1. Frontiers Science Center for Flexible Electronics Institute of Flexible Electronics Northwestern Polytechnical University Xi'an 710072 P. R. China
2. Key Laboratory of Flexible Electronics of Zhejiang Province Ningbo Institute of Northwestern Polytechnical University 218 Qingyi Road Ningbo 315103 China
3. Department of Materials Science and Engineering National University of Singapore Singapore 117576 Singapore
4. School of Electrical and Electronic Engineering Nanyang Technological University Singapore 639798 Singapore