1. National Laboratory for Infrared Physics; Shanghai Institute of Technical Physics; Chinese Academy of Sciences; 500 Yutian Road Shanghai 200083 China
2. Department of Physics and Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education; Wuhan University; Wuhan 430072 China
3. Department of Physics and Materials Science; City University of Hong Kong; Hong Kong SAR China
4. Material Engineering; The University of Queensland; St Lucia QLD 4072 Australia
5. Centre for Microscopy and Microanalysis; The University of Queensland; St Lucia QLD 4072 Australia
6. State Key Laboratory of Surface Physics and Department of Physics; Fudan University; Shanghai 200433 China