1. Qingdao Institute for Theoretical and Computational Sciences, Institute of Frontier and Interdisciplinary Science, Shandong University, Qingdao, Shandong 266237, P. R. China
2. Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA
3. Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
4. Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA