How Growing Conditions and Interfacial Oxygen Affect the Final Morphology of MgO/Ag(100) Films

Abstract

In spite of the relevance of ultrathin MgO films for the study of model systems as well as for technological applications, great difficulties have been found so far in the growth of extended, well-ordered, ultrathin films. Combining scanning tunneling microscopy, X-ray photoemission spectroscopy, and high-resolution electron energy loss spectroscopy experiments with ab initio calculations, we demonstrate here that the structure of sub-monolayer MgO films grown on Ag(100) by reactive deposition is strongly affected not only by the growth conditions but also by after-growth treatments. The latter ones allow one to quench the thermodynamically most stable configuration at the deposition temperature or let the system evolve toward the low-temperature equilibrium state. Moreover, we give experimental and theoretical evidence of the accumulation of oxygen atoms at the MgO/Ag interface at the highest deposition temperature, which reduces the stress of the oxide film favoring the formation of extended terraces. The result is the possibility to tune the morphology of the films from small islands with corrugated borders, to perfectly square islands of larger size, to MgO terraces several tens of nanometers wide

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen

“…The quality of the film is related to the accumulation of oxygen atoms at the MgO/Ag interface, which reduces the stress of the oxide layer favoring the formation of extended terraces. 21 A large, flat MgO monolayer is an ideal substrate for NP deposition, such as Ni clusters. We investigated this system with respect to cluster size and geometry by low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT) calculations, focusing our attention on the smallest NPs, up to six Ni atoms in size.…”

Morphology, Work Function, and Silver Ad-Structures of High-Temperature Grown Ultrathin MgO Films on Ag(001)

“…If we assume an amount of interface oxygen of 30%, as experimentally found in refs 13 and 14, theory predicts a WF reduction of around Δϕ MgOa−Ag = −500 meV for 1 and 2 ML thin films, 14 which is not comparable to our values from above; however, the concentration of the interface oxygen is unknown in our case, and if we assume a higher concentration of ∼50%, predicted WF differences on the order of |Δϕ MgO−Ag | ≈ 50 meV can be principally obtained. 14 Closed thin MgO Films. To check whether a closed atomic thin MgO film can perfectly cover the Ag(001) surface, 13 we increased the nominal quantity of MgO during the growth at 783 K. Because of the appearance of large silver objects on the surface (see below) the imaging in the STM mode has been shown to be more stable than in the nc-AFM mode, in particular when scanning wide regions of the surface.…”

A DFT study of (WO3)3nanoclusters adsorption on defective MgO ultrathin films on Ag(001)

“…[21][22][23][24][25][26] These systems exhibit unusual chemical and electronic properties with respect to thicker lms or single crystal oxide surfaces, due to the presence of the dielectric boundary and the reduced dimensionality of the insulating lms, offering new opportunities for the design of new functional materials. [27][28][29][30] One specic and crucial property of ultrathin oxide lms is the occurrence of a spontaneous charge transfer from the metal support to an adsorbed species through the thin insulating layer (or vice versa). 24,28,[31][32][33][34][35] This has been done recently for various systems, including adsorbed Au atoms, 36,37 Au clusters, [38][39][40] (WO 3 ) 3 clusters, 34 or simple molecules like O 2 , 41 NO 2 35,42 when adsorbed on MgO/Ag(001) lms becoming negatively charged.…”

“…[27][28][29][30] One specic and crucial property of ultrathin oxide lms is the occurrence of a spontaneous charge transfer from the metal support to an adsorbed species through the thin insulating layer (or vice versa). 24,28,[31][32][33][34][35] This has been done recently for various systems, including adsorbed Au atoms, 36,37 Au clusters, [38][39][40] (WO 3 ) 3 clusters, 34 or simple molecules like O 2 , 41 NO 2 35,42 when adsorbed on MgO/Ag(001) lms becoming negatively charged. Therefore, the possibility to grow or deposit monodispersed oxide clusters on different kinds of supports with diverse nature of defects and the forms of the supported material (thin lms, single crystal) open new scenarios for the understanding of the fundamental properties of these systems.…”