Abstract
We have performed what we believe to be the first time-resolved absorption studies of C60 solid films. The films were fabricated by the evaporation of highly purified C60 onto a sapphire substrate in UHV conditions (<10-10 Torr) at 550 degrees C. Measurements were performed using a CPM laser (620 nm, pulsewidth <100 fsec) in a standard pump-probe transmission geometry. The excitation wavelength corresponds to excitation of carriers across the HOMO-LUMO gap of solid C60, a normally symmetry-forbidden transition in the isolated molecule. A nonexponential decay of optical transmission was observed on a picosecond timescale, with no subpicosecond transient characteristic of hot carrier thermalization. The nonexponential decay of the transmission signal is indicative of a geminate carrier recombination process, usually seen in amorphous semiconductors. The optical absorption spectrum of C60 also possesses an exponential rise at the band edge (Urbach tail), also characteristic of amorphous semiconductors. The observed carrier recombination can be theoretically described by a diffusion/neutralization model from which a diffusion length can be obtained. The theoretically obtained mobility is in agreement with conductivity measurements for undoped 60C films on insulating substrates.
© 1991 Optical Society of America
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