Photoluminescence and optical transitions in C60 fullerene thin films deposited on glass, silicon and porous silicon

The C60/glass, C60/Si, C60/porous Si/Si as well as porous Si/Si thin film structures for comparison were prepared and studied by photoluminescence spectroscopy at room temperature. Along with a broad-band low-energy emission at 1.774 eV in the photon energy range of the nominaly forbidden optical transitions, the C60 films on glass substrate exhibit high-energy emissions at 2.115 eV and 2. 342 eV in the range of allowed dipole transititions. On the other hand, C60 films on Si or porous Si/Si display only the 1.774 eV emission band. The intensity of this emission in C60/porous Si/Si is 200 times higher than that in C60/Si and at least 20 times exceeds the one for the emission observed on porous Si/Si. Such a dramatic increase in emission intensity is indicative of the relaxation of the selection rules for nominally forbidden optical transitions and suggests strong interaction of C60 molecules with porous silicon walls. Molecular signatures of the C60 films are clearly manifested by the vibronic transitions with 60 meV energy separation (Ag-mode energy) between the irradiative levels, observed on 1.774 eV emission band in C60/porous Si/Si. The origin of the 2.115 and 2.342 eV emission bands is analyzed within a standard band insulator approach, together with the obtained ellipsometric data. The first corresponds to the band-to-band transitions at band gap. The appearance of the second can be related to the splitting of the valence band in the X point of the Brillouin zone of C60 film due to the crystal field. The strength of the crystal field splitting is then ~ 200 meV.