* Corresponding authors

^a Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China
E-mail: rong.sun@siat.ac.cn

^b Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China

^c College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, China
E-mail: xz.fu@szu.edu.cn
Fax: +86-755-86392299
Tel: +86-755-86392151

^d Department of Electronics Engineering, The Chinese University of Hong Kong, Hong Kong, China

^e School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA