基金We thank Prof. Yang Gan of Harbin Institute of Technology, and Prof. Fu-long Yuan of Heilongjiang University for the help in the experiments. This work was supported by the Center for Precision Engineering of Harbin Institute of Technology, the Youth the Colleges and Universities in Heilongjiang Province in 2010 （No.1155G54）, the Training Fund Project of Jiamusi University （No.RC2009-037）, and the National Natural Science Foundation of China （No.51105174）.
