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1.3 PHYSICAL SCIENCES
This work primarily focuses on the archetypal fullerene, which is C60. C60 is electron rich and yet can be six fold reduced, taking on up to six additional electrons. Interest in the electrical properties of the solid state C60 were intensified when it was observed that it could be chemically doped with alkali metals, producing a metallic state A1C60, and even a superconducting state A3C60, where A indicates an alkali metal such as potassium or rubidium. Previous work has demonstrated that such a conductive state can also be achieved by optical pumping or by electron injection. The electronic properties of Fullerene films have, however, been shown to be highly dependent on film morphology. In a molecular solid, conductance in a perfect crystal is determined by intermolecular electron hopping, but in a polycrystalline film it is determined by interdomain hopping. In this study of thin films of fullerenes were sublimated on indium tin oxide coated glass substrates. The transparent indium tin oxide electrode allowed for in situ spectroscopic characterisation of the films while a think aluminium top electrode completed the sandwich geometry for electrical characterisation. The thickness and optical absorption spectra of the deposited films were examined as a function of deposition films were examined as a function of deposition time and deposition rate. Deposition rates were varied by altering the surface area of the evaporation boat. At low deposition rates, the absorption spectral profile was found to vary considerably and was not well correlated with deposition time. The spectral variations are compatible to those observed in annealed fullerene thin films and this it was concluded that use of the small area evaporation boats, resulting in a low deposition rate, resulted in an effective annealing of the material prior to evaporation. Progressive increase of the surface area of the evaporation boat resulted in a progressive increase of the evaporation rate. The absorption spectral profile then becomes better defined and correlates well with the deposition rate. Current-Voltage (IV) measurements were carried out on a variety of films ranging in thickness from 100 nm to 800 nm, from each of the different boats. The IV characteristics showed that as the area of the boat increased the conductivity of the films decreased. The smaller boat area produced highly conductive films consistent with reported higher conductivities for annealed films. On the other hand the larger boat area produced lower conductivity films consistent with pristine C60 films. The temperature dependence of the conductivity, conducted under vacuum, indicated that at high deposition rates, the transport is a thermally activated hopping process, typical of molecular solids and therefore pristine C60 films. Activation energies of between 0.3 eV to 1.1 eV have previously been reported for polycrystalline fullerene films. Analysis of the temperature dependence indicates a better fit to a variable range hopping process, however, indicative of amorphous films. In the films produced by low deposition rates, the conductivity is less temperature dependent, consistent with the behaviour of annealed fullerene films. The conductivities of the films produced by varying the deposition range from 1.8 x10-6S cm-1 to 7.1 x10-8S cm-1, the higher deposition rates producing lower conductivities. Although the higher conductivities may be attractive for some applications, the lower conductivities are more typical of pristine fullerene films. It is concluded that the electronic and optical properties of fullerene films are highly sensitive to deposition parameters. The ability to produce polycrystalline films of well-defined physical characteristics requires precise control of many conditions and the variability of film morphology may be the root of the variability of reported electronic properties of fullerenes.
Brant, N. (2006). Electronic and optical processes in Fullerene films. Masters dissertation. Technological University Dublin. doi:10.21427/D75K6W.