[1] CALTECH, Dept Elect Engn, Pasadena, CA 91125 USA
[2] Nanjing Univ, Nanjing Natl Lab Microstruct, Nanjing 210093, Jiangsu, Peoples R China
[3] Univ Calif San Diego, Dept Elect & Comp Engn, La Jolla, CA 92093 USA
[4] CALTECH, Kavli Nanosci Inst, Pasadena, CA 91125 USA