1. Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, United States
2. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
3. NSF Nanoscale Science and Engineering Center, University of California, Berkeley, California 94720, United States
4. Faculties of Sciences and Engineering, The University of Hong Kong, Hong Kong, China
5. Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, United States