Rationalizing Linear and Nonlinear Optical Properties of Few-Layer Graphene–WSe₂ Heterostructures

The electronic structure and properties of 2D–2D interfaces formed by monolayer (1L) and bilayer (2L) graphene (G) and tungsten diselenide (WSe2) materials are probed within the framework of state-of-the-art density functional theory (DFT)-based methods. The electronic structure theory calculations untangle the intricate photophysics of G-supported WSe2 heterostructures influenced by crucial factors, such as intra- and interlayer coupling, symmetry breaking, and spin–orbit coupling (SOC). The charge transport from G to WSe2 layers creates an interface dipole, and the interlayer coupling breaks the centrosymmetric environment of the pristine 2L materials, which may contribute to enhancing linear and nonlinear optical responses. The extent of sublattice symmetry breaking is more pronounced for heterostructures comprising 2L G with AB (Bernal) stacking, leading to the band gap opening of the G layer on the order of 15–24 meV. As revealed by the calculated optical functions employing DFT combined with the Kubo–Greenwood formalism, attaching 2L WSe2 to 2L G results in stronger absorption over a wider spectrum of wavelengths. The second-harmonic generation (SHG) signal is intensified for the 2L G-supported WSe2 heterostructures and impacted by the extent of inversion symmetry breaking, the SOC effect, and the stacking order of G layers. The G-WSe2 heterostructures are predicted to enable more effective charge separation from excitons compared to pristine materials. The estimation of photoresponsivity and external quantum efficiency for a heterojunction G-WSe2 p-n diode further confirms the efficacy of the G-WSe2 heterostructure in photodetectors, especially for the detection of high-energy photons in the ultraviolet spectrum. The acquired information is highly beneficial in designing functional 2D–2D heterostructures as an alternative to traditional metal–semiconductor interfaces for its application in electronic devices such as field-effect transistors.