Optical properties of fullerene-based hetero-multilayers grown by molecular beam deposition

We report measurements on a multilayer of alpha-sexithienyl (T6) and C60 grown in ultra-high vacuum. The photoluminescence spectrum of the multilayer is made up of T6 emission at high energies, and C60 emission at low energies. The T6 emission has more structure than that of bulk T6, even after taking into account reabsorption by the C60. However, the spectrum can be simulated quite well by assuming that there is both emission from bulk T6 and isolated molecules of T6, based on the vibronic structure that we would expect from isolated T6 molecules. Molecules of T6 may conceivably become isolated at the C60 interface, if they migrate into gaps between C60 grains, for example. Thus we cannot conclude that any of the new peaks we see are due to charge transfer betwen the C60 and T6. The low energy fluorescence is that of C60, but its efficiency is orders of magnitude larger than is seen in films of C60. We cannot explain this yet.