seen to sporadically emit white light at randomly localized points analogous to the much documented electroluminescence in single crystals. Moreover the evidence suggests that this highly conducting species maybe similar in nature to a high intensity optically excited species. It is further speculated that the species recently reported in the superconducting lattice expanded C60 single crystals may also be analogous to the highly conducting species observed here.
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Siobhan Bridget Phelan, Barry Stephan O'Connell, Garrett F. Farrell, Gordon Chambers, and Hugh James Byrne "In-situ Raman spectroscopy of electrically generated species in fullerene thin films", Proc. SPIE 4876, Opto-Ireland 2002: Optics and Photonics Technologies and Applications, (27 August 2003);

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