1. Department of physics, Chong Yuet Ming Physics Building, The University of Hong Kong, Pokfulam Road, Hong Kong
2. College of Science, China University of Petroleum, Qingdao 266580, China
3. Jiangsu Engineering Laboratory for Bio-sensor and chip technology, Jiangsu Key Laboratory of Micro-Nano Heat Fluid Flow Technology and Energy Application, School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou, Jiangsu 215009, PR China