Z-contrast Imaging Research Articles

Effect of Solid-Solution W Addition on the Nanostructure of Electrodeposited Ni

ABSTRACTElectrodeposition method was employed to produce freestanding Ni-W alloy foils. The foils consist of nanograins. The structure of the foil, e.g. texture, grain morphology, size distribution, and the nature of grain boundaries, were characterized using X-ray diffraction and high-resolution electron microscopy. The deposited foils exhibit an equiaxed nanocrystalline structure having a grain size value of about 6 nm. Two types of grain boundary structure were observed. One type of grain boundary is essentially one atomic layer thin and another type consists of a structureless layer of about 0.5–1 nm in thickness. Angular dark field (Z-contrast) image of the deposited foils showed an inhomogeneous distribution of W solutes. In some local regions, the W content actually exceeds the equilibrium solid solution limit. Many grain boundaries with a structureless layer of about 0.5–1 nm are probably a result of local supersaturation of W.

Development of a High Lateral Resolution Electron Beam Induced Current Technique for Electrical Characterization of InGaN-Based Quantum Well Light Emitting Diodes

ABSTRACTElectron Beam Induced Current (EBIC) is a Scanning Electron Microscope (SEM)-based technique that can provide information on the electrical properties of semiconductor materials and devices. This work focuses on the design and implemenation of an EBIC system in a dedicated Scanning Transmission Electron Microscope (STEM). The STEM-EBIC technique was used in the characterization of an Indium Gallium Nitride (InGaN) quantum well Light Emitting Diode (LED). The conventional “H-bar” Transmission Electron Microscopy (TEM) sample preparation method using Focused Ion Beam Micromachining (FIBM) was adapted to create an electron-transparent membrane approximately 300 nm thick on the sample while preserving the electrical activity of the device. A STEM-EBIC sample holder with two insulated electrical feedthroughs making contact to the thinned LED was designed and custom made for these experiments. The simultaneous collection of Z-contrast images, EBIC images, and In and Al elemental images allowed for the determination of the p-n junction location, AlGaN and GaN barrier layers, and the thin InGaN quantum well layer within the device. The relative position of the p-n junction with respect to the thin InGaN quantum well was found to be (19 ± 3) nm from the center of the InGaN quantum well.

Nanoscale Structure/Property Correlation Through Aberration-Corrected Stem And Theory

The combination of atomic-resolution Z-contrast microscopy, electron energy loss spectroscopy and first-principles theory has proved to be a powerful means for structure property correlations at interfaces and nanostructures. The scanning transmission electron microscope (STEM) now routinely provides atomic-sized electron beams, allowing simultaneous Z-contrast imaging and EELS as shown in Fig. 1. The feasiblity of correcting the inherently large spherical aberration of microscope objective lenses promises to at least double the achievable resolution. The potential benefits for the STEM, however, may turn out to be much greater than those for the conventional TEM because it is very much less sensitive to chromatic instabilities. The 100 kV VG Microscopes HB501UX at Oak Ridge National Laboratory (ORNL) is now fitted with an aberration corrector constructed by Nion Co., which improved its resolution from 2.2 Å (full-width-half-maximum probe intensity) to around 1.3 Å. It is now very comparable in performance to the uncorrected 300 kV HB603U STEM at ORNL which, before correction, also had a directly interpretable resolution of 1.3 Å, although information transfer had been demonstrated down to 0.78 Å8. Initial results after installing an aberration corrector on the 300 kV STEM indicate a resolution of 0.84 Å. The theoretically achievable probe size in the absence of instabilities is predicted to be 0.5 Å.

Atomic Scale Observations of the Chemistry at the Metal–Oxide Interface in Heterogeneous Catalysts

Here we present a preliminary analysis of the atomic scale interface chemistry in a model heterogeneous catalyst system, Pt/SiO2. We show that the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) exhibits a sensitivity to surface oxidation and changes in the interfacial chemistry that is higher than that of conventional analytical methods such as energy-dispersive X-ray spectroscopy (EDS), chemisorption, and X-ray absorption near edge structure analysis (XANES). In particular, the presence of a few monolayers of platinum oxide can be clearly seen, and changes in the chemistry of the SiO2 support within ∼1 nm of the metal–oxide interface can be characterized as a function of the catalyst preparation conditions. These results demonstrate that this combination of novel techniques can provide unparalleled information that is potentially the key to understanding the activity and selectivity of heterogeneous catalyst systems.

Internal self-ordering in In(Sb,As), (In,Ga)Sb, and (Cd,Zn,Mn)Se nano-agglomerates/quantum dots

Nano-agglomerates of In(Sb,As) in InAs, (In,Ga)Sb in GaSb, and (Cd,Zn,Mn)Se in (Zn,Mn)Se are classified by transmission electron microscopy. In scanning transmission electron microscopy, atomic resolution Z-contrast images reveal different modes of internal compositional modulation on the atomic length scale, resulting for all three material systems in nano-agglomerates of an appropriate size that may constitute a new type of quantum dot. For other nano-agglomerates of In(Sb,As) in InAs and (In,Ga)Sb in GaSb, we observed a second type of nanoscale ordering that results in nano-agglomerates with an internal compositional modulation on a length scale of a few nm. Both types of compositional modulation are discussed as having arisen from a rather long-term structural response to a combination of internal and external strains.

The Si/SiO2 Interface: Atomic Structures, Composition, Strain and Energetics

A number of recent studies of grain boundaries and heterophase interfaces have demonstrated the power of combining Z-contrast STEM imaging, EELS and first-principles theoretical modeling to give an essentially complete atomic scale description of structure, bonding and energetics. Impurity sites and valence can be determined experimentally and configurations determined through calculations.Here we present an investigation of the Si/SiO2 interface. The Z-contrast image in Fig. la, taken with the VG Microscopes HB603U STEM, shows that the atomic structure of Si is maintained up to the last layers visible. The decrease in intensity near the interface could originate from interfacial roughness of around one unit cell (∼0.5 nm), or may represent dechanneling in the slightly buckled columns induced by the oxide. Fig. lb, taken from a sample with ∼1 nm interface roughness, shows a band of bright contrast near the interface. This is not due to impurities or thickness variation since it disappears on increasing the detector angle from 25 mrad to 45 mrad (Fig. lc), and is therefore due to induced strain.

Non-Stoichiometry at Dislocation Cores in Perovskites and Related Materials

Z-contrast images and electron energy loss spectra (EELS) were obtained from low angle grain boundaries in SrTiO3 and YBa2Cu3O7-x (YBCO). Z-contrast images are easy to interpret and especially useful for positioning the beam to acquire EELS data from small sample areas [1], because both these techniques can be performed simultaneously.In high-temperature superconductors even a single grain boundary can reduce the critical current by up to four orders of magnitude. The band-bending model can quantitatively explain this phenomenon. YBCO is a hole-doped superconductor with about one hole per unit cell for optimum doping at x close to zero. It has a structure closely related to the perovskite structure, and Z-contrast images have shown that the dislocation cores are made up of similar structural units as in SrTiO3.[2,3] Our EELS measurements show clear evidence for band bending effects around isolated dislocation cores in an undoped 8° low angle grain boundary.

Atomic Scale Characterization of Oxygen Vacancy Ordering in Oxygen Conducting Membranes By Z-Contrast IMAGING and EELS

Mixed conductors have been the focus of many studies in the last decade, leading to a detailed understanding of many of the macroscopic bulk properties of these materials. in particular, although the reduced low temperature phase in rare earth perovskite oxides is commonly explained in terms of ordered brownmillerite structured micro domains, its transition to the high temperature phase remains elusive. in this presentation an investigation of (La, Sr)FeO3, prepared under different reducing conditions through correlated atomic resolution annular dark field imaging and electron energy loss spectroscopy will be shown.We investigate the (La, Sr)FeO3 material by atomic resolution Z-contrast imaging and EELS using a 200 keV STEM/TEM JEOL2010F with a post column GIF. The combination of these techniques allows us to obtain direct images from the atomic structure of the bulk sample and to correlate this with the atomically resolved EELS information. In-situ heating of the material in a heating double tilt holder in the microscope columns allows us to simulate the highly reducing operating conditions for this oxygen conducting membrane material.

Characterization of Antimony-Doped TIN Oxide Catalysts

Antimony-doped tin oxide (ATO) catalysts are used for the oxidation of propylene to acrolein, the ammoxidation of propylene to acrylonitrile and the oxidative dehydrogenation of butanes to 1,3- butadiene. The distribution and valence states of Sb in ATOs are key in determining their catalytic activities. While these materials have been subjects of intensive studies for more than 20 years, X-ray photoelectron spectroscopy, Mössbauer spectrometry, and X-ray absorption spectroscopy4 have so far provided only indirect data for the distribution of Sb and its valence states. in particular, while has been hypothesized that the tin (IV) oxide contains Sb (V) within the bulk lattice and Sb (III) located at surface sites, no direct experimental evidence for this has been provided.Here we use electron energy loss spectroscopy (EELS) combined with Z-contrast imaging in a JEOL 2010F field emission STEM/TEM operating at 200 KV to analyze ATO catalysts.

Structural Insights of Supported Nanoparticles By Z-Contrast and High Resolution Electron Microscopy

A vital component to nanoparticle science will be the three dimensional (3-D) characterization of both structure and chemistry of these nanoparticles on their supports at the nanometer scale and below. to achieve this goal, quantitative Z-contrast and atomic resolution will provide essential information about their structure. Z-contrast imaging is ideal for imaging these large Z nanoparticles on low Z supports. in this proceedings, we present a quantitative Z-contrast method to determine number of atoms and a few examples of a combination of electron microscopy methods to gain structural insights into supported nanoparticle, such as Pt on different support materials, PtRu5 on C and Pt-Sn on SiO2.A relatively new and powerful method is to determine the number of atoms in a nanoparticle, by very high angle annular dark-field (HAADF) imaging or Z-contrast technique [1, 2]. We have shown that quantification of the absolute image intensity from very HAADF microscopy will provide the number of atoms in very small particles of high atomic number to ±2 atoms for Re6 nanoparticles supported on carbon [3].

Contrast Effect of Strain Fields in ADF-STEM Imaging

STEM is an important tool for the study of material by forming Z-contrast ADF images. Studies of strain fields suggest that strain can cause extra contrast in ADF images. Since strain fields exist in many TEM samples, especially interfaces of two different materials, it is necessary to study the contrast effect of strain fields in more detail. in this paper, we study the contrast effect of strain fields by STEM experiments and multislice simulations.Sample thickness and the dimensions of ADF detector can both affect strain contrast. We first study the influence of sample thickness. A cross-section TEM sample of an interface of crystal silicon and amorphous silicon was prepared by typical tripod polishing and was studied with the 100 kV Cornell VG 5.0 UHV STEM. The sample was tilted so that the crystal was on the (110) zone axis and ADF images of the interface were recorded.

Atomic Scale Analysis of a YSZ Bicrystal Grain Boundary

Yttria-stabilized zirconia (YSZ) has been the subject of many experimental and theoretical studies, due to the commercial applications of zirconia-based ceramics in solid state oxide fuel cells. Since the grain boundaries usually dominate the overall macroscopic performance of the bulk material, it is essential to develop a fundamental understanding of their structure-property relationships. Previous research has been performed on the atomic structure of grain boundaries in YSZ, but no precise atomic scale compositional and chemistry characterization has been carried out. Here we report a detailed analytical study of an [001] symmetric 24° bicrystal tilt grain boundary in YSZ prepared with ∼10 mol % Y2O3 by Shinkosha Co., Ltd by the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS).The experimental analysis of the YSZ sample was carried out on a 200kV Schottky field emission JEOL 201 OF STEM/TEM4.

Quantitative HREM: Reliable Structure Determination of Grain Boundaries in SrTiO3

High resolution transmission electron microscopy (HREM) is an excellent experimental method to image grain boundary structures with atomic resolution. The advantage of the method is the short exposure time of only about one second that is needed to record an image. Other methods like Z-contrast imaging require much longer exposure times and are therefore much more prone to specimen drift during recording. However there is the remaining difficulty to HREM that the evaluation of experimental images is not straightforward and a thorough analysis of the images is necessary in order to deduce quantitative information with small error bars of only a few pm (10-15m). A second inherent difficulty common to all atomic resolution imaging techniques is that the information is retrieved from a very small area of a specimen. The question arising from that is: can we nevertheless be sure to obtain a representative answer to a “real world” material science problem? A positive answer to this question is given by the investigations presented here.

Analysis of The Electronic Structure of Threading Dislocations in GaN Using Multiple Scattering Simulations

Abstract The promise of advanced technological applications in optical and electronic devices has led to a significant recent research effort in the structure-property relationships of defects in GaN. in particular, the major scientific issues arise from the high density of threading dislocations induced during thin film growth by film-substrate lattice mismatch. There is still debate as to the exact effect of these dislocations on the overall properties; they may or may not be electrically active and are thought to decrease the lifetime of devices. As such, a fundamental understanding of the electronic properties of these defects will facilitate the development of new and improved devices. The analysis of the electronic structure of dislocations in GaN is performed here by a combination of atomic resolution electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) and multiple scattering (MS) simulations. Experimentally, a Z-contrast image of the dislocation core is obtained first1 and used to position the probe for EELS.

Atomic Scale Analysis of Oxygen Vacancy Segregation At Grain Boundaries in Ceramic Oxides

The properties of ceramic oxides being developed for such varied applications as fuel cells, ionic transporting membranes, high-Tc superconductors, ferroelectrics and varistors are dominated by the presence of grain boundaries. Key to controlling the electronic properties of the grain boundaries in these materials is a fundamental understanding of the complex relationship between structure, composition and local electronic structure. The ability to characterize and directly correlate these parameters on the atomic scale is afforded by the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). Furthermore, the recent development of in-situ heating capabilities in the JEOL 201 OF STEM/TEM permits atomic resolution analysis to be performed at elevated temperatures and the interactions of grain boundaries with the oxygen vacancies determined.Figure 1 shows an example of the type of experiment that can be performed using these methods.

Direct Observation of Changes of the Metal-Substrate Interface Chemistry in Supported Heterogeneous Catalysts

The heterogeneous catalytic system Pt/SiO2 is widely used in “three-way” catalysts, because of its highly selective catalytic reduction of NO by hydrocarbons at low operating temperatures. Although used effectively for more than a decade, in recent years it has become clear that the core phenomena of heterogeneous catalysis can occur at interfaces. in the work presented here, we seek to better understand the role of the atomic and electronic structure of interfaces in making particular reactions facile and moderating the stability and selectivity of a catalytic system.We investigate model supported platinum catalysts by atomic resolution Z-contrast imaging and EELS using a 200 kV STEM/TEM JEOL2010F with a post column GIF. The combination of these techniques allows us to obtain direct images of the metal particle and its interface with the supporting SiO2, and to correlate that with the modulation of the Si L-edge fine structure.

Analysis of the Defect Chemistry at Gd-Doped Ceria Grain Boundaries

Abstract Gd3+ doped Ce oxides are very promising candidates as electrolytes for solid oxide fuel cells operating at ∼ 500 °C. For their successful commercial implementation, a full understanding of the defect chemistry in the bulk and at grain boundaries is essential. in particular, the contribution of the grain boundaries to the total ionic conductivity through such effects as the segregation of impurities, dopants and vacancies is of crucial importance. Here the effect of the atomic structure on the local electronic properties, i.e. oxygen coordination and cation valence at grain boundaries of the fluorite structured Gd0.2Ce0.8O2-x ceramic electrolyte is investigated by a combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the JEOL 201 OF STEM (operating at 200keV, and aligned for a probe size ∼ 0.2 nm). Preliminary O K- and Ce M45-edges were acquired from points in the grains (A and B) and grain boundary shown in Figure 1.

Direct atomic scale characterization of interfaces and doping layers in field-effect transistors

Atomic-resolution Z-contrast imaging and electron energy loss spectroscopy combined with energy dispersive x-ray spectroscopy are used to investigate the structure-property relationships in an ideal metal–oxide–semiconductor device structure. Arsenic segregation with a very narrow profile occurring precisely at the silicide/Si interface was identified. Images show that the As is substitutional on the Si lattice sites, implying that it remains electrically active. These structural results imply desirable electronic properties for the device and are consistent with electrical measurements showing a decrease in contact resistance for these samples.

Ho arrangement in the Zn6Mg3Ho icosahedral quasicrystal studied by atomic-resolution Z-contrast STEM.

The atomic structure of the Zn6Mg3Ho icosahedral quasicrystal has been studied by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with Z-contrast (Z: atomic number). We demonstrate that in particular Z-contrast imaging is quite powerful for specifying heavy atom positions in the quasicrystalline compound, as shown by a comparison with high-resolution phase-contrast imaging. It is confirmed that the observed Z-contrast images are fairly well explained by the projected potential of only the Ho atomic arrangement, which was recently proposed by X-ray diffraction analysis; Ho occupies an even-body-center site of the 3-dimensional Penrose lattice. Consequently, the present direct structural observation strongly supports the validity of the proposed Ho site.

On the origin of transverse incoherence in Z-contrast STEM.

We use a Bloch wave approach to further investigate the origins of the incoherent nature of Z-contrast imaging using an ADF detector in a STEM. We discuss how, although at high angles the collected electrons will be mostly thermally scattered in addition to the elastic scattering, it is not the thermal scattering that destroys the coherence, rather the combination of the large detector with the high-angle elastic scattering. This incoherent nature of the elastic scattering arises through the filtering of the 1s-type Bloch states by the detector geometry. We show that it is this filtering that renders an atomic column an independent scatterer insensitive to the configuration of neighbouring columns. It also makes the image contrast insensitive to the effects of beam spreading onto neighbouring columns as the probe propagates through the crystal. We also discuss the implications of this for previous calculations of the intensity of Z-contrast images.