Wilfried Sigle

Role of space charge in the grain boundary blocking effect in doped zirconia

Abstract Although the presence of siliceous phases is a major cause of the grain boundary blocking effect in doped zirconia (ZrO 2 ), the grain boundary resistivity also appears to be significantly influenced by space charges. The ionic transport across the grain boundaries occurs solely through direct grain-to-grain contacts which are themselves blocking in nature. Such a blocking effect can be consistently accounted for by the oxygen vacancy depletion in the grain boundary space charge layer. The Al 2 O 3 addition markedly modified the grain boundary properties, which are also consistently explained by the space charge effect.

Direct imaging of surface plasmon resonances on single triangular silver nanoprisms at optical wavelength using low-loss EFTEM imaging.

Using low-loss energy-filtering transmission electron microscopy (EFTEM) imaging, we map surface plasmon resonances (SPRs) at optical wavelengths on single triangular silver nanoprisms. We show that EFTEM imaging combining high spatial sampling and high energy resolution enables the detection and for the first time, to the best of our knowledge, mapping at the nanoscale of an extra multipolar SPR on these nanoparticles. As illustrated on a 276.5 nm long nanoprism, this eigenmode is found to be enhanced on the three edges where it exhibits a two-lobe distribution.

Toroidal Plasmonic Eigenmodes in Oligomer Nanocavities for the Visible

Plasmonics has become one of the most vibrant areas in research with technological innovations impacting fields from telecommunications to medicine. Many fascinating applications of plasmonic nanostructures employ electric dipole and higher-order multipole resonances. Also magnetic multipole resonances are recognized for their unique properties. Besides these multipolar modes that easily radiate into free space, other types of electromagnetic resonances exist, so-called toroidal eigenmodes, which have been largely overlooked historically. They are strongly bound to material structures and their peculiar spatial structure renders them practically invisible to conventional optical microscopy techniques. In this Letter, we demonstrate toroidal modes in a metal ring formed by an oligomer of holes. Combined energy-filtering transmission electron microscopy and three-dimensional finite difference time domain analysis reveal their distinct features. For the study of these modes that cannot be excited by optical far-field spectroscopy, energy-filtering transmission electron microscopy emerges as the method of choice. Toroidal moments bear great potential for novel applications, for example, in the engineering of Purcell factors of quantum-optical emitters inside toroidal cavities.

Melt-spun precipitation-hardened Sm2(Co, Cu, Fe, Zr)17 magnets with abnormal temperature dependence of coercivity

Rapidly quenched Sm(CobalCu0.08Fe0.22Zr0.02)8.5 (Cu-/Fe-rich) and Sm(CobalCu0.05Fe0.10Zr0.03)8.5 (Cu-/Fe-poor) ribbons have been prepared by means of the melt-spinning technique. By applying an appropriate annealing procedure a microstructure similar to that of sintered magnets can be obtained. The energy dispersive x-ray microanalysis of the compositional dependence near the cell boundaries suggests a model for the profile of the crystal anisotropy constants responsible for the magnetic hardening. The Cu-/Fe-rich alloy shows a normal temperature dependence of coercivity with a negative temperature coefficient, but the Cu-/Fe-poor ribbons show a positive temperature coefficient in the temperature range from 400–700 K. The different temperature coefficients are discussed in terms of a pinning model.

SESAM: Exploring the frontiers of electron microscopy

We report on the sub-electron-volt-sub-angstrom microscope (SESAM), a high-resolution 200-kV FEG-TEM equipped with a monochromator and an in-column MANDOLINE filter. We report on recent results obtained with this instrument, demonstrating its performance (e.g., 87-meV energy resolution at 10-s exposure time, or a transmissivity of the energy filter of T1 ev = 11,000 nm2). New opportunities to do unique experiments that may advance the frontiers of microscopy in areas such as energy-filtered TEM, spectroscopy, energy-filtered electron diffraction and spectroscopic profiling are also discussed.

Ultrathin 2D coordination polymer nanosheets by surfactant-mediated synthesis

Low-dimensional nanostructures offer a host of intriguing properties which are distinct from those of the bulk material, owing to size-confinement effects and amplified surface areas. Here, we report on the scalable, bottom-up synthesis of ultrathin coordination polymer nanosheets via surfactant-mediated synthesis and subsequent exfoliation. Layers of a two-dimensional (2D) zinc coordination polymer are self-assembled in the interlamellar space of a reverse microemulsion mesophase into stacks of nanosheets interleaved with cethyltrimethylammonium bromide (CTAB) at regular intervals, thus giving rise to a lamellar hybrid mesostructure with a lattice period of ~8 nm and an underlying highly crystalline substructure. The basic structural motif is composed of 2D acetato-benzimidazolato-zinc layers of tetrahedrally coordinated zinc joined together by anionic acetate and benzimidazolate ligands. The hierarchical structure was studied by PXRD, TEM, EDX, EELS, AFM, and solid-state NMR spectroscopy, revealing a high level of order on both the atomic and mesoscale, suggesting fairly strong interactions along the organic-inorganic hybrid interface. Exfoliation of the hybrid material in organic solvents such as THF and chloroform yields sheet- and belt-like nanostructures with lateral sizes between 10s and 100s of nanometers and a height of about 10 nm measured by AFM, which precisely maps the basal spacing of the lamellar mesostructure; further exfoliation results in nanobelts with minimum sizes around 4 nm. Finally, the sheetlike nanostructures behave as morphological chameleons, transforming into highly regular multiwalled coordination polymer nanotubes upon treatment with organic solvents.

Phase Boundary Propagation in Large LiFePO4 Single Crystals on Delithiation

Large single crystals of LiFePO(4) have been chemically delithiated. The relevance of chemical oxidation in comparison with electrochemical delithiation is discussed. Analyses of the Li content and profiles were done by electron energy loss spectroscopy and secondary ion mass spectrometry. The propagation of the FePO(4) phase growing on the surface of the large single crystal was followed by in situ optical microscopy as a function of time. The kinetics were evaluated in terms of linear irreversible thermodynamics and found to be characterized by an induction period followed by parabolic growth behavior of the FePO(4) phase indicating transport control. The growth rate was shown to depend on the crystallographic orientation. Scanning electron microscopy images showed cracks and a high porosity of the FePO(4) layer due to the significant changes in the molar volumes. The transport was found to be greatly enhanced by the porosity and crack formation and hence greatly enhanced over pure bulk transport, a result which is supposed to be very relevant for battery research if coarse-grained powder is used.

Experimental realization of graded L10-FePt/Fe composite media with perpendicular magnetization

A concept is suggested and experimentally realized to fabricate graded media for ultrahigh density magnetic recording where the material parameters vary gradually in the interfacial region between the hard magnetic part and the soft magnetic part of epitaxial L10-FePt/Fe exchange spring nanocomposites with perpendicular magnetization. A graded interface between the L10-FePt phase and the Fe phase is formed by depositing part of the Fe layer at elevated temperatures. The existence of the graded interface is verified by electron energy-loss spectroscopy. The influence of the character of the graded interface on the magnetic properties is studied. With increasing thickness of the graded interface the coercivity continuously decreases, which can be used for a fine tuning of the coercivity of exchange spring composite media.

High-resolution electron microscopy and molecular dynamics study of the (a/2)[111] screw dislocation in · molybdenum

The structure of the (a/2)[111] screw dislocation in Mo is studied by high-resolution transmission electron microscopy (HRTEM) and by molecular dynamics (MD) simulation. Detailed analysis of HRTEM images reveals, for the first time, the nonplanar dissociation of the screw dislocation core which is responsible for the high flow stress of bcc metals at low temperatures. It is shown that image contrast is dominated by surface relaxations induced by the Eshelby twist. These relaxations do not exactly correspond to those given theoretically by Eshelby and Stroh but are influenced by the nonplanar core dissociation. It is concluded that MD simulations are important for the interpretation of HRTEM images of defects which lead to shear stress components on the surface.

ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY

■ Abstract Chemical analysis at high spatial resolution is the domain of analytical transmission electron microscopy. Owing to rapid instrumental developments during the past decade, electron energy-loss spectroscopy offers now a spatial resolution close to 0.1 nm and an energy resolution close to 0.1 eV. This development has been accompanied by the introduction of numerous new techniques and methods for data acquisition and analysis, which are outlined in the present article. Recent results for a wide range of material systems are addressed. These comprise first-principles calculations, which have contributed to enormous progress in the calculation of near-edge fine structures, and fingerprinting methods, which are still important for the interpretation of experimental data.