To calculate the band gap and optical absorption spectrum of a graphene monolayer and a MoS2 monolayer, we can use density functional theory DFT calculations. DFT is a widely used quantum chemical method to study the electronic structure of materials.Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. It is a zero-gap semiconductor, meaning that its conduction and valence bands touch each other at the Dirac points. As a result, the band gap of graphene is essentially zero. Due to this property, graphene exhibits very weak optical absorption, which is around 2.3% for visible light.On the other hand, MoS2 is a layered material composed of molybdenum Mo and sulfur S atoms. In its monolayer form, MoS2 has a direct band gap of about 1.8 to 2.0 eV, depending on the calculation method and experimental conditions. This band gap falls within the visible light range, allowing MoS2 to absorb a significant portion of the visible light spectrum. The optical absorption spectrum of monolayer MoS2 shows strong peaks corresponding to excitonic transitions, which are a result of the strong Coulomb interaction between electrons and holes in this material.Comparing the two materials, it is clear that MoS2 has a more suitable band gap and optical absorption properties for optoelectronic devices than graphene. The nonzero band gap of MoS2 allows for efficient light absorption and emission, making it a promising candidate for applications such as photodetectors, solar cells, and light-emitting diodes LEDs . In contrast, the zero band gap of graphene limits its potential for optoelectronic applications, as it cannot efficiently absorb or emit light.In conclusion, based on the calculated band gap and optical absorption spectrum, MoS2 monolayer is more suitable for optoelectronic devices than a graphene monolayer.