[1] Nanyang Technol Univ, Sch Mat Sci & Engn, Singapore 639798, Singapore
[2] Nanyang Technol Univ, Sch Phys & Math Sci, Singapore 637371, Singapore
[3] Natl Ctr Nanosci & Technol, Beijing 100190, Peoples R China
[4] Zhejiang Univ, State Key Lab Silicon Mat, Hangzhou 310027, Zhejiang, Peoples R China
[5] Zhejiang Univ, Sch Mat Sci & Engn, Hangzhou 310027, Zhejiang, Peoples R China
[6] Natl Univ Singapore, Ctr Adv Mat 2D, Singapore 117542, Singapore
[7] Natl Univ Singapore, Dept Phys, Singapore 117542, Singapore
[8] Natl Tsing Hua Univ, Dept Phys, Hsinchu 30013, Taiwan
[9] Nanyang Technol Univ, Nanoelect Ctr Excellence, Sch Elect & Elect Engn, NOVITAS, Singapore 639798, Singapore
[10] US Army, Res Labs, Sensors & Electron Devices Directorate, Adelphi, MD 20783 USA
[11] Rice Univ, Dept Mat Sci & Nanoengn, Houston, TX 77005 USA
[12] Oak Ridge Natl Lab, Div Mat Sci & Technol, Oak Ridge, TN 37831 USA