Overlayers

Overlayers are thin film materials deposited on a solid surface, forming a distinct layer on top of the substrate. They typically range from sub-nanometer to a few nanometers in thickness and can be crystalline, polycrystalline, or amorphous. Overlayers may be epitaxial, matching the substrate lattice, or non-epitaxial, with arbitrary orientation.

Growth depends on interfacial energies, lattice mismatch, temperature, and deposition environment. Common growth modes include layer-by-layer growth (Frank-van der Merwe), island growth (Volmer-Weber), and mixed growth with an initial layer followed by islands (Stranski-Krastanov). Strain from lattice mismatch can induce intermixing, relaxation, or roughening, influencing stability and properties.

Overlayers are deposited by various techniques, such as physical vapor deposition, chemical vapor deposition, atomic layer deposition, and electrochemical deposition. Atomic layer deposition, in particular, can produce conformal, ultrathin overlayers with precise thickness control, while PVD and CVD are used for metals, oxides, and organic films. Substrate preparation affects adhesion and uniformity.

Characterization methods assess thickness, composition, structure, and interfaces. X-ray photoelectron spectroscopy, low-energy electron diffraction, scanning probe microscopy, ellipsometry, and reflection high-energy electron diffraction are commonly employed. These tools help determine coverage, crystallography, roughness, and interfacial reactions that govern overlayer performance.

Overlayers have broad implications across catalysis, electronics, optics, and corrosion resistance. They can modify catalytic activity and selectivity, tune electronic or optical properties, protect surfaces from wear or corrosion, or serve as dielectric or barrier layers in devices. Examples include metal or oxide overlayers on catalysts, graphene or transition metal dichalcogenide layers on substrates, and polymer coatings.