1. Hunan Key Laboratory for Micro–Nano Energy Materials and Devices, School of Physics and Optoelectronics, Xiangtan University, Hunan 411105, P. R. China
2. School of Materials Science and Engineering, Central South University, Changsha 410083, China
3. School of Physics and Electronics, Central South University, Changsha 410083, China
4. Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan