1. NANOTAM-Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey
2. Department of Electrical and Electronics Engineering, Bilkent University, Ankara 06800, Turkey
3. Department of Physics, Bilkent University, Ankara 06800, Turkey
4. UNAM−National Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey