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

The C 60 /glass, C 60 /Si, C 60 /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 C 60 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, C 60 films on Si or porous Si/Si display only the 1.774 eV emission band. The intensity of this emission in C 60 /porous Si/Si is 200 times higher than that in C 60 /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 C 60 molecules with porous silicon walls. Molecular signatures of the C 60 films are clearly manifested by the vibronic transitions with 60 meV energy separation (A g -mode energy) between the irradiative levels, observed on 1.774 eV emission band in C 60 /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 C 60 film due to the crystal field. The strength of the crystal field splitting is then ~ 200 meV. • C 60 fullerene layers on various substrates are obtained and thoroughly characterized. • C 60 thin film is a direct gap band insulator with band gap energy of 2.115 eV. • Vibronic levels of singlet Frenkel excitons are involved in emission below 2 eV. • 200 times enhanced emission below 2 eV is observed on C 60 film/porous Si/Si structures. [ABSTRACT FROM AUTHOR]