1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA.
2 Molecular Foundry Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720, USA.
3 Key Laboratory of Computational Physical Sciences (Ministry of Education), Institute of Computational Physical Sciences, State Key Laboratory of Surface Physics, and Department of Physics, Fudan University, Shanghai 200433, China.
4 Shanghai Qi Zhi Institute, Shanghai 200030, China.
5 Physics Department, University of Arkansas, Fayetteville, Arkansas 72701, USA.
6 Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA.