High-angle Dark-field Imaging Research Articles

Transmission electron microscopy study of the MgS–Tm2S3 system

This work presents the structural–microstructural characterization of the NaCl-derivative MgS–Tm2S3 system, which can be formulated by the expression Mg(1−x)Tm(2/3)x□(1/3)xS (□→cation vacancy). Transmission electron microscopy observations show the transition between NaCl-type and spinel-type structures when 0 ≤x≤ 0.75. The increase of Tm content in the solid solution provokes the increase of the spinel-type phase proportion, which intergrows with the NaCl-type crystals. When x≥0.75, some phases derived from NaCl-type structure through the chemical twinning at the unit cell level crystallographic operation are observed, such as CT-MgTm2S4 and CT-MgTm4S7. The existence and nature of the extended defects observed along the c direction of these structures are characterized by means of Scanning-Transmission electron microscopy high-angle dark field imaging, which allows observing the presence of quasi ordered crystals with new possible complex stoichiometries at atomic resolution.

High-precision scanning transmission electron microscopy at coarse pixel sampling for reduced electron dose

Abstract Determining the precise atomic structure of materials’ surfaces, defects, and interfaces is important to help provide the connection between structure and important materials’ properties. Modern scanning transmission electron microscopy (STEM) techniques now allow for atomic resolution STEM images to have down to sub-picometer precision in locating positions of atoms, but these high-precision techniques generally require large electron doses, making them less useful for beam-sensitive materials. Here, we show that 1- to 2-pm image precision is possible by non-rigidly registering and averaging a high-angle dark field image series of a 5- to 6-nm Au nanoparticle even though a very coarsely sampled image and decreased exposure time was used to minimize the electron dose. These imaging conditions minimize the damage to the nanoparticle and capture the whole nanoparticle in the same image. The high-precision STEM image reveals bond length contraction around the entire nanoparticle surface, and no bond length variation along a twin boundary that separates the nanoparticle into two grains. Surface atoms at the edges and corners exhibit larger bond length contraction than atoms near the center of surface facets.

High Angle Dark Field Imaging of Two-Dimensional Crystals

Two-dimensional (2D) materials, graphene, hexagonal boron nitride (h-BN) and transition metal dichalcogenides (TMD) have been investigated by means of Scanning Transmission Electron Microscopy (STEM), in particular via High Angle Annular Dark Field (HAADF) imaging technique. They are compared in terms of their structure and durability under intense electron beams.

Ion Implantation of Graphene—Toward IC Compatible Technologies

Doping of graphene via low energy ion implantation could open possibilities for fabrication of nanometer-scale patterned graphene-based devices as well as for graphene functionalization compatible with large-scale integrated semiconductor technology. Using advanced electron microscopy/spectroscopy methods, we show for the first time directly that graphene can be doped with B and N via ion implantation and that the retention is in good agreement with predictions from calculation-based literature values. Atomic resolution high-angle dark field imaging (HAADF) combined with single-atom electron energy loss (EEL) spectroscopy reveals that for sufficiently low implantation energies ions are predominantly substitutionally incorporated into the graphene lattice with a very small fraction residing in defect-related sites.

Interaction of Metals with Suspended Graphene Observed by Transmission Electron Microscopy.

In this Perspective, we present an overview of how different metals interface with suspended graphene, providing a closer look into the metal-graphene interaction by employing high-resolution transmission electron microscopy, especially using high-angle dark field imaging. All studied metals favor sites on the omnipresent hydrocarbon surface contamination rather than on the clean graphene surface and present nonuniform distributions, which never result in continuous films but instead in clusters or nanocrystals, indicating a weak interaction between the metal and graphene. This behavior can be altered to some degree by surface pretreatment (hydrogenation) and high-temperature vacuum annealing. Graphene etching is observed in a scanning transmission electron microscope (STEM) under high vacuum and 60 kV electron beam acceleration voltage conditions for all metals, except for Au. This unusual metal-mediated etching sheds new light on the metal-graphene interaction; it might explain the observed higher frequency of cluster nucleation for certain transition metals and might have implications regarding controlled nanomanipulation, that is, for self-assembly and sculpturing of future graphene-based devices.

Erbium environments in erbium-silicon/silica light emitting nanostructures

Co-doping of SiO2 with Si and Er to achieve silica fibre amplifiers has resulted in encouraging levels of light emission, much above those of Er-only doped SiO2. However, different fabrication methods, i.e., co-implantion and sequential implantation of Er and Si, has led to several factors difference in light levels. This paper looks into the reasons for these differences by establishing structure and local stoichiometry of the created entities via analytical transmission electron microscopy. In both cases Si-nanocrystals (NCs) have formed in the SiO2 matrix. In the former case Er-ions are co-located with /integrated within the NCs, in the latter case NCs and Er are separate. By assessing the NCs' internal and interfacial structure with the surrounding material, we attempt to identify chemical/structural Er-phases/defects and their effect on the sensitising efficiency in the Er:Si-NCs system; high resolution phase contrast- and high angle dark field imaging as well as nano-scale spatially resolved electron energy core loss- and plasmon-spectroscopy carried out in an aberration corrected dedicated STEM lend valuable support to these studies.

Metal−Graphene Interaction Studied via Atomic Resolution Scanning Transmission Electron Microscopy

Distributions and atomic sites of transition metals and gold on suspended graphene were investigated via high-resolution scanning transmission electron microscopy, especially using atomic resolution high angle dark field imaging. All metals, albeit as singular atoms or atom aggregates, reside in the omni-present hydrocarbon surface contamination; they do not form continuous films, but clusters or nanocrystals. No interaction was found between Au atoms and clean single-layer graphene surfaces, i.e., no Au atoms are retained on such surfaces. Au and also Fe atoms do, however, bond to clean few-layer graphene surfaces, where they assume T and B sites, respectively. Cr atoms were found to interact more strongly with clean monolayer graphene, they are possibly incorporated at graphene lattice imperfections and have been observed to catalyze dissociation of C-C bonds. This behavior might explain the observed high frequency of Cr-cluster nucleation, and the usefulness as wetting layer, for depositing electrical contacts on graphene.

Scanning electron microscope observation of dislocations in semiconductor and metal materials

Scanning electron microscope (SEM) image contrasts have been investigated for dislocations in semiconductor and metal materials. It is revealed that single dislocations can be observed in a high contrast in SEM images formed by backscattered electrons (BSE) under the condition of a normal configuration of SEM. The BSE images of dislocations were compared with those of the transmission electron microscope and scanning transmission electron microscope (STEM) and the dependence of BSE image contrast on the tilting of specimen was examined to discuss the origin of image contrast. From the experimental results, it is concluded that the BSE images of single dislocations are attributed to the diffraction effect and related with high-angle dark-field images of STEM.

Nanotopography of graphene

AbstractMicroscopically clean graphene surfaces are scrutinized via atomic resolution high angle dark field imaging and electron energy loss spectroscopy for presence of adsorbed atoms. Carbon ad‐atoms can be observed in dark field images, whereas hydrogen is revealed in electron loss spectra. Spectrum images show the spatial distribution of hydrogen in pristine and deliberately H‐dosed graphene; hydrogen is found on all graphene surfaces but the coverage is higher and more uniform in dosed samples. On clean surfaces the energy loss is characteristic of the ground level excitation in atomic hydrogen, whereas a spread in the hydrogen energy levels is found in hydrocarbon deposits. A rippling effect in the graphene sheets is revealed when lattice images are processed using fast Fourier transform (FFT) techniques. This contrast effect originates from changes in the bond length projection, due to inclinations of the sheet. It is suggested that point defects – vacancies and ad‐atomes – influence the ripple patterns.

Enhanced Activity for Oxygen Reduction Reaction on “Pt3Co” Nanoparticles: Direct Evidence of Percolated and Sandwich-Segregation Structures

Atomically resolved structures and compositions of Pt alloy nanoparticles were obtained using aberration-corrected high-angle dark field imaging, which was correlated to specific ORR activity based on a Pt surface area. The enhanced specific ORR activity (approximately 2 times relative to Pt) of acid-treated "Pt3Co" nanoparticles can be related to composition variations at the atomic scale and the formation of percolated Pt-rich and Pt-poor regions within individual particles. Upon annealing, we show direct evidence of surface Pt sandwich-segregation structures, which correspond to a specific ORR activity approximately 4 times relative to Pt.

Electron microscopy nanoscale characterization of ball-milled Cu–Ag powders. Part I: Solid solution synthesized by cryo-milling

We present a study of nanostructured Cu–Ag material obtained by low temperature ball milling. The microstructural characterization is carried out using a wide variety of transmission electron microscopy (TEM) techniques from conventional dark-field imaging, selected area diffraction and energy dispersive X-ray spectrometry (EDS) to more modern techniques of scanning TEM high-angle dark-field imaging (HAADF), nanodiffraction and high resolution imaging (HREM). A novel method of HREM image analysis is also presented which consists of calculating geometric phase images by Fourier filtering. Real-space maps of lattice spacings and lattice rotations are thus obtained. The analysis addresses the following essential points of nanostructural characterization: dislocation densities, grain sizes and morphologies, grain boundaries and local lattice rotations, local textures, local variations in lattice parameters. A new description of the microstructure emerges from the observations, quite different to that expected. Analytical modeling suggests that large lattice rotations can be expected in nanomaterials produced by intense deformation.

Aberration-Corrected STEM: the Present and the Future

Surprising as it may seem, aberration correction for the scanning transmission electron microscope (STEM) is now a practical proposition. The first-ever commercial spherical aberration corrector for a STEM was delivered by Nion to IBM Research Center in June 2000, and other deliveries have taken place since or are imminent. At the same time, the development of corrector hardware and software is still proceeding at full speed, and our understanding of what are the most important factors for the successful operation of a corrector is deepening continuously.Fig. 1 shows two high-angle dark field (HADF) images of [110] Si obtained with the IBM VG HB501 STEM operating at 120 kV, about 2 weeks after we fitted a quadrupole-octupole corrector into it. Fig. 1(a) shows the best HADF image that could be obtained with the corrector's quadrupoles on but its octupoles off. Sample structures were captured down to about 2.5 Å detail, easily possible in a STEM with a high resolution objective lens with a spherical aberration coefficient (Cs) of 1.3 mm. Fig. 1(b) shows a HADF image obtained after the Cs-correcting octupoles were turned on and the corrector tuned up. The resolution has now improved to 1.36 Å. This is sufficient to resolve the correct separation of the closely-spaced Si columns.

High angle annular dark field imaging of stacking faults

High angle annular dark field imaging has been extensively applied to high resolution imaging of crystalline materials. Dislocations have also been imaged using the high angle dark field detector, even when the lattice has not been directly resolved. Diffraction contrast, as employed in transmission electron microscopy analysis of defects, is a possible mechanism for dislocation contrast. Stacking faults should also show diffraction contrast and a Bloch wave theory is developed for the high angle dark field image. The results are compared with an experiment which shows, in agreement with the theory, that the strongest contrast is found when the fault is close to the surface and the objective aperture is small.

The Analytical Limits: HADF (High Angle Dark Field Imaging)

Abstract High Angle Dark Field Imaging, or Z contrast imaging, is an Imaging method. It takes advantage of the useful fact that if one uses the high angle scattering intensities and eliminates the elastic scattered (diffracted) beams from the image (by using a Howie type angular dark field detector), the remaining image will be characterized by, if the probe used is on the order of the atomic dimensions, intensity modulations that reveal atom positions and relative atomic number. In simple terms, the image will display Z-contrast at the atomic level and can differentiate columns of heavy atoms from columns of lighter ones.

A characterisation of dark-field imaging of colloidal gold labels in a scanning transmission X-ray microscope

While X-ray microscopes provide images of biological specimens for which the contrast is mainly due to the difference in the absorption of carbon and oxygen when X-rays transmitted through the specimen are detected, signals other than absorption can also be used to form images. Using the Stony Brook scanning transmission X-ray microscope at the National Synchrotron Light Source, high-angle dark-field images have been formed of cells labelled with colloidal gold, with and without silver enhancement. The high density of the colloidal gold particles, or the silver particles seeded by the gold, leads to a large scattering signal, and the fact that the particle diameters are comparable to the width of the microscope point spread function results in good localisation of the label with high contrast. The dark-field images can have a greater signal to noise ratio than bright-field images acquired with the same incident X-ray dose. The theory of dark- and bright-field imaging is reviewed. Theoretical calculations of scattering from gold and silver particles are presented and good agreement is found between these and experimental dark-field images of 30 nm diameter gold particles. The signal to noise ratios of experimental bright- and dark-field images are measured and found to be in agreement with theory. Images are presented of cells labelled by immunolabelling and in situ hybridisation.

Instrumental developments for electron microscopes: A STEM detector and correctors for a LVSEM and a 200 kV TEM

One advantage of scanning transmission electron microscopy (STEM) over conventional TEM which is often cited is the capability to simultaneously record the various scattered electrons with properly designed detectors. So far, this advantage has only been utilized to record bright-field or inelastic and dark-field (low and high angle) images in parallel. However, it has not been used to record all transmitted electrons separately according to their scattering angle. We developed a flexible multichannel detector system based on a silicon chip which has been fabricated to our specifications. This detector consists of 30 rings which are split into 4 quadrants (see Fig. 1), and is operated in an electron counting mode. The rings can be used to separate the electrons according to their scattering angle for low and high angle dark-field images, to obtain the various phase-contrast images and to normalize the signals by the sum of all detectors. The system records the signals of the 120 channels in parallel and the counts of each channel can be combined in an integer processing unit in order to form 8 different images.