1. College of Microelectronics; Department of Information Science and Electronic Engineering; Zhejiang University; Hangzhou 310027 P. R. China
2. State Key Laboratory of Modern Optical Instrumentation; Zhejiang University; Hangzhou 310027 P. R. China
3. State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering; Xiamen University; Xia-men 361005 P. R. China