A stable BiI 3 monosheet has been grown for the first time on the (0001) surface of the topological insulator Bi 2 Se 3 as confirmed by scanning tunnelling microscopy, surface X-ray diffraction, and X-ray photoemision spectroscopy. BiI 3 is deposited by molecular beam epitaxy from the crystalline BiTeI precursor that undergoes decomposition sublimation. The key fragment of the bulk BiI 3 structure, a∞2[I—Bi—I] layer of edge-sharing BiI 6 octahedra, is preserved in the ultra-thin film limit, but

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... 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 BiI 3 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 BiI 3 film that could be interpreted as an interface state.