Variations in the Nature of Metal Adsorption on Ultrathin Al2O3

Abstract

First-principles density-functional calculations are used to study metal adsorption (Li, K, Y, Nb, Ru, Pd, Pt, Cu, Ag, Au, and A1 at 1/3-4 monolayer coverages) atop 5 A A1203 films on Al( oxide-metal bond is ionic at low coverages but, with interesting exceptions, caused by PO at high coverages where the overlayer is metallic. Binding trends are explained in terms concepts. Increasing Few fields in materials science are so technologically important, yet poorly understood, as metal-ceramic interfaces. Microchip packaging, catalysis support, corrosion and electrical protection, and biophysical implants are a few areas that would benefit from an atomicscale understanding of oxide-metal bonding. Experimental [l,2] and theoretical approaches [3] have been limited by the inherent complexity of oxides. Many different adhesion mechanisms have been proposed, including van der Waals, covalent, ionic, and polarization [3-51, but there is a profound lack of systematic insight. This unsatisfaeory situation suggests that any fundamental and methodical understanding of oxide-metal interactions must include a detailed knowledge of the electronic structure at the interface. Density-functional theory (DFT) [6,7] provides an accurate basis for attacking this complex task from first principles, avoiding problems that have rendered several previous oxide calculations disputable (as discussed in Refs. [3,4]). Progress in algorithms and computer power now make this total-energy method feasible for this class of materials at a time when there is a surge of meticulous oxide experiments [8,9].In this Letter, we report adsorption properties of metal adsorbates, spanning the periodic table (PT), on ultrathin A1203 films on Al(111). This allows us to study binding trends due to variations in ionic radii, metal interactions, valence configurations, and ionization potentials. We make several contacts with experiment, and address issues such as wettability and epitaxial growth modes. The ultrathin film is directly relevant to understanding corrosion of NiAl and the Ni3Al family of "superalloys". It is a good model system for bulk sapphire [lo], and has the additional advantage that it can be characterized with scanning tunneling microscopy (STM) and ionizing probes.The ionicity of an oxide and the magnitude of adsorbate-induced relaxations largely determine how metal atoms bind to its surface. The Madelung potential in the near-surface region gives a further indication of what bond type is to be expected. In oxides with lack of strict layer-by-layer neutrality, it can be strong enough to ionize an isolated metal atom [4]. In MgO, NiO, and other oxides with strict layer neutrality, this potential is considerably weaker. Relaxations are small compared with alumina (where they are critically important, see below), and soft van der Waals or stronger covalent oxidemetal bonds have been reported [5]. As the metal coverage increases, adatom-adatom interactions strengthen at the expense of adatom-oxide bonds, and the metal overlayer can liter...

Adhesion of ultrathin ZrO2(111) films on Ni(111) from first principles

“…Our observation that a single ceramic layer bonds strongly, whereas the bonding is weakened for many ceramic layers is a mirror of the trend noticed for metals deposited onto ceramics; many metals are predicted to wet, but then ball up for more than one monolayer deposited, 46,68,69 i.e., growing in a Stranski-Krastanov fashion. Note that the trend that the ceramic favors interceramic bonds when the thickness of the ceramic layer is increased is opposite of that expected from the simplest image charge interaction model, 67 which states that the major contribution to metalceramic bonding is the electrostatic attraction between the ceramic and its ͑oppositely-charged͒ image in the metal: a thicker ceramic layer would produce a larger ͑oppositely charged͒ electrostatic image in the metal and thus bond stronger.…”

“…However, in all cases, we find very small dipole energy corrections, of order 9 mJ/m 2 or less. Finally, we stress that ionic relaxations in all cases are performed using forces corresponding to the total energy density functional used; i.e., we do not evaluate the GGA energies by static ͑ion͒ calculations of the structure obtained by ionic relaxation using forces derived from the LSDA density functional; this would be a bad approximation, 46 considering the differences in predicted equilibrium bond lengths, as will be discussed in Secs. II D and III B.…”

Ab initiostudies of Al, O, andO2adsorption onα−Al2O3Wallin1, Andersson2, Münger3 et al. 2006Phys. Rev. B445390View full textAdd to dashboardCite

“…The ionic type of bonding is consistent with what has been observed previously for adsorption of other metals at low coverages on this surface. 21,24 A clear difference in the distance between the adsorbate and the surface can be made in the three cases. For adsorption in position E, the atom relaxes down to a position almost at the same level as for the relaxed AlO-terminated surface ͑a relaxation in the Al-O layer distance of −65%͒.…”

Mechanism of Cu Deposition on theα−Al2O3(0001) Surface

“…In the past few years experimental studies concerning deposition of metals have been reported; however, the complexity of these systems makes it difficult to obtain direct, structural, or electronic information even in an ultrahigh vacuum and well-defined, controlled, experimental conditions, and interpretations are not exempt of contradictions. Theoretical approaches have proved useful tools for understanding the nature and mechanism of the deposition and nucleation of atoms and clusters [2,3].…”