1. National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Eco-Materials and Renewable Energy Research Center (ERERC), College of Engineering and Applied Sciences, Nanjing University, Nanjing, Jiangsu 210093, P. R. China
2. Jiangsu Province Key Laboratory for Nanotechnology, School of Physics, Nanjing University, Nanjing, Jiangsu 210093, P. R. China