Pristine Interlayer Coupling for Strain Engineering of WS2/WSe2 Nanosheet-Based van der Waals Heterostructures

Two-dimensional (2D) van der Waals heterostructures (vdw-HSs) have attracted extensive attention as a versatile platform for exploring fundamental physics and potential applications in nanoelectronics and optoelectronics in view of the different band alignments and the unique interlayer couplings. In particular, strain engineering has become an effective approach to modulate the electronic structures and interlayer coupling of nanosheet-based vdw-HSs. However, there are very few works that concern the effect of the pristine coupling strength of the HSs on the process of strain engineering. Here, we demonstrate a comprehensive investigation of the strain engineering on the nanoscale for a bilayer WS2/WSe2 vertical HS containing certain regions with different coupling strengths through Raman and photoluminescence spectroscopy. We compare the responses of intralayer excitons to tensile strain between the HSs and their constituent monolayers. The obtained results demonstrate that the original coupling strength of the HS can significantly affect the electronic band structure evolution under the same strain which mainly depends on whether the coupling strength of the HS is further enhanced during the strain process. Meanwhile, we also pay attention to the evolution of interlayer excitons at different coupling regions under tensile strain. A direct-to-indirect transition process for interlayer exciton is observed at the weak-coupling region of the HS but lost at the strong-coupling region, which further confirms that the strain engineering of the HS can be impacted by its original coupling levels. Our findings provide an intensive understanding of the strain engineering of the nanosheet-based vdwHSs and broaden the technological innovation of nanodevices, such as flexible photodetectors and exciton-emitting devices.