1. Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA.
2. Resnick Sustainability Institute, California Institute of Technology, Pasadena, CA 91125, USA.
3. Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, USA.
4. Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, USA.
5. Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA.
6. Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, CA 91125, USA.