This work was supported by MOE under AcRF Tier 1 (M4011782.070 RG4/17 and M4011993.070 RG7/18), AcRF Tier 2 (2015-T2-2-007, 2016-T2-1-131, 2016-T2-2-153 and 2017-T2-2-136) and AcRF Tier 3 (2018-T3-1-002), and A*Star QTE programme. This work was also supported by MOE under AcRF Tier 2 (2015-T2-2-057, 2016-T2-2-103, 2017-T2-1-162) and AcRF Tier 1 (2016-T1-002-051, 2017-T1-001-150, 2017-T1-002-119), and NTU under Start-Up Grant (M4081296.070.500000) in Singapore. H.Z. thanks the support from ITC via Hong Kong Branch of National Precious Metals Material Engineering Research Center, and the Start-Up Grant from City University of Hong Kong. We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy facilities. Q.J.W. acknowledges the support of the Ministry of Education- Singapore grant (MOE2016-T2-1-128) and National Research Foundation-Competitive Research Program (NRF-CRP18-2017-02). Z.W.S. acknowledges the support of the Institute of Materials Research and Engineering, A*STAR (IMRE/17-1R1211).