Enhanced interlayer coupling and efficient photodetection response of in-situ grown MoS₂-WS₂ van der Waals heterostructures

Currently, 2D layered material (2DLM) based heterostructures, also known as van der Waals (vdW) heterostructures, are actively pursued owing to their great potential for optoelectronic applications. They are produced either by vertical stacking of individual layers or via in-plane stitching of 2DLMs. Producing vdW heterostructures with clean interfaces and growing them using direct growth methods are challenging. Herein, we report successful growth of large-area MoS₂-WS₂ vdW heterostructures (single- to few-monolayer, ML, the thickness of MoS₂ and WS₂) on oxidized Si (100) substrates using the catalyst-free Pulsed Laser Deposition technique. The in-plane (E¹_2g) and out-of-plane (A_1g) Raman-active phonon modes are used to probe the interlayer interactions between the constituent 2D layers. We observe a blueshift of 4.73 (1.63) cm⁻¹ of the A_1g peak corresponding to MoS₂ (WS₂) in the MoS₂(1ML)-WS₂(1ML) heterostructure compared to 1.27 (0.88) cm⁻¹ for the homo-bilayers. The E¹_2g mode also exhibited blueshift for the heterostructure and redshift for the bilayer of the constituent material. We show that the broadband photodetectors fabricated utilizing in situ grown MoS₂-WS₂ heterostructures exhibit responsivity, specific detectivity, and current on/off ratio as high as 2.51 × 10⁵ A/W, 4.20 × 10¹⁴ Jones, and 1.05 × 10⁵, respectively, under 24 μW/cm² at 405 nm excitation. The successful fabrication of vdW heterostructures using a simple and scalable direct growth method and excellent photodetector performance pave the way for exploitation of their application potential and offer a playground to test some of the theoretical predictions.