Z-contrast Imaging Research Articles

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems

The new possibilities of aberration-corrected scanning transmission electron microscopy (STEM) extend far beyond the factor of 2 or more in lateral resolution that was the original motivation. The smaller probe also gives enhanced single atom sensitivity, both for imaging and for spectroscopy, enabling light elements to be detected in a Z-contrast image and giving much improved phase contrast imaging using the bright field detector with pixel-by-pixel correlation with the Z-contrast image. Furthermore, the increased probe-forming aperture brings significant depth sensitivity and the possibility of optical sectioning to extract information in three dimensions. This paper reviews these recent advances with reference to several applications of relevance to energy, the origin of the low-temperature catalytic activity of nanophase Au, the nucleation and growth of semiconducting nanowires, and the origin of the eight orders of magnitude increased ionic conductivity in oxide superlattices. Possible future directions of aberration-corrected STEM for solving energy problems are outlined.

Relationship between 4H-SiC∕SiO2 transition layer thickness and mobility

The interfacial region between silicon carbide (SiC) and its native oxide contains a high density of interfacial traps, which is considered a major problem leading to a lower mobility that has hindered SiC metal oxide semiconductor field effect transistors from reaching their theoretical expectations. We investigate the microstructure and chemistry of the 4H-SiC∕SiO2 interface due to variations in nitric oxide annealing and aluminum implantation using Z-contrast imaging and electron energy loss spectroscopy. A transition layer with a carbon to silicon ratio greater than 1 is consistently observed on the SiC side of the interface in each of these samples, and the width of this transition layer is found to be inversely related to the effective channel mobility measured on fabricated devices.

Is Pd2(DBA)3 a Feasible Precursor for the Synthesis of Pd Nanoparticles?

The organometallic reagent tris(dibenzylideneacetone) dipalladium(0) [Pd2(DBA)3] has been used as a reagent for synthesis of hexagonal microstructures ranging from 0.5−10 μm. We have shown elsewhere that Pd2(DBA)3 spontaneously forms hexagonal and cubic microstructures. The results obtained herein confirm that microstructures formed by spontaneous aggregation of Pd2(DBA)3 produce the same morphology and carbonaceous chemical composition as those produced with a process that has been called RNA-mediation by pyridyl-modified RNA (pmRNA). Synthesis products from both methods of preparation were analyzed using a combination of scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction. Indexing of electron diffraction patterns from hexagonal molecular crystals and metallic Pd nanoparticles combined with STEM Z-contrast imaging and X-ray dot maps were used to positively differentiate the two materials. On the...

Electron microscopy study of CeOx–Pd/α-Al2O3 catalysts for methane dry reforming

We have investigated the interaction between Pd and Ce in a (0.47 wt %) CeOx–Pd(1 wt %)/α-Al2O3 catalyst that is used in the reforming reaction of CH4 with CO2. The freshly reduced catalyst was characterized by various electron microscopy techniques, such as elemental mapping, Z-contrast imaging, and electron energy-loss spectroscopy to understand the role of Ce on a microscopic scale. The high spatial resolution elemental mapping indicates that CeOx is located in close proximity of the palladium nanoparticles. High-resolution lattice images and energy-loss spectra obtained in the vicinity of the Pd particles show an anisotropic distribution of CeOx crystallites limited to the interface region between Pd and the substrate but not covering the surface of the Pd nanoparticles. Energy-loss near edge fine structure of Pd M edges reveals that the Pd nanoparticles are not oxidized.

Application of two-dimensional crystallography and image processing to atomic resolution Z-contrast images

Zone axis images recorded using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM or Z-contrast imaging) reveal the atomic structure with a resolution that is defined by the probe size of the microscope. In most cases, the full images contain many sub-images of the crystal unit cell and/or interface structure. Thanks to the repetitive nature of these images, it is possible to apply standard image processing techniques that have been developed for the electron crystallography of biological macromolecules and have been used widely in other fields of electron microscopy for both organic and inorganic materials. These methods can be used to enhance the signal-to-noise present in the original images, to remove distortions in the images that arise from either the instrumentation or the specimen itself and to quantify properties of the material in ways that are difficult without such data processing. In this paper, we describe briefly the theory behind these image processing techniques and demonstrate them for aberration-corrected, high-resolution HAADF-STEM images of Si(46) clathrates developed for hydrogen storage.

Direct measurement of Co-ion spin state transitions in Ca3Co4O9 using variable-temperature electron energy-loss spectroscopy

Previous studies of Ca3Co4O9 suggested that the abrupt changes in the magnetic susceptibility and electrical resistivity at ∼420 K can be attributed to Co spin state transitions. In this letter, we study the possible transitions above 420 K by variable-temperature Z-contrast imaging and electron energy-loss spectroscopy. We find that there is no observable change in the structure and Co valence state upon in situ heating to 500 K, compared to room temperature. However, an intensity decrease of the prepeak in the O K-edge near edge fine structure indicates a Co3+-ion spin state transition has occurred from a low to an intermediate spin state.

In situ TEM of radiation effects in complex ceramics

In situ transmission electron microscopy (TEM) has been extensively applied to study radiation effects in a wide variety of materials, such as metals, ceramics and semiconductors and is an indispensable tool in obtaining a fundamental understanding of energetic beam-matter interactions, damage events, and materials' behavior under intense radiation environments. In this article, in situ TEM observations of radiation effects in complex ceramics (e.g., oxides, silicates, and phosphates) subjected to energetic ion and electron irradiations have been summarized with a focus on irradiation-induced microstructural evolution, changes in microchemistry, and the formation of nanostructures. New results for in situ TEM observation of radiation effects in pyrochlore, A(2)B(2)O(7), and zircon, ZrSiO(4), subjected to multiple beam irradiations are presented, and the effects of simultaneous irradiations of alpha-decay and beta-decay on the microstructural evolution of potential nuclear waste forms are discussed. Furthermore, in situ TEM results of radiation effects in a sodium borosilicate glass subjected to electron-beam exposure are introduced to highlight the important applications of advanced analytical TEM techniques, including Z-contrast imaging, energy filtered TEM (EFTEM), and electron energy loss spectroscopy (EELS), in studying radiation effects in materials microstructural evolution and microchemical changes. By combining ex situ TEM and advanced analytical TEM techniques with in situ TEM observations under energetic beam irradiations, one can obtain invaluable information on the phase stability and response behaviors of materials under a wide range of irradiation conditions.

Quantitative Structure Analysis of Nano-sized Materials by Transmission Electron Microscopy

AbstractNanostructured materials from almost all classes of materials are of great interest because the reduced dimensionality may drastically change the physical properties. In general, these properties are a function of size, shape, arrangement and chemical composition of the nano-sized materials. Transmission electron microscopy (TEM) is a powerful tool to get a detailed insight into the material characteristics. To correlate microstructure as well as microchemistry and materials properties the various TEM techniques for imaging, diffraction and spectroscopy have to be combined. The potential applicability of quantitative TEM will be demonstrated for different nano-sized objects, particularly for semiconductor islands, nanowires, quantum dots and for soft magnetic materials. The classical diffraction contrast method of conventional TEM is applied to analyse the size, shape and arrangement of nano-sized structures, where a quantitative analysis often requires image simulations of diffraction contrast for theoretical structure models. An alternative and powerful method is the three-dimensional reconstruction of the shape from two-dimensional phase mapping by means of electron holography. This reqires the exact calculation of the mean inner potential of the specimen. Quantitative high-resolution transmission electron microscopy (qHRTEM) has to be applied to analyse structure and chemical composition on an atomic scale of magnitude. Particularly the application of aberration-corrected HRTEM offers new possibilities for quantitative structure analysis due to a contrast transfer by means of negative spherical aberration imaging (NCSI) and the resulting strong suppression of image delocalisation effects. An example for quantitative composition analysis will be demonstrated for ternary semiconductor quantum structures by means of a combined analysis of dark-field imaging and qHRTEM. The results will be compared with analytical TEM data (energy-dispersive X-ray spectroscopy (EDXS), electron energy-loss spectroscopy (EELS), and energy-filtered TEM (EFTEM)). The retrieval of chemical information with atomic resolution will be illustrated for III-V semiconductor nanostructures using STEM (scanning TEM) Z-contrast imaging. The correlation of structure and magnetic properties of soft magnetic materials will be demonstrated by combined application of Lorentz microscopy and electron holography. The potential applicability of the different quantitative TEM methods will be shown for following systems:(i) (Si,Ge) islands(ii) ZnTe and (Zn,Mn)Te nanowires(iii) Ga(As,Sb) quantum dots (QDs) on GaAs substrate(iv) nc softmagnetic FeCo alloysThe possibilities and limitations of the various methods applied will be critically evaluated.

Aberration-corrected Z-contrast imaging of SrTiO3 dislocation cores

Many fundamental problems in materials science, physics, and particular nanotechnology rely on the direct determination and characterization of atomic arrangements and electronic environments of individual interfaces or defects. In this paper, we will show how aberration-corrected Z-contrast imaging in combination with electron energy-loss spectroscopy can be used to directly measure the local atomic and electronic structures of dislocation cores in low-angle SrTiO3 [001] tilt grain boundaries. In particular, we will study two types of dislocation cores in a 3 degrees tilt grain boundary, a pure edge dislocation, and a dissociated dislocation core. While it is energetically favorable for an edge dislocation to dissociate into two partial dislocations in such a low-angle grain boundary, we can find pure edge dislocations that show a higher O vacancy concentration than the dissociated cores. We suggest that the increased oxygen vacancy concentration might help stabilizing the pure edge dislocations in 3 degrees tilt grain boundaries of SrTiO3.

Column-by-column compositional mapping by Z-contrast imaging

A phenomenological method is developed to determine the composition of materials, with atomic column resolution, by analysis of integrated intensities of aberration-corrected Z-contrast scanning transmission electron microscopy images. The method is exemplified for InAs x P 1− x alloys using epitaxial thin films with calibrated compositions as standards. Using this approach we have determined the composition of the two-dimensional wetting layer formed between self-assembled InAs quantum wires on InP(0 0 1) substrates.

High angle annular dark field and electron energy loss spectroscopy study on the substitution and distribution of cobalt in ZnO by multilayer growth

Co-doped ZnO films were synthesized by ion beam sputtering using multilayer (ZnO/Co) growth. Both the distribution and the chemical states of Co in ZnO can be well controlled by varying the ratio of the nominal layer thickness of ZnO to Co. Transmission electron microscopy indicated that all of the as-deposited Zn1−x(Co)xO films were polycrystalline with a (0002) preferred orientation. The local microstructures and chemical states were identified by Z-contrast imaging and electron energy loss spectroscopy. In ZnO (1.5 nm)/Co (0.1 nm), homogeneous Co-doped ZnO was observed to have been formed through interdiffusion. However, decreasing or increasing the thickness of ZnO leads to the formation of Co clusters in the ZnO matrix or Zn1−x(Co)xO multilayers, respectively. For ZnO thickness≧1.5 nm, Co is substituted for Zn, and its valence state is 2+. All Co-doped ZnO films show room-temperature ferromagnetic behavior, which appears to depend strongly on the Co distribution.

High Resolution ADF-STEM Imaging Application for Organic Crystals

High-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging was applied for structural observations of organic crystals so as to achieve quantitative atomically resolved images. In particular, a low angle annular dark-field method (LAADF-STEM) was found to be highly advantageous for radiation sensitive organic materials, because stronger thermal diffuse scatterings can be utilized for imaging. So called quantitative Z-contrast images of organic molecules at atomic resolution were demonstrated in several organic crystals.

Detailed arsenic concentration profiles at Si/SiO2 interfaces

The pile-up of arsenic at the Si/SiO2 interface after As implantation and annealing was investigated by high resolution Z-contrast imaging, electron energy-loss spectroscopy (EELS), grazing incidence x-ray fluorescence spectroscopy (GI-XRF), secondary ion mass spectrometry, x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, as well as Hall mobility and four-point probe resistivity measurements. After properly taking into account their respective artifacts, the results of all methods are compatible with each other, with EELS and GI-XRF combined with etching providing similar spatial resolution on the nanometer scale for the dopant profile. The sheet concentration of the piled-up As at the interface was found to be ∼1×1015 cm−2 for an implanted dose of 1×1016 cm−2 with a maximum concentration of ∼10 at. %. The strain observed in the Z-contrast images also suggests a significant concentration of local distortions within 3 nm from the interface, which, however, do not seem to involve intrinsic point defects.

Investigation of the effect of growth interruption on the formation of InAs/GaAs quantum dot superlattice near the InAs critical thickness

Two kinds of superlattices (i) with and (ii) without growth interrupt (GI) after deposition of 1.77 monolayers (ML) of InAs on GaAs (001) were grown by solid-source molecular beam epitaxy (MBE) and assessed by transmission electron microscopy (TEM) techniques, double crystal X-ray diffraction (DCXRD) and photoluminescence (PL) measurements in order to gain an understanding of the structural and compositional properties. In case (i) formation of coherent dislocation free self-organized quantum dots (SOQDs) with 2.8–3.2nm height and 13–16nm lateral size was observed, whereas in case (ii) no quantum dots had formed. In order to better understand the implication of growth interruption for the formation mechanism, highly localised assessment of the composition of the QD was carried out via atomic resolution Z-contrast imaging and electron energy loss spectroscopy (EELS).

Atomic resolution study of the interfacial bonding at Si3N4/CeO2−δ grain boundaries

Using a combination of atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope, we examine the atomic and electronic structures at the interface between Si3N4 (101¯0) and CeO2−d intergranular film (IGF). Ce atoms are observed to segregate to the interface in a two-layer periodic arrangement, which is significantly different from the structure observed in a previous study. Our EELS experiments show (i) oxygen in direct contact with the terminating Si3N4 open-ring structures, (ii) a change in the Ce valence from a nominal oxidation state of +3 to almost +4 moving from the interface into the IGF, and (iii) a uniform concentration of Si in the film.

On the accuracy of maximum entropy reconstruction of high-resolution Z-contrast STEM images

The accuracy of maximum entropy reconstruction of Z-contrast STEM images has been evaluated with the effects of experimental variables and noise taken into account by the means of image simulation. As the specimen contains atom species of greatly different atomic numbers, special attention is given to the reliability of the position and composition of lighter atoms that are determined from Z-contrast images in the presence of heavier atoms. When the noise is moderate (SNR >2.5), the position of atom columns can be measured within an accuracy of 0.03 nm. With a higher signal-to-noise ratio (SNR >5) the composition of lighter atoms can be resolved reliably from the Z-contrast images. However, when image noise increases, the relative intensity of lighter atoms may deviate from the actual value in the specimen object function.

Z-Contrast Imaging and EELS Study of Supported CoPd Nano-Catalyst

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Investigation of the Atomic Structures of Si3N4/CeO2-δ Interfaces using Atomic Resolution Z-contrast Imaging and EELS combined with First-Principles Methods

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Experimental quantification of annular dark-field images in scanning transmission electron microscopy

This paper reports on a method to obtain atomic resolution Z-contrast (high-angle annular dark-field) images with intensities normalized to the incident beam. The procedure bypasses the built-in signal processing hardware of the microscope to obtain the large dynamic range necessary for consecutive measurements of the incident beam and the intensities in the Z-contrast image. The method is also used to characterize the response of the annular dark-field detector output, including conditions that avoid saturation and result in a linear relationship between the electron flux reaching the detector and its output. We also characterize the uniformity of the detector response across its entire area and determine its size and shape, which are needed as input for image simulations. We present normalized intensity images of a SrTiO 3 single crystal as a function of thickness. Averaged, normalized atom column intensities and the background intensity are extracted from these images. The results from the approach developed here can be used for direct, quantitative comparisons with image simulations without any need for scaling.

Structure of Al–Co–Ni pentagonal quasicrystal studied by HAADF-STEM and ab initio calculations

Using a combination of atomic-resolution high-angle annular dark-field (HAADF) Z-contrast imaging and ab initio calculations, atomic models of clusters 2 nm in diameter and 0.8 nm in height are proposed for the Al–Co–Ni pentagonal quasicrystal. This quasicrystal has 5-fold symmetry (the so-called 5f state) without superstructures, and is one of numerous modifications of the Al–Co–Ni decagonal quasicrystal. HAADF results reveal that the two-dimensional quasi-periodic lattice contains mainly Penrose pentagonal tiling. The centres within the decorated pentagonal tiles, i.e. the so-called pentagonal super-clusters, show structural characteristics having both a satellite-orbit shape and a pentagon-symmetry shape. The proposed atomic models, based directly on the HAADF images, are subjected to ab initio total energy calculations. After relaxation, the calculations demonstrate that the models with 5-fold symmetry are energetically more favourable than those with 10-fold symmetry.