1. Micro and Nanotechnology Laboratory, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 208 North Wright Street, Urbana, Illinois 61801, USA
2. Department of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, USA
3. Mechanical Science and Engineering Department, University of Illinois at Urbana-Champaign, 154 Mechanical Engineering Building, Urbana, Illinois 61801, USA
4. Department of Bioengineering, University of Illinois at Urbana-Champaign, 1270 Digital Computer Laboratory, MC-278, Urbana, Illinois 61801, USA