1. Department of Physics, Soongsil University, Seoul 156-743, Korea
2. Department of Physics, Sogang University, Seoul 121-742, Korea
3. Department of Chemical Engineering, Soongsil University, Seoul 156-743, Korea
4. Department of Chemical Engineering, SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nano Technology (HINT), Sungkyunkwan University, Suwon 440-476, Korea