A monolayer is a single, closely packed layer of entities, commonly atoms or molecules. Monolayers can also be made out of cells. Self-assembled monolayers form spontaneously on surfaces. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials.

Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS2.