1. California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA.
2. Department of Chemistry & Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA
3. Department of Materials Science & Engineering, University of California, Los Angeles, Los Angeles, CA 90095, USA
4. Department of Engineering Science & Mechanics, The Pennsylvania State University, University Park, PA 16802, USA
5. Department of Engineering Science & Mechanics, The Pennsylvania State University, University Park, PA 16802, USA.