Buckminsterfullerene and related systems possess a strong chromophoric character resulting from the sp2 hybridization of their valence electrons. As a result, Raman scattering can be used to reveal details of the physical properties and doping dynamics, also allowing the study of the effects of chemical modifications of these systems. Due to the high symmetry of the molecules the spectra at first appear rather simple. However, crystal field effects and general symmetry breaking conditions can also be studied.
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...
From in situ Raman measurements during potassium doping of pure C60 films to a structure K C60 in the range of 0(x(6 we found a continuous change of line intensities for the Ag modes of the stable phases in this material. No shift of these modes with doping was observed, which proves a strict phase separation. The Raman line at 1467 cm for pristine C60 is due to oxygen contamination and is assigned to positively charged fullerene states. The intrinsic pinch mode of the pristine material is at 1458 cm ' and has a linewidth of 4 cm Since the preparation of large quantities of Csti fullerene' and the subsequent demonstration of superconducting properties after doping this material became one of the most challenging subjects in solid-state and molecular physics and in synthetic chemistry. At room temperature the C60 molecules form a crystal with free rotating molecular units on an fcc Bravais lattice. ' The pristine solid C&n is an insulator with a calculated energy gap of about 2 eV between a narrow valence band of h" symmetry and a narrow conduction band of ti"symmetry. 6 The crystals can be doped to a metallic state A, C&tt using strongly reducing atoms A like alkali or alkalineearth metals. ' Doping beyond the metallic regime leads again to semiconducting material.Several phases of A, C60 have been identified by x-ray measurements. ' For x=3 the structure is still fcc but with a slightly modified lattice parameter, x =4 has a bct and x=6 a bcc structure. With x increasing from zero the conductivity increases and reaches a maximum at x=3, where the conduction band is half filled. Thus, the materials are metallic with conductivities between 10 and 5 x 10 Scm '. Further doping decreases the conductivity and A"C&n becomes again semiconducting at x =6, where the t1"band is full and the calculated energy gap to the next band is 0.48 eV. " Raman experiments have been reported for the pristine materials in some detail. ' From the 174 vibrational modes of the C60 molecule only the two As and the eight Hg modes of the icosahedral factor group are Raman active. The most characteristic As mode is the pinch mode, where the shorter bonds between two hexagons and the longer bonds between hexagons and pentagons are vibrating in opposite phase. This mode is similar to an amplitude mode (oscillation in the bond alternation) in conjugated polymers' which is usually observed at around 1500 cm '. A resonance enhancement of a characteristic mode at 1467 cm ' was reported recently. ' This mode was originally identified with the pinch mode but a very recent investigation showed that it is oxygen induced. ' Raman scattering proved also to be very useful for monitoring the doping process as was demonstrated already in the initial work by Haddon ef al. Recently the two phases of the equilibrated doped state were identified in the spectra from samples grown and doped in a sealed pyrex tube' and in a sandwich C60-K-C60 structure. ' From in situ experiments on very thin films a continuous transition between the undoped state, the metallic...
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