A type-I van der Waals heterostructure formed by monolayer WS2 and trilayer PdSe2

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.