1. Stanford Nano Shared Facilities, Stanford University 1 , Stanford, California 94305, USA
2. Department of Physics, Kaunas University of Technology 2 , LT-51368 Kaunas, Lithuania
3. Department of Mechanical Engineering and Mechanics, Drexel University 3 , Philadelphia, Pennsylvania 19104, USA
4. 4 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA