Metal-doped C-60 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 C-60 films are considerably enhanced. PL peaks of the electron transition between the highest occupied molecular orbital and the lowest unoccupied molecular orbital states of C-60 molecules are confirmed in Al-doped and Ga-doped C-60 films, but not in Ge-doped C-60 films. Optimized bonding structures of these impurity atoms to C-60 molecules are determined by using ab initio calculations. Stable covalent bonds between impurities and C-60 molecules are verified to be formed. The impurity atoms may act as bridges between C-60 molecules. The distortioh of C-60 cages due to the bonding with metals is confirmed. In the Al- and Ga-doped C-60 films, this distortion probably makes the dipole forbidden transition relieved. The binding energies are found to be related to the experimentally determined Vickers hardness. (c) 2007 Elsevier B.V. All rights reserved.