Large Area Van der Waals MoS₂-WS₂ Heterostructures for Visible-Light Photocatalysis and Energy Conversion

Two-dimensional Transition Metal Dichalcogenides semiconductors (2D-TMDs) have recently collected a strong scientific and technological interest over a broad community due to their peculiar optoelectronic response within a broadband spectrum, and to their tunable bandgap that enables promising optoelectronic and photonic functionalities. Additionally, the superior optical absorption coefficients in the Visible spectrum, enhanced by about two orders of magnitude with respect to conventional bulk semiconductors, qualify TMDs as optimal candidates for photodetection and photoconversion applications [1].<br/>Owing to their atomically smooth surfaces and fully saturated in-plane chemical bonds, an opportunity to further engineer the optoelectronic response of a 2D device is offered by the combination of two different TMDs layers to form a van der Waals heterostructure, in which new optoelectronic properties arise from the band structure coupling of the junction [2].<br/>Here, we demonstrate homogeneous growth of large area 2D-TMDs films by means of a physical deposition process based on Ion Beam Sputtering, followed by high temperature recrystallization in a sulfur enriched atmosphere [3,4]. The fabrication process has been subsequently optimized for the sequential synthesis of vertically stacked large area MoS₂-WS₂ van der Waals heterostructures supported on a large area graphene layer which acts as a transparent conductive window [5]. Vertical stacking of MoS₂-WS₂ heterostructure ensures a type-II staggered band alignment which in turn allows to split excitons across the junction, thus increasing the lifetime of the photogenerated carriers compared to the individual TMD components. Charge separation can be exploited both for energy conversion, since recombination rate is reduced, and for photocatalytic applications, since the separated charges on the surface can boost specific reactions. This is reflected in the superior photodissociation rate of molecular dye probes in solution near the heterostructure sample, and in the photovoltage and photocurrent measured upon illumination of the large area device.<br/>These results show that TMDs van der Waals heterostructures represent optimal building blocks for the fabrication of self-powered photodetectors, by exploiting the charge separation of the photogenerated carriers at the 2D interface without any external bias.<br/><br/>[1] M. Bhatnagar et al., ACS Appl. Mater. Interfaces 2021, 13, 13508−13516<br/>[2] D. Jariwala et al., ACS Photonics 2017, 4, 12, 2962–2970<br/>[3] M.C. Giordano et al., Adv. Mater. Interfaces 2022, 2201408<br/>[4] M. Gardella et al., “Maskless synthesis of van der Waals heterostructure arrays engineered for light harvesting on large-area templates”, under submission<br/>[5] M. Gardella et al., “Large area van der Waals MoS2-WS2 heterostructures for visible-light energy conversion”, submitted at npj 2D materials and applications