Abstract
We present an electron spectroscopy study of phase-pure AC60 thin films (A= Rb, Cs) in their monomer (face-centred cubic (fcc)) and polymer phases. A surface electronic reconstruction is observed in polymeric RbC60, analogous to that reported for the fcc phase. As for pristine C60, the occupied electronic states of AC60 fullerides are not dramatically affected by polymerization. The energy separation between the leading feature in photoemission and inverse photoemission is similar in both stable AC60 phases. These observations suggest that electron correlation effects are similar in the two phases, and that their different electronic behaviour is mainly related to the reduction of degeneracy of the polymer frontier states. Photoemission and electron-energy loss spectroscopy data show that the thin-film form of the RbC60 polymer is metallic at room temperature, and that it undergoes a metal–insulator transition at around 100 K. This transition temperature is much higher than that reported for the corresponding bulk phase and signals a poorer screening of Coulomb interactions at the film surface.
Highlights
Theoretical investigations [32]–[35] on polymerized C60 have shown that, except for the region near 8–9 eV below the Fermi level which contains the contribution of the σ orbitals corresponding to the covalent intermolecular bonds [32], the occurrence of [2 + 2] cycloaddition bonds does not lead to a dramatic modification of the electronic density of states (DOS) with respect to monomeric C60
The PES and inverse photoemission (IPES) measurements on CsC60 were carried out at RT on films prepared under the same conditions and thoroughly characterized by Auger electron spectroscopy and low-energy electron diffraction (LEED) to ensure that the same phase was obtained
We have investigated phase-pure RbC60 and CsC60 thin films by means of photoemission, inverse photoemission and electron energy loss spectroscopy
Summary
Theoretical investigations [32]–[35] on polymerized C60 have shown that, except for the region near 8–9 eV below the Fermi level which contains the contribution of the σ orbitals corresponding to the covalent intermolecular bonds [32], the occurrence of [2 + 2] cycloaddition bonds does not lead to a dramatic modification of the electronic density of states (DOS) with respect to monomeric C60 This has been confirmed by photoemission spectroscopy (PES) studies of the orthorhombic C60 polymer phase [36, 37] as well as x-ray emission spectroscopy characterizations of the 2D-polymer phases [16, 38]. These similarities suggest that the difference in electronic behaviour between the two phases originates only from the reduced degeneracy of the frontier polymer states [32]
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