The luminescence of C^ was investigated as a function of temperature between 300 and 10 K and for various oxygen concentrations. Its intensity was found to increase strongly with decreasing temperature down to about T\ -100 K and to decrease again for a further reduction of T. Below 100 K, new structures appeared in the spectra. The experiments are described by a self-localized polaron exciton and by a diffusing free exciton above and below Tj, respectively. The important difference between the two regions is the free rotation of the CM above and the definite orientation below T\. PACS numbers: 71.35,+z, 71.50.+t, 78.55.KzThe dramatic increase of interest from material scientists in the fullerenes originates from the very unusual properties of these materials. Very high stability, many possibilities for chemical modifications, high optical nonlinearity, metallic conductivity, and even superconductivity at rather high temperatures are just a few of them. Looking at all these properties and considering the quasiaromatic structure of the molecules render the fullerenes as the missing link between the various lowdimensional systems such as high-temperature superconductors, charge-transfer crystals, and conducting polymers.From a detailed study of the luminescence of the undoped C6o and C70 compounds of this system we can add another relation between this material and the lowdimensional systems mentioned above. The luminescence of C60 has been reported already in the early days of studying the fullerenes [l], but no satisfying interpretation of the phenomenon could be given. From a detailed study of the temperature dependence and from the influence of impurity effects on the luminescence we suggest that the luminescence originates from a self-localized exciton which is created at ambient temperature on the quasi-two-dimensional surface of the spinning C60 molecules. Because of the conjugated character of the bonds this exciton can be expected to be very similar to the polaron exciton in conjugated polymers. As the temperature is lowered the spinning speed slows down or the spinning proceeds discontinuously until at the final lock-in temperature three-dimensional order is obtained. As a consequence the probability for creating a self-localized polaron exciton is reduced and the luminescence is quenched.Thin films of standard mixtures of C60/C70, purified C60, and purified C70 were grown by sublimation on a silicon wafer to a thickness of about 0.4 jim. The C60/C70 mixture was commercial and has a C70 concentration of about 15%. The purification effect during the sublimation process gave a final concentration of 4% C70 in the films as determined from Raman line intensities. The luminescence spectrum was excited with various laser lines at temperatures between 320 and 10 K. In order to remove the oxygen contamination a few samples were stored in argon immediately after preparation and were further purified from remaining oxygen by laser irradiation in high vacuum before the luminescence was investigated.The general appea...
The electronic structure of fullerenes and fullerene compounds from high-energy spectroscopy Golden, M.S.; Knupfer, M.; Fink, J.; Armbruster, J.F.; Cummins, T.R.; Romberg, H.A.; Roth, M.; Sing, M.; Schmidt, M. Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. Abstract The high-energy spectroscopic study of the elecuonic smcture of fullerenes and their compounds in the solid staie is reviewed. it is shown that photoemission and electron enera/-loss spectroscopy in transmission provide powerful tools for the~experimental study of the electronic structure of solid Cao, C7a and higher fullerenes. as well as the compounds formed by their intercalation wilh alkali and alkaline-earth metals. Topical issues in the field are addressed. such 3s the role played by elecmn conelmion, disorder and eleciron-phonon coupling.
The electronic structure of undoped and K-doped C60 has been investigated by high-energy electron energy-loss spectroscopy in transmission. Information on the filling and energetic position of π*-levels as a function of dopant concentration is obtained. From momentum-dependent measurements it is concluded that for undoped and doped C60 the π-bands are narrow indicating a weak interaction between the C60 molecules.
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