Electronic Structure of a Polar Ceramic/Metal Interface: {222} MgO/Cu
R. Benedek and D. N. Seidman, Northwestern University
L. H. Yang, Lawrence Livermore National Laboratory
A. Alavi, The Queen’s University of Belfast
Research Objectives
This project is part of an effort to characterize on an atomic scale the structure and physical properties of ceramic-metal interfaces. When such interfaces are formed internally within a specimen, for example by internal oxidation, nitridation, or carburization, polar orientations typically occur, i.e., the ceramic facets at precipitate interfaces with the metal matrix are exclusively either anion or cation. Although such polar interfaces are more strongly bonded than neutral ceramic-metal interfaces (which are produced, for example, by molecular beam epitaxy deposition of metal onto a cleaved ceramic substrate), they have received much less attention theoretically. This research intends to elucidate the electronic structure and the chemical bonding at a model polar ceramic-metal interface on which considerable experimental information exists, {222} MgO/Cu.
Computational Approach
Local density functional theory (LDFT) is a first-principles computational framework that gives a physically accurate description of atomic and electronic structure of condensed matter. A convenient and numerically efficient inplementation of LDFT is the planewave pseudopotential representation.
A sequential planewave pseudopotential code (developed by L. H. Yang) based on a preconditioned conjugate gradient optimization algorithm is used on the Cray J90; and a parallel planewave code (developed by A. Alavi et al.), based on the Lanczos algorithm, is employed on the Cray T3E.
Accomplishments
Calculations are performed for a {222} MgO/Cu interface to elucidate the electronic structure in the vicinity and show how the electronic states at the interface differ from those in bulk MgO and bulk Cu. The results are analyzed to determine the layer-projected electronic density of states in the vicinity of the interface or free surface (see figure). The densities of states for the layers other than the CuO bilayer at the interface are relatively bulklike, which indicates that the interface perturbation is essentially confined to the interface bilayer. The panels labeled "bulk" represent layers two layers removed from the interface.
A noteworthy feature of the densities of states of the interface bilayer (second through fifth panels from the top of the figure) is the appearance of a peak a few tenths of an eV below the Fermi level, within the bulk MgO gap. This peak is absent from the surface O-layer spectrum at an MgO free surface (bottom panel), and results from antibonding hybrid states that mix Cu 3d and O 2p character. These are metal-induced gap states, which decay exponentially with distance from the interface.
The presence of such high-lying antibonding states of d-character (as well as corresponding low-lying bonding states) is typical of copper-oxide bonding, and is found, for example, in crystalline Cu2O and O adsorbed onto Cu {100} substrates. Some characteristics of the bonding of these three systems are qualitatively similar, in spite of the differences in their detailed geometries.
Calculated layer-projected densities of states for unreconstructed {222} MgO free surface (bottom panel), oxygen-terminated MgO/Cu interface (second, fourth, fifth, and seventh panels), and Cu monolayer on oxygen-terminated MgO substrate (third and sixth panels).
The theoretical effort is complemented by experimental investigations by atom-probe-field-ion microscopy, high-resolution and scanning-electronic microscopy, and electron energy loss (EELS) spectroscopy. The electronic density of states in the oxygen layer above the Fermi energy is found to agree closely with the EELS O-K edge spectrum.
Significance
Ceramic-metal interfaces are prominent in many advanced materials, including high-temperature alloys, sensors, electronic components, and medical prostheses. It is of both practical and scientific interest to characterize the structure and properties of such interfaces.
Publications
D. A. Muller, D. A. Shashkov, R. Benedek, L. H. Yang, D. N. Seidman, and J. Silcox, "Chemistry and bonding at {222} MgO/Cu heterophase interfaces," in Proc. Microscopy Society of America, August 1997, edited by G. W. Bailey et al. (Springer, New York, 1997), 647.
R. Benedek, D. A. Shashkov, D. N. Seidman, D. A. Muller, J. Silcox, M. F. Chisholm, and L. H. Yang, "Atomic structure of a polar ceramic/metal interface: {222} MgO/Cu," Materials Research Society Symposium Proceedings 492, 103 (1998).
D. A. Muller, D. A. Shashkov, R. Benedek, L. H. Yang, J. Silcox, and D. N. Seidman, "Atomic scale observations of metal-induced gap states at {222} MgO/Cu interfaces," Phys. Rev. Lett. 80, 4741-4744 (1998).