A bismuth triiodide monosheet on Bi₂Se₃(0001)

A stable BiI₃ monosheet has been grown for the first time on the (0001) surface of the topological insulator Bi₂Se₃ as confirmed by scanning tunnelling microscopy, surface X-ray diffraction, and X-ray photoemision spectroscopy. BiI₃ is deposited by molecular beam epitaxy from the crystalline BiTeI precursor that undergoes decomposition sublimation. The key fragment of the bulk BiI₃ structure, [Formula: see text][I-Bi-I] layer of edge-sharing BiI₆ octahedra, is preserved in the ultra-thin film limit, but exhibits large atomic relaxations. The stacking sequence of the trilayers and alternations of the Bi-I distances in the monosheet are the same as in the bulk BiI₃ structure. Momentum resolved photoemission spectroscopy indicates a direct band gap of 1.2 eV. The Dirac surface state is completely destroyed and a new flat band appears in the band gap of the BiI₃ film that could be interpreted as an interface state.