1. a Department of Physics, Faculty of Science and Arts , University of Gazi , Teknikokullar, 06500, Ankara, Turkey
2. b Nanotechnology Research Center, Bilkent University , Bilkent, 06800, Ankara, Turkey
3. c Department of Physics and Department of Electrical and Electronics Engineering , Bilkent University , Bilkent, 06800, Ankara, Turkey