Optical Spectra of Fullerenes: C 60 a new amorphous semiconductor?
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Optical Spectra of Fullerenes: C60 a new amorphous semiconductor? ANDREW SKUMANICH IBM Almaden Research Center, K67/802, 650 Harry Rd. San Jose, CA 95120. ABSTRACT The optical absorption spectra for thin films of sublimed C 60 and C 70 are presented. Both transmission spectroscopy and photothermal deflection spectroscopy are used to obtain the spectrum over a wide energy range (0.4 to 6.2 eV) for the same film. The optical gap varies for the different fullerenes. The value for C 60 is - 1.6 eV and the corresponding transition appears to be optically forbidden given the weakness of the absorption. Structure is observed on the "gap edge" which can be attributed to vibronic transitions. The gap region can be described in terms used for amorphous semiconductors, having features such as an Urbach edge, and sub-gap defect absorption. The similarity suggests that the nature of fullerene thin films may be analogous to amorphous silicon and carbon films. These films have both a molecular and an amorphous semiconductor nature.
(1) Introduction Some of the intriguing questions about the fuUerenes (C60 , C 70 , etc.) have to do with the nature of the solid state of these large molecules. Are these films simply collections of molecules, are they solid thin films with intermolecular bonding, are they similar to amorphous semiconductors with extended states? Previous photoemission studies when compared to electronic-structure calculations imply that films of C 60 can be explained using a picture of "molecular clusters" with weak van der Waals attractive forces. 1,2 An interesting question is whether there exists any evidence for extended bandlike states? The cohesiveness of the thin films might seem to imply a molecular interaction stronger than a van der Waals force. An important related question is how do the deposition conditions influence the thin film properties, and is there a source for inhomogeneity even when considering only a single fullerene? In an attempt to examine these questions in more detail, the fundamental optical properties were measured using both transmission and photothermal deflection spectroscopy (PDS) for thin films of C 60 and C 70 . The data show that thin films of these fullerenes can be described as having both molecular and semiconductor characteristics. There is evidence of extended electronic states. Further, sample preparation can influence the band structure by introducing low energy states.
(2) Film synthesis and characterization The samples were generated from a carbon arc, subsequently purified, and then sublimed onto glass. For the purification process, the arc generated material was dissolved in toluene, collected and then dried. Columnar chromatography was used to separate out the C60 , and this was collected and placed in a small resistively heated stainless steel tube oven which was placed in a bell jar and evacuated to _ 10-6 Torr. After heating the oven to 150 C for five minutes, a substrate wvindow was placed 2-5 mm above the Imm opening in the tube. Heating the tube further (to -55
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