Thickness-dependent excitonic properties of WSe/FePS van der Waals heterostructures.

van der Waals heterostructures (vdWHs), with their flexible combination of various two-dimensional (2D) materials, are continuously revealing new physics and functionalities. 2D magnetic materials have recently become a focus due to their fascinating electronic and spintronic properties. However, there has rarely been any investigation of the optical properties of 2D magnetic materials-based heterostructures. Herein, we construct a new WSe/FePS heterostructure, in which WSe works as a "sensor" to visualize the thickness-dependent properties of FePS. As characterized by photoluminescence (PL) spectra, whether under or on top of the FePS, the PL intensity of the monolayer WSe is strongly quenched. The quenching effect becomes more obvious as the FePS thickness increases. This is because of the efficient charge transfer process occurring at the WSe/FePS interface with type II band alignment, which is faster for thicker FePS, as is evident from transient absorption measurements. The thickness-dependent charge transfer process and corresponding excitonic properties are further revealed in low-temperature photoluminescence spectra of WSe/FePS heterostructures. Our results show that the thickness of 2D magnetic materials can work as an experimental tuning knob to manipulate the optical performance of conventional 2D semiconductors, endowing van der Waals heterostructures with more unexpected properties and functionalities.