Journal of Electron Microscopy - recent issues

Advanced electron microscopy in materials physics

Zhu, Y., Jarausch, K. — 2009-05-18

Historical aspects of aberration correction

Rose, H. H. — 2009-05-18

A brief history of the development of direct aberration correction in electron microscopy is outlined starting from the famous Scherzer theorem established in 1936. Aberration correction is the long story of many seemingly fruitless efforts to improve the resolution of electron microscopes by compensating for the unavoidable resolution-limiting aberrations of round electron lenses over a period of 50 years. The successful breakthrough, in 1997, can be considered as a quantum step in electron microscopy because it provides genuine atomic resolution approaching the size of the radius of the hydrogen atom. The additional realization of monochromators, aberration-free imaging energy filters and spectrometers has been leading to a new generation of analytical electron microscopes providing elemental and electronic information about the object on an atomic scale.

Atomic-resolution spectroscopic imaging: past, present and future

Pennycook, S. J., Varela, M., Lupini, A. R., Oxley, M. P., Chisholm, M. F. — 2009-05-18

This review examines the development of atomically resolved electron energy loss spectroscopy from the first demonstration of plane-by-plane compositional profiling, through column-by-column spectroscopy to full two-dimensional and potentially three-dimensional spectroscopic imaging. Examples will be presented to highlight the increasing analytical sensitivity and image contrast obtained through each generation of aberration correction, moving towards the ultimate goal of mapping electronic structure inside materials with atomic resolution.

Review of recent advances in spectrum imaging and its extension to reciprocal space

Maigne, A., Twesten, R. D. — 2009-05-18

Using examples from various domains of science, this review covers some recent developments in spectrum imaging (SI) using scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). Advanced multi-dimensional acquisition methods allow the acquisition of STEM–EELS data with other complementary data such as energy dispersive X-ray spectroscopy (EDS), cathodoluminescence and even combining them with reciprocal space analysis through a new method called diffraction imaging. This method allows real and reciprocal space information to be mixed to get a more complete description of the electron–sample interaction. The developments in SI data analysis such as multiple linear least-squares fitting, non-linear least-squares fitting and multivariate analysis allow for a robust extraction not only of each elemental distribution but also of each chemical phase in a sample through an intuitive computer-assisted method.

Performance and image analysis of the aberration-corrected Hitachi HD-2700C STEM

Inada, H., Wu, L., Wall, J., Su, D., Zhu, Y. — 2009-05-18

We report the performance of the first aberration-corrected scanning transmission electron microscope (STEM) manufactured by Hitachi. We describe its unique features and versatile capabilities in atomic-scale characterization and its applications in materials research. We also discuss contrast variation of the STEM images obtained from different annular dark-field (ADF) detectors of the instrument, and the increased complexity in contrast interpretation and quantification due to the increased convergent angles of the electron probe associated with the aberration corrector. We demonstrate that the intensity of atomic columns in an ADF image depends strongly on a variety of imaging parameters, sample thickness, as well as the nuclear charge and the deviation from their periodic position of the atoms we are probing. Image simulations are often required to correctly interpret the atomic structure of an ADF-STEM image.

Control of parasitic aberrations in multipole optics

Batson, P. E. — 2009-05-18

A method is described to find the optimal fourth order setup of a quadrupole–octupole third-order aberration corrector. Given accurate measurements of aberrations to fifth order, stimulus/response experiments can be used to synthesize pure controls for each measured aberration up to fourth order, including those which are caused by parasitic effects – symmetry violations, misalignments, construction mistakes, post-construction drift or other problems.

Local crystal structure analysis with 10-pm accuracy using scanning transmission electron microscopy

2009-05-18

We demonstrate local crystal structure analysis based on annular dark-field (ADF) imaging in scanning transmission electron microscopy (STEM). Using a stabilized STEM instrument and customized software, we first realize high accuracy of elemental discrimination and atom-position determination with a 10-pm-order accuracy, which can reveal major cation displacements associated with a variety of material properties, e.g. ferroelectricity and colossal magnetoresistivity. A-site ordered/disordered perovskite manganites Tb_0.5Ba_0.5MnO₃ are analysed; A-site ordering and a Mn-site displacement of 12 pm are detected in each specific atomic column. This method can be applied to practical and advanced materials, e.g. strongly correlated electron materials.

Phase TEM for biological imaging utilizing a Boersch electrostatic phase plate: theory and practice

2009-05-18

A Boersch electrostatic phase plate (BEPP) used in a transmission electron microscope (TEM) system can provide tuneable phase shifts and overcome the low contrast problem for biological imaging. Theoretically, a pure phase image with a high phase contrast can be obtained using a BEPP. However, a currently available TEM system utilizing a BEPP cannot achieve sufficiently high phase efficiency for biological imaging, owing to the practical conditions. The low phase efficiency is a result of the blocking of partial unscattered electrons by BEPP, and the contribution of absorption contrast. The fraction of blocked unscattered beam is related to BEPP dimensions and to divergence of the illumination system of the TEM. These practical issues are discussed in this paper. Phase images of biological samples (negatively stained ferritin) obtained by utilizing a BEPP are reported, and the phase contrast was found to be enhanced by a factor of ~1.5, based on the calculation using the Rose contrast criterion. The low gain in phase contrast is consistent with the expectation from the current TEM/BEPP system. A new generation of phase TEM utilizing BEPP and designed for biological imaging with a high phase efficiency is proposed.

First application of Cc-corrected imaging for high-resolution and energy-filtered TEM

2009-05-18

Contrast-transfer calculations indicate that C_c correction should be highly beneficial for high-resolution and energy-filtered transmission electron microscopy. A prototype of an electron optical system capable of correcting spherical and chromatic aberration has been used to verify these calculations. A strong improvement in resolution at an acceleration voltage of 80 kV has been measured. Our first C_c-corrected energy-filtered experiments examining a (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7/LaCoO₃ interface demonstrated a significant gain for the spatial resolution in elemental maps of La.

Aberration-corrected ADF-STEM depth sectioning and prospects for reliable 3D imaging in S/TEM

Xin, H. L., Muller, D. A. — 2009-05-18

The short depth of focus of aberration-corrected scanning transmission electron microscopes (STEMs) could potentially enable 3D reconstruction of nanomaterials through acquisition of a through-focal series. However, the contrast transfer function of annular dark-field (ADF)-STEM depth sectioning has a missing-cone problem similar to that of tilt-series tomography. The elongation as a function of the probe-forming angle is found to be . For existing aberration-corrected STEMs operated at optimal imaging conditions, the elongation factor for depth sectioning is larger than 30. This large elongation factor results in highly distorted shapes of 3D objects and unexpected artifacts due to the loss of information. Depth-sectioning experiments using a 33-mrad 100 keV C₅-corrected aberration-corrected STEM demonstrate the elongation effect and the missing-cone problem in real and reciprocal space. The performance limits of different S/TEM-based imaging modes are compared. There is a missing cone of information for bright-field S/TEM, ADF-STEM, hollow-cone ADF-STEM and coherent scanning confocal electron microscopy (SCEM). Only incoherent SCEM fills the missing cone.

Three-dimensional shapes and structures of lamellar-twinned fcc nanoparticles using ADF STEM

Gontard, L. C., Dunin-Borkowski, R. E., Gass, M. H., Bleloch, A. L., Ozkaya, D. — 2009-05-18

Small particles with face-centred cubic structures can have non-single-crystallographic shapes. Here, an approach based on annular dark-field scanning transmission electron microscopy (STEM) is used to obtain information about the crystal sub-units that make up supported and unsupported twinned Pt, Pt alloy and Au nanoparticles. The three-dimensional shapes of two types of lamellar-twinned particles (LTPs) of Pt are obtained using high-angle annular dark-field STEM. Possible growth mechanisms of the LTPs and origins for the contrast features in the recorded images are discussed.

Three-dimensional electron microscopy of individual nanoparticles

Jarausch, K., Leonard, D. N. — 2009-05-18

The characterization of nanomaterials with complex three-dimensional (3D) geometries is required to further research and enable the continuing development of nanotechnology. In this manuscript, we report a protocol which combines focused ion beam (FIB) milling, thin film deposition and solution chemistry to optimize a rotation holder for 3D structural and chemical analysis of nanoparticles. This protocol is used to customize the geometry, surface and chemistry of a scanning transmission electron microscope (STEM) or transmission electron microscope (TEM) rotation holder for the nanoparticle system of interest. To illustrate this concept, rotation holder stubs were optimized to facilitate the 3D STEM imaging and analysis of core-shell nanoparticles used for DNA detection. Using this approach, it was possible to characterize the morphology, optoelectronic properties and chemical composition of individual core-shell nanoparticles in 3D. STEM images were captured at regular angular intervals over a complete 360° rotation to eliminate missing wedge artifacts. Electron energy-loss spectroscopy (EELS) spectrum images were acquired intermittently for comparative chemical analysis. This approach allows the 3D STEM/TEM analysis to be performed with the nanoparticle of interest cantilevered over vacuum to minimize substrate effects. Standard tomography techniques were used to reconstruct the 3D structure of the individual nanoparticles from the STEM HAADF rotation series. EELS spectrum imaging was used to determine the local material properties such as composition, band-gap and plasmon energy. The nanoparticle analysis protocol reported here can easily be adapted to facilitate 3D TEM/STEM analysis of other nanomaterial systems.

Aberration-corrected Z-contrast imaging of SrTiO3 dislocation cores

Klie, R. F., Walkosz, W., Yang, G., Zhao, Y. — 2009-05-18

Many fundamental problems in materials science, physics, and particular nanotechnology rely on the direct determination and characterization of atomic arrangements and electronic environments of individual interfaces or defects. In this paper, we will show how aberration-corrected Z-contrast imaging in combination with electron energy-loss spectroscopy can be used to directly measure the local atomic and electronic structures of dislocation cores in low-angle SrTiO [001] tilt grain boundaries. In particular, we will study two types of dislocation cores in a 3 tilt grain boundary, a pure edge dislocation, and a dissociated dislocation core. While it is energetically favorable for an edge dislocation to dissociate into two partial dislocations in such a low-angle grain boundary, we can find pure edge dislocations that show a higher O vacancy concentration than the dissociated cores. We suggest that the increased oxygen vacancy concentration might help stabilizing the pure edge dislocations in 3° tilt grain boundaries of SrTiO .

Aberration-corrected STEM investigation of the M2 phase of MoVNbTeO selective oxidation catalyst

Blom, D. A., Pyrz, W. D., Vogt, T., Buttrey, D. J. — 2009-05-18

The ‘M2’ phase of the oxidation catalyst MoVNbTeO was studied with an aberration-corrected STEM using HAADF imaging, and three 60° twin orientations were identified. Comparisons between the experimental HAADF images and image simulations suggest that the there are two different Te sites, as has been previously reported; however, there are differences between the structure proposed in DeSanto et al. (2004) Z. Kristogr. 219: 152 and the experimental HAADF images.

Evolution of gold structure during thermal treatment of Au/FeOx catalysts revealed by aberration-corrected electron microscopy

2009-05-18

High-resolution aberration-corrected electron microscopy was performed on a series of catalysts derived from a parent material, 2 at.% Au/Fe₂O₃ (WGC ref. no. 60C), prepared by co-precipitation and calcined in air at 400°C, and a catalyst prepared by leaching surface gold from the parent catalyst and exposed to various treatments, including use in the water–gas shift reaction at 250°C. Aberration-corrected JEOL 2200FS (JEOL USA, Peabody, MA) and Vacuum Generators HB-603U STEM instruments were used to image fresh, reduced, leached, used and re-oxidized catalyst samples. A new in situ heating technology (Protochips Inc., Raleigh, NC, USA), which permits full sub-Ångström imaging resolution in the JEOL 2200FS was used to study the effects of temperature on the behavior of gold species. A remarkable stability of gold to redox treatments up to 400°C, with atomic gold decorating step surfaces of iron oxide was identified. On heating the samples in vacuum to 700°C, it was found that monodispersed gold began to sinter to form nanoparticles above 500°C. Gold species internal to the iron oxide support material was shown to diffuse to the surface at elevated temperature, coalescing into discrete nanocrystals. The results demonstrate the value of in situ heating for understanding morphological changes in the catalyst with elevated temperature treatments.

Electron beam coherence measurements using diffracted beam interferometry/holography

Herring, R. A. — 2009-05-18

The intensity and coherence of elastically and inelastically scattered electrons have been studied by the interference of electron-diffracted beams using a method of diffracted beam interferometry/holography (DBI/H). In the interferograms produced, fringes were found to exist from low to high scattering angles. The intensity and coherence of the fringes are useful for understanding the contrast mismatch between experimental and simulated images found in atomic resolution images of crystals produced by transmission electron microscopy (TEM) and annular dark-field (ADF) scanning transmission electron microscopy (STEM). The fringes disappear when the interfering beams are separated from an exact overlay position, which produces a measurement of the beam's lateral coherence and holds promise for measuring the coherence of the respective quasi-particles associated with the energy loss electrons.

Application of two-dimensional crystallography and image processing to atomic resolution Z-contrast images

Morgan, D. G., Ramasse, Q. M., Browning, N. D. — 2009-05-18

Zone axis images recorded using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM or Z-contrast imaging) reveal the atomic structure with a resolution that is defined by the probe size of the microscope. In most cases, the full images contain many sub-images of the crystal unit cell and/or interface structure. Thanks to the repetitive nature of these images, it is possible to apply standard image processing techniques that have been developed for the electron crystallography of biological macromolecules and have been used widely in other fields of electron microscopy for both organic and inorganic materials. These methods can be used to enhance the signal-to-noise present in the original images, to remove distortions in the images that arise from either the instrumentation or the specimen itself and to quantify properties of the material in ways that are difficult without such data processing. In this paper, we describe briefly the theory behind these image processing techniques and demonstrate them for aberration-corrected, high-resolution HAADF-STEM images of Si₄₆ clathrates developed for hydrogen storage.

Beam damage suppression of low-{kappa} porous Si-O-C films by cryo-electron-energy loss spectroscopy (EELS)

Otsuka, Y., Shimizu, Y., Tanaka, I. — 2009-03-09

Porous Si–O–C films with lower dielectric constant () relative to silicon dioxide have been widely studied as inter-layer dielectrics in new-generation microelectronic devices. On the analysis of the film by transmission electron microscopy (TEM), it is susceptible of beam damage during both sample preparation by a focused ion beam (FIB) technique and TEM observation. We use electron energy loss spectroscopy (EELS) to quantify the magnitude of the beam damage during these processes. The intensity of the 285-eV peak in C-K electron energy loss near edge structures (ELNES) is enhanced by the damage, which can be ascribed to the formation of the C=C double bonds as a result of the decomposition of the methyl groups by the beam. The use of cryo-holder for TEM at 100 K is found to be essential to reduce the damage of the low- layers. The lowering of the acceleration voltage of FIB down to 5 keV does not change the spectra. Since the FIB damage is localized at the surface, the use of thick regions in the TEM foil such as 130 nm is preferred to reduce the superposition of EELS of the damaged region on those from the sample of interest.

High-contrast imaging of plastic-embedded tissues by phase contrast electron microscopy

2009-03-09

Phase contrast electron microscopy utilizing phase plates has been considered suitable for high-contrast observation of weak phase objects. This novel technique was newly applied to histochemically stained strong phase objects of osmificated biological specimens. Sections of various thicknesses, specifically stained for the Golgi apparatus by the ZIO technique using the heavy metals Zn and Os, were observed with a phase contrast electron microscope in Zernike and Hilbert imaging modes. Quantitative analysis of image contrast in real space and the power spectrum in Fourier space showed a high-contrast gain even for strong phase objects. This result clearly indicates that phase contrast electron microscopy can be effectively used not only for weak phase objects but also for strong phase objects in biology.

Visualization of the ultrastructural interface of cells with the outer and inner-surface of coral skeletons

Jeger, R., Lichtenfeld, Y., Peretz, H., Shany, B., Vago, R., Baranes, D. — 2009-03-09

Crystalline, porous biomaterials, such as marine invertebrate skeletons, have been widely used for functional reconstruction of human tissues like bone and dental implants. Since in such an abrasive microenvironment adequate cell–material interactions are crucial for a successful treatment, it is of great importance to improve the means to examine these interactions. We developed a method that reveals the ultrastructure of the interface between coral skeletons and cultured neural cells to a higher quality than do traditional methods as it does not include damaging procedures like decalcification or sectioning non-decalcified skeletons. It is rather based on generating two electron opacity distinct Araldite masks, of the skeleton and its surrounding, by polymerizing them to different durations. The contrast created at the border of the two masks outlined the fine and fragile crystals of the coral skeleton's outer and inner surfaces and their contact sites with the cells. The skeleton's internal structure contains a mesh of narrow (few microns wide) and large channel-shaped gaps interrupted by irregular-shaped crystalline material. Neural cells grew on the skeleton surface by stretching between crystal tips, with occasional rearrangements of cytoskeletal fibers located near the anchorage focal adherence points. Cell processes infiltrated the skeleton interior by stretching between inter-surface crystals and by adjusting their volume to the space of the conduits they grew into. The technique advances the study of coral biology and of neural cells–hard biomaterial interaction; it can be applied to other biomaterials and cell types and open new ways for studying tissue development and engineering.

Warfarin administration disrupts the assembly of mineralized nodules in the osteoid

2009-03-09

This study aimed to elucidate the ultrastructural role of Gla proteins in bone mineralization by means of a warfarin-administration model. Thirty-six 4-week-old male F344 rats received warfarin (warfarin group) or distilled water (control group), and were fixed after 4, 8 and 12 weeks with an aldehyde solution. Tibiae and femora were employed for histochemical analyses of alkaline phosphatase, osteocalcin and tartrate-resistant acid phosphatase, and for bone histomorphometry and electron microscopy. After 4, 8 and 12 weeks, there were no marked histochemical and histomorphometrical differences between control and warfarin groups. However, osteocalcin immunoreactivity was markedly reduced in the warfarin-administered bone. Mineralized nodules and globular assembly of crystalline particles were seen in the control osteoid. Alternatively, warfarin administration resulted in crystalline particles being dispersed throughout the osteoid without forming mineralized nodules. Immunoelectron microscopy unveiled lower osteocalcin content in the warfarin-administered osteoid, which featured scattered crystalline particles, whereas osteocalcin was abundant on the normally mineralized nodules in the control osteoid. In summary, Gla proteins appear to play a pivotal role in the assembly of mineralized nodules.

A small lamellar corpuscle within a small nerve bundle outside the tendon of the rat soleus muscle

Desaki, J., Nishida, N. — 2009-03-09

A small nerve bundle outside the tendon of the adult rat soleus muscle contained a small lamellar corpuscle similar in structural organization to the ordinary paciniform corpuscle. A terminal axon composing this corpuscle was originated from a side branch of an afferent nerve fiber and surrounded by a number (~15) of closely packed flattened lamellae of modified Schwann cells, while the stem nerve fiber freely terminated within the nerve bundle. These findings suggested that an afferent nerve fiber retracted after degeneration might extend a new branch within the nerve bundle and unexpectedly form a lamellar corpuscle within it.

Morphology and chemical composition analysis of inorganic nanosheets by the field-emission scanning electron microscope system

2009-01-20

Nanosheets can be used as building blocks to fabricate versatile nanostructured materials. In this paper, morphology of the Cs₄W₁₁O₃₆ and Nb₃O₈ and TaO₃ sheets with different layers are analyzed by different field-emission scanning electron microscopes (FE-SEMs). Chemical composition of the single-layered Cs₄W₁₁O₃₆ with thickness of about 2 nm, and multilayered Nb₃O₈ nanosheets with thickness of less than 14 nm are analyzed by both the Si(Li) solid-state detector and transition edge sensor (TES) microcalorimeter, successfully. The effects of energy resolution, accelerating voltage and substrate on the quantitative analysis are discussed briefly.

Observations of a magnetic microstructure in a Co-CoO obliquely evaporated tape using electron holography

2009-01-20

The magnetic microstructure in a Co-CoO obliquely evaporated tape that was subjected to a recording bit length of 250 nm was studied using electron holography. The reconstructed phase image demonstrated a periodic pattern of magnetic flux loops that were inclined to the film normal due to a well-developed columnar structure. When a magnetic field was applied to the tape for observing the remanent state by holography, the periodic pattern of the flux loops gradually disappeared. Interestingly, on applying a large magnetic field, the contour lines in the reconstructed phase image became approximately parallel to the longitudinal axis of the sliced tape, i.e. the contour lines were made to virtually deviate from the easy magnetization axis. The observations were supported by a computer simulation in which the effect of the stray magnetic field was considered.

Contrast mechanism due to interface trapped charges for a buried SiO2 microstructure in scanning electron microscopy

Zhang, H.-B., Li, W.-Q., Wu, D.-W. — 2009-01-20

We clarify the scanning electron microscopic contrast mechanism for imaging a buried SiO₂ trench microstructure with interface trapped charges by simulating both electron scattering and transport. Here, the interface trapped charges make the SiO₂ film more negatively charged and increase excess holes in the space charge distribution of the electron scattering region. The generated positive surface electric field thus redistributes some emitted secondary electrons and results in the dark contrast. This contrast mechanism is validated by comparing with experiments, and it may also provide an interesting approach for imaging and detecting deep interface trapped charges in insulating films.

Scanning electron and light microscopy study of the cervical mucus in women with polycystic ovary syndrome

Vigil, P., Cortes, M. E., Zuniga, A., Riquelme, J., Ceric, F. — 2009-01-20

Two types of cervical mucus are recognized, oestrogenic and gestagenic. These are constituted by different subtypes, and their characteristics change depending on variations in the hormonal levels and on the existence of several pathologies. Our aim was to identify the ultrastructure and crystallization characteristics of the cervical mucus in women suffering from polycystic ovary syndrome, and to compare these characteristics with those of normal control women. Cervical mucus samples were taken from 10 women, 4 control group women (with normal ovulatory menstrual cycles) and 6 suffering from polycystic ovary syndrome (2 with ovulatory and 4 with anovulatory cycles). This mucus was characterized according to its ultrastructure and crystallization. The type of mucus obtained was related to the levels of oestradiol and progesterone present when the samples were taken. As regards mucus ultrastructure, differences were found between the control women and those with polycystic ovary syndrome and anovulatory menstrual cycles. Such variations were evident in the type of mesh and the average diameter of the mucus pores. Mucus crystallization in control women showed the usual oestrogenic disposition: fern-like (L, P2), rectilinear (S) or a hexagonal structure (P6). On the other hand, in women with polycystic ovary syndrome, indefinite mucus crystallizations were found, as well as crystallization patches resembling oestrogenic and gestagenic-like mucus. This study shows that the ultrastructure and crystallization characteristics of the cervical mucus in polycystic ovary syndrome women are different from those of control women. The latter would be dependent on their levels of oestradiol and progesterone.