1. Institute for Integrative Nanosciences, Leibniz IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany
2. Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong 999077, China
3. Department of Physics, Southeast University, Nanjing 211189, P. R. China
4. Material
Systems for Nanoelectronics, Chemnitz University of Technology, Reichenhainer
Str. 70, 09107 Chemnitz, Germany