1. Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization; The University of Toledo; Toledo OH 43606 USA
2. Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China; School of Physics and Technology; Wuhan University; Wuhan 430072 China
3. National Renewable Energy Laboratory; 15013 Denver West Parkway Golden CO 80401 USA
4. Department of Mechanical Engineering and Materials Science; Duke University; Durham NC 27708 USA
5. Department of Chemistry; Duke University; Durham NC 27708 USA