1. Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0390, United States
2. Graduate School of Nanoscience and Technology, World Class University, KAIST, Daejeon 305-701, Korea
3. Key Laboratory of Automobile Materials (MOE) and College of Materials Science and Engineering, Jilin University, Changchun 130012, China
4. Department of Chemistry and Biochemistry and Liquid Crystal Materials Research Center, University of Colorado, Boulder, Colorado 80309-0215, United States