1. Department of Physics, University of Texas at Dallas, Richardson, TX 75080, USA.
2. Department of Materials Science and Engineering and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
3. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
4. Department of Chemistry, University of Houston, Houston, TX 77204, USA.