heterostructures probed by first-principles calculations and ultrafast laser spectroscopy*

2D layered materials have attracted significant scientific attention, as their 2D nature allows the stacking of different materials leading to a control of the functionality. However, a detailed understanding of the properties of such heterostructures is still under development. Here we compare the photocarrier dynamics of heterostructures of monolayer MoS₂ with monolayer or trilayer WSe₂ using first-principles modeling based on hybrid density functional theory and ultrafast laser spectroscopy. 

Our computational results show that the conduction-band minimum (CBM) at K in monolayer WSe₂ is higher in energy than the CBM of MoS₂. So when carriers are excited in WSe₂, electrons will transfer from WSe₂ to MoS₂. Secondary transfer processes to the Λ_min valley of MoS₂ are also possible, but those require momentum changes through phonons, and are therefore slower. The valence band maximum of WSe₂ is higher in energy than in MoS₂, so holes do not transfer to MoS₂. When a heterostructure with a trilayer MoS₂ is formed, the CBM in WSe₂ shifts to the Λ_min valley. Charge carriers excited at the K point therefore have more transfer pathways (including to the Λ_min valley in either MoS₂ and WSe₂), leading to interesting carrier dynamics as confirmed by ultrafast laser spectroscopy measurements.