1. Optoelectronics Research Centre, University of Southampton, Highfield, Southampton SO17 1BJ, UK.
2. Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA.
3. Department of Physics, Pennsylvania State University, University Park, PA 16802, USA.
4. Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA.
5. Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA.