adlayer

An adlayer is a layer of atoms or molecules that are adsorbed onto a solid surface, forming a two-dimensional film on the substrate. The term is used in surface science and catalysis to distinguish the adsorbed overlayer from the underlying substrate. Adlayers may be composed of individual atoms (adatoms) or small molecules and are typically described by their coverage, often expressed as a fraction of a monolayer.

Adlayers can be physisorbed, held by weak van der Waals forces, or chemisorbed, involving chemical bonds to surface atoms. They can range from sub-monolayer to multiple layers, though a true adlayer usually refers to at least one complete adsorbed layer on the surface. The structural arrangement may be commensurate with the substrate lattice or incommensurate, and adsorbates can induce reconstruction of the surface.

Formation occurs by adsorption from gas or liquid phases and is governed by temperature, pressure, and the chemical potential of the adsorbate. The properties of the adlayer—such as stability, binding energy, and electronic influence—determine changes in surface reactivity, work function, friction, and optical response.

Adlayers play a central role in catalysis research, corrosion protection, lubrication, and the formation of self-assembled monolayers on metals. Common examples include hydrogen or carbon monoxide adlayers on platinum group surfaces, water adlayers on oxide or metal surfaces, and organic SAMs on gold or silicon interfaces.

Techniques to study adlayers include LEED, STM, AFM, XPS, IRRAS, and thermal desorption spectroscopy, among others. Understanding adlayers helps interpret surface reactions and engineer interfaces in sensors, electronics, and energy devices.