1. Department of Electron Microscopy, Institute of Scientific Instruments of the Czech Academy of Sciences, Brno 61200, Czech Republic
2. Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
3. Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China