Adatom

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. ′′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. If crystallography is used, or if the growth temperatures are higher, which would give an entropy effect, the crystal surface becomes rough, causing greater number of kinks. This means that adatoms have a greater chance of arriving at a kink site, to become part of the crystal.