Charge transfer, doping, and interface morphologies for Al-${\mathrm{C}}_{60}$

The electronic properties and morphologies of aluminum-${\mathrm{C}}_{60}$ interfaces have been studied with photoelectron spectroscopy. The formation of an interface dipole between the first layer of ${\mathrm{C}}_{60}$ and Al establishes the Fermi level as the common energy-level reference. This dipole layer reflects the transfer of ∼0.2 electrons per first-layer fullerene. Deposition of Al onto ${\mathrm{C}}_{60}$ at 300 K results in the formation of Al clusters on the surface and also limited Al diffusion into the fullerene lattice. These interstitial Al atoms create a solid solution and form donor levels that are thermally ionized. Aluminum overlayer growth at 60 K results in smaller islands and suppressed doping because both surface and bulk diffusion are thermally activated processes. There is no evidence of bulk Al-${\mathrm{C}}_{60}$ compound formation.