1. Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
2. Physics Department, Faculty of Science, Damietta University, New Damietta 34517, Egypt
3. Guangdong University Key Laboratory for Advanced Quantum Dot Displays and Lighting and Department of Electrical & Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China