A type-I van der Waals heterostructure formed by monolayer WS ₂ and trilayer PdSe ₂

Two-dimensional (2D) heterostructures, formed by stacking 2D semiconductors through the van der Waals force, have been extensively studied recently. However, the majority of the heterostructures discovered so far possess type-II interfaces that facilitate interlayer charge separation. Type-I interfaces, on the other hand, confine both electrons and holes in one layer, which is beneficial for optical applications that utilize electron-hole radiative recombination. So far, only a few type-I 2D heterostructures have been achieved, which has limited the construction of multilayer heterostructures with sophisticated band landscapes. Here, we report experimental evidence of a type-I interface between monolayer WS₂ and trilayer PdSe₂. Two-dimensional PdSe₂ has emerged as a promising material for infrared optoelectronic and other applications. We fabricated the heterostructure by stacking an exfoliated monolayer WS₂ flake on top of a trilayer PdSe₂ film, synthesized by chemical vapor deposition. Photoluminescence spectroscopy measurements revealed that the WS₂ exciton peak is significantly quenched in the heterostructure, confirming efficient excitation transfer from WS₂ to PdSe₂. Femtosecond transient absorption measurements with various pump/probe configurations showed that both electrons and holes photoexcited in the WS₂ layer of the heterostructure can efficiently transfer to PdSe₂, while neither type of carriers excited in PdSe₂ can transfer to WS₂. These experimental findings establish a type-I band alignment between monolayer WS₂ and trilayer PdSe₂. Our results further highlight PdSe₂ as an important 2D material for constructing van der Waals heterostructures with emergent electronic and optoelectronic properties.