1. Frontier Institute of Science and Technology jointly with College of Science; State Key Laboratory for Strength and Vibration of Mechanical Structures; Xi'an Jiaotong University; Xi'an Shaanxi 710054 China
2. Department of Nuclear Science and Engineering and Department of Materials Science and Engineering; MIT; Cambridge MA 02139 USA
3. School of Electrical Engineering, The Center of Nanomaterials for Renewable Energy; Xi'an Jiaotong University; Xi'an Shaanxi 710054 China