1. Department of Materials Science and Engineering, National Laboratory of Solid State Microstructure, Nanjing University, Nanjing 210093, People's Republic of China, Hewlett-Packard Laboratories, Hewlett-Packard Company, Palo Alto, California 94304, and Department of Electrical and Computer Engineering, University of Waterloo, 200 University Avenue, West Waterloo, Ontario N2L3G1, Canada