Characteristics of multivalent impurity doped C₆₀ films grown by MBE

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.