The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor1,2,3,4. In this scheme, ‘on’ is the ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator5,6,7,8,9, and ‘off’ is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels. Such a topological transistor is promising for low-energy logic circuits4, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far6,7,8. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases3,10,11,12,13,14,15,16. Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal3,17 Na3Bi are two-dimensional topological insulators with bulk bandgaps greater than 300 millielectronvolts owing to quantum confinement in the absence of electric field. On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90 millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25 millielectronvolts), suggest that ultrathin Na3Bi is suitable for room-temperature topological transistor operation.
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The data that support the findings of this study are available from the corresponding author upon reasonable request.
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M.T.E. was supported by ARC DECRA fellowship DE160101157. M.T.E., J.L.C., C.L. and M.S.F. acknowledge funding support from CE170100039. J.L.C., J.H. and M.S.F. are supported by M.S.F.’s ARC Laureate Fellowship (FL120100038). S.A. acknowledges funding support from ARC Discovery Project DP150103837. M.T.E. and A.T. acknowledge travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron, part of ANSTO, and funded by the Australian Government. M.T.E. acknowledges funding from the Monash Centre for Atomically Thin Materials Research and Equipment Scheme. S.A.Y. and W.W. acknowledge funding from Singapore MOE AcRF Tier 2 (grant no. MOE2015-T2-2-144). S.A. acknowledges the National University of Singapore Young Investigator Award (R-607-000-094-133). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. Part of this research was undertaken on the soft X-ray beamline at the Australian Synchrotron, part of ANSTO. The authors acknowledge computational support from the National Supercomputing Centre, Singapore.