1. Materials Research Laboratory, University of California, Santa Barbara, CA 93106.
2. Department of Chemical Engineering, University of California, Santa Barbara, CA 93106.
3. Materials Department, University of California, Santa Barbara, CA 93106.
4. Physics Department, University of California, Santa Barbara, CA 93106.
5. Exxon Research and Engineering Company, Annandale, NJ 08801, USA.