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Molybdenum disulfide is a semiconducting compound with a black metallic luster found in nature in the mineral molybdenite. It is one of several TMDCs (transition metal dichalcogenides), which are compounds with the chemical formula MX2, where M is a transition metal and X is a chalcogen.
The crystal structure of molybdenum disulfide is made up of a hexagonal plane of S atoms on both sides of a hexagonal plane of Mo atoms, with strong covalent bonds holding layers together and weak van der Waals forces that allow layers to be mechanically separated. This allows for the formation of monolayers of MoS2 that exhibit different optical properties and are often used in a wide range of applications.
When monolayers of MoS2 are exposed to light, they exhibit a direct band gap. This means that photons can be absorbed without the need for electron tunneling, which significantly increases the photoluminescence (PL) efficiency. This effect is known as excitonic polarons, and it can be enhanced by doping the material with either electrical  or chemical  dopants. The resulting trions are red-shifted by approximately 40meV with respect to the A exciton peak (which can be tunable through doping concentration), and they exhibit nonlinear optical properties that make them very attractive for a variety of nonlinear optical applications, including gas sensors.
MoS2 has been used for a number of electronic, sensing and terahertz applications. However, many articles have not fully compared the different properties of bulk and monolayer MoS2. This article aims to highlight some key differences between these two forms of the material, particularly their optical properties.