1. Shaanxi Key Laboratory of Optical Information TechnologySchool of Physical Science and TechnologyNorthwestern Polytechnical University Xi'an 710129 P. R. China
2. Shenzhen Engineering Center for the Fabrication of Two-Dimensional Atomic CrystalsShenzhen Institutes of Advanced TechnologyChinese Academy of Sciences Shenzhen 518055 P. R. China
3. Center for Opto-Electronic Engineering and TechnologyShenzhen Institutes of Advanced TechnologyChinese Academy of Sciences Shenzhen 518055 P. R. China