1. State Key Laboratory of Chemical Resource Engineering; P.O. Box 98; Beijing University of Chemical Technology; Beijing 100029 P. R. China
2. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province; College of Optoelectronic Engineering; Shenzhen University; Shenzhen 518060 P. R. China
3. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore