154 Materials Science and Engineering, B19 (1993) 154-161 Optical studies of fullerene-based solids P. C. Eklund, A. M. Rao, Ping Zhou, Ying Wang, Kai-An Wang, G. T. Hager and J. M. Holden Department of Physics and Astronomy and Center for Applied Energy Research, University of Kentucky, Lexington, KY 40506 (USA) Abstract Results of optical studies on oxygen-doped, alkali-metal(M)-doped and photopolymerized solid C6o films are reported. Results of Raman scattering studies of MxCto films are presented, but discussion is focused on recent Raman scattering studies of superconducting RbaCt0 films (Tc-28 K) from T=300 to 9 K. The effects of the exposure of C6o to light and/or oxygen are also presented. If pristine C6o is exposed at T= 300 K to visible or UV light, a photoinduced polymerization of the structure is obtained, which is consistent with results from laser desorption mass spectroscopy and optical spectroscopy. If oxygen is also present, the photoinduced polymerization is suppressed, and the oxygen is observed to diffuse about 2000/~ in about 1 h, eventually leading to oxidation of the structure. 1. Introduction The discovery by Smalley and coworkers [1] of large carbon cage molecules CN, referred to as 'fullerenes', has led to a new class of carbon-based solids which exhibit a wide variety of unusual physical and chemical properties [2--4]. In this paper, we review some of our recent work on C6o-based solid films. In particular, we will present results on oxygen- and alkali-metal(M)- doped C60, including Raman scattering [5, 6] studies of the superconducting compounds M3C6o first discov- ered by Haddon and coworkers at AT&T Bell labo- ratories [2, 7]. We first discuss the results of our studies on the photopolymerization of solid C60 [8]. As is now well known, pristine solid C60 is a van- der-Waals-bonded molecular solid whose electronic and vibrational properties are strongly connected to the properties of the C60 molecule itself, which exhibits icosahedral symmetry. This symmetry, associated with the pattern of periodically placed hexagonal and pen- tagonal carbon rings on a nearly spherical shell, is quite high and this leads to a dramatic simplification of the vibrational and electronic states [9]. The form of the experimental optical dielectric function E(w) = el(w) + i ez(to) is particularly simple, exhibiting a few IR active modes and narrow electronic absorption bands in the visible and UV (vis-UV) regions of the spectrum, as shown in Fig. 1 for e2(w) [10]. The data in Fig. 1 were obtained from spectra taken at temperature T--300 K on thin solid films of C6o, vacuum deposited onto various substrates, e.g. Si(100), KBr, fused quartz, etc., depending on the experiment and region of the spectrum. (The turbopumped, liquid nitrogen trapped deposition apparatus now resides in our helium atmosphere glove box. Samples can be doped in this deposition apparatus (e.g. alkali metals) and removed into a helium atmosphere (H20, Oz< 1 ppm) for fast transfer into the appropriate cell for study.) The data at high energy (E> 1.5 eV) in the figure were obtained from variable angle spectroscopic ellipsometry (VASE) [11] and a similar study was also carried out for solid C7o films [12]. For solid C6o, the vis-UV region contains four sharp, reasonably well- resolved, electronic bands with a full width at half- maximum (FWHM) of about 0.5 eV [13]. We identify these features with dipole-allowed transitions between narrow electronic energy bands derived from the ap- propriate molecular orbitals. Below this energy (about 1.5 eV), data were also taken by conventional near- normal incidence reflection and transmission spectros- copy. Using the quantity A = 1-(R + T), or the absor- bance, we detected an electronic absorption edge at 1.7 eV [13] associated with the highest occupied mo- lecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gap. The weak absorption above the edge region is assigned to transitions between molecular states with strong vibronic coupling. In the mid-IR region, the material exhibits weak vibrational mode activity. For an isolated C6o molecule there are only 14 optically active modes, owing to the high symmetry of the icosahedral group. Of the 180-186 possible vibrational modes, only 10 are Raman active modes 0921-5107/93/$6.00 © 1993- Elsevier Sequoia. All rights reserved