Optical transitions and electronic interactions in self-assembled cobalt-fullerene mixture films

Optical spectra of Co_xC₆₀ mixture films (x  <  10) were studied in order to obtain insights into electronic interactions in an organometallic system that is attractive for spin-transfer electronics. The optical absorption (OA) spectra were described in terms of the Lorentz oscillators model, allowing the quantification of the spectral variations driven by Co concentration x. The variations are found to be most pronounced for 0  <  x  <  2–4, suggesting the effects of the electronic interactions in the Co₂C₆₀ fulleride formed in the mixture. The Co–C₆₀ electronic interactions evidenced by the OA edge shift and by the OA peak detected at approximately 1 eV give rise to the Jahn–Teller (JT) splitting of the t _1u electronic levels in C₆₀ occupied by the electrons transferred from Co. The JT effect was confirmed by observation of the downshifted T _1u(4) infrared-active mode modified by electron–phonon coupling at the t _1u levels. (paper)