Charge transfer at aluminum-C60 interfaces in thin-film multilayer structures

Thin-film multilayer structures with up to 20 repeat layers have been grown in a high-vacuum chamber by sequential deposition of aluminum (Al) and fullerene (C60) onto room-temperature substrates. The periodicity of the layers is confirmed by x-ray-diffraction and in situ resistance measurements. The presence of underlying layers of C60 reduces the critical thickness at which Al becomes conducting from ∼35 to ∼20. In addition, there is a sudden increase in resistance that occurs when each Al layer is covered by a monolayer of C60. These observations, together with the measurement of a downward shift in frequency of a considerably broadened Raman-active Ag(2) pentagonal-pinch mode, imply that up to six electrons per C60 are transferred from the Al to the C60 layer. This demonstration of charge transfer across planar metal-C60 interfaces suggests that multilayers may be a useful vehicle for forming fullerene interface compounds in two-dimensional structures.