Physics Tomorrow Letters

Introduction

The discovery of van der Waals (vdW) materials, including two-dimensional (2D) transition metal dichalcogenides (TMDs) and graphene, has brought about a revolution in the assembly of artificial heterostructures by allowing for the combination of two different materials without the constraints of lattice matching. Such an unprecedented level of flexibility in heterostructure design has led to the emergence of novel properties not seen in individual materials. A prime example is twisted bilayer graphene at magic angles, which exhibits exotic phases such as correlated insulating states [1] and superconductivity [2]. Another notable development is the stacking of two TMDs, resulting in the observation of unique excitons known as interlayer excitons, characterized by electrons and holes located in separate layers [3–6]. The spatially indirect nature of interlayer excitons imparts them with distinct properties, including long exciton lifetimes [3], extended diffusion lengths [7], large valley polarization [8], and significant modulation by moir´e potentials [9, 10]. The existing vdW heterostructures comprise of 2D materials with similar lattice structure, excitonic characteristics, and inherently identical dimensions. Development of vdW heterostructures that encompass lower dimen- sional materials may give rise to unique interface exciton states resulting from the mixed dimensionality. Carbon nanotubes (CNTs), a typical one-dimensional (1D) material, are ideal for such heterostructures as they have all bonds confined to the tube itself [11, 12]. CNTs interact with 2D materials through weak vdW forces, resulting in well-defined, atomically sharp interfaces [13, 14]. The chirality-dependent bandgap of CNTs can be utilized to tune the band alignment [15], allowing for unambiguous identification of excitonic states at the 1D-2D interface.