1. Department of Applied Physics and Materials Research Center The Hong Kong Polytechnic University Kowloon Hung Hom Hong Kong
2. Shenzhen Research Institute The Hong Kong Polytechnic University Shenzhen Guangdong 518057 P. R. China
3. Ningbo Research Institute Zhejiang University Ningbo 315100 P. R. China
4. Department of Materials Science and Engineering City University of Hong Kong Kowloon Hong Kong
5. Department of Electrical and Computer Engineering University of California Davis CA 95616 USA