1. School of Materials Science and Engineering National Institute for Advanced Materials Nankai University Tianjin 300350 P. R. China
2. Key Laboratory for the Physics and Chemistry of Nanodevices and Center for Carbon‐based Electronics Department of Electronics Peking University Beijing 100871 P. R. China
3. Tianjin Key Laboratory for Rare Earth Materials and Applications Nankai University Tianjin 300350 P. R. China