* Corresponding authors

^a Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
E-mail: csun@iphy.ac.cn
Fax: +86-10-82649901
Tel: +86-10-82649901

^b Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing Key Laboratory for New Energy Materials and Devices, Beijing 100190, China

^c Richard G. Lugar Center for Renewable Energy, Department of Mechanical Engineering, Indiana University Purdue University, Indianapolis 46202, USA
E-mail: yk35@iupui.edu
Fax: +1-317-274-9744
Tel: +1-317-274-9711

^d Texas Materials Institute, The University of Texas at Austin, Texas 78712, USA