**Acknowledgements**

This work was funded by the grant of the National Science Council 102-2622-E-007-021-CC2, 101-2221-E-007-103-MY3, and 101-2112-M-007-019-MY3. The authors would like to thank Dr. Wei-Jen Chen for many useful discussions. They would also like to thank Prof. Ru-Pin Pan for the use of the microscope.
