Enhanced nonlinear saturable absorption from Type III van der Waals heterostructure Bi2S3/MoS2 by interlayer electron transition

Abstract

Nonlinear optical properties of van der Waals heterostructures (vdWHs) are important for new-generation nanophotonics and integrating photonics. However, type-III vdWHs are extremely neglected in the nonlinear optical research. Herein, MoS2 and Bi2S3 are proposed to form mixed vdWHs by liquid-phase exfoliation method combined with a vacuum filtration technique. Type-III vdWHs Bi2S3/MoS2 is identified and the band alignment is confirmed by XPS and UV–Vis absorption spectra successfully. The nonlinear responses of the vdWHs nanofilms given by Z-scan method indicate that the saturable absorption response of Bi2S3/MoS2 is enhanced obviously compared with that of pure Bi2S3 and MoS2. Furthermore, an energy level model is proposed and the results suggest that the quick interlayer electron transition from the conduction band of Bi2S3 to the valence band of MoS2 plays a crucial role for the observed enhancement. This interlayer electron transition is driven by an oriented built-in electric field due to the band-to-band tunneling transition in type-III vdWHs. Our results make clear the mechanism of type-III heterostructures in nonlinear optical process and pave the way for applications of type-III vdWHs in nonlinear optoelectronics.