Xuemei Hu ,a,b Weizhu Xu,a,b Qingbin Fan,a,b Tao Yue,a,b Feng Yan,a,b Yanqing Lu,a,c and Ting Xua,c,* a Nanjing University, Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid-State Microstructures, Nanjing, China b Nanjing University, School of Electronic Sciences and Engineering, Nanjing, China c Nanjing University, College of Engineering and Applied Sciences, Jiangsu Key Laboratory of Artificial Functional Materials, Nanjing, China