1. College of Material Science and EngineeringTaiyuan University of Technology Taiyuan 030024 China
2. Institute of Physical Chemistry & Center for Materials ResearchJustus-Liebig-University Gießen Heinrich-Buff-Ring 17 Gießen 35392 Germany
3. College of Physics and OptoelectronicsTaiyuan University of Technology Taiyuan 030024 China
4. Department of PhysicsThe University of the West Indies Mona Kingston 7 Jamaica