1. School of Electrical and Electronic Engineering; Nanyang Technological University; 50 Nanyang Avenue 639798 Singapore
2. Department of Materials Science and Engineering; National University of Singapore; 9 Engineering Drive 1 117576 Singapore
3. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue 639798 Singapore
4. Department of Electrical and Electronic Engineering; South University of Science and Technology of China; Shenzhen 518055 P. R. China