1. State Key Lab of Crystal Materials, Shandong University; 27 Shandanan Road Jinan 250100 China
2. Biosystems and Biomaterials Division, The National Institute of Standards and Technology; MD 20899-8300 USA
3. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; Beijing China
4. Department of Physics and Astronomy; Bowling Green State University; Bowling Green OH 43403 USA