A monolayer is a single-molecule thick film or layer of closely packed atoms or molecules at an interface.

Monolayers are often formed at the air-water interface as a Langmuir film. Monolayers are often located on a solid surface or floating on a gas-liquid interface. However, some monolayers can also be formed at a liquid-liquid interface.

Many materials can be used to form monolayers including lipids, nanoparticles, polymers and proteins to name a few. Monolayers are expected to be highly useful components in many practical and commercial applications such as sensors, detectors, displays and electronic circuit components. The possibility to synthesize organic molecules and develop new inorganic materials has enabled the production of electrical, optical and biological nano-components. 

Monolayer properties depend on many criteria including the material used, the film structure and the packing density of the monolayer. Langmuir film and Langmuir-Blodgett film deposition are techniques most used for the controlled study and fabrication of well organized monolayers at gas-liquid, liquid-liquid and gas-solid interfaces. Langmuir troughs are used to fabricate monolayers in a controlled way. Other methods such as Dip coating or spin-coating can be used but they do not enable as much control of the film structure and packing density.

Floating monolayers at the air-liquid interface (top). Monolayers on solids (bottom).