[1] Penn State Univ, Dept Phys, University Pk, PA 16802 USA
[2] Univ Twente, Fac Sci & Technol, NL-7500 AE Enschede, Netherlands
[3] Univ Twente, MESA Inst Nanotechnol, NL-7500 AE Enschede, Netherlands
[4] Penn State Univ, Dept Mat Sci & Engn, University Pk, PA 16802 USA