1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, United States
2. Kavli Energy NanoScience Institute at the University of California, Berkeley, Berkeley, California 94720, United States
3. Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States
4. Institute for Nanoelectronic Devices and Quantum Computing, Fudan University, Shanghai 200438, China