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Monolayer 1T' WSe₂ has recently been predicted to be a two-dimensional topological insulator (2D TI) with a larger bulk bandgap than 1T' WTe₂, more preferable for practical applications. However, the energetically stable structure of WSe₂ is the 2H phase, and hence the 1T' WSe₂ doesn’t exist in the bulk. Here, we report the molecular beam epitaxial growth of monolayer 1T' WSe₂ on epitaxial graphene/SiC(0001). We find the growth of homogeneous monolayer WSe₂ is strongly dependent on the substrate temperature: while the 1T' phase can be grown below 300, a mixed 1T’ and 1H is obtained with increasing substrate temperature, with a complete 1H phase formation above 400^oC. Using scanning tunneling microscopy/spectroscopy and angle resolved photoemission spectroscopy, we determine a bulk bandgap of 130 meV in monolayer 1T' WSe₂, almost twice that of 1T' WTe₂ (70 meV). We further observe one-dimensional edge states within the bulk bandgap in tunneling spectroscopy, confirming that monolayer 1T' WSe₂ is indeed a 2D TI.
This research is supported by NSF (DMR-1734017).