1. Department of Electrical Engineering, California Institute of Technology, Pasadena, California 91125, United States
2. Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States
3. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States
4. Thomas J. Watson Laboratories of Applied Physics, California Institute of Technology, Pasadena, California 91125, United States