Atomic-Scale Chemical Conversion of Single-Layer Transition Metal Dichalcogenides

Chemical conversion by atomic substitution offers a powerful route toward the creation of unusual structures and functionalities. Here, we demonstrate the progressive transformation of single-layer TiTe₂ into TiSe₂ by reaction with a Se flux in vacuum. Angle-resolved photoemission spectroscopy and scanning tunneling microscopy reveal intriguing reaction patterns involving TiSe₂ island ingrowth starting from the TiTe₂ island edges, while the band structure and core level signatures of TiSe₂ grow in intensity at the expense of those corresponding to TiTe₂. Lattice mismatch between TiTe₂ and TiSe₂ results in misfit holes and lattice distortions over a distance behind a seamless fingerlike reaction front. The regions of TiSe₂ and TiTe₂ are distinguished by a height difference and a charge density wave (CDW) at different transition temperatures. The method of in situ chemical conversion offers opportunities for atomic-scale engineering of layered transition metal dichalcogenides that host useful properties arising from CDW, Dirac, Weyl, superconducting, spin-valley, and magnetic structures.