1. Department of Physics, Sogang University, Seoul 04107, South Korea
2. Department of Physics and Astronomy, Sejong University, Seoul 05006, South Korea
3. Department of Physics and Astronomy, California State University Long Beach, Long Beach, California 90840, USA
4. National NanoFab Center, Daejeon 34141, South Korea
5. Institute of Physics, Johannes Gutenberg University Mainz, Mainz 55128, Germany