1. Department of Electrical Engineering, Stanford University 1 , Stanford, California 94305, USA
2. Department of Mechanical Engineering, Stanford University 2 , Stanford, California 94305, USA
3. Department of Materials Science and Engineering, Stanford University 3 , Stanford, California 94305, USA
4. Department of Applied Physics, Stanford University 4 , Stanford, California 94305, USA
5. Precourt Institute for Energy, Stanford University 5 , Stanford, California 94305, USA