1. Department of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan
2. Institute of Electronics, National Yang Ming Chiao Tung University, Hsinchu 300, Taiwan
3. Centre of Nanotechnology, I. I. T. Roorkee, Roorkee 247667, India
4. Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia