High-resolution scanning transmission electron microscopy

We investigated high-resolution scanning transmission electron microscope (STEM) images obtained from a microscope equipped with a spherical aberration corrector. The probe size (full-width at half-maximum) is reduced to 0.76 A at 200 kV by assuming the fifth-order spherical aberration coefficient C5 = 100 mm. For the simulation we have used the recently developed scheme for a STEM image simulation based on the Fast Fourier Transform (FFT) multislice algorithm. The peak-to-background (P/B) ratio of the high-angle annular dark-field (HAADF) image is significantly improved at a thin specimen region. Although the P/B ratio becomes worse at a thicker region, the resolution is kept high even at such a region. An almost true HAADF signal will be obtained even from a weak-scattering phosphorous column in InP [001] when the background is subtracted. In the bright-field image the coherent character of elastic scattering is suppressed by averaging over a large convergence angle, making the specimen effectively self-luminous. The claim that HAADF imaging is relatively insensitive to a defocus as well as a specimen thickness is valid only qualitatively, and a detailed image simulation will be required for a quantitative analysis as in the case of the conventional transmission electron microscope. It was noted that the delta function approximation for the object function may not be applicable for a very fine probe, and that the achievable resolution of the HAADF imaging will be limited by the widths of the high-angle thermal diffuse scattering potential.

Imaging with high-angle scattered electrons and secondary electrons in the STEM

Scanning transmission electron microscopy (STEM) has been shown as powerful tools for material characterization, especially after the appearance of aberration-corrector which greatly enhances the resolution of STEM. High angle annular dark field (HAADF) and annular bright field (ABF) imaging of the aberration-corrected STEM are widely used due to their high-resolution capabilities and easily interpretable image contrasts. However, HAADF mode of the STEM is still limited in detecting light elements due to the weak electron-scattering power. ABF mode of the STEM could detect light and heavy elements simultaneously, providing unprecedented opportunities for probing unknown structures of materials. Atomic-level structure investigation of materials has been achieved by means of these imaging modes, which is invaluable in many fields for either improving properties of materials or developing new materials. This paper aims to provide a introduction of HAADF and ABF imaging techniques and reviews their applications in characterization of cathode materials, study of electrochemical reaction mechanisms, and exploring the effective design of lithium-ion batteries (LIBs). The future prospects of the STEM are also discussed.

Aberration‐corrected scanning transmission electron microscopy (STEM) is providing previously unattainable views of materials at the atomic scale. The quality of STEM data is now often limited by environmental and experimental factors instead of instrument factors (e.g. electron optics). Some of these environmental limitations can be overcome by collecting and processing STEM data using new data science techniques. These techniques expose new atomic‐scale materials information by improving our ability to measure atomic column positions, 3D structure, single point defects, and atomic‐scale composition. The precision in locating atomic column positions in STEM images is fundamentally limited by the image signal to noise ratio (SNR), but typically practical limits including sample and microscope instabilities that produce distortions in STEM images are encountered before reaching the SNR limit. We have developed a non‐rigid registration (NRR) technique that corrects image distortion of all length scales and enables averaging to enhance the SNR.[1–2] Sub‐pm precision images of single crystal materials have been achieved by NRR and averaging high angle annular dark field (HAADF) STEM images. NRR has allowed measurements of pm‐scale bond length variations of Pt nanocatalyst atoms that may help explain their catalytic activity. Figure 1 shows NRR and averaged HAADF STEM data of a Pt nanocatalyst on an alumina support from a side‐view that exhibits ˜2 pm precision. Displacement measurements of each atomic column position reveal moderate Pt surface bond length contraction and strong but localized strain of Pt atoms near nanoparticle‐support interface. Some of the interface strain is transferred up the twin boundary. Determining the three dimensional atomic structure of materials from two dimensional S/TEM images is a major hurdle. The standardless atom counting technique is one promising route to measure local sample thickness by quantitatively comparing experimental and simulated HAADF STEM images and can be used to deduce 3D structure.[3] Unlike previous examples, NRR and averaging STEM images have allowed standardless atom counting with the uncertainty no longer dominated by Poisson noise [1]. This should allow the unique determination of the number of atoms in atomic columns, although this has not yet been demonstrated due to other sample limitations. Point defects are critical to the properties of a wide range of materials, but imaging single defects is challenging. Quantitative STEM has allowed imaging single substitutional and interstitial dopant impurity atoms[4], but experimentally imaging single vacancies has remained elusive. We have used HAADF STEM frozen phonon multislice simulations to predict the detectability of La vacancies in LaMnO 3 by the reduced atomic column intensity and atomic column distortions around the vacancy. NRR and averaging HAADF STEM images of LaMnO 3 improves the SNR and the image precision sufficiently to potentially detect single La vacancies. Experimental images contain candidate single La vacancies that have local atomic column distortions and intensity variations which match simulated predictions. Atomic‐resolution composition maps can be created using STEM energy dispersive x‐ray spectroscopy (EDS) spectrum imaging (SI). However, long total dwell times that may introduce spatial distortions are required because of low x‐ray production and collection efficiency. The most common approach to minimize distortions is to sum multiple SIs using online drift‐correction software that discards the individual SIs and HAADF images. The quality of EDS SIs of a Nd 2/3 TiO 3 sample was improved by saving the simultaneously acquired raw HAADF and EDS SI series, and applying post acquisition NRR and averaging. The resulting elemental maps show less spatial distortions and more atomic localization of x‐rays. In addition, a novel non‐local principle component analysis further enhances the quality of EDS SIs compared to conventional denoising methods.[5]

Multislice frozen phonon high angle annular dark-field image simulation study of Mo–V–Nb–Te–O complex oxidation catalyst “M1”

An analytical technique with high spatial resolution is required to investigate the composition of thin layers in compound semiconductor multilayer specimens. The techniques currently in use in TEM can broadly be divided into those in which element-specific signals are collected (e.g. EDX) and those in which compositional information is inferred from the variation in the position or magnitude of some contrast feature in the image (e.g. the displacement of thickness fringes in a wedge-shaped sample ). In this paper we report on the determination of the atomic fraction of Al, denoted x, in the Alx Ga1-x As/GaAssystem by analysis of high-angle annular dark field (HADF) images. The principal advantage of using an imaging technique is that the required information is contained in the variation of a signal proportional to an elastic scattering or a thermal-diffuse scattering cross-section rather than one related to an inner shell excitation. As the former are generally a few orders of magnitude larger than the latter it is possible to obtain an image from a wide area of specimen in the time that is required to collect an adequate EDX spectrum from a single position. Thus accumulation of the required information is much more rapid. Instances where it is desirable to perform a quantitative assay over a wide area include specimens in which there are graded interfaces where the atomic fraction varies continuously (e.g. to obtain specific light-guiding properties) or specimens which are deposited at elevated temperatures where Ga desorption can lead to significant variations from the expected composition. In both circumstances it is useful to make measurements at several locations in planes perpendicular to the growth direction. This is possible using techniques where cross-sectional specimens are employed but not where the presence of a wedge is essential.

Determination of the chemical composition of GaNAs using STEM HAADF imaging and STEM strain state analysis

Graphene–graphene oxide (GO) hybrid liquid cells (LCs) for liquid-phase scanning transmission electron microscopy (STEM) were fabricated using a facile method with commercial graphene on a polymethyl methacrylate sheet and GO on a TEM grid. LCs containing Pt nanoparticles (NPs) and pure water were efficiently produced and observed via STEM. Their composition and thickness were characterized by STEM-electron energy-loss spectroscopy. High-resolution (HR) STEM revealed slow-moving Pt NPs’ atomic structures and fast-moving single Pt atoms at the LC’s thin edges. Minimal damage during HR STEM indicated stable LCs because of their excellent electrical and thermal conductivities and radiolysis species scavenging ability.

Quantitative comparisons between experimental and simulated high angle annular dark field (HAADF) images of SrTiO3, PbTiO3, InP and In0·53Ga0·47As in the scanning transmission electron microscope (STEM) are presented. Significant discrepancies are found in the signal to background ratios. For these strongly scattering samples, spatial incoherence and instabilities cannot account for the bulk of this discrepancy, as they could in previous explorations on silicon. The authors discuss where additional experiments and theoretical studies are needed for developing a quantitative understanding of HAADF image contrast.

Role of surface amorphous film in high-resolution high-angle annular dark field STEM imaging

In the past years lead free K x Na 1‐x NbO 3 ceramics with huge d 33 ‐values, comparable to that of PbZr 1‐x Ti x O 3 , have been demonstrated [1]. The high piezoelectric coefficients are generally attributed to the presence of a monoclinic phase at the morphotropic phase boundary and the associated symmetry reduction. A recent work by Schwarzkopf et al. [2] reports that monoclinic phases can also be stabilized in ferroelectric perovskite thin films by utilizing anisotropic epitaxial strain. Another advantage of using epitaxial strain is to engineer by an appropriate choice of the substrate and film composition the arrangement and size of domains, which in turn influences on the piezo‐ and ferroelectric material properties. Ferroelectric thin films and their domain structure are commonly characterized by combining X‐ray diffraction (XRD) and piezoresponse force microscopy (PFM). While XRD gives useful crystallographic information, its local resolution is limited and a detailed study of the domains is impossible. On the other hand PFM has sufficient spatial resolution but does not provide structural information. Plan‐view transmission electron microscopy (TEM) with its ability to study structural data with high accuracy at atomic resolution is in principle ideally suited to gain detailed information on the domain structure of the film. There is, however, one major challenge. In order to preserve the as‐grown strain state of the film and thus its domain structure plan‐view TEM samples with a thick substrate layer are necessary. Since TEM is a projective method it images typically the projected potential averaged along the beam direction including both film and substrate for a plan‐view observation geometry. In this paper we will show that this challenge can be overcome by appropriate imaging conditions using high resolution scanning TEM (STEM) annular dark field (ADF) imaging. Due to the channeling effect and the reduced depth of focus in STEM mode we are able to obtain information mainly from the monoclinic distorted film, while the substrate gives rise to a uniform background intensity. As an example we study a 26 nm thin film of K 0.95 Na 0.05 NbO 3 , which was epitaxially grown on top of a (110) NdScO 3 substrate and shows giant piezoelectricity. PFM reveals a complex periodic domain structure with the coexistence of two different monoclinic domains (yellow and purple stripes in Fig. 2 a). According to XRD these domains differ from each other by the orientation of the in‐plane monoclinic distortion. Based on the structural data provided by XRD and PFM we created a supercell consisting of a K 0.95 Na 0.05 NbO 3 film, which contains two domains with a difference of the in‐plane monoclinic distortion of 2β=8 mrad, on top of a 150 nm thick rigid NdScO 3 substrate without any lattice distortion (Fig. 1 a). The chosen thickness of the substrate is a minimum value necessary to preserve the as‐grown strain state of the film. Frozen phonon simulations reveal that if the incident electron probe is transmitted first through the film, then the final high resolution STEM ADF image pattern directly reflects the domain structure of the K 0.95 Na 0.05 NbO 3 film with the evaluated shear angle 2β as in the supercell (Fig. 1 b). However, if the electron beam direction is inverted, i.e. the beam is transmitted first through the substrate, then no lattice distortion corresponding to the domain structure of the film is found in the obtained high resolution STEM ADF image. An analysis of the evolution of the electron probe inside the supercell (Fig. 1 c) shows that this 3‐dimensional sensitivity is caused by the electron channeling effect. In the present case the high resolution pattern in the final image is produced basically in the first 20‐30 nm of transmitted material. After this thickness the wave function of the electron beam is widely dispersed, irrespectively of the initial position of the electron probe. Thus the subsequent part of the specimen mainly contributes to a uniform background intensity in the final image. Consequently, the frozen phonon simulations demonstrate the feasibility of our approach to study domains in epitaxially strained ferrolectic films by plan‐view high resolution STEM ADF imaging. An experimental example for the STEM analysis of the domain structure for the above mentioned K 0.95 Na 0.05 NbO 3 film is shown in Fig 2 c. A model of the complex domain pattern including the sub‐domain structure will be discussed in the paper.

First experimental test of a new monochromated and aberration-corrected 200 kV field-emission scanning transmission electron microscope

The growth and microstructure of the thermally grown oxide (TGO) underneath the electron beam physical vapor deposited (EB-PVD) yttria stabilized zirconia (YSZ) topcoat were examined after short-term isothermal oxidation (1100 °C, up to 50 hours) for the as-coated and gritblasted (Ni,Pt)Al bondcoats. Microstructural analysis was carried out by a high resolution scanning transmission electron microscope equipped with bright/dark field imaging, high angle annular dark field imaging, and nano-spot energy dispersive spectroscopy. Presence of mixed oxide zone (MOZ) and a continuous Al2O3 oxide zone (COZ) was observed on the thermal barrier coating (TBC) with the as-coated (Ni,Pt)Al bondcoat. However, on the TBC with grit-blasted bondcoat, only the continuous-columnar Al2O3 scale was observed. For the as-coated type bondcoat, numerous voids were observed near the interface between MOZ and COZ after isothermal oxidation. On the other hand, COZ showed parabolic growth without any formation of voids for the grit-blasted specimens.

A novel approach for 3D morphological characterization of silica nanoparticle population through HAADF-STEM

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