Films

Platinum ditelluride (PtTe₂), a type-II Dirac semimetal in the bulk form, is made of Te-Pt-Te tri-atomic layers (TLs) loosely bonded together by van der Waals interactions. It remains semimetallic in ultrathin films down to just two TLs, but a further reduction of the film thickness to just a single TL induces a Lifshitz electronic transition to a semiconductor with a sizable gap. This transition is evidenced by experimental mapping of the band structure by angle-resolved photoemission spectroscopy for films of various thicknesses prepared by molecular beam epitaxy on a bilayer-graphene-terminated SiC substrate. Layer-by-layer evolution of the band structure is well resolved, which facilitates absolute layer counting. The measured semiconducting band structure for the single TL is in excellent agreement with theoretical calculations. Our results demonstrate a novel electronic transition at the single-layer, or two-dimensional limit through film thickness control.