1. Department of Materials Science and Engineering; University of California; Berkeley CA 94720 USA
2. School of Geography and Biological Information; Nanjing University of Posts and Telecommunications; Nanjing 210023 P. R. China
3. School of Electronic Science and Engineering; Nanjing University; Nanjing 210093 P. R. China
4. Materials Sciences Division; Lawrence Berkeley National Laboratory; Berkeley CA 94720 USA