82 Sur les Différents Types de Liaisons lors de l'Adsorption Chimique sur des Semi-Conducteurs

Abstract

One of the characteristics of chemisorption is that it permits the formation of different types of bonds between a given adsorbed species and the same adsorbent. Thus, an atom can be attached to an ionic crystal by a “weak” covalent bond, a “strong” covalent bond, or an ionic bond. The first is characterized by a localized electron and an induced dipole moment that may be larger by several orders of magnitude than the moment due to physical adsorption. When bonding is augmented by a free electron from the crystal lattice, the adsorbed atom (in the case of monovalent electropositive atom) is held by a “strong” covalent bond. On the other hand, localization of a hole near a weakly adsorbed atom leads to the formation of an ionic bond. Thus the same atom can represent an acceptor or a donor at the same time. Whereas the type of bonding in normal compounds is determined by the nature of the reactants, the types and relative amounts of adsorption bonds on semiconductors are governed by the presence of free electrons and holes, by the concentration of adsorbate on the surface, and by the nature and concentration of impurities in the bulk of the crystal. Furthermore, bonding of an adsorbate can change from one type to another. The chemical potential determines the equilibrium concentration of each type. As the potential changes from high values (conduction band) to low values (valence band), the fraction of electron-bound atoms decreases, that of hole-bound atoms increases, and that of electrically neutral ones passes through a maximum. The value of the potential depends on the surface coverage, and the number of adsorbed atoms thus determines the equilibrium and the surface charge. This change of bonding character after adsorption is not predicted by the surface layer theories of Aigrain and Dugas, Weiz, Germain, Engel, and Hauffe, who considered only ionic adsorption. As a first approximation, one may assume that only weakly bonded atoms can enter into chemical reaction, because only such atoms have an unsaturated valence. Their relative concentration is affected by impurities that change the chemical potential, the so-called catalyst promoters and poisons. This is another consequence not foreseen by the above theories.