1. Department of Physics and Astronomy and Petersen Institute of NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
2. Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China (USTC), Hefei 230026, China
3. Department of Physics, University of Science and Technology of China (USTC), Hefei 230026, China
4. Department of Electrochemistry and Conducting Polymers, Leibniz-Institute for Solid State and Materials Research (IFW), Dresden, Germany
5. Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China