Non-Additive Optical Response in Transition Metal Dichalcogenides Heterostructures.

Van der Waals (vdW) heterostructures pave the way to achieve the desired material properties for a variety of applications. In this way, new scientific and industrial challenges and fundamental questions arise. One of them is whether vdW materials preserve their original optical response when assembled in a heterostructure. Here, we resolve this issue for four exemplary monolayer heterostructures: MoS 2 /Gr, MoS 2 /hBN, WS 2 /Gr, and WS 2 /hBN. Through joint Raman, ellipsometry, and reflectance spectroscopies, we discovered that heterostructures alter MoS 2 and WS 2 optical constants. Furthermore, despite the similarity of MoS 2 and WS 2 monolayers, their behavior in heterostructures is markedly different. While MoS 2 has large changes, particularly above 3 eV, WS 2 experiences modest changes in optical constants. We also detected a transformation from dark into bright exciton for MoS 2 /Gr heterostructure. In summary, our findings provide clear evidence that the optical response of heterostructures is not the sum of optical properties of its constituents.