This work was supported by the Singapore National Research Foundation under NRF award number NRF-NRFF2013-08, Tier 2 MOE2016-T2-2-153, MOE2016-T2-1-131, MOE2015-T2-2-007, Tier 1 RG164/15, Tier 1 RG4/17, CoE Industry Collaboration Grant WINTECH-NTU and the A*Star QTE programme. T.Y. acknowledges MOE Tier 1 RG100/15. J.L. and K.S. acknowledge the financial support of JST-ACCEL and JSPS KAKENHI (JP16H06333 and P16382). The work in SICCAS was supported by the National Key Research and Development Program of China (2016YFB0700204) and the National Natural Science Foundation of China (51502327). The work at IOP was supported by the Ministry of Science and Technology of China (grant numbers 2014CB920904, 2015CB921101 and 2016YFA0300600), the National Natural Science Foundation of China (grant numbers 11174340, 912212012, 11527806 and 91421303) and the Chinese Academy of Sciences (grant numbers XDB07010100). H.L. acknowledges the Singapore National Research Foundation for support under NRF award number NRF-NRFF2013-03. The work at Rice was supported by the US Department of Energy (DE-SC0012547) and by the R. Welch Foundation (C-1590).

Reviewer information

Nature thanks D. Akinwande, M. Terrones and the other anonymous reviewer(s) for their contribution to the peer review of this work.