1. SNTG Laboratory, Department of Physics Engineering, Hacettepe University, 06800 Ankara, Turkey
2. National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, USA
3. Nanotechnology Research Center, Bilkent University, 06800 Ankara, Turkey
4. Department of Physics, Bilkent University, 06800 Ankara, Turkey
5. Department of Electrical and Electronics Engineering, Bilkent University, 06800 Ankara, Turkey
6. National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA