1. Department of Biological Systems Engineering, University of Nebraska—Lincoln, 3605 Fair Street, Lincoln, Nebraska 68583-0726, United States
2. Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln, 855 N 16th Street, Lincoln, Nebraska 68588-0298, United States
3. Department of Physics, Creighton University, 2500 California Plaza, Omaha, Nebraska 68178, United States