1. Department of Physics and Astronomy & Nebraska Center for Materials and Nanoscience, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United States
2. Department
of Materials Science and Engineering, University of Wisconsin—Madison, Madison, Wisconsin 53706, United States
3. Department of Physics & NUSNNI-NanoCore, Faculty of Science, National University of Singapore, Singapore 117411, Singapore
4. ICREA-Institut Catala de Recerca i Estudis Avançats, Barcelona, Spain
5. ICN2-Institut Catala
de Nanociencia i Nanotecnologia, Campus de Bellaterra, Barcelona, Spain