Exploring the Role of Flexoelectric Effect in Band Modulation in 1D MoS₂/Boron Phosphide Nanotube Heterostructures

Designing and discovering superior type-II band alignment are crucial for advancing optoelectronic device technologies. Here, we employ first-principles calculations to investigate the evolution of band edges in monolayer MoS₂, boron phosphide (BP), and MoS₂/BP heterostructures before and after their rolling into nanotubes. Our research results indicate that the intrinsic MoS₂/BP vertical heterostructures exhibit a type-II direct bandgap, but this feature is not robust under strain. For MoS₂/BP coaxial heterotubes, the type of bandgap is influenced by both chirality and diameter. Specifically, when the diameter exceeds 19 Å under zigzag chirality, the system undergoes a transition from a type-I direct bandgap to a type-II direct bandgap, which remains stable within a strain range of -6 to 6%. Furthermore, we delve into the alterations in band edge positions in single-walled nanotubes induced by curvature-driven flexoelectric effects and circumferential tensile strain. In coaxial heterotubes, the transfer of electrons between the inner and outer tubes forms a cylindrical capacitor-like structure. Incorporating the inherent flexoelectric voltage in single-walled nanotubes, we have derived a functional relationship between the counteracting voltage (V_hyb) and their diameter. Finally, the system was explored for its strong light absorption capabilities with absorption levels up to 10⁵, and it was found that strain can effectively modulate the range of light absorption. The findings of this research contribute to new insights and theoretical foundations for the development of novel one-dimensional (1D) van der Waals (vdW) optoelectronic devices.