Kai-An Wang

Optical properties of C60- and C70-based solid films

Abstract We review results obtained from our laboratory regarding the optical properties of C 60 - and C 70 -based thin solid films, including alkali metal (M) doped compounds. The films were deposited on various substrates (e.g. KBr, Si(100), quartz, Cu) by vacuum sublimation. First- and higher-order IR and Raman spectroscopies were used to determine values for most of the 46 intra-molecular vibrational modes of the pristine solid, and the results are compared with theory. Doping (M)-induced shifts and splittings of these mode frequencies are also discussed. Finally, results of UV-visible-IR reflection-transmission spectroscopies and ellipsometry are presented which reveal structure in the dielectric function due to electronic absorption. For pristine C 60 , the Clausius-Mossotti relation is used to extract the frequency-dependent molecular polarizability. Results from these studies of the electronic contribution to the optical dielectric function are discussed in connection with recent calculations.

Optical studies of fullerene-based solids

Results of optical studies on oxygen-doped, alkali-metal(M)-doped and photopolymerized solid C60 films are reported. Results of Raman scattering studies of MxC60 films are presented, but discussion is focused on recent Raman scattering studies of superconducting Rb3C60 films (Tc ≈ 28 K) from T = 300 to 9 K. The effects of the exposure of C60 to light and/or oxygen are also presented. If pristine C60 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 A in about 1 h, eventually leading to oxidation of the structure.

Physical Properties of Nanocrystalline MoS 2 and WS 2 Particles Produced by CO 2 Laser Pyrolysis

Nanocrystalline powders of 2H-MoS 2 and 2H-WS 2 with average particle size 5 and 9 nm, respectively, have been produced using C0 2 laser pyrolysis. Typical production rate for these nanoparticles is 2g/hr. Particle size, crystallite size, and the structural phase were determined using X-ray diffraction(XRD), transmission electron microscopy(TEM), and Raman scattering. Particle size effects may have been observed in the Raman-active modes (WS 2 and MoS 2 ) and in the band-edge excitons (MoS 2 ).

Size-effect Stabilization of the Low-T Ferroelectric Phase in Nanocrystalline WO 3

Using a CO 2 laser to drive the pyrolysis of W(CO) 6 and O 2 , we have synthesized nanocrystalline WO 3-x particles. These nanopowders are found to exhibit a narrow size distribution with an average particle size produced in the range 5 to 10 nm, depending on the experimental conditions. Typical production rates are ∼ 2 g/h. Results from a Raman scattering study on WO 3 nanopowder samples annealed in O 2 indicate that a smaller particle size appears to stabilize the low-T ferroelectric phase (

Vibrational Spectroscopic Studies of Solid C60 Films

Experimental studies of the vibrational modes of pristine and photopolymerized solid C60 films are presented. A modest flux of visible or ultraviolet light is shown in the temperature range 250 > T > 373 K to transform C60 into a second solid phase which is identified with a polymeric form. A photochemical “2+2 cycloaddition” reaction mechanism is proposed to form the C60 polymer.