1. Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea
2. Institute of Materials Science, Vietnam Academy of Science and Technology, 18-Hoang Quoc Viet, Hanoi, Vietnam
3. Department of Mechanical and Aerospace Engineering, University of California, San Diego, California 92093-0411, USA
4. Institute of Chemistry, Vietnam Academy of Science and Technology, 18-Hoang Quoc Viet, Hanoi, Vietnam
5. Faculty of Engineering Physics and Nanotechnology, VNU University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam
6. Department of Nanoscience and Nanotechnology, Advanced Institute for Science and Technology, Hanoi University of Science and Technology, 01 Dai Co Viet, Hai Ba Trung, Hanoi, Vietnam