1. Department of Physics and Astronomy University of Pennsylvania Philadelphia PA 19104 USA
2. State Key Lab of New Ceramics and Fine Processing School of Materials Science and Engineering Tsinghua University Beijing 100084 China
3. Department of Electrical and System Engineering University of Pennsylvania 209 South 33rd Street Philadelphia PA 19104 USA
4. Department of Mechanical Engineering & Applied Mechanics University of Pennsylvania Philadelphia PA 19104 USA
5. Department of Chemistry University of Pittsburgh Pittsburgh PA 15260 USA