1. Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, P.R. China, Ecomaterials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing University, Nanjing 210093, P.R. China, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P.R. China, and Ecomaterials Center, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan