1. Departments of Physics, Materials and Science Engineering, Chemistry, and Electrical and Computer Engineering, Beckman Institute, and Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana-Champaign 61801, School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907-1285, and Department of Materials Science and Nanotechnology, Bilkent University, 06800 Bilkent, Ankara, Turkey