thumb|Adatom according to the [[Terrace ledge kink|TLK model]]
An adatom is an atom that lies on a crystal surface, and can be thought of as the opposite of a surface vacancy. This term is used in surface chemistry and epitaxy, when describing single atoms lying on surfaces and surface roughness. The word is a portmanteau of "adsorbed atom". A single atom, a cluster of atoms, or a molecule or cluster of molecules may all be referred to by the general term "adparticle". This is often a thermodynamically unfavorable state. However, cases such as graphene may provide counter-examples.
Growth
″Adatom″ is a portmanteau word, short for adsorbed atom. When the atom arrives at a crystal surface, it is adsorbed by the periodic potential of the crystal, thus becoming an adatom. The minima of this potential form a network of adsorption sites on the surface. There are different types of adsorption sites. Each of these sites corresponds to a different structure of the surface. There are five different types of adsorption sites, which are: on a terrace, where the adsorption site is on top of the surface layer that is growing; at the step edge, which is next to the growing layer; in the kink of a growing layer; in the step edge of a growing layer, and in the surface layer, where the adsorption site is inside the lower layer.
Out of these adsorption site types, kink sites play the most important role in crystal growth. Kink density is a major factor of growth kinetics. Attachment of an atom to the kink site, or removal of the atom from the kink, does not change the free surface energy of the crystal, since the number of broken bonds does not change. This gives that the chemical potential of an atom in the kink site is equal to that of the crystal, which means that the kink site is the one adsorption site type where an adatom becomes a part of the crystal. In total there are five different types of layer growth: normal growth, step-flow growth, layer-by-layer growth, multilayer (or three-dimensional island) growth, and spiral growth.
The adatom can create more than one bond with the crystal, depending on the structure of the crystal. If it is a simple cubic lattice, the adatom can have up to 6 bonds, whereas in a face-centered cubic lattice, it can have up to 12 nearest neighbors. The more bonds created, the more energy is confined, making it harder to desorb the adatom.
A special site for an adatom is a kink, where exactly half of the bonds with the surface can be created, also called the "half-crystal position".
Magnetic adatoms
Adatoms, due to having fewer bonds than the other atoms in the crystal, have unbound electrons. These electrons have spin and therefore a magnetic moment. This magnetic moment has no preference for orientation until an external influence, like a magnetic field, is present. The structure of the adatoms on a surface can be adjusted by changing the external magnetic field. Through this method theoretical situations, such as the atomic chain, can be simulated. Quantum mechanics needs to be taken into account when using adatoms due to the small scale.
The magnetic field created by an atom is caused mostly by the orbit and spin of the electrons. The proton's and neutron's magnetic moment are negligible when compared to that of the electron due to their larger masses. When an atom with free electrons is inside an external magnetic field, its magnetic moment aligns with the external field because this lowers its energy. This is why bound electrons do not display this magnetic moment, they already have a favorable energy state and it is unfavorable to change. The magnetization of an (magnetically aligned) atom is given by:
:<math>M = \frac{N g_j^2 \mu_{\mathrm B}^2B}{3k_{\mathrm B}T}j(j+1)</math>
Where N is the number of electrons, g<sub>j</sub> is the g-factor, μ<sub>B</sub> is the Bohr magneton, k<sub>B</sub> is the Boltzmann constant, T is the temperature and j is the total angular momentum quantum number. This formula holds under the assumption that the magnetic energy of an electron is given by <math>E = m_j g_j \mu_{\mathrm B} B</math> and there is no exchange interaction.
Movement across a surface
The movement of adatoms across a surface can be described by the Burton–Cabrera–Frank (CBF) model by Keith Burton, Nicolás Cabrera and Charles Frank. The model treats adatoms as a 2D gas on top of the surface. The adatoms diffuse with a diffusion constant D; they are desorbed back to the medium above with a rate of <math>1/\tau_{\mathrm{des</math> per atom and adsorbed with flux F.
With the technology available nowadays it is possible to create a linear chain of adatoms on top of an epitaxial film. With this, one can analyse theoretical situations.
Furthermore, Usami et al. were able to create quantum wells by adding Si atoms to a SiGe bulk crystal. Within these wells they observed photoluminescence of excitons that were confined in these wells.