Oanh, Le Thi Mai, Center for Nano Science and Technology, Hanoi University of Education, 136 Xuan Thuy Road, Cau Giay District, Hanoi, 100000, Vietnam and Department of Physics, Hanoi National University of Education, 136 Xuan Thuy Road, Cau Giay District, Hanoi, 100000, Vietnam, Viet Nam