monolayer

A monolayer is a layer that is one molecule thick, covering a surface or an interface. In chemistry and materials science, monolayers can form on solid substrates, at gas–liquid interfaces, or between immiscible liquids. The defining feature is thickness on the order of a single molecule or atom, yielding a continuous film that is the thinnest possible for that material. Monolayers may be organic, inorganic, or elemental; a well-known example in materials science is the graphene monolayer, a single layer of carbon atoms that serves as the fundamental two-dimensional form of carbon. Another classic example is a Langmuir monolayer, composed of amphiphilic molecules at the air–water interface, whose surface pressure can be controlled and transferred to substrates via Langmuir–Blodgett deposition.

Monolayers can arise from self-assembly, adsorption, or deposition processes, and their properties depend strongly on the molecular structure and substrate. Because the thickness is fixed, properties such as conductivity, optical response, or barrier behavior can differ markedly from bulk materials. They are widely used in sensors, protective coatings, corrosion resistance, catalysis, energy storage, and nanoelectronics, as well as in biological research to model cell membranes with lipid monolayers.