1. Department of Physics/Pen-Tung Sah Institute of Micro-Nano Science and Technology; Xiamen University; Xiamen Fujian 361005 China
2. Department of Mechanical Engineering; University of California; Berkeley California 94720 USA
3. State Key Lab of Physical Chemistry of Solid Surfaces and Department of Chemistry; College of Chemistry and Chemical Engineering; Xiamen University; Xiamen Fujian 361005 China