1. School of Physics, Key Laboratory of Cluster Science of Ministry of Education, Beijing Institute of Technology, Beijing 100081, People's Republic of China
2. Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, People's Republic of China
3. Physics Department, University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, Norway