1. Department of Mechanical and Materials Engineering; University of Nebraska; Lincoln NE 68588 USA
2. Nebraska Center for Materials Nanoscience; University of Nebraska; Lincoln NE 68588 USA
3. Department of Physics and Astronomy; University of Nebraska; Lincoln NE 68588-0299 USA
4. State Key Laboratory of Supramolecular Structure and Materials; Jilin University; Changchun 130012 P. R. China