1. Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
2. Department of Material Science and Engineering, Stanford University, Stanford, California 94305, USA
3. Department of Mechanical Engineering, Stanford University, Stanford, California 94305, USA
4. School of Energy and Power Engineering, Xi'an Jiatong University, Xi'an, Shaanxi 710049, China
5. Hewlett-Packard Labs, 1501 Page Mill Rd., Palo Alto, California 94304, USA