Adhesive energy and charge transfer for MgO/Cu heterophase interfaces

Characterization and Simulation of Nanoscale Catastrophic Failure of Metal/Ceramic Interfaces

The catastrophic failure of metal/ceramic interfaces is a complex process involving the energy transfer between accumulated elastic strain energy and many types of energy dissipation. To quantify the contribution of bulk and interface cohesive energy to the interface cleavage fracture without global plastic deformation, we characterized the quasi-static fracture process of both coherent and semi-coherent fcc-metal/MgO(001) interface systems using a spring series model and molecular static simulations. Our results show that the theoretical catastrophe point and spring-back length by the spring series model are basically consistent with the simulation results of the coherent interface systems. For defect interfaces with misfit dislocations, atomistic simulations revealed an obvious interface weakening effect in terms of reduced tensile strength and work of adhesion. As the model thickness increases, the tensile failure behaviors show significant scale effects-thick models tend to catastrophic failure with abrupt stress drop and obvious spring-back phenomenon. This work provides insight into the origin of catastrophic failure at metal/ceramic interfaces, which highlights a pathway by combining the material and structure design to improve the reliability of layered metal-ceramic composites.

Enhanced wetting of Cu on ZnO by migration of subsurface oxygen vacancies

Metal adhesion on metal oxides is strongly controlled by the oxide surface structure and composition, but lack of control over the surface conditions often limits the possibilities to exploit this in opto- and micro-electronics applications and heterogeneous catalysis where nanostructural control is of utmost importance. The Cu/ZnO system is among the most investigated of such systems in model studies, but the presence of subsurface ZnO defects and their important role for adhesion on ZnO have been unappreciated so far. Here we reveal that the surface-directed migration of subsurface defects affects the Cu adhesion on polar ZnO(0001) in the technologically interesting temperature range up to 550 K. This leads to enhanced adhesion and ultimately complete wetting of ZnO(0001) by a Cu overlayer. On the basis of our experimental and computational results we demonstrate a mechanism which implies that defect concentrations in the bulk are an important, and possibly controllable, parameter for the metal-on-oxide growth.

Comprehensive elucidation of metal-support interactions is important for controlling and improving their performances in a range of pertinent technologies. Here, the authors reveal how subsurface defects influence the adhesion and wetting of a metal on the surface of a metal oxide.

Non-uniform Solute Segregation at Semi-Coherent Metal/Oxide Interfaces

The properties and performance of metal/oxide nanocomposites are governed by the structure and chemistry of the metal/oxide interfaces. Here we report an integrated theoretical and experimental study examining the role of interfacial structure, particularly misfit dislocations, on solute segregation at a metal/oxide interface. We find that the local oxygen environment, which varies significantly between the misfit dislocations and the coherent terraces, dictates the segregation tendency of solutes to the interface. Depending on the nature of the solute and local oxygen content, segregation to misfit dislocations can change from attraction to repulsion, revealing the complex interplay between chemistry and structure at metal/oxide interfaces. These findings indicate that the solute chemistry at misfit dislocations is controlled by the dislocation density and oxygen content. Fundamental thermodynamic concepts – the Hume-Rothery rules and the Ellingham diagram – qualitatively predict the segregation behavior of solutes to such interfaces, providing design rules for novel interfacial chemistries.

Epitaxial CdSe-Au nanocrystal heterostructures by thermal annealing

The thermal evolution of a collection of heterogeneous CdSe-Au nanosystems (Au-decorated CdSe nanorods, networks, vertical assemblies) prepared by wet-chemical approaches was monitored in situ in the transmission electron microscope. In contrast to interfaces that are formed during kinetically controlled wet chemical synthesis, heating under vacuum conditions results in distinct and well-defined CdSe/Au interfaces, located at the CdSe polar surfaces. The high quality of these interfaces should make the heterostructures more suitable for use in nanoscale electronic devices.

Layer-by-layer growth of polar MgO(111) ultrathin films

By alternate deposition of Mg and exposure of O2, layer-by-layer growth, polar MgO(111) ultrathin films with Mg-terminated or O-terminated surfaces have been successfully fabricated on Mo(110) substrate. The surface geometric structure and electronic structures of the polar MgO(111) films were investigated using surface analysis techniques including low-energy electron diffraction and photoelectron emission and electron energy loss spectroscopies. The results indicate that the O-terminated surface is of an insulating character, while for Mg-terminated surface, a prominent new surface state at 2-3 eV and appreciable density of states near Fermi level have been observed. The polar oxide films provide ideal model surfaces for further investigation of support-particle system.

Polar oxide interface stabilization by formation of metallic nanocrystals

In situ x-ray photoelectron spectroscopy and ex situ transmission electron microscopy and diffraction studies of a model Fe3O4(111)/MgO(111) polar oxide interface exclude stabilization by interface faceting, reconstruction, or by formation of a continuous interfacial layer with altered stoichiometry, and uncover stabilization by dominant formation of metallic Fe(110) nanocrystals. The iron nanocrystals nucleate both at the interface and within the magnetite film and grow in a Nishiyama-Wasserman orientation relationship with a bimodal size distribution related to twinning. Minority magnetite nanocrystals were also observed, growing in the less polar (100) orientation than the magnetite (111) film. Electron transfer and bond hybridization mechanisms are likely at the metal/oxide and oxide/oxide interfaces and remain to be explored.

Subnanometer three-dimensional atom-probe investigation of segregation at MgO/Cu ceramic/metal heterophase interfaces

Three-dimensional atom-probe (3DAP) microscopy has been applied to the study of segregation at ceramic/metal (C/M) interfaces. In this article, results on the MgO/Cu(X) (where X = Ag or Sb) systems are summarized. Nanometer-size MgO precipitates with atomically clean and atomically sharp interfaces were prepared in these systems by internal oxidation. Segregation of the ternary component (Ag or Sb) at the MgO/Cu heterophase interface was enhanced by extended low-temperature anneals. Magnesia precipitates in the 3DAP reconstructions were delineated as isoconcentration surfaces, and segregation of each ternary component at the C/M interfaces was analyzed with the proximity histogram method developed at Northwestern University. This method allows the direct extraction of the Gibbsian interfacial excess of solute at the C/M interfaces from the experimental data. A value of (3.2+/-2.0) x 10(17)m(-2) at 500 degrees C is obtained for the segregation of Ag at a MgO/Cu(Ag) interface, while a value of (2.9+/-0.9) x 10(18) m(-2) at 500 degrees C is obtained for the segregation of Sb at a MgO/Cu(Sb) interface. The larger Gibbsian excess for Sb segregation at this ceramic/metal heterophase interface is most likely due to the so-called pdeltaV effect.

First principles simulation of a ceramic /Metal interface with misfit

The relaxed atomic structure of a model ceramic/metal interface, 222MgO/Cu, is simulated, including lattice constant mismatch, using first principles local-density functional theory plane wave pseudopotential methods. The 399-atom computational unit cell contains 36 O and 49 Cu atoms per layer in accordance with the 7/6 ratio of MgO to Cu lattice constants. The atomic layers on both sides of the interface warp to optimize the local bonding. The interface adhesive energy is calculated. The interface electronic structure is found to vary appreciably with the local environment.