Chemisorption of ammonia molecules on ytterbium nanofilms deposited on silicon Si(111) at room temperature

The interaction of ammonia molecules with the surface of ytterbium nanofilms of uniform thickness grown on a Si(111) 7 $\times$ 7 substrate and forming a chemically sharp interface with it has been investigated. It has been found that this interaction depends significantly on the thickness of nanofilms. For a film thickness equal to 5 monoatomic layers, chemisorption of ammonia has an essentially nondissociative character and the molecules form a donor-acceptor bond with ytterbium; as a result, the metal transforms from the divalent state into a new nonautonomous trivalent state. For other film thicknesses, the adsorbed molecules dissociate, nitrogen atoms diffuse into the bulk of the film and enter into the reaction with silicon, and ytterbium remains in the initial divalent state. It has been shown that the character of the interaction of ammonia molecules with ytterbium nanofilms and the stability of the NH$_3$–Yb–Si(111) structures are determined by the standing waves of electron density (Friedel oscillations) generated by the interface between the metal and silicon substrate.