The authors would like to apologize for an error in the Acknowledgements of their article. The correct Acknowledgement should read as follows:
The authors would like to thank the Center for Micro/Nano Technology Research, National Cheng Kung University, Tainan, Taiwan, Republic of China for access to their equipment and for their technical support. The funding from the Ministry of Education and the National Science Council of Taiwan, Republic of China under contract no. NSC 93-2320-B-006-015 is gratefully acknowledged. Funding to pay the Open Access publication charges for this article was provided by National Science Council of Taiwan.