1. Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
2. Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing, Jiangsu 210093, China.
3. Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, USA.
4. Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093, USA.