Yong-Jun Yuan1, Zhen-Tao Yu, Xiao-Yu Chen, Ji-Yuan Zhang, Zhi-Gang Zou. 1. Eco-Materials and Renewable Energy Research Center, National Laboratory of Solid State Microstructures, Department of Materials Science and Engineering, Nanjing University, NO. 22, Hankou Road, Nanjing, Jiangsu 210093, PR China.