1. State Key Laboratory of Chemical Resource Engineering; Beijing University of Chemical Technology; P.O. Box 98 Beijing 100029 P.R. China
2. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
3. SZU-NUS Collaborative Innovation Centre for Optoelectronic Science & Technology and College of Optoelectronic Engineering; Shenzhen University Shenzhen 518060 P.R. China