1. Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Viet Nam
2. Department of Physics, Hankuk University of Foreign Studies, Yongin 449-791, South Korea
3. Faculty of Engineering Physics and Nanotechnology, VNU-University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Ha Noi, Viet Nam
4. Department of Science Education, Chungbuk National University, Cheongju 360-763, South Korea