Unexpected 4 × 4 Charge Density Wave in 1H-NbTe2 Monolayer

Abstract

Transition metal ditellurides exhibit structural and electronic properties distinct from their selenide and sulfide counterparts, including the stabilization of multiple charge density wave (CDW) phases. Within the niobium compound family, notable differences in structure are also observed: bulk NbTe₂ crystallizes in the 1T phase, distinguishing it from NbSe₂ and NbS₂, which typically adopt both the 2H and 1T structures. However, in this work, we report the unexpectedly successful synthesis of two-dimensional (2D) islands of the metastable 1H-NbTe₂ phase via molecular beam epitaxy on epitaxial graphene supported on Ir(111). The phase is identified by its characteristic hexagonal lattice in combination with distinct spectroscopic features and the presence of mirror twin boundaries. A comprehensive analysis combining scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations confirms the metallic character and structural stability of the 1H-NbTe₂ phase. Furthermore, low-temperature STS measurements reveal the formation of a 4 × 4 CDW modulation, which is explained based on a phonon instability. These findings motivate the exploration of the complex phase behavior and electronic properties of NbTe₂ in the 2D limit.