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[Display omitted] • DFT electronic and optical properties calculations were performed for MoS 2 -graphene-Au complexes. • Tunable optical properties of MoS 2 due to underlying graphene (Gr) in vertical vdWH are reported. • Decreased work function of MoS 2 /Gr suggested modulated electronic properties attributed to interlayer coupling albeit weaker. • Local photoconductivity confirmed MoS 2 /Gr heterojunction behavior, which boosts modern optoelectronic devices. We present a computational and experimental study for various heterostructures and heterointerfaces comprising two-dimensional (2D) molybdenum disulfide (MoS 2) and graphene (Gr) supported on gold (Au). These structures are useful for photo-electrochemical and nanophotonic applications, as well as for hydrogen generation. The frequency-dependent dielectric function, the refractive index, and the reflectivity are calculated using light polarization parallel and perpendicular to the respective monolayer c axis. We identify the transitions within MoS 2 and graphene, which correspond to peaks in the imaginary part of the frequency-dependent dielectric function. The Gr-MoS 2 dielectric function appears as a composition of the corresponding dielectric functions from the isolated monolayers with their peaks being shifted. However, for the Gr-MoS 2 -Au heterointerfaces, some of these peaks from the isolated monolayers are no longer detected. Charge transfers and work function calculations show that MoS 2 and graphene are n and p -type semiconductors, respectively, which agree with our experimental local photoconductivity measurements. The heterojunction behavior for Gr-MoS 2 is attributed to the interlayer electronic coupling, while minimizing Fermi level pinning at the MoS 2 /Au interface and charge transfers from graphene to MoS 2. Thus, interfacing MoS 2 with graphene signifies substrate engineering, allowing tunable MoS 2 physical properties for diverse applications across the electromagnetic spectrum. [ABSTRACT FROM AUTHOR]