Interfacial excitons across dimensional boundaries: a mixed-dimensional SnS/BNNT heterostructure.

Interfacial excitons are commonly associated with type-II two-dimensional (2D) van der Waals (vdW) heterostructures, while their behavior in mixed-dimensional systems remains largely unexplored. In this letter, we demonstrate the formation of an interdimensional exciton (IDE) in a SnS/BNNT mixed-dimensional heterostructure (MDH), using many-body perturbation theory within the GW approximation combined with the Bethe-Salpeter equation (GW + BSE). The IDE arises from a Γ-point transition in which an electron is excited from the 2D valence band of SnS to the confined one-dimensional (1D) conduction band of the BNNT. Owing to enhanced quantum confinement in the nanotube, the conduction band minimum exhibits a pronounced asymmetric E-k dispersion, resulting in a highly anisotropic, direction-dependent exciton effective mass. These results establish a microscopic framework for understanding excitonic behavior in mixed-dimensional vdW heterostructures and highlight their potential for anisotropic optoelectronic applications.