1. College of Chemistry and Chemical Engineering; Chongqing University; Chongqing 400044 China
2. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332 USA
3. College of Physics; Chongqing University; Chongqing 400044 China
4. School of Materials Science and Engineering; South-Central University for Nationalities; Wuhan 430074 China