Metal-doped C60 films (aluminum, gallium and germanium) are grown on GaAs and quartz glass substrates by solid source molecular beam epitaxy. Mechanical and optical properties of the films are investigated by Vickers hardness test and photoluminescence (PL) measurement. Vickers hardness values of all the impurity-doped C60 films are considerably enhanced. PL peaks of the electron transition between the highest occupied molecular orbital and the lowest unoccupied molecular orbital states of C60 molecules are confirmed in Al-doped and Ga-doped C60 films, but not in Ge-doped C60 films. Optimized bonding structures of these impurity atoms to C60 molecules are determined by using ab initio calculations. Stable covalent bonds between impurities and C60 molecules are verified to be formed. The impurity atoms may act as bridges between C60 molecules. The distortion of C60 cages due to the bonding with metals is confirmed. In the Al- and Ga-doped C60 films, this distortion probably makes the dipole forbidden transition relieved. The binding energies are found to be related to the experimentally determined Vickers hardness.
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