First-principles study of size- and edge-dependent properties of MXene nanoribbons

We present results and analyses from first-principles calculations aimed at exploring the size- and edge-dependent properties of a wide range of MXene nanoribbons cut from two-dimensional (2D) semiconducting MXenes. The nanoribbons are classified by their edge types (armchair versus zigzag), the composition and sequencing of the terminating atomic lines, and the lowest-energy structural models of their 2D counterparts. The semiconducting versus metallic nature of the nanoribbons is well explained using an electron counting rule for the edge dangling bonds. For semiconducting nanoribbons, the band-gap evolution as a function of ribbon size is shown to be dependent on the lowest-energy structural model, and determined by a combination of factors such as quantum confinement, the energetic location of the edge states, and the strength of the $d\text{\ensuremath{-}}d$ hybridization. Nanoribbons cut from 2D MXenes with asymmetric surfaces are found to have bent ground-state structures with curvatures increasing as the size of the ribbon decreases.