Atomic Columns Research Articles

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples.

We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a β-Si3N4 wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and lattice fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.

Elemental redistributions at structural defects in Cu(In,Ga)Se2 thin films for solar cells

The microstructural evolution of Cu(In,Ga)Se2 absorber layers during a three-stage-type co-evaporation process was studied to elucidate the effect of a Cu-rich stage on the formation of extended structural defects. Defect densities for two Cu-poor samples, one interrupted before and one after this crucial Cu-rich composition stage, were investigated by scanning transmission electron microscopy (STEM) imaging. The structure and chemical nature of individual defects were investigated by aberration-corrected high-resolution STEM in combination with electron energy-loss spectroscopy on the atomic-scale. In spite of the different defect densities between the two samples, most of the individual defects exhibited similar chemistry. In particular, the elemental distributions of atomic columns at {112} twin planes, which are very frequent in Cu(In,Ga)Se2 thin films, were found to be the same as in the defect-free grain interiors. In contrast, within grain boundaries, dislocation cores, and other structurally more complex defects, elemental redistributions of Cu and In were observed.

Analytical ABF-STEM imaging of Li ions in rechargeable batteries.

Rechargeable batteries are being intensively investigated in an attempt to solve the energy issues while meeting the environmental demands. Even though Li-ion batteries (LIB) with high energy and light weight have been commercialized within the last 20 years, these devices currently require higher energy density, output power and sustainability characteristics. The atomic behavior of Li ion that determines LIB's performance is hardly characterized by transmission electron microscopy (TEM) owing to its weak electron-scattering power. In this sense, annular bright-field (ABF) scanning TEM (STEM), in which the contrast has a low scaling rate with the atomic number, has been proven to be a robust technique for simultaneous imaging of light and heavy elements. The s-state model, in which electron channeling along the atomic column allows the intensity to be focusing in the forward direction, has successfully explained the theory of ABF contrast. Furthermore, the detector angle range, the defocus-thickness dependence and the accelerating voltage (among other parameters) were discussed for optimized imaging conditions. ABF-STEM has shown powerful capabilities in resolving the atomic structure and the chemistry of electrodes (e.g. Li-ion occupation and diffusion, phase transformation and interface reaction), thereby providing critical insights into the physical properties, the battery performance and the design guidance of LIB. The future directions of ABF imaging for the characterization of LIB materials were also reviewed.

Direct visualization of the periodic displacement of Ta atoms in the commensurate charge density wave phase of1T−TaSe2in real space

In $1T\ensuremath{-}\mathrm{TaS}{\mathrm{e}}_{2}$, annular dark-field scanning transmission electron microscopy (ADF-STEM) of the commensurate charge density wave (CCDW) phase shows the periodic image contrast superimposed on the atomic columns. The periodicity of the image contrast is consistent with the superlattice formed by the periodic displacement of Ta atoms by the CCDW. Comparing the ADF-STEM with simulated images shows that the image contrast is formed by the displacement of Ta atoms. This result means that the displacement of Ta atoms by the CCDW is directly observed in real space and suggests that a coherent CDW forms on every layer of $1T\ensuremath{-}\mathrm{TaS}{\mathrm{e}}_{2}$.

Beam-induced atomic migration at Ag-containing nanofacets at an asymmetric Cu grain boundary

Abstract

Depth sectioning combined with atom-counting in HAADF STEM to retrieve the 3D atomic structure

Aberration correction in scanning transmission electron microscopy (STEM) has greatly improved the lateral and depth resolution. When using depth sectioning, a technique during which a series of images is recorded at different defocus values, single impurity atoms can be visualised in three dimensions. In this paper, we investigate new possibilities emerging when combining depth sectioning and precise atom-counting in order to reconstruct nanosized particles in three dimensions. Although the depth resolution does not allow one to precisely locate each atom within an atomic column, it will be shown that the depth location of an atomic column as a whole can be measured precisely. In this manner, the morphology of a nanoparticle can be reconstructed in three dimensions. This will be demonstrated using simulations and experimental data of a gold nanorod.

Atomic Resolution Imaging of Halide Perovskites

The radiation-sensitive nature of halide perovskites has hindered structural studies at the atomic scale. We overcome this obstacle by applying low dose-rate in-line holography, which combines aberration-corrected high-resolution transmission electron microscopy with exit-wave reconstruction. This technique successfully yields the genuine atomic structure of ultrathin two-dimensional CsPbBr3 halide perovskites, and a quantitative structure determination was achieved atom column by atom column using the phase information of the reconstructed exit-wave function without causing electron beam-induced sample alterations. An extraordinarily high image quality enables an unambiguous structural analysis of coexisting high-temperature and low-temperature phases of CsPbBr3 in single particles. On a broader level, our approach offers unprecedented opportunities to better understand halide perovskites at the atomic level as well as other radiation-sensitive materials.

Atom-counting in High Resolution Electron Microscopy:TEM or STEM – That's the question

In this work, a recently developed quantitative approach based on the principles of detection theory is used in order to determine the possibilities and limitations of High Resolution Scanning Transmission Electron Microscopy (HR STEM) and HR TEM for atom-counting. So far, HR STEM has been shown to be an appropriate imaging mode to count the number of atoms in a projected atomic column. Recently, it has been demonstrated that HR TEM, when using negative spherical aberration imaging, is suitable for atom-counting as well. The capabilities of both imaging techniques are investigated and compared using the probability of error as a criterion. It is shown that for the same incoming electron dose, HR STEM outperforms HR TEM under common practice standards, i.e. when the decision is based on the probability function of the peak intensities in HR TEM and of the scattering cross-sections in HR STEM. If the atom-counting decision is based on the joint probability function of the image pixel values, the dependence of all image pixel intensities as a function of thickness should be known accurately. Under this assumption, the probability of error may decrease significantly for atom-counting in HR TEM and may, in theory, become lower as compared to HR STEM under the predicted optimal experimental settings. However, the commonly used standard for atom-counting in HR STEM leads to a high performance and has been shown to work in practice.

Atomic scale imaging of structural variations in La(1-x)/3LixNbO3 (0 ≤ x ≤ 0.13) solid electrolytes

Solid-state Li-ion battery electrolyte materials La(1-x)/3LixNbO3 (LLNbO) are layered A-site-deficient perovskites with complex structural features resulting from their high intrinsic cation vacancy concentrations. We report an atomic-scale study of a series of single crystals of LLNbO with Li contents x = 0, 0.04, 0.07, and 0.13 using state-of-the-art scanning transmission electron microscopy. By combining high angle annular dark field and annular bright field imaging techniques, columns of heavy and light atoms could be imaged simultaneously with atomic resolution. Structure modulation within La-rich layers, observed in all samples, was strongest for Li content x = 0.07, the content which has been reported to exhibit the highest conductivity. Unlike for end member La1/3NbO3 (x = 0), for Li content x = 0.04, significant tilting of NbO6 octahedra occurs, with regions of different tilting directions corresponding to nanodomains within the crystal. This tilting and the associated nanodomains are absent when x = 0.07, but occur again when x = 0.13, with even greater distortion of NbO6 octahedra. These structural differences help explain the changes in Li-ion conductivity with Li content in LLNbO.

Peak separation method for sub-lattice strain analysis at atomic resolution: Application to InAs/GaSb superlattice

We report on a direct measurement of cation and anion sub-lattice strain in an InAs/GaSb type-II strained layer superlattice (T2SLs) using atomic resolution imaging and advanced image processing. Atomic column positions in InAs and GaSb are determined by separating the cation and anion peak intensities. Analysis of the InAs/GaSb T2SLs reveals the compressive strain in the nominal GaSb layer and tensile strain at interfaces between constituent layers, which indicate In incorporation into the nominal GaSb layer and the formation of GaAs like interfaces, respectively. The results are compared with the model-dependent X-ray diffraction measurements in terms of interfacial chemical intermixing and strain. Together, these techniques provide a robust measurement of atomic-scale strain which is vital to determine T2SL properties.

Three-dimensional imaging of individual point defects using selective detection angles in annular dark field scanning transmission electron microscopy

We propose a new scanning transmission electron microscopy (STEM) technique that can realize the three-dimensional (3D) characterization of vacancies, lighter and heavier dopants with high precision. Using multislice STEM imaging and diffraction simulations of β-Ga2O3 and SrTiO3, we show that selecting a small range of low scattering angles can make the contrast of the defect-containing atomic columns substantially more depth-dependent. The origin of the depth-dependence is the de-channeling of electrons due to the existence of a point defect in the atomic column, which creates extra “ripples” at low scattering angles. The highest contrast of the point defect can be achieved when the de-channeling signal is captured using the 20–40mrad detection angle range. The effect of sample thickness, crystal orientation, local strain, probe convergence angle, and experimental uncertainty to the depth-dependent contrast of the point defect will also be discussed. The proposed technique therefore opens new possibilities for highly precise 3D structural characterization of individual point defects in functional materials.

Visualization of lithium ions by annular bright field imaging.

The detection of lithium ions is required for characterization of lithium ion batteries, since the movement of lithium ions in the battery is one of the key ways to improve the performance. Annular bright field (ABF) imaging enables us to visualize individual lithium atomic columns simultaneously with heavy elements. Furthermore, it has been found that the number of lithium ions at the column is countable when the specimen is thin. These results suggest that movement of lithium ions in the material can be observed by taking consecutive ABF images during operation or in situ ABF observation. Actually, the spinel structure of L2V4O crystals was directly observed to be transformed into the defective NaCl structure at the moment when lithium ions were extracted from the original position during electron beam irradiation. We clarify the features of ABF imaging by comparing it with HAADF imaging in order to understand what information can be obtained by ABF imaging directly.

Locating light and heavy atomic column positions with picometer precision using ISTEM

Recently, imaging scanning transmission electron microscopy (ISTEM) has been proposed as a promising new technique combining the advantages of conventional TEM (CTEM) and STEM (Rosenauer et al., 2014 [1]). The ability to visualize light and heavy elements together makes it a particularly interesting new, spatially incoherent imaging mode. Here, we evaluate this technique in term of precision with which atomic column locations can be measured. By using statistical parameter estimation theory, we will show that these locations can be accurately measured with a precision in the picometer range. Furthermore, a quantitative comparison is made with HAADF STEM imaging to investigate the advantages of ISTEM.

(Invited) Transistor Strain Measurement Using Electron Beam Techniques

Strain and the related stress in nanoelectronic devices play a critical role in semiconductor technologies. In general, strain arises from the lattice mismatch between dissimilar materials and thus strain is critical for the design and growth of epitaxial heterostructures in micro- and nano-electronics. In transistors, strained silicon channels provide a major boost to the performance of metal-oxide-semiconductor field-effect transistors (MOSFETs), e.g., the use of epitaxial strain of Si and Ge alloys in PMOS [1]. In the near term, the transistor technology is pushing towards the sub 10 nm technology node. At this length scale, resolving strain in the devices requires a resolution of sub nanometer with not only high resolution, also high accuracy and reliability. The use of novel architectures and alternative materials in the transistor technology also demands metrology capable of handling these structural and material changes. High resolution strain measurement can be performed using high energy electrons either in reciprocal space using convergent beam electron diffraction (CBED), scanning electron nanodiffraction (SEND), nanobeam electron diffraction (NBED) [2], or scanning precession electron diffraction (SPED) or in real space through direct imaging using high resolution electron microscopy (HREM), scanning transmission electron microscopy (STEM) [3] or dark-field electron holography (DFEH)[4]. The resolution and strain measurement sensitivities range from Å to several nm and 10-3 to 10-4respectively. Recent developments in aberration corrected STEM especially have improved the spatial resolution to sub- Å and the precision of measuring atomic column positions to picometers [3]. This talk will review high resolution and high sensitivity electron beam techniques for strain characterization in nanodevices and device materials. Examples include planar and a finfet transistors and III-V heterostructures. For finfet transistors, we demonstrate that NBED provides an effective approach for strain mapping (See figure below) and some of challenges will also be highlighted. In III-V heterostructures, we will describe atomic resolution Z-contrast imaging based methods for high resolution interfacial strain analysis and its applications for the investigation of interfacial intermixing and atomic layer-by-layer strain profiles in InAs/GaSb and InAs/InAsSb superlattices, targeted for middle- and long-wavelength IR detection.

Investigation of Pristine Li1.2Ni0.13Mn0.56Co0.13O2 By Advanced TEM

Layered Li-transition metal (TM) oxides are very promising materials for new Li ion battery cathodes. Compounds with an increased content of Li and Mn are particularly interesting because they exhibit a high capacity of 200-300 mAhg-1 even after an initial drop in capacity during the first charging cycle.[1] The structure of such compounds and their capacity degradation over multiple cycles is not fully understood due to the complexity of the crystallographic phases, the ambiguities in the diffraction data and the presence of additional phases. The investigated compound Li1.2Ni0.13Mn0.56Co0.13O2 is derived from LiNi0.33Mn0.33Co0.33O2, (NMC). Due to the higher capacity of the Li-rich compound, it will be referred to as high energy NMC (HENMC).[2] The chemical formula of HENMC can be rewritten as Li2MnO3 · LiNi0.33Mn0.33Co0.33O2 which implies that this compound is a mixture of trigonal NMC and monoclinic Li2MnO3. In NMC, there are two different cation layers which are exclusively occupied by Li and TM respectively. In Li2MnO3 one third of the TM sites are occupied by Li. HENMC must be either a phase mixture with distinct domains of the two oxides or a solid solution of the two, with “Li rich” positions in the TM layers and an overall reduced, monoclinic symmetry (Figure 1).[3] The exact nature of HENMC and similar materials is crucial to understand the de-lithiation processes during charging cycles. The layered structure and its relation to other phases was investigated with a combination of electron diffraction in bright-field and scanning modes with high resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) imaging. Due to the high structural similarity of NMC and Li2MnO3, all lattice distances which can be found for NMC can also be found for Li2MnO3; as seen in the respective electron diffraction patterns (Figure 2). HAADF-STEM imaging reveals the presence of Li-rich positions in the TM layers as indicated by a lower intensity of the related atom columns (Figure 3). Due to the stacking faults of the TM layers, the interpretation of contrast in any other orientation becomes by far less intuitive as we will demonstrate. A LiTM2O4 spinel phase is found on the surface quite frequently. Its relation to the bulk phase was derived from high resolution images. STEM-EELS revealed the presence of surface reduction due to oxygen depletion in HENMC. This work thus provides structural information about the synthesized material in its pristine state to facilitate understanding the changes the material undergoes during the (de-)lithiation segments of electrochemical cycling. Further investigations on such effects are currently being carried out and will be discussed. [1] B. L. Ellis, K. T. Lee, L. F. Nazar, Chem. Mater. 2010, 22, 691-714. [2] C. S. Johnson, N. Li, C. Lefief, J. T. Vaughey, M. M. Thackeray, Chem. Mater. 2008, 20, 6095-6106. [3] H. Yu, H. Zhou, J. Phys. Chem. Lett. 2013, 4, 1268−1280. Figure 1

StatSTEM: An efficient approach for accurate and precise model-based quantification of atomic resolution electron microscopy images

An efficient model-based estimation algorithm is introduced to quantify the atomic column positions and intensities from atomic resolution (scanning) transmission electron microscopy ((S)TEM) images. This algorithm uses the least squares estimator on image segments containing individual columns fully accounting for overlap between neighbouring columns, enabling the analysis of a large field of view. For this algorithm, the accuracy and precision with which measurements for the atomic column positions and scattering cross-sections from annular dark field (ADF) STEM images can be estimated, has been investigated. The highest attainable precision is reached even for low dose images. Furthermore, the advantages of the model-based approach taking into account overlap between neighbouring columns are highlighted. This is done for the estimation of the distance between two neighbouring columns as a function of their distance and for the estimation of the scattering cross-section which is compared to the integrated intensity from a Voronoi cell. To provide end-users this well-established quantification method, a user friendly program, StatSTEM, is developed which is freely available under a GNU public license.

Electron-Beam-Induced Antiphase Boundary Reconstructions in a ZrO2-LSMO Pillar-Matrix System.

The availability of aberration correctors for the probe-forming lenses makes simultaneous modification and characterization of materials down to atomic scale inside a transmission electron microscopy (TEM) realizable. In this work, we report on the electron-beam-induced reconstructions of three types of antiphase boundaries (APBs) in a probe-aberration-corrected TEM. With the utilization of high-angle annular dark-field scanning transmission electron microscopy (STEM), annular bright-field STEM, and electron energy-loss spectroscopy, the motion of both heavy element Mn and light element O atomic columns under moderate electron beam irradiation are revealed at atomic resolution. Besides, Mn segregated in the APBs was observed to have reduced valence states which can be directly correlated with oxygen loss. Charge states of the APBs are finally discussed on the basis of these experimental results. This study provides support for the design of radiation-engineering solid-oxide fuel cell materials.

Influence of experimental conditions on atom column visibility in energy dispersive X-ray spectroscopy

Here we report the influence of key experimental parameters on atomically resolved energy dispersive X-ray spectroscopy (EDX). In particular, we examine the role of the probe forming convergence semi-angle, sample thickness, lattice spacing, and dwell/collection time. We show that an optimum specimen-dependent probe forming convergence angle exists to maximize the signal-to-noise ratio of the atomically resolved signal in EDX mapping. Furthermore, we highlight that it can be important to select an appropriate dwell time to efficiently process the X-ray signal. These practical considerations provide insight for experimental parameters in atomic resolution energy dispersive X-ray analysis.

Dynamical Scattering and Electron Channeling in Orthorhombic and Tetragonal LaFeAsO

In the study of LaFeAsO and doped compounds thereof, high-resolution transmission electron microscopy (TEM) has been used to characterize the structural and morphological properties, while cryo-TEM has been used to purportedly observe the structural phase transition occurring at 160 K. Often, the appearance and disappearance of Bragg spots in diffraction patterns, as well as changes in diffraction contrast in bright-field images, have been pointed to as indicators of the phase transition. Here we show that effects not related to the transition can produce signatures reminiscent of those typically associated with the symmetry change. In particular, we demonstrate the effects of electron channeling and multiple scattering on intensity modulation of atomic columns and Bragg spots in high-angle annular dark-field scanning TEM (HAADF-STEM) images and parallel-beam electron diffraction (PBED) patterns, respectively, from both tetragonal and orthorhombic LaFeAsO. From electron-transparent lamellae, we quantify t...

Influence of grain boundary characteristics on thermal stability in nanotwinned copper.

High density grain boundaries provide high strength, but may introduce undesirable features, such as high Fermi levels and instability. We investigated the kinetics of recovery and recrystallization of Cu that was manufactured to include both nanotwins (NT) and high-angle columnar boundaries. We used the isothermal Johnson-Mehl-Avrami-Kolmogorov (JMAK) model to estimate activation energy values for recovery and recrystallization and compared those to values derived using the non-isothermal Kissinger equation. The JMAK model hinges on an exponent that expresses the growth mechanism of a material. The exponent for this Cu was close to 0.5, indicating low-dimensional microstructure evolution, which is associated with anisotropic twin coarsening, heterogeneous recrystallization, and high stability. Since this Cu was of high purity, there was a negligible impurity-drag-effect on boundaries. The twin coarsening and heterogeneous recrystallization resulted from migration of high-angle columnar boundaries with their triple junctions in one direction, assisted by the presence of high concentration vacancies at boundaries. Analyses performed by electron energy loss spectroscopy of atomic columns at twin boundaries (TBs) and in the interior showed similar plasma peak shapes and L3 edge positions. This implies that values for conductivity and Fermi level are equal for atoms at TBs and in the interior.