1. School of Materials Science and Engineering; Georgia Institute of Technology; Atlanta GA 30332-0245 USA
2. NanoGenerators and NanoEngineering Laboratory; School of Mechanical and Industrial Engineering; University of Toronto; Toronto ON M5S 3G8 Canada
3. Design and Production Engineering Department; Faculty of Engineering; Ain Shams University; Cairo 11535 Egypt
4. Beijing Institute of Nanoenergy and Nanosystems; Chinese Academy of Sciences; Beijing 100083 China