Exploring the Role\nof Flexoelectric Effect in Band\nModulation in 1D MoS<sub>2</sub>/Boron Phosphide Nanotube Heterostructures

Designing and discovering superior type-II band alignment\nare crucial\nfor advancing optoelectronic device technologies. Here, we employ\nfirst-principles calculations to investigate the evolution of band\nedges in monolayer MoS<sub>2</sub>, boron phosphide (BP), and MoS<sub>2</sub>/BP heterostructures before and after their rolling into nanotubes.\nOur research results indicate that the intrinsic MoS<sub>2</sub>/BP\nvertical heterostructures exhibit a type-II direct bandgap, but this\nfeature is not robust under strain. For MoS<sub>2</sub>/BP coaxial\nheterotubes, the type of bandgap is influenced by both chirality and\ndiameter. Specifically, when the diameter exceeds 19 Å under\nzigzag chirality, the system undergoes a transition from a type-I\ndirect bandgap to a type-II direct bandgap, which remains stable within\na strain range of −6 to 6%. Furthermore, we delve into the\nalterations in band edge positions in single-walled nanotubes induced\nby curvature-driven flexoelectric effects and circumferential tensile\nstrain. In coaxial heterotubes, the transfer of electrons between\nthe inner and outer tubes forms a cylindrical capacitor-like structure.\nIncorporating the inherent flexoelectric voltage in single-walled\nnanotubes, we have derived a functional relationship between the counteracting\nvoltage (<i>V</i><sub>hyb</sub>) and their diameter. Finally,\nthe system was explored for its strong light absorption capabilities\nwith absorption levels up to 10<sup>5</sup>, and it was found that\nstrain can effectively modulate the range of light absorption. The\nfindings of this research contribute to new insights and theoretical\nfoundations for the development of novel one-dimensional (1D) van\nder Waals (vdW) optoelectronic devices.