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1.3 PHYSICAL SCIENCES
This study attempts to develop an understanding of the electronic processes active within the solid state of C60. The emphasis throughout the work has been upon the generation and spectroscopic identification of any species, which could potentially contribute to electronic conduction in thin films of C60. The relative importance of these inter-and intramolecular processes in terms of their contribution to the electronic transport is discussed through the comparison of the properties of the molecule with the properties of the bulk solid
Initially the low intensity optical properties of the C60 molecule in solution and in solid were assessed. Vibrational spectroscopy of C60 in the solid state indicated that C60 was predominantly molecular in character, however electronic spectroscopy revealed features, which were specific to the solid. These features have been attributed to an intermolecular charge transfer state, which can potentially contribute to the generation of photocarriers. Further optical analysis at higher intensities revealed that the excited state properties of molecular C60 were dominated by an extremely fast intersystem crossing rate (~1.2ns) to the first triplet state manifold. This was indicated by the rapid evolution of a triplet – triplet absorption at ~750nm in the transient absorption measurements. This molecular triplet was assigned the Raman signature 166cm-1 and was seen to play an integral part in the 2+2 –cycloaddition mechanism proposed for the photopolymerisation of C60 from solution. The molecular triplet was identified for the first time in the solid state using Raman spectroscopy and was seen to be positioned at 1466cm-1 as in solution. In addition to the molecular triplet a second excited state species positioned at 14663cm-1 was observed in the solid state. It was proposed that the species positioned at 1463cm-1 was an excited co-operative involving two or more molecules in the solid. A temperature dependence study of the excited state species in the solid indicated that the co-operative species was extremely non-linear and intrinsic to the material below the orientational phrase transition, which occurs at 249K in C60. Both the molecular triplet state and the excited states specific to the solid were seen to have active roles in the photopolymerisation and depolymerisation of C60. The Raman signature of the photopolymer of C60, seen at ~1458cm-1, completed the optical characterisation of all the photophysical states of C.60. This characterisation raised a number of questions regarding the triplet excited state and the role of excited state species in the conduction process.
The electrical generation of ionic species that can contribute to the electronic transport process in solid state C60 were also examined using electron injection techniques. In solution it was seen that the C60-1 and C60-3 ionic species, which are analogues to the highly conducting and superconducting salts KC60 and K3C60 respectively, could be readily produced by the application of moderate voltages. Raman spectroscopy indicated that the C60-1 species was analogous to the aforementioned excited state co-operative observed in the solid. In the solid state structural arrangements and subsequent electronic interactions resulted in the formation of polymeric species. The formation of these polymeric species complicated the generation of anionic species and inhibited the conductivity of the solid state. The effect of this was evident from reported conductivity measurements on C60 drop cast films, and the current-voltage characteristics of vacuum evaporated C60 films incorporated into sandwich type structures. < The effect of this structural rearrangements and electronic interactions was overcome for the sandwich type structure by cooling the arrangement down to 20K. At this low temperature a relatively stable, highly conducting film was produced. However the nature of the conducting species remains to be determined. It is speculated to be a reduced form of C60, which may resemble the excited states co-operative species, which is intrinsic to the material at these low temperatures
It was proposed that the key to preserving the conducting species was the stabilisation of the C60 lattice against a polymeric breakdown, thereby shifting the phase transition at 249K to higher temperatures. This was done through the incorporation of neutral solvent molecules into the lattice, which then behave as ‘molecular spacers’, inhibiting the formation of polymeric bonds. The incorporation of the solvent molecules was seen to result in a change in the crystal packing with the emergence of a new phase as evident from X-ray diffraction data. This type of solvent inclusion compound is referred to as a clathrate. Alternatively structural enhancement was also achieved by thermal annealing. The annealing process also resulted in an apparent change in phase. This new phase in both techniques appears to be resistant to photodegradation indicating an enhanced lattice stability at room temperature. In the future this enhanced stability should allow a more detailed exploration of the highly conducting species for application in optical and electronic devices. A number novel thin film devices have been discussed as well as the potential of this excited material for non-linear optical applications.
Chambers, G. (2001). In-situ spectroscopic studies of elecronic processes in Buckminsterfullerene thin films. Doctoral thesis. Technological University Dublin. doi:10.21427/D79W33