Annular Dark-field Scanning Transmission Electron Microscopy Imaging Research Articles

The Formation and Utility of Sub-Angstrom to Nanometer-Sized Electron Probes in the Aberration-Corrected Transmission Electron Microscope at the University of Illinois

We evaluate the probe forming capability of a JEOL 2200FS transmission electron microscope equipped with a spherical aberration (Cs) probe corrector. The achievement of a real space sub-Angstrom (0.1 nm) probe for scanning transmission electron microscopy (STEM) imaging is demonstrated by acquisition and modeling of high-angle annular dark-field STEM images. We show that by optimizing the illumination system, large probe currents and large collection angles for electron energy loss spectroscopy (EELS) can be combined to yield EELS fine structure data spatially resolved to the atomic scale. We demonstrate the probe forming flexibility provided by the additional lenses in the probe corrector in several ways, including the formation of nanometer-sized parallel beams for nanoarea electron diffraction, and the formation of focused probes for convergent beam electron diffraction with a range of convergence angles. The different probes that can be formed using the probe corrected STEM opens up new applications for electron microscopy and diffraction.

Mn Valency at La0.7Sr0.3MnO3/SrTiO3 (0 0 1) Thin Film Interfaces

This article presents a (scanning) transmission electron microscopy (TEM) study of Mn valency and its structural origin at La 0.7 Sr 0.3 MnO 3/SrTiO3(0 0 1) thin film interfaces. Mn valency deviations can lead to a breakdown of ferromagnetic order and thus lower the tunneling magnetoresistance of tunnel junctions. Here, at the interface, a Mn valency reduction of 0.16 +/- 0.10 compared to the film interior and an additional feature at the low energy-loss flank of the Mn-L3 line have been observed. The latter may be attributed to an elongation of the (0 0 1) plane spacing at the interface detected by geometrical phase analysis of high-resolution images. Regarding the interface geometry, high-resolution high-angle annular dark-field scanning TEM images reveal an atomically sharp interface in some regions whereas the transition appears broadened in others. This can be explained by the presence of steps. The performed measurements indicate that, among the various structure-related influences on the valency, the atomic layer termination and the local oxygen content are most important.

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.

Analytical TEM observation of Au–Pd nanoparticles prepared by sonochemical method

The Au–Pd nanoparticles prepared by sonochemical technique were investigated by the analytical transmission electron microscopy. The core (Au)–shell (Pd) structure without apparent mixing layers and with similar thickness of Pd shells was clearly confirmed by the image and intensity profile of annular dark field scanning transmission electron microscopy (ADF-STEM) and also by the line-scan analysis of energy dispersive X-ray spectroscopy (EDS) in STEM mode. Detailed analyses of the ADF-STEM imaging, the electron diffraction pattern and the FFT pattern of HRTEM images complementarily show that the Pd shell epitaxially grows on the Au core expanded with the same lattice constant as Au and with the coherent Pd/Au interface structure.

Effects of specimen tilt in ADF-STEM imaging of a-Si/c-Si interfaces

Annular dark field scanning transmission electron microscopy (ADF-STEM) imaging of a crystal depends strongly on specimen orientation, but for an amorphous sample it is insensitive to orientation changes. To fully investigate the effects of specimen tilt, an interface of amorphous Si (a-Si) and crystalline Si (c-Si) was rotated systematically off a zone axis in a STEM equipped with low-angle ADF (LAADF) and high-angle ADF (HAADF) detectors. The change of relative intensity across the interface shows very different trends in the LAADF and the HAADF images upon tilting. More importantly, it is found that the HAADF signal decreases much more rapidly when tilted off a zone axis than does the LAADF signal. The high-resolution lattice fringes also disappear much faster in the HAADF image than in the LAADF image. These trends reflect the fact that the channeling peaks that are responsible for scattering into the HAADF detector decrease more quickly upon tilting than the lower angle scattering to the LAADF detector does.

Structural analysis of strained p-type AlGaN∕GaN superlattice

We investigated the nanostructure of AlGaN∕GaN strained-layer superlattice (SLS) cladding in the GaN-based violet laser diode (LD) and the AlInGaN-based ultraviolet (UV) light emitting diode (LED) with a scanning transmission electron microscope (STEM). In the p-SLS cladding, comprising 34 pairs of p-Al0.1Ga0.9N∕p-GaN:Mg layers in the GaN-based LD, the Al0.1Ga0.9N and GaN layers were distinguished as dark and bright bands ∼6nm wide in the high-angle annular dark-field (HAADF) STEM images. Threading dislocations (TDs) were observed. Among TDs that came from the underlying layer, some run outside through the SLS, and the others disappeared within the SLS, which discloses a role of the SLS in suppressing defect propagation. A HAADF-STEM image of the TD with a dark line along the center of a bright contour was found. The dark line, which was darker than the surrounding matrix, is striking. One of the probable explanations for the dark line that may be considered is local segregation of light atoms (Mg or Al) in Cottrell atmosphere around the dislocation core. In the HAADF-STEM image of the UV LED wafer, the AlInGaN and AlInGaN:Si layers in the MQW were definitely resolved, appearing as dark and bright bands. HAADF-STEM also distinguished between the AlGaN and GaN layers in the p-SLS cladding in the UV LED wafer.

Subtleties in ADF imaging and spatially resolved EELS: A case study of low-angle twist boundaries in SrTiO 3

A screw dislocation network at the low-angle SrTiO 3/Nb:SrTiO 3 twist grain boundary has been analyzed by annular dark field (ADF) imaging and spatially resolved electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). The cores of one set of dislocations running parallel to the beam direction appear dark in the ADF STEM images. EELS on the dislocation core reveals a reduced Sr/Ti ratio compared to the bulk suggesting Sr-deficient cores. The second set of dislocations, orthogonal to the latter, is imaged by its strain field using low-angle annular dark field (LAADF) imaging. Multislice image simulations suggest channeling of the electron probe on the atomic columns for small tilts, θ<1°, where the Sr columns act as beam guides. Only for larger tilts is the channeling effect strongly reduced and the fringe contrast approaches the value predicted by a purely incoherent imaging model. Ti-L 2,3 EELS across the dislocation core shows an asymmetry between the EELS and the ADF signal which cannot be explained by the geometry or beam broadening. This asymmetry might be explained by an effective nonlocal potential representing inelastic scattering in EELS.

Deposition of ultrathin rare-earth dopedY2O3 phosphor films on alumina nanoparticles

Ultrathin films of Eu3+ doped Y2O3 were deposited onto alumina nanoparticles using a unique solution synthesis method. Thesurface structure, composition, and morphology of the thin films deposited were analysedusing high resolution transmission electron microscopy (TEM) and high angle annular darkfield scanning TEM imaging and energy dispersive x-ray measurements. The filmsdeposited were extremely thin, on the order of 3–5 nm, and uniformly covered allthe alumina nanoparticles. X-ray diffraction was used to investigate the phasesand structures of the thin films deposited. At the heat treatment temperature of600 °C,cubic Y2O3 nanocrystals were found in the film while as-coated layers exhibited mainlyamorphous features. As the heat treatment temperature increased to750 °C, the amorphous thin film became well crystallized. Optical properties ofEu3+ doped Y2O3 films were characterized by fluorescence spectroscopy. Strongphotoluminescence was observed in the sample annealed at750 °C, from thefluorescence of Eu3+ ions in a well-crystallized film, consistent with the x-ray diffraction and TEM observations.

IMAGE-WARP: A real-space restoration method for high-resolution STEM images using quantitative HRTEM analysis

We have developed a new method for processing distorted high-resolution scanning transmission electron microscopy (STEM) images. The method is based on finding the displaced vertices in the experimental STEM image and warping to geometrically correct reference grid of the object. As a reference grid for warping a structural model obtained using a high-resolution transmission electron microscopy (HRTEM) analysis of the area of interest is utilised. Combined with quantitative HRTEM analysis the IMAGE-WARP method provides a real-space restoration of high-resolution high-angle annular dark-field (HAADF) STEM images without affecting the original Z-contrast information. The method can be applied to extract valuable compositional atomic-column data from any HAADF-STEM image of any kind of bulk crystals with local occupancy or chemistry fluctuations, stacking faults, special grain boundaries or interfaces, for which we have an available structural model. After the warping, distortion-corrected images can be further enhanced using conventional image-filtering techniques, and finally quantified with HAADF-STEM image simulations. The applicability of the IMAGE-WARP method was illustrated using experimental HAADF-STEM images of a strontium titanate crystal disrupted with a Ruddlesden-Popper-type antiphase boundary.

Lattice imaging in low-angle and high-angle bright-field scanning transmission electron microscopy

Atomic resolution low-angle bright-field (LABF) scanning transmission electron-microscope (STEM) images and high-angle bright-field (HABF) STEM images of [011]-orientated Si have been experimentally obtained together with high-angle annular dark-field (HAADF) STEM images. The contrast formation mechanisms of the LABF STEM and HABF STEM images are examined in comparison with HAADF STEM images. The HABF STEM images independent of defocus and thickness have spatial resolution comparable with HAADF STEM images, and are shown to be given as a simple convolution under the non-dispersion approximation of localized Bloch waves.

Symmetries in BF and HAADF STEM image calculations

Reductions in bright-field (BF) scanning transmission electron microscopy (STEM) and high-angle annular dark-field (HAADF) STEM image calculations with the aid of Bloch wave symmetry are discussed under assumptions that an absorption potential is written by a local potential and a zero-order Laue zone lies parallel to the crystal surface. Translational symmetry allows us to take only partial incident beams in the first Brillouin zone instead of enormous number of partial incident beams in a large convergent disk. Two dimensional point group confines partial incident beams to an irreducible area in addition to factoring a dispersion matrix into noninteracting submatrices on a high symmetry line using the projection operator. The drastic reductions in computing time and memory enable us to readily calculate various BF STEM and HAADF STEM images. The validity and accuracy are demonstrated in comparisons with high resolution experimental BF STEM and HAADF STEM images.

Direct observation of a stacking fault in Si(1 - x)Ge(x) semiconductors by spherical aberration-corrected TEM and conventional ADF-STEM.

Spherical aberration (C(S))-corrected transmission electron microscopy (TEM) and annular dark-field scanning TEM (ADF-STEM) are applied to high-resolution observation of stacking faults in Si(1 - x)Ge(x) alloy films prepared on a Si(100) buffer layer by the chemical vapor deposition method. Both of the images clarify the individual nature of stacking faults from their directly interpretable image contrast and also by using image simulation in the case of the C(S)-corrected TEM. Positions of the atomic columns obtained in the ADF-STEM images almost agree with a projection of the theoretical model studied by Chou et al. (Phys. Rev. B 32(1985): 7979). Comparison between the C(S)-corrected TEM and ADF-STEM images shows that their resolution is at a similar level, but directly interpretable image contrast is obtained in ultrathin samples for C(S)-corrected TEM and in slightly thicker samples for ADF-STEM.

Atomic structure of a (2 x 1) reconstructed NiSi2/Si(001) interface.

Nickel disilicide/silicon (001) interfaces were investigated by aberration corrected scanning transmission electron microscopy (STEM). The atomic structure was derived directly from the high spatial resolution high angle annular dark field STEM images without recourse to image simulation. It comprises fivefold coordinated silicon and sevenfold coordinated nickel sites at the interface and shows a 2 x 1 reconstruction. The proposed structure has not been experimentally observed before but has been recently predicted theoretically by others to be energetically favored.

Artifacts in aberration-corrected ADF-STEM imaging

The introduction of an experimental black level may introduce unintended artifactual details into high-resolution annular dark field scanning transmission electron microscopy (ADF-STEM) lattice images. This article presents the multislice simulation results of such possible situations. Three simulated scanning transmission electron microscopy (STEM) probes of sizes 0.8, 1.2 and 2.0 Å are scanned on the surface of a 〈 1 ̄ 10〉 oriented Si/Ge crystal. The simulation results suggest that high-frequency artifact peaks will appear in the power spectra when an artificial black level clips the lowest (background) signal. The lowest signal in an ADF-STEM image decreases as the incident probe shrinks in size. Therefore, care must be taken when interpreting the resolution limit of the microscope from images taken with nonzero black level setting, especially in case of sub-Å microscope. The simulation result is compared with an experimental image and they agree with each other. The analysis suggests that aberration corrected STEM provide sensitive low level detail.

Imaging and X-ray Microanalysis of a Poly(ethylene-ran-methacrylic acid) Ionomer Melt Neutralized with Sodium

The ionic aggregates in an as-extruded and a recrystallized poly(ethylene-ran-methacrylic acid) ionomer melt neutralized with sodium have been imaged using scanning transmission electron microscopy (STEM). The recrystallized sample exhibits a macrophase-separated structure with three phases with distinct boundaries. Phase I does not contain aggregates > 1 nm, phase II contains small spherical aggregates of ∼2−15 nm in diameter, and phase III contains large aggregates with diameters ranging from ∼20 to 160 nm. Contrast reversal between bright field and annular dark field STEM images indicates that the aggregates are Na-rich. In contrast, the as-extruded sample is featureless on the STEM length scale. This study shows that thermal history has a major impact on ionomer morphology. X-ray energy dispersive spectroscopy (XEDS) using a 1 nm probe confirms higher Na contents in the ionic aggregates and detects Na in the matrix. XEDS also finds a significant amount of C in the ion-rich aggregates due to the incorp...

Atomic-resolution annular dark-field STEM image calculations

A method based on the three-dimensional Bloch wave description has been developed for the simulation of atomic-resolution annular dark-field (ADF) scanning transmission electron microscopy (STEM) images, which includes the coherent Bragg reflection and the incoherent thermal diffuse scattering (TDS). The contribution of TDS is estimated using two kinds of optical potentials. The validity and accuracy of the method are demonstrated by comparisons in focus dependence between experimental and calculated high-angle (HA) ADF STEM images and in thickness dependence between experimental and calculated ADF STEM intensities. The method reduces the computing time for a HAADF STEM image calculation to about one-tenth of that required for the usual three-dimensional Bloch wave method.

Multiple inelastic scattering in high-energy electron diffraction and imaging

A dynamical multiple elastic and inelastic electron scattering theory is proposed and is applied to the plural scattering cases of phonon, single-electron and valence (or plasmon) excitations. The incoherence of all the possible inelastic scattering processes of different energies and momenta is evaluated analytically before any numerical calculations. The effects of multiple scattering are equivalent partially to the broadening of the scattering function of a single inelastic process by those of others and partially to the re-scattering of the Kikuchi pattern produced in one inelastic process by others. The final diffraction pattern is a convoluted result of those Kikuchi patterns produced by different inelastic scattering processes. All these characteristics can be considered in just one single formula. The theory of multiple-phonon excitations in simulating high-angle annular-dark-field (ADF) scanning transmission electron-microscopy (STEM) images is proposed. It is shown that the single-phonon scattering model is a good approximation except at the points close to atomic nuclei if the electron probe is comparable in size to that of an atom. The higher-order phonon scattering may improve the resolution of the ADF STEM images of thin crystals.

Dynamical inelastic scattering in high-energy electron diffraction and imaging: A new theoretical approach.

A generalized multislice theory is proposed from quantum mechanics to approach the multiple elastic and multiple inelastic scattering of high-energy electrons in a solid. The nonperiodic structure of crystals can be introduced in the calculations for the scattering geometries of transmission electron microscopy and reflection electron microscopy. Detailed applications of this generalized theory will be given for calculating (a) the energy-filtered-plasmon energy-loss diffraction patterns and images, (b) the energy-filtered diffraction patterns from atomic inner-shell losses, and (c) the contribution of thermal diffuse scattering to the high-angle annular-dark-field (ADF) scanning-transmission-electron-microscopy (STEM) lattice images. An ``incoherent'' imaging theory is presented for simulating the ADF STEM images and the detailed calculations are addressed for Ge/Si interfaces.

Single atom visibility on (111) silicon in simulated ADF STEM images

The multislice method has been adapted to simulate annular dark field (ADF) scanning transmission electron microscope (STEM) images. In the STEM image simulation, a highly focused electron probe is scanned across a specimen and the scattered intensity, accumulated over an annular detector, is recorded as a function of probe position. Each pixel in the STEM image is determined by an entire multislice calculation for a particular position of the incident probe. This N4 process is very computationally expensive and currently requires the use of a supercomputer to achieve runtimes of less than a day.The simulated specimen consisted of a (111) silicon crystal substrate, which was a multiple of 94 Å (30 slices) thick, followed by an additional slice containing a single gold atom. Slice potentials were 38.4 Å x 39.9 Å (256 x 256 pixels) in size, which set the maximum included scattering angle to 79 mrad.

Stem Observations of the Microstructures Present in Polymer Blend Thin Films.

AbstractThis paper examines the effectiveness of scanning transmission electron microscopy (STEM) as an analytical tool for determining composition in multi-phase polymer blend microstructures. The polymer blend polystyrene (PS) - polyether sulphone (PES) thin film used in this study exhibited a two-phase microstructure consisting of PES-rich inclusions, ranging from 0.2μm to 1.2μm in diameter, in a PS-rich matrix. Emphasis in this presentation is placed on the use of annular dark-field (ADF) STEM image contrast to infer information concerning the local composition of adjacent microstructural features.