[1] Guangxi Univ, Ctr Nanoenergy Res, Sch Phys Sci & Technol,Guangxi Key Lab Relat Astr, Guangxi Key Lab Proc Nonferrous Met & Featured Ma, Nanning 530004, Peoples R China
[2] Basque Ctr Mat Applicat & Nanostruct, UPV EHU Sci Pk,Barrio Sarriena S-N, Leioa 48940, Spain
[3] Hong Kong Polytech Univ, Dept Appl Phys, Kowloon, Hong Kong, Peoples R China
[4] Natl Univ Singapore, Dept Mech Engn, Singapore 117575, Singapore
[5] Nanjing Univ Posts & Telecommun, Coll Elect & Opt Engn, Nanjing 210026, Peoples R China
[6] Nanjing Univ Posts & Telecommun, Coll Microelect, Nanjing 210026, Peoples R China
[7] Guilin Univ Technol, Guangxi Key Lab Opt & Elect Mat & Devices, Guilin 541004, Peoples R China
[8] Georgia Inst Technol, Sch Mat Sci & Engn, Atlanta, GA 30332 USA