Factors limiting quantitative phase retrieval in atomic-resolution differential phase contrast scanning transmission electron microscopy using a segmented detector.

Quantitative differential phase contrast imaging of materials in atomic-resolution scanning transmission electron microscopy using segmented detectors is limited by various factors, including coherent and incoherent aberrations, detector positioning and uniformity, and scan-distortion. By comparing experimental case studies of monolayer and few-layer graphene with image simulations, we explore which parameters require the most precise characterisation for reliable and quantitative interpretation of the reconstructed phases. Coherent and incoherent lens aberrations are found to have the most significant impact. For images over a large field of view, the impact of noise and non-periodic boundary conditions are appreciable, but in this case study have less of an impact than artefacts introduced by beam deflections coupling to beam scanning (imperfect tilt-shift purity).

Improving the depth resolution of STEM-ADF sectioning by 3D deconvolution.

Although the possibility of locating single atom in three dimensions using the scanning transmission electron microscope (STEM) has been discussed with the advent of aberration correction technology, it is still a big challenge. In this report we have developed deconvolution routines based on maximum entropy method (MEM) and Richardson-Lucy algorithm (RLA), which are applicable to the STEM-annular dark-field (ADF) though-focus images to improve the depth resolution. The new three-dimensional (3D) deconvolution routines require a limited defocus-range of STEM-ADF images that covers a whole sample and some vacuum regions. Since the STEM-ADF probe is infinitely elongated along the optical axis, a 3D convolution is performed with a two-dimensional (2D) convolution over xy-plane using the 2D fast Fourier transform in reciprocal space, and a one-dimensional convolution along the z-direction in real space. Using our new deconvolution routines, we have processed simulated focal series of STEM-ADF images for single Ce dopants embedded in wurtzite-type AlN. Applying the MEM, the Ce peaks are clearly localized along the depth, and the peak width is reduced down to almost one half. We also applied the new deconvolution routines to experimental focal series of STEM-ADF images of a monolayer graphene. The RLA gives smooth and high-P/B ratio scattering distribution, and the graphene layer can be easily detected. Using our deconvolution algorithms, we can determine the depth locations of the heavy dopants and the graphene layer within the precision of 0.1 and 0.2 nm, respectively. Thus, the deconvolution must be extremely useful for the optical sectioning with 3D STEM-ADF images.

Atomic resolution electron microscopy in a magnetic field free environment.

Atomic-resolution electron microscopes utilize high-power magnetic lenses to produce magnified images of the atomic details of matter. Doing so involves placing samples inside the magnetic objective lens, where magnetic fields of up to a few tesla are always exerted. This can largely alter, or even destroy, the magnetic and physical structures of interest. Here, we describe a newly developed magnetic objective lens system that realizes a magnetic field free environment at the sample position. Combined with a higher-order aberration corrector, we achieve direct, atom-resolved imaging with sub-Å spatial resolution with a residual magnetic field of less than 0.2 mT at the sample position. This capability enables direct atom-resolved imaging of magnetic materials such as silicon steels. Removing the need to subject samples to high magnetic field environments enables a new stage in atomic resolution electron microscopy that realizes direct, atomic-level observation of samples without unwanted high magnetic field effects.

Large angle illumination enabling accurate structure reconstruction from thick samples in scanning transmission electron microscopy.

Most reconstructions of the electrostatic potential of a specimen at atomic resolution assume a thin and weakly scattering sample, restricting accurate quantification to specimens only tens of Ångströms thick. We demonstrate that using large-angle-illumination scanning transmission electron microscopy (STEM)-a probe forming aperture with convergence angle larger than about 50 mrad-allows us to better meet the weak phase object approximation and thereby accurately reconstruct the electrostatic potential in samples thicker than the order of 100 Å.

On the quantitativeness of grain boundary chemistry using STEM EDS: A ZrO2 Σ9 model grain boundary case study.

Atomic-resolution energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) has recently been shown to be a powerful approach to investigate local chemistry of nanoscale structures quantitatively. While most of the studies have been focused on the quantification of the chemical composition in bulk crystals, few were discussed on interfaces. In this study, we theoretically explored the applicability of STEM EDS for the quantification of local chemistry in grain boundaries (GBs), where the electron channeling can be dramatically changed compared with the bulk due to non-periodic atomic arrangement. We find that: (1) line scan analysis across the GBs or mapping analysis, which have been widely used for interface analysis, sometimes leads to misinterpretation of true interface chemistry. (2) Tilting the specimen, which is effective to reduce the effects of scattering, is not always useful for the quantification of GBs. (3) EDS analysis covering the whole GB structure unit, such as using a box scan, can provide true chemical information. Our study provides useful insights into characterization of interface chemistry using STEM EDS.

A new method to detect and correct sample tilt in scanning transmission electron microscopy bright-field imaging.

Important properties of functional materials, such as ferroelectric shifts and octahedral distortions, are associated with displacements of the positions of lighter atoms in the unit cell. Annular bright-field scanning transmission electron microscopy is a good experimental method for investigating such phenomena due to its ability to image light and heavy atoms simultaneously. To map atomic positions at the required accuracy precise angular alignment of the sample with the microscope optical axis is necessary, since misalignment (tilt) of the specimen contributes to errors in position measurements of lighter elements in annular bright-field imaging. In this paper it is shown that it is possible to detect tilt with the aid of images recorded using a central bright-field detector placed within the inner radius of the annular bright-field detector. For a probe focus near the middle of the specimen the central bright-field image becomes especially sensitive to tilt and we demonstrate experimentally that misalignment can be detected with a precision of less than a milliradian, as we also confirm in simulation. Coma in the probe, an aberration that can be misidentified as tilt of the specimen, is also investigated and it is shown how the effects of coma and tilt can be differentiated. The effects of tilt may be offset to a large extent by shifting the diffraction plane detector an amount equivalent to the specimen tilt and we provide an experimental proof of principle of this using a segmented detector system.

Atomistic origin of an ordered superstructure induced superconductivity in layered chalcogenides.

Interplay among various collective electronic states such as charge density wave and superconductivity is of tremendous significance in low-dimensional electron systems. However, the atomistic and physical nature of the electronic structures underlying the interplay of exotic states, which is critical to clarifying its effect on remarkable properties of the electron systems, remains elusive, limiting our understanding of the superconducting mechanism. Here, we show evidence that an ordering of selenium and sulphur atoms surrounding tantalum within star-of-David clusters can boost superconductivity in a layered chalcogenide 1T-TaS2-xSex, which undergoes a superconducting transition in the nearly commensurate charge density wave phase. Advanced electron microscopy investigations reveal that such an ordered superstructure forms only in the x area, where the superconductivity manifests, and is destructible to the occurrence of the Mott metal-insulator transition. The present findings provide a novel dimension in understanding the relationship between lattice and electronic degrees of freedom.

On the quantitativeness of EDS STEM.

Chemical mapping using energy dispersive X-ray spectroscopy (EDS) in scanning transmission electron microscopy (STEM) has recently shown to be a powerful technique in analyzing the elemental identity and location of atomic columns in materials at atomic resolution. However, most applications of EDS STEM have been used only to qualitatively map whether elements are present at specific sites. Obtaining calibrated EDS STEM maps so that they are on an absolute scale is a difficult task and even if one achieves this, extracting quantitative information about the specimen - such as the number or density of atoms under the probe - adds yet another layer of complexity to the analysis due to the multiple elastic and inelastic scattering of the electron probe. Quantitative information may be obtained by comparing calibrated EDS STEM with theoretical simulations, but in this case a model of the structure must be assumed a priori. Here we first theoretically explore how exactly elastic and thermal scattering of the probe confounds the quantitative information one is able to extract about the specimen from an EDS STEM map. We then show using simulation how tilting the specimen (or incident probe) can reduce the effects of scattering and how it can provide quantitative information about the specimen. We then discuss drawbacks of this method - such as the loss of atomic resolution along the tilt direction - but follow this with a possible remedy: precession averaged EDS STEM mapping.

Observations of crack propagation along a Zr-doped alumina grain boundary.

Structural ceramics are typically used in polycrystalline form. It is well known that polycrystalline ceramics often show the intergranular fracture. To improve their mechanical properties, transition metals can be used as dopants into a bulk material, which tend to segregate into the grain boundaries[1]. However, the effect of dopant segregation on grain boundary fracture is still uncertain. In order to investigate the fracture behavior of a dopant-segregated grain boundary, we observed the crack propagation of a Zr-doped alumina grain boundary by in situ nanoindentation in a transmission electron microscope (TEM), and characterized the fracture surface by scanning TEM (STEM).An alumina bicrystal with a Zr-doped Σ13 grain boundary was fabricated by diffusion bonding at 1500(o)C for 10 hours in air, where a face of one crystal was coated by Zr metal in advance to the bonding (Fig. 1a). A TEM sample was prepared from the bicrystal by mechanical grinding and Ar ion milling. For in situ indentation, the sample had a free edge perpendicular to the grain boundary (Fig. 1b). The indentation experiment was performed by using a double-tilt indentation holder (Nanofactory) and JEM-2010 (200kV, JEOL). The fracture surface was further observed by high angle annular dark field (HAADF) STEM (ARM-200F, 200kV, JEOL).jmicro;63/suppl_1/i20-a/DFU064F1F1DFU064F1Fig. 1.(a) Schematic illustrations of bicrystal fabrication by diffusion bonding and (b) Bright field TEM image showing the geometric arrangement of the in situ nanoindentation experiment In the in situ TEM nanoindentation experiment, at first a crack was introduced in bulk close to the grain boundary and propagated with the amount of indentation. After the crack reached the grain boundary, it preferentially propagated along the grain boundary. To identify the crack pass at the atomic level, the STEM analysis was performed. We found that three-atomic-layer Zr was formed in the unbroken region of the grain boundary, whereas one to three Zr layers remained on the fracture surface. This indicates that the crack propagated within the segregation region of Zr in the grain boundary. In the presentation, we will discuss the crack propagation behavior and the atomic structure of the fracture surfaces in detail.

Enhanced light element imaging in atomic resolution scanning transmission electron microscopy.

We show that an imaging mode based on taking the difference between signals recorded from the bright field (forward scattering region) in atomic resolution scanning transmission electron microscopy provides an enhancement of the detectability of light elements over existing techniques. In some instances this is an enhancement of the visibility of the light element columns relative to heavy element columns. In all cases explored it is an enhancement in the signal-to-noise ratio of the image at the light column site. The image formation mechanisms are explained and the technique is compared with earlier approaches. Experimental data, supported by simulation, are presented for imaging the oxygen columns in LaAlO3. Case studies looking at imaging hydrogen columns in YH2 and lithium columns in Al3Li are also explored through simulation, particularly with respect to the dependence on defocus, probe-forming aperture angle and detector collection aperture angles.

The effect of vacancies on the annular dark field image contrast of grain boundaries: a SrTiO(3) case study.

The analysis of grain boundary structure in high resolution electron microscopy is often hindered by contrast variation within the grain boundary region which is not explained by simple models of the grain boundary structure. Recent work suggests that structural disorder along the beam direction and the presence of vacancies contribute significantly to this effect. One might expect a significant reduction in contrast in a Z-contrast image of a grain boundary would imply that vacancies present must result from the absence of heavier elements. Using a [001](210) Σ5 grain boundary in SrTiO(3) as a test case and first principles structure relaxation to calculate stable defect structures, we show that the reduction in the intensity from fully occupied Sr columns due to the structural distortion resulting from a nearby O vacancy can be as great as that due to introducing a Sr vacancy in the column itself. The effect on energy dispersive X-ray spectroscopy signals is also considered, but found to be smaller than that on Z-contrast images.

Scanning transmission electron microscopy imaging dynamics at low accelerating voltages.

Motivated by the desire to minimize specimen damage in beam sensitive specimens, there has been a recent push toward using relatively low accelerating voltages (<100 kV) in scanning transmission electron microscopy. To complement experimental efforts on this front, this paper seeks to explore the variations with accelerating voltage of the imaging dynamics, both of the channelling of the fast electron and of the inelastic interactions. High-angle annular-dark field, electron energy loss spectroscopic imaging and annular bright field imaging are all considered.

Prospects for lithium imaging using annular bright field scanning transmission electron microscopy: a theoretical study.

There is strong interest in lithium imaging, particularly because of its significance in battery materials. However, light atoms only scatter electrons weakly and atomic resolution direct imaging of lithium has proven difficult. This paper explores theoretically the conditions under which lithium columns can be expected to be directly visible using annular bright field scanning transmission electron microscopy. A detailed discussion is given of the controllable parameters and the conditions most favourable for lithium imaging.

Electrically induced ferromagnetism at room temperature in cobalt-doped titanium dioxide.

The electric field effect in ferromagnetic semiconductors enables switching of the magnetization, which is a key technology for spintronic applications. We demonstrated electric field-induced ferromagnetism at room temperature in a magnetic oxide semiconductor, (Ti,Co)O(2), by means of electric double-layer gating with high-density electron accumulation (>10(14) per square centimeter). By applying a gate voltage of a few volts, a low-carrier paramagnetic state was transformed into a high-carrier ferromagnetic state, thereby revealing the considerable role of electron carriers in high-temperature ferromagnetism and demonstrating a route to room-temperature semiconductor spintronics.

Direct oxygen imaging within a ceramic interface, with some observations upon the dark contrast at the grain boundary.

Annular bright field scanning transmission electron microscopy, which has recently been established to produce directly interpretable images with both light and heavier atomic columns visible simultaneously, is shown to allow directly interpretable imaging of the oxygen columns within the Σ13[12¯10](101¯4) pyramidal twin grain boundary in α-Al(2)O(3). By using information in the high-angle annular dark field image and annular bright field images simultaneously, we estimate the specimen thickness and finite source size, and use them to explore in simulation the issue of dark contrast in the vicinity of the grain boundary in the annular dark field image.

Dynamics of annular bright field imaging in scanning transmission electron microscopy.

We explore the dynamics of image formation in the so-called annular bright field mode in scanning transmission electron microscopy, whereby an annular detector is used with detector collection range lying within the cone of illumination, i.e. the bright field region. We show that this imaging mode allows us to reliably image both light and heavy columns over a range of thickness and defocus values, and we explain the contrast mechanisms involved. The role of probe and detector aperture sizes is considered, as is the sensitivity of the method to intercolumn spacing and local disorder.

Quantitative analyses of oxidation states for cubic SrMnO(3) and orthorhombic SrMnO(2.5) with electron energy loss spectroscopy.

The oxidation state of Mn in cubic SrMnO(3) and orthorhombic SrMnO(2.5) was investigated by electron energy loss (EEL) spectroscopy. Change in the oxidation state of Mn produced some spectral changes in the O-K edge as well as in the Mn-L(2,3) edge EEL spectra. This study demonstrated that the oxidation state of Mn and the amount of oxygen vacancies in cubic SrMnO(3) and orthorhombic SrMnO(2.5) could be quantified by analyzing the features of the O-K edge spectrum and the Mn L(3)∕L(2) ratio in the Mn-L(2,3) edge spectrum. Our quantitative analysis showed that the spectral changes in the Mn-L(2,3) edge were mainly caused by the oxidation state of Mn, whereas those in the O-K edge could be sensitive to both the oxidation state of Mn and to lattice distortions.

What atomic resolution annular dark field imaging can tell us about gold nanoparticles on TiO(2) (110).

Annular dark field scanning transmission electron microscopy imaging was recently applied to a catalyst consisting of gold nanoparticles on TiO(2) (110), showing directly that the gold atoms in small nanoparticles preferentially attach to specific sites on the TiO(2) (110) surface. Here, through simulation, a parameter exploration of the imaging conditions which maximise the visibility of such nanoparticles is presented. Aberration correction, finite source size and profile imaging are all considered while trying to extracting the maximum amount of information from a given sample. Comment is made on the role of the thermal vibration of the atoms in the nanoparticle, the magnitude of which is generally not known a priori but which affects the visibility of the nanoparticles in this imaging mode.

Atomic-scale imaging of individual dopant atoms in a buried interface.

Determining the atomic structure of internal interfaces in materials and devices is critical to understanding their functional properties. Interfacial doping is one promising technique for controlling interfacial properties at the atomic scale, but it is still a major challenge to directly characterize individual dopant atoms within buried crystalline interfaces. Here, we demonstrate atomic-scale plan-view observation of a buried crystalline interface (an yttrium-doped alumina high-angle grain boundary) using aberration-corrected Z-contrast scanning transmission electron microscopy. The focused electron beam transmitted through the off-axis crystals clearly highlights the individual yttrium atoms located on the monoatomic layer interface plane. Not only is their unique two-dimensional ordered positioning directly revealed with atomic precision, but local disordering at the single-atom level, which has never been detected by the conventional approaches, is also uncovered. The ability to directly probe individual atoms within buried interface structures adds new dimensions to the atomic-scale characterization of internal interfaces and other defect structures in many advanced materials and devices.

Interface structures of gold nanoparticles on TiO2 (110).

Scanning transmission electron microscopy and density functional theory are used to characterize atomic structures of nanoscale heterointerfaces between gold nanoparticles and a TiO2 (110) surface. It is found that when the gold nanoparticle size is smaller than a few nanometers, gold atoms preferentially attach to specific sites on the TiO2 surface and thus form an epitaxial and coherent heterointerface. Conversely, as the gold size becomes larger, the gold-TiO2 interface loses lattice coherency in order to accommodate the large lattice mismatch between the two dissimilar crystals.