[1] Fudan Univ, State Key Lab Surface Phys, Shanghai 200433, Peoples R China
[2] Fudan Univ, Dept Phys, Shanghai 200433, Peoples R China
[3] Ningbo Univ, Dept Microelect Sci & Engn, Fac Sci, Ningbo 315211, Peoples R China
[4] Yunnan Univ, Sch Mat Sci & Engn, Kunming 650091, Yunnan, Peoples R China