1. State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University 1 , Shapingba District, Chongqing 400044, People's Republic of China
2. Department of Physics and Astronomy, University of California 2 , Irvine, California 92697, USA
3. The Molecular Foundry, Lawrence Berkeley National Laboratory 3 , Berkeley, California 94720, USA