Structural properties of C60-multivalent metal composite layers grown by molecular beam epitaxy

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C 60 -multivalent metal composite layers (aluminum, gallium, and germanium) are grown on GaAs and quartz glass substrates by molecular beam epitaxy. The structural properties of the C60-metal composite layers are investigated by reflection high-energy electron diffraction and transmission electron microscopy measurements, and it is confirmed that these layers have an amorphous structure. Mechanical properties of the layers are investigated by Vickers hardness test, and the values of the C60-metal composite layers are confirmed to be dramatically increased. The structural change and the hardness enhancement are induced by the bonding between C60 molecules and multivalent metal atoms. Optical properties of the layers are measured by the absorption coefficient spectra. The absorption peaks in C60–Ge composite layers become less pronounced with increasing Ge concentration and the intensity in visible light spectrum is increased. Pure C60, C60–Al, and C60–Ga composite layers are confirmed to be insulators in air. In contrast, the conductivity of a C60–Ge composite layer is found to be 0.02 Ω−1 cm−1 at room temperature with an activation energy of 120 meV. These enhancements of absorption coefficient and conductivity are very important for solar cells applications.