1. Key Laboratory of Cluster Science (Beijing Institute of Technology), Ministry of Education of China, School of Science, Beijing Institute of Technology, Beijing 100081, P. R. China
2. College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China
3. Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
4. Department of Chemical Engineering, Case School of Engineering, Case Western Reserve University, Cleveland, Ohio 44106