Effects of post-transfer annealing and substrate interactions on the photoluminescence of 2D/3D monolayer WS2/Ge heterostructures

The ultraflat and dangling bond-free features of two-dimensional (2D) transition metal dichalcogenides (TMDs) endow them with great potential to be integrated with arbitrary three-dimensional (3D) substrates, forming mixed-dimensional 2D/3D heterostructures. As examples, 2D/3D heterostructures based on monolayer TMDs (e.g., WS2) and bulk germanium (Ge) have become emerging candidates for optoelectronic applications, such as ultrasensitive photodetectors that are capable of detecting broadband light from the mid-infrared (IR) to visible range. Currently, the study of WS2/Ge(100) heterostructures is in its infancy and it remains largely unexplored how sample preparation conditions and different substrates affect their photoluminescence (PL) and other optoelectronic properties. In this report, we investigated the PL quenching effect in monolayer WS2/Ge heterostructures prepared via a wet transfer process, and employed PL spectroscopy and atomic force microscopy (AFM) to demonstrate that post-transfer low-pressure annealing improves the interface quality and homogenizes the PL signal. We further studied and compared the temperature-dependent PL emissions of WS2/Ge with those of as-grown WS2 and WS2/graphene/Ge heterostructures. The results demonstrate that the integration of WS2 on Ge significantly quenches the PL intensity (from room temperature down to 80 K), and the PL quenching effect becomes even more prominent in WS2/graphene/Ge heterostructures, which is likely due to synergistic PL quenching effects induced by graphene and Ge. Density functional theory (DFT) and Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations show that the interaction of WS2 and Ge is stronger than in adjacent layers of bulk WS2, thus changing the electronic band structure and making the direct band gap of monolayer WS2 less accessible. By understanding the impact of post-transfer annealing and substrate interactions on the optical properties of monolayer TMD/Ge heterostructures, this study contributes to the exploration of the processing-properties relationship and may guide the future design and fabrication of optoelectronic devices based on 2D/3D heterostructures of TMDs/Ge.