1. Department of Physics and Institute of Advanced Materials; Hong Kong Baptist University; Kowloon Tong Hong Kong P. R. China
2. Institute for Chemical Process and Environmental Technology; National Research Council of Canada; 1200 Montreal Road Ottawa Canada ON K1A 0R6
3. Department of Materials Science and Engineering; University of Florida; Gainesville FL 32611-6400 USA
4. Department of Physics and Materials Science; City University of Hong Kong; Kowloon Tong Hong Kong P. R. China