S. J. Pennycook

Colossal Ionic Conductivity at Interfaces of Epitaxial ZrO2:Y2O3/SrTiO3 Heterostructures

The search for electrolyte materials with high oxygen conductivities is a key step toward reducing the operation temperature of fuel cells, which is currently above 700°C. We report a high lateral ionic conductivity, showing up to eight orders of magnitude enhancement near room temperature, in yttria-stabilized zirconia (YSZ)/strontium titanate epitaxial heterostructures. The enhancement of the conductivity is observed, along with a YSZ layer thickness–independent conductance, showing that it is an interface process. We propose that the atomic reconstruction at the interface between highly dissimilar structures (such as fluorite and perovskite) provides both a large number of carriers and a high-mobility plane, yielding colossal values of the ionic conductivity.

Incoherent imaging using dynamically scattered coherent electrons

Abstract We use a Bloch wave approach to show that, even for coherent dynamical scattering from a stationary lattice with no absorption, annular dark-field imaging in a scanning transmission electron microscope gives a direct incoherent structure image of the atomic-column positions of a zone-axis-aligned crystal. Although many Bloch waves may be excited by the probe, the detector provides a filtering effect so that the 1s-type bound states are found to dominate the image contrast for typical experimental conditions. We also find that the column intensity is related to the transverse kinetic energy of the 1s states, which gives atomic number, Z , contrast. The additional effects of phonon scattering are discussed, in particular the reasons why phonon scattering is not a prerequisite for transverse incoherence.

Direct imaging of the atomic configuration of ultradispersed catalysts

Direct imaging of individual catalyst metal atoms on the insulating surface of an industrial support is demonstrated. Individual platinum and rhodium atoms ultradispersed on γ-Al2O3 supports were imaged by high-resolution Z-contrast (atomic number Z) microscopy in a 300-kilovolt scanning transmission electron microscope. Within small clusters, the configuration of the metal atoms was seen to be constrained to match the surface structure of the γ-Al2O3, from which likely surface adsorption sites were deduced. A thin, extended raft of rhodium atoms was observed, mostly corresponding to one monolayer. Occasional two-atom features suggested partial dissolution into the top layers of the γ-Al2O3 support.

Detection of Single Atoms and Buried Defects in Three Dimensions by Aberration-Corrected Electron Microscope with 0.5-Å Information Limit

The ability of electron microscopes to analyze all the atoms in individual nanostructures is limited by lens aberrations. However, recent advances in aberration-correcting electron optics have led to greatly enhanced instrument performance and new techniques of electron microscopy. The development of an ultrastable electron microscope with aberration-correcting optics and a monochromated high-brightness source has significantly improved instrument resolution and contrast. In the present work, we report information transfer beyond 50 pm and show images of single gold atoms with a signal-to-noise ratio as large as 10. The instruments new capabilities were exploited to detect a buried Sigma3 {112} grain boundary and observe the dynamic arrangements of single atoms and atom pairs with sub-angstrom resolution. These results mark an important step toward meeting the challenge of determining the three-dimensional atomic-scale structure of nanomaterials.

Enhanced tunnelling electroresistance effect due to a ferroelectrically induced phase transition at a magnetic complex oxide interface

The range of recently discovered phenomena in complex oxide heterostructures, made possible owing to advances in fabrication techniques, promise new functionalities and device concepts. One issue that has received attention is the bistable electrical modulation of conductivity in ferroelectric tunnel junctions (FTJs) in response to a ferroelectric polarization of the tunnelling barrier, a phenomenon known as the tunnelling electroresistance (TER) effect. Ferroelectric tunnel junctions with ferromagnetic electrodes allow ferroelectric control of the tunnelling spin polarization through the magnetoelectric coupling at the ferromagnet/ferroelectric interface. Here we demonstrate a significant enhancement of TER due to a ferroelectrically induced phase transition at a magnetic complex oxide interface. Ferroelectric tunnel junctions consisting of BaTiO3 tunnelling barriers and La(0.7)Sr(0.3)MnO3 electrodes exhibit a TER enhanced by up to ~10,000% by a nanometre-thick La(0.5)Ca(0.5)MnO3 interlayer inserted at one of the interfaces. The observed phenomenon originates from the metal-to-insulator phase transition in La(0.5)Ca(0.5)MnO3, driven by the modulation of carrier density through ferroelectric polarization switching. Electrical, ferroelectric and magnetoresistive measurements combined with first-principles calculations provide evidence for a magnetoelectric origin of the enhanced TER, and indicate the presence of defect-mediated conduction in the FTJs. The effect is robust and may serve as a viable route for electronic and spintronic applications.

Direct determination of grain boundary atomic structure in SrTiO3

An atomic structure model for a 25� [001] symmetric tilt grain boundary in SrTiO3 has been determined directly from experimental data with the use of high-resolution Z-contrast imaging coupled with electron energy loss spectroscopy. The derived model of the grain boundary was refined by bond-valence sum calculations and reveals candidate sites for dopant atoms in the boundary plane. These results show how the combined techniques can be used to deduce the atomic structure of defects and interfaces without recourse to preconceived structural models or image simulations.

The principles and interpretation of annular dark-field Z-contrast imaging

Publisher Summary This chapter describes the way in which an annular dark-field (ADF) image is formed in a scanning transmission electron microscope (STEM). ADF imaging refers to the use of particular detector geometry in STEM. A geometrically large annular detector is placed in the optical far field beyond the specimen. The total intensity detected over the whole detector is recorded and displayed as a function of the position of the illuminating probe. Because the detector only receives a signal when the specimen is present, the vacuum appears dark, hence the name, and the heavier the atom, the higher the intensity of the scattering, which leads to atomic number (Z) contrast in the image. The most important feature of ADF imaging is that it can be described as being incoherent that has many advantages at atomic resolution. The chapter explains the way in which the image data may be used to provide atomic-resolution information about the specimen.

Direct observation of the core structures of threading dislocations in GaN

Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.

On the origin of the high coarsening resistance of Ω plates in Al–Cu–Mg–Ag Alloys

The thickening kinetics of Ω plates in an Al–4Cu–0.3Mg–0.2Ag (wt. %) alloy have been measured at 200, 250 and 300°C using conventional transmission electron microscopy techniques. At all temperatures examined the thickening showed a linear dependence on time. At 200°C the plates remained less than 6 nm in thickness after 1000 h exposure. At temperatures above 200°C the thickening kinetics are greatly increased. Atomic resolution Z-contrast microscopy has been used to examine the structure and chemistry of the (001)Ω‖(111)α interphase boundary in samples treated at each temperature. In all cases, two atomic layers of Ag and Mg segregation were found at the broad face of the plate. The risers of the thickening ledges and the ends of the plates were free of Ag segregation. The necessary redistribution of Ag and Mg accompanying a migrating thickening ledge occurs at all temperatures and is not considered to play a decisive role in the excellent coarsening resistance exhibited by the Ω plates at temperatures up to 200°C. Plates transformed at 200°C rarely contained ledges and usually exhibited a strong vacancy misfit normal to the plate. A large increase in ledge density was observed on plates transformed at 300°C, concomitant with accelerated plate thickening kinetics. The high resistance to plate coarsening exhibited by Ω plates at temperatures up to 200°C, is due to a prohibitively high barrier to ledge nucleation in the strong vacancy field normal to the broad face of the plate. Results also suggest that accommodation of the large misfit that exists normal to the broad face of the plate is unlikely to provide the driving force for Ag and Mg segregation.

Growth and relaxation mechanisms of YBa2Cu3O7−x films

Abstract Using a combination of Z-contrast scanning transmission electron microscopy, scanning tunneling microscopy, and plan view diffraction contrast imaging, we have studied the growth and relaxation mechanisms of YBa2Cu3O7−x deposited on MgO and SrTiO3 substrates. Two-dimensional island growth occurs on SrTiO3 substrates, with relaxation through the nucleation of dislocation half-loops. The threading dislocation segments then have a screw component and can lead to kinetic roughening through the development of growth pyramids. In contrast, growth on MgO occurs by true three-dimensional island growth (with no wetting layer), most of the interface being incommensurate with the substrate (although crystallographically aligned). Dislocations with both edge and screw components are generated on island coalescence. A highly anisotropic surface energy is shown to be responsible for cell-by-cell c⊥ growth being thermodynamically preferred, although at high supersaturations a transition to a⊥ growth occurs.