[1] Nanyang Technol Univ, NOVITAS, Nanoelect Ctr Excellence, Sch Elect & Elect Engn, Singapore 639798, Singapore
[2] Xiangtan Univ, Key Lab Quantum Engn & Micronano Energy Technol, Sch Mat Optoelect & Phys, Xiangtan 411105, Peoples R China
[3] Nanyang Technol Univ, Sch Mat Sci & Engn, Singapore 639798, Singapore