1. Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
2. School of Energy and Chemical Engineering; Ulsan National Institute of Science and Technology (UNIST); Ulsan 689-798 Republic of Korea
3. Department of Electrical and Computer Engineering; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
4. Department of Mechanical Science and Engineering; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
5. Department of Physics; University of Illinois at Urbana-Champaign; Urbana IL 61801 USA
6. Department of Physics, College of Sciences; China University of Mining and Technology; Xuzhou Jiangsu 221116 P. R. China
7. Department of Electrical Engineering and Computer Sciences; University of California, Berkeley; Berkeley CA 94720 USA