1. Center for Nano-imaging Technology, Korea Research Institute of Standards and Science, 267 Gajeong-Ro, Yuseong, Daejeon 305-340, Korea
2. Center for Nano-Bio Technology, Korea Research Institute of Standards and Science, 267 Gajeong-Ro, Yuseong, Daejeon 305-340, Korea
3. Research Institute for Technical Physics and Materials Science, H-1525 Budapest, POB 49, Hungary