1. School of Materials Science and Engineering; Nanjing University of Science and Technology; Nanjing 210094 P. R. China
2. School of Materials Science and Engineering; Nanyang Technological University; Singapore 639798 Singapore
3. Department of Materials Science and Engineering; National Chiao Tung University; Hsinchu 30010 Taiwan
4. National Laboratory of Solid State Microstructures; Nanjing University; Nanjing 210093 P. R. China