Aberration-corrected Electron Microscopy Research Articles

Atomic structure characterization of an incommensurate grain boundary

The structure of an incommensurate 90°〈110〉 tilt grain boundary in gold was characterized by atomic resolution aberration-corrected electron microscopy and compared with atomistic simulations. Based on a periodic hyperspace description, the non-periodic structure can be described by the Aubry hull function, which plots atomic relaxations at the core of the boundary relative to an unrelaxed structure, folded into a single repeat unit of the neighboring grain. By measuring the hull functions from atomic resolution images, we were able to make quantitative comparisons of experimental observations with molecular statics simulations of this boundary. The results show good agreement in the pattern of atomic relaxations, replicate features of the hull functions that are characteristic of a boundary with superglide behavior and demonstrate the experimental feasibility of this approach for analysis of interfaces.

Revisiting the “In-clustering” question in InGaN through the use of aberration-corrected electron microscopy below the knock-on threshold

The high intensity of light emitted in InxGa1−xN/GaN heterostructures has been generally attributed to the formation of indium-rich clusters in InxGa1−xN quantum wells (QWs). However, there is significant disagreement about the existence of such clusters in as-grown InxGa1−xN QWs. We employ atomically resolved CS-corrected scanning transmission electron microscopy and electron energy loss spectroscopy at 120 kV—which we demonstrate to be below the knock-on displacement threshold—and show that indium clustering is not present in as-grown In0.22Ga0.78N QWs. This artifact-free, atomically resolved method can be employed for investigating compositional variations in other InxGa1−xN/GaN heterostructures.

Interfacial defects in YBa2Cu3O7-δ/SrTiO3(0 0 1) heterostructures studied by aberration-corrected ultrahigh-resolution electron microscopy

The microstructure of interfacial defects in YBa2Cu3O7-δ/SrTiO3(0 0 1) heterostructures has been investigated by aberration-corrected ultrahigh-resolution electron microscopy. We determine that c-axis-oriented YBa2Cu3O7-δ thin films epitaxially grow on SrTiO3(0 0 1) with two types of interface structure. The coalescence of nucleation sites with different types of interface structure leads to the formation of antiphase domain boundaries in YBa2Cu3O7-δ thin films, which terminate at planar faults with different configurations near the interface. Stand-off misfit dislocations are observed and the dislocation core structure is explored. Based on the interface structure and interfacial defects, the initial growth mode of YBa2Cu3O7-δ thin films on SrTiO3(0 0 1) is discussed.

Metadislocation core structure and atomic model for metadislocation motion

We have analyzed the atomic structure of a metadislocation core using aberration-corrected high-resolution electron microscopy. It is shown that the metadislocation core is highly ordered and based on atomic clusters. It possesses a multiscale structure with an inner core comprising the full strain of the metadislocation, and an outer core which is not strained but has a cluster structure modified with respect to the bulk. We have developed the first atomic model for metadislocation motion based on repetitive jump patterns along various crystallographic directions. It involves the movement of about 100 atomic positions per step of metadislocation motion. Characteristic parallel “paired jumps” of neighboring atomic columns lead to a net flux of atoms towards the metadislocation core which is consistent with the negative climb mode of metadislocation motion.

Crystallization of NiSix in a Body-Centered Cubic Structure during Solid-State Reaction between an Ultrathin Ni Film and Si(001) Substrate at 150–350 °C

We investigate ultrathin silicide formation during a solid-state reaction between Ni layers and Si(001) substrates by aberration-corrected electron microscopy. Interdiffusion of two nm thick (equivalent) Ni layers with Si during magnetron-sputter deposition results in an amorphous Ni–Si solid solution. Upon annealing at 150–350 °C, a novel body-centered cubic (bcc) NiSix phase is found to grow epitaxially with a crystallographic relationship {100}<001>bcc-NiSix//{100}<001>Si. bcc-NiSix belongs to the space group I4̅3m (217) with random Ni and Si distribution. The cell parameter is 0.272 nm, which is approximately half that of NiSi2. Further annealing transforms bcc-NiSi to NiSi2 with an activation energy of 0.6 ± 0.1 eV.

Atomic Resolution Analysis of Microporous Titanosilicate ETS‐10 through Aberration Corrected STEM Imaging

It′s all your fault! Microporous titanosilicate ETS-10 crystals have been analyzed by advanced electron microscopy techniques. With the last generation of spherical aberration corrected electron microscopes, truly atomic resolution images have been recorded. Owing to the extremely high-resolution images that have been obtained, the multiple defects (stacking faults, lack of porosity and “double-pores”) present in this type of material can be analyzed in great detail.

Exploring aberration-corrected electron microscopy for compound semiconductors

The development of aberration-corrected electron microscopes (ACEMs) has made it possible to resolve individual atomic columns ('dumbbells') with correct interatomic spacings in elemental and compound semiconductors. Thus, the latest generations of ACEMs should become powerful instruments for determining detailed structural arrangements at defects and interfaces in these materials. This paper provides a short overview of off-line ('software') and on-line ('hardware') ACEM techniques, with particular reference to characterization of elemental and compound semiconductors. Exploratory probe-corrected studies of ZnTe/InP and ZnTe/GaAs epitaxial heterostructures and interfacial defects are also described. Finally, some of the associated problems and future prospects are briefly discussed.

Atomic-Scale Measurement of Structure and Chemistry of a Single-Unit-Cell Layer of LaAlO3 Embedded in SrTiO3

A single layer of LaAlO3 with a nominal thickness of one unit cell, which is sandwiched between a SrTiO3 substrate and a SrTiO3 capping layer, is quantitatively investigated by high-resolution transmission electron microscopy. By the use of an aberration-corrected electron microscope and by employing sophisticated numerical image simulation procedures, significant progress is made in two aspects. First, the structural as well as the chemical features of the interface are determined simultaneously on an atomic scale from the same specimen area. Second, the evaluation of the structural and chemical data is carried out in a fully quantitative way on the basis of the absolute image contrast, which has not been achieved so far in materials science investigations using high-resolution electron microscopy. Considering the strong influence of even subtle structural details on the electronic properties of interfaces in oxide materials, a fully quantitative interface analysis, which makes positional data available with picometer precision together with the related chemical information, can contribute to a better understanding of the functionality of such interfaces.

Electron Vortex Production and Control Using Aberration Induced Diffraction Catastrophes

An aberration corrected electron microscope is used to create electron diffraction catastrophes, containing arrays of intensity zeros threading vortex cores. Vortices are ascribed to these arrays using catastrophe theory, scalar diffraction integrals, and experimentally retrieved phase maps. From measured wave function phases, obtained using focal-series phase retrieval, the orbital angular momentum density is mapped for highly astigmatic electron probes. We observe vortex rings and topological reconnections of nodal lines by tracking the vortex cores using the retrieved phases.

収差補正電子顕微鏡と表面・界面科学

Aberration corrected electron microscopy has revealed various aspects of materials and their functions, i.e., interfacial, ionic, chemical, photonic, plasmonic, magnetic and electronic aspects. A 50 pm resolution microscopy, particularly, enabled us to observe lithium ions in LiMn2O4, a material used as lithium ion battery electrodes. The 50 pm electron probe scanning over the surface is sensitive enough to detect a single adatom/vacancy similarly to STM. The 50 pm microscopy, in transmission mode of observation, is useful for study of catalysis, as demonstrated for Au/TiO2 in O2 and/or CO environment. Aberration corrected in-situ electron microscopy provides atomic, ionic, and electronic properties, in relation to structures at “nano in macro”.

Advanced Characterization Techniques in High-Temperature Oxidation and Corrosion Studies

Recent improvements in characterization techniques have greatly advanced fundamental understanding of the complex processes associated with high temperature oxidation and corrosion. These techniques allow for chemical and structural analyses at high resolutions, now clearly in the micrometer range, but also submicron for some and even down to the atomic scale with such techniques as atom probe and aberration corrected electron microscopy. Added to these is the possibility to conduct advanced analyses in situ. In this present issue, it will be clearly demonstrated that substrate and oxide scale microstructures, playing a key role in a number of systems, can now be characterized with exquisite detail and often in three-dimensions, such as by the coupling of FIB, MEB/FEG and even EBSD. In addition, residual stress mapping by Raman spectroscopy, the study of semiconductor properties with photoelectrochemistry techniques, and isotopic tracer experiments in conjunction with SIMS analysis to identify principal reacting and transport processes, all provide excellent examples of what are available to better understand the mechanisms involved in oxide-scale formation at elevated temperatures.

Three-dimensional analysis of nanoparticles on carbon support using aberration-corrected scanning confocal electron microscopy

We observed Pt nanoparticles on carbon nanohorn aggregates by annular dark-field scanning confocal electron microscopy (ADF-SCEM) with an aberration-corrected microscope for three-dimensional (3D) imaging. The object elongation length along an optical axis that corresponds to the depth resolution was independent of the lateral size. Furthermore, the nanoparticle elongation length was reduced to 34 nm as compared with that by an uncorrected microscope. Hence, aberration-corrected ADF-SCEM provides structural information not only on the nanoparticles but also on the large carbon supports such as projecting nanohorns and hollow structures. Finally, aberration-corrected ADF-SCEM is effective for the 3D analysis of nanoparticle-supported composites.

Fe3O4(1 1 1) thin films with bulk-like properties: growth and atomic characterization

Growth defects, domain structures and off-stoichiometric compositional deviation are the cause of significant deviation of magnetic and transport properties of magnetite (Fe3O4) thin films. We show that these anomalous properties can be fully controlled by postannealing in CO/CO2 atmosphere. Aberration corrected electron microscopy has revealed a full structural transformation from Fe/FeOx to Fe3O4 and a drastic reduction of growth defects in the postannealed films. The magnetic and magnetotransport measurements show that postannealed film properties are bulk-like: the magnetization was fully saturated with ∼480 emu cm−3, as in bulk single crystal, and the negative magnetoresistance was suppressed by an order of magnitude.

Progress toward an aberration-corrected low energy electron microscope for DNA sequencing and surface analysis.

Monochromatic, aberration-corrected, dual-beam low energy electron microscopy (MAD-LEEM) is a novel imaging technique aimed at high resolution imaging of macromolecules, nanoparticles, and surfaces. MAD-LEEM combines three innovative electron-optical concepts in a single tool: a monochromator, a mirror aberration corrector, and dual electron beam illumination. The monochromator reduces the energy spread of the illuminating electron beam, which significantly improves spectroscopic and spatial resolution. The aberration corrector is needed to achieve subnanometer resolution at landing energies of a few hundred electronvolts. The dual flood illumination approach eliminates charging effects generated when a conventional, single-beam LEEM is used to image insulating specimens. The low landing energy of electrons in the range of 0 to a few hundred electronvolts is also critical for avoiding radiation damage, as high energy electrons with kilo-electron-volt kinetic energies cause irreversible damage to many specimens, in particular biological molecules. The performance of the key electron-optical components of MAD-LEEM, the aberration corrector combined with the objective lens and a magnetic beam separator, was simulated. Initial results indicate that an electrostatic electron mirror has negative spherical and chromatic aberration coefficients that can be tuned over a large parameter range. The negative aberrations generated by the electron mirror can be used to compensate the aberrations of the LEEM objective lens for a range of electron energies and provide a path to achieving subnanometer spatial resolution. First experimental results on characterizing DNA molecules immobilized on Au substrates in a LEEM are presented. Images obtained in a spin-polarized LEEM demonstrate that high contrast is achievable at low electron energies in the range of 1-10 eV and show that small changes in landing energy have a strong impact on the achievable contrast. The MAD-LEEM approach promises to significantly improve the performance of a LEEM for a wide range of applications in the biosciences, material sciences, and nanotechnology where nanometer scale resolution and analytical capabilities are required. In particular, the microscope has the potential of delivering images of unlabeled DNA strands with nucleotide-specific contrast. This simplifies specimen preparation and significantly eases the computational complexity needed to assemble the DNA sequence from individual reads.

Atomic-scale determination of surface facets in gold nanorods

It is widely accepted that the physical properties of nanostructures depend on the type of surface facets. For Au nanorods, the surface facets have a major influence on crucial effects such as reactivity and ligand adsorption and there has been controversy regarding facet indexing. Aberration-corrected electron microscopy is the ideal technique to study the atomic structure of nanomaterials. However, these images correspond to two-dimensional (2D) projections of 3D nano-objects, leading to an incomplete characterization. Recently, much progress was achieved in the field of atomic-resolution electron tomography, but it is still far from being a routinely used technique. Here we propose a methodology to measure the 3D atomic structure of free-standing nanoparticles, which we apply to characterize the surface facets of Au nanorods. This methodology is applicable to a broad range of nanocrystals, leading to unique insights concerning the connection between the structure and properties of nanostructures.

Intrinsic Instability of Aberration-Corrected Electron Microscopes

Aberration-corrected microscopes with subatomic resolution will impact broad areas of science and technology. However, the experimentally observed lifetime of the corrected state is just a few minutes. Here we show that the corrected state is intrinsically unstable; the higher its quality, the more unstable it is. Analyzing the contrast transfer function near optimum correction, we define an "instability budget" which allows a rational trade-off between resolution and stability. Unless control systems are developed to overcome these challenges, intrinsic instability poses a fundamental limit to the resolution practically achievable in the electron microscope.

Optimization and stability of the contrast transfer function in aberration-corrected electron microscopy

The Contrast Transfer Function (CTF) describes the manner in which the electron microscope modifies the object exit wave function as a result of objective lens aberrations. For optimum resolution in C₃-corrected microscopes it is well established that a small negative value of C₃, offset by positive values of C₅ and defocus C₁ results in the most optimal instrument resolution, and optimization of the CTF has been the subject of several studies. Here we describe a simple design procedure for the CTF that results in a most even transfer of information below the resolution limit. We address not only the resolution of the instrument, but also the stability of the CTF in the presence of small disturbances in C₁ and C₃. We show that resolution can be traded for stability in a rational and transparent fashion. These topics are discussed quantitatively for both weak-phase and strong-phase (or amplitude) objects. The results apply equally to instruments at high electron energy (TEM) and at very low electron energy (LEEM), as the basic optical properties of the imaging lenses are essentially identical.

Reversible contrast in focus series of annular bright field images of a crystalline LiMn2O4 nanowire

A through-focus series of annular bright field (ABF) images were observed simultaneously with high-angle annular dark field (HAADF) images of very thin lithium manganese oxide (LiMn₂O₄), a typical cathode material used in lithium ion batteries, using a spherical aberration corrected electron microscope with a 50 pm resolution (R005). The ABF images showed dark dips at the positions of Li and Mn--O atomic columns, which reversed to bright peaks when the defocus sign was changed, as commonly observed in phase contrast images. The optimal defocus for the ABF images was about 2 nm of over-focus, while that for the HAADF images was 2 nm of under-focus for an incident probe with a convergent semi-angle of 30 mrad. These experimental results are interpreted based on a weak-phase-object approximation.

Recent Advances in the Application of Electron Tomography to Materials Chemistry

Nowadays, tomography plays a central role in pureand applied science, in medicine, and in many branches of engineering and technology. It entails reconstructing the three-dimensional (3D) structure of an object from a tilt series of two-dimensional (2D) images. Its origin goes back to 1917, when Radon showed mathematically how a series of 2D projection images could be converted to the 3D structural one. Tomographic X-ray and positron scanning for 3D medical imaging, with a resolution of ∼1 mm, is now ubiquitous in major hospitals. Electron tomography, a relatively new chemical tool, with a resolution of ∼1 nm, has been recently adopted by materials chemists as an invaluable aid for the 3D study of the morphologies, spatially-discriminating chemical compositions, and defect properties of nanostructured materials. In this Account, we review the advances that have been made in facilitating the recording of the required series of 2D electron microscopic images and the subsequent process of 3D reconstruction of specimens that are vulnerable, to a greater or lesser degree, to electron beam damage. We describe how high-fidelity 3D tomograms may be obtained from relatively few 2D images by incorporating prior structural knowledge into the reconstruction process. In particular, we highlight the vital role of compressed sensing, a recently developed procedure well-known to information theorists that exploits ideas of image compression and "sparsity" (that the important image information can be captured in a reduced data set). We also touch upon another promising approach, "discrete" tomography, which builds into the reconstruction process a prior assumption that the object can be described in discrete terms, such as the number of constituent materials and their expected densities. Other advances made recently that we outline, such as the availability of aberration-corrected electron microscopes, electron wavelength monochromators, and sophisticated specimen goniometers, have all contributed significantly to the further development of quantitative 3D studies of nanostructured materials, including nanoparticle-heterogeneous catalysts, fuel-cell components, and drug-delivery systems, as well as photovoltaic and plasmonic devices, and are likely to enhance our knowledge of many other facets of materials chemistry, such as organic-inorganic composites, solar-energy devices, bionanotechnology, biomineralization, and energy-storage systems composed of high-permittivity metal oxides.

A new aberration-corrected, energy-filtered LEEM/PEEM instrument II. Operation and results

In Part I we described a new design for an aberration-corrected Low Energy Electron Microscope (LEEM) and Photo Electron Emission Microscope (PEEM) equipped with an in-line electron energy filter. The chromatic and spherical aberrations of the objective lens are corrected with an electrostatic electron mirror that provides independent control of the chromatic and spherical aberration coefficients Cc and C3, as well as the mirror focal length. In this Part II we discuss details of microscope operation, how the microscope is set up in a systematic fashion, and we present typical results.