1. State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. Department of Chemistry and the Materials Research Center Northwestern University 2145 Sheridan Road Evanston IL 60208 USA
3. School of Physics State Key Laboratory of Crystal Materials Shandong University Jinan 250100 P. R. China
4. School of Automation Engineering University of Electronic Science and Technology of China (UESTC) Chengdu Sichuan 611731 P. R. China
5. Flexterra Corporation Skokie IL 60077 USA