edge have been identified primarily with Herzberg-Teller vibronic coupling of the respective singlet states to vibrational modes. This basic interpretation of the optical spectrum near the absorption edge (~1.7 eV) in the solid state is based on the similarity between the OA spectra in the film (as for the spectra at 80 K) and in solution (see Fig. 13.6). This similarity suggests the same origin for the transitions in both CM solid films and C60 in solution and provides strong evidence for the molecular character of the solid. Therefore the features (Fr) in the OA spectrum of the film (see Fig. 13.6) are classified using the index i' to relate the Fr labels to the M(, labels for the OA spectrum of the Cm/MCH (methylcyclohexane) solution. Although the features in the room temperature OA spectrum of the film are very broad relative to those in the T = 300 K C60/MCH solution, the 80 K OA spectrum in the film near the optical threshold is remarkably similar to the T — 300 K OA spectrum for C60 in solution (C^/MCH). The entire 80 K OA spectrum of the C6q solid film is, however, red-shifted by ~430 cm 1 with respect to that of C60 in solution, corresponding to a shift in the zero-phonon transition energy . This shift in is not due to changes in the vibrational mode frequencies themselves, which are observed to exhibit little change between the solid phase and that of C60 in solution [13.88]. Furthermore, the red shift of the 80 K OA film spectrum relative to the 300 K solution spectrum is four times larger than the ~100 cm-1 blue shift of this spectrum due to lowering the temperature.
It is also instructive to correlate the features in the film PL spectrum (labeled by symbols FA to F8 [Fig. 13.6(b)]) with the corresponding features observed in the solution PL spectrum [Fig. 13.6(a)]. Luminescence spectra appear to be more sensitive to sample quality and to show wider sample-to-sample variation than absorption spectra [13.89]. The 80 K film PL spectrum downshifts by ~ 200 cm-1 with respect to the 77 K solution
PL spectrum, yielding Egg, = 15,200 cm 1 (1.88 eV) for C60 in solution and Egjj, = 15,000 cm-1 (1.86 eV) for the C60 solid film. It is found that the PL-deduced values for the vibrational frequencies wvib in the film are close to those deduced from the PL spectrum of C60 in solution for PL transitions from the Eg\y vibrationless initial state to the various final states in the spectrum.
Despite the similarities between the film and solution PL and OA spectra, there are three noteworthy differences: (1) the film OA and PL spectra are red-shifted in energy with respect to the solution OA and PL spectra and this red shift is attributed to solid state interactions; (2) the solution (Qq/MCH) features labeled M0 to M3 are not observed in the PL spectrum of the film, but the corresponding Cv C2 and C3 features are indeed found in the single-crystal PL spectra [13.28] (the features C, for the single-crystal PL data in Fig. 13.6 are labeled similarly to those in solution); (3) the PL features in the solid film [Fig. 13.6(b)] are broadened considerably with respect to those in the solution spectrum [Fig. 13.6(a)]. This broadening is not simply due to intermolecular C60-C60 interactions in the solid state, as can be seen by comparing the 10 K single-crystal PL data [13.28] to that of the 80 K film and the 77 K solution PL data shown in Fig. 13.6(b). If the broadening were due to C60-C60 interactions, the single-crystal PL spectrum would not be so remarkably similar to that of C60 in solution. Since the PL spectrum of the single crystal not only contains most of the features seen in the solution PL spectrum, but also exhibits similar relative intensities of the various features, a clear connection is seen between the molecular (H-T vibronic) origin of the PL structure in the solution PL spectrum and that in the crystal and film PL spectra. Furthermore, the single-crystal PL spectrum shows that the intrinsic intermolecular interactions in fee C60 are not responsible for the broadening of the vibronic features in the film PL spectrum. The observed broadening is therefore identified with grain boundary defects in the film. Consistent with this view is the observation that the PL spectrum of a polycrystalline powder[13.90,91] also shows similar broad vibronic features.
Since the PL spectra of both C60 single crystals and films downshift by about the same amount relative to the solution spectrum, it is concluded that it is the lattice potential that introduces the spectral downshift in the solid-state spectra. In aromatic, molecular crystals, a spectral shift relative to solution data has been connected with two mechanisms [13.6]: the solvent shift and the exciton shift. The solvent shift is due to the polarizability of the environment, which is quite different for C60 in solution and for C60 in the pristine solid. The exciton shift is due to the electron-hole interaction spread over translationally equivalent molecules. Within a given set of vibronic transitions, one usually finds that the solvent shift is constant.
Since the crystal grain size in the solid films of Fig. 13.6(b) is ~ 20 nm [13.15], many C60 molecules are within one or next-neighbor distances of a grain boundary (the C60 diameter ~1 nm). Grain boundary defects will lower the local symmetry of C60 and lift the high degeneracy of the intramolecular modes. Therefore, for molecules located near grain boundaries, many more vibrational modes might participate in vibronic transitions, thus accounting for the broadening of the PL film spectrum. Another factor which might contribute to the broadening of the film PL spectrum is the presence of X-traps [13.9,28,92] associated with grain boundary defects. An X-trap yields a PL spectrum with the same vibronic features as the intrinsic (undisturbed) crystal, but the features are red-shifted by the trap depth AE [13.28]. A range of trap depths AE (25 < AE < 200 cm"1) could also explain the observed broadening of the PL spectra of the films.
As we move away from the absorption edge to higher photon energies, we see a large increase in absorption coefficient. In this higher energy regime the electronic energy band dispersion is expected to broaden the optical absorption bands in the solid significantly over those observed in solution or in the gas phase. In Fig. 13.13 we compare the UV-visible optical absorption of C60 in solution (decalin) to that in a polycrystalline Qq film on a Suprasil substrate [13.73] in a frequency range higher than the data shown near the absorption edge in Fig. 13.12. As can be seen in Fig. 13.13, the optical spectra of C60 in solution and in the solid state are quite similar, and the solid-state broadening is not dramatic. Four strong optical absorption bands, identified with dipole-allowed transitions, exhibit peaks at ~3.0, 3.6, 4.7, and 5.7 eV for both the solid film and for C60 in solution [13.73]. Note that the three highest-energy optical bands exhibit an increase in their FWHM linewidths (full widths at half-maximum) of ~0.3 to 0.5 eV and the peak at 3.0 eV is also broadened. Furthermore, the intensities of the lowest two bands at 3.0 and 3.6 eV, measured relative to the higher two bands at 4.7 and 5.7 eV, exhibit a significant increase in intensity in the solid phase. EELS spectra (taken at 60 meV resolution) in the region 1 to 8 eV on C^q in the gas phase and in the solid phase are quite similar to the UV-visible optical spectrum of C60 in solution, exhibiting similar width peaks in both the gas and solid phases at 2.24, 2.9, 3.8, 4.9, and 5.8 eV [13.93]. Consistent with the similarity of the UV-visible spectra for C60 in solution and in the solid state, the EELS data also indicate that most of the intrinsic width in the optical absorption bands stems from intramolecular processes, most likely due to electron-molecular vibration scattering, as suggested by theoretical calculations in both CgQ and C7q, where intramolecular bond disorder is introduced to simulate the effect of molecular vibrations on the dipole absorption [13.94,95]. By carrying out in situ optical absorption measurements in thin films of C60 as a function of alkali metal doping, it has been possi-
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