1. Materials Science and Engineering Department, University of Pittsburgh, Pittsburgh, PA 15261, USA.
2. Department of Materials Science and Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794-2275, USA.
3. Bellcore, Red Bank, NJ 07701-7040, USA.
4. Exxon Research and Engineering Company, Corporate Research Science Laboratory, Annandale, NJ 08801, USA.