1. Department of Electrical and Computer Engineering, and N. Holonyak Jr. Micro & Nanotechnology Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
2. Laboratory of Nanoscale Biology, Institute of Bioengineering, School of Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH 1015, Switzerland