1. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100 P R China, Department of Applied Physics, Chongqing University, Chongqing 400044, China, College of Engineering, Peking University, Beijing, China, and National Center for Nanoscience and Technology of China, Beijing, 100080, China