1. Department of Materials Science and Engineering University of California Berkeley CA 94720 USA
2. Materials Sciences Division Lawrence Berkeley National Laboratory Berkeley CA 94720 USA
3. State Key Laboratory of Precision Measurement Technology and Instruments Department of Precision Instrument Tsinghua University Beijing 100084 P. R. China
4. Department of Electrical Engineering and Computer Sciences University of California Berkeley CA 94720 USA
5. Department of Molecular and Cell Biology University of California Berkeley CA 94720 USA