1. School of Electrical Engineering and Automation Hefei University of Technology Hefei 230009 P. R. China
2. School of Electronic Science and Applied Physics Hefei University of Technology Hefei 230009 P. R. China
3. School of Materials Science and Technology Hefei University of Technology Hefei 230009 P. R. China
4. Key Laboratory of Photovoltaic and Energy Conservation Materials Institute of Solid State Physics Hefei Institutes of Physical Science Chinese Academy of Sciences Hefei 230031 China
5. Nanomaterials Centre Australian Institute for Bioengineering and Nanotechnology and School of Chemical Engineering The University of Queensland St Lucia Queensland 4072 Australia