Conventional transmission electron microscopy imaging beyond the diffraction and information limits.

The energy filtered imaging technique has so far been carried out in a scanning transmission electron microscope (STEM) fitted with a sector type energy analyzer. The STEM has a disadvantage of low beam parallelity because it uses a convergent beam, while the conventional transmission electron microscope (CTEM) allows good phase contrast and diffraction contrast to be obtained because of the high parallelity of the beam, and allows also high resolution images to be obtained. The technique to obtain energy filtered CTEM images has thus far been carried out by a Castaing-Henry type filter or an Ω type filter. However, these filters have the disadvantage of lower energy resolution than conventional sector type energy analyzer at the present time. This paper reports energy filtered CTEM images of MgO smoke, obtained using a new scanning CTEM image technique and a high energy resolution sector type energy analyzer which can resolve bulk and surface plasmon energy.

IntroductionCurrently, the resolution of structural detail in conventional transmission electron microscopy (CTEM) of organic materials is limited by the electron beam damage suffered by the sample. It has recently been shown that the imaging modes now available in scanning transmission electron microscopy (STEM) provide the potential for improvements in resolution over CTEM methods. However, the effect of the sensitivity of an organic sample itself to beam damage has not been compared in STEM versus CTEM. In STEM each element of the sample area is briefly illuminated with a small diameter, high flux, scanning electron beam, whereas in CTEM the entire sample area is continuously illuminated with a large diameter, lower flux, static electron beam. Although it has been shown that the amount of sample damage in CTEM is independent of dose rate(2), the electron flux in STEM may range from 3 to 4 orders of magnitude greater than that in CTEM, with a possible influence on damage rate. Generally, the effects of beam size, high flux, and short dwell time of the scanning electron beam on sensitivity of the sample to damage in STEM are not known.

Multiple electron scattering and the nonintuitive nature of image formation with coherent radiation complicate the interpretation of conventional transmission electron microscopy images. Precession of the illuminating beam in transmission electron microscopy (TEM) can lead to more robust and interpretable images with some penalty to image contrast, a technique known as dynamic hollow-cone illumination TEM. We demonstrate direct and robust imaging of light and heavy atoms in a crystalline environment with this technique. This method is similar to the annular bright-field technique in scanning transmission electron microscopy, via the principle of reciprocity. Dynamic hollow-cone illumination TEM is challenging in practice due to sensitivity to the misalignment of the precession axis, microscope objective aperture, and crystal zone axis.

The evolution of hydrogen from many hydrated cryo-preserved soft materials under electron irradiation in the transmission electron microscope can be observed at doses of the order of 1000 e nm(-2) and above. Such hydrogen causes artefacts in conventional transmission electron microscope or scanning transmission electron microscopy (STEM) imaging as well as in analyses by electron energy-loss spectroscopy. Here we show that the evolution of hydrogen depends on specimen thickness. Using wedge-shaped specimens of frozen-hydrated Nafion, a perfluorinated ionomer, saturated with the organic solvent DMMP together with both thin and thick sections of frozen-hydrated porcine skin, we show that there is a thickness below which hydrogen evolution is not detected either by bubble observation in transmission electron microscope image mode or by spectroscopic analysis in STEM electron energy-loss spectroscopy mode. We suggest that this effect is due to the diffusion of hydrogen, whose diffusivity remains significant even at liquid nitrogen temperature over the length scales and time scales relevant to transmission electron microscopy analysis of thin specimens. In short, we speculate that sufficient hydrogen can diffuse to the specimen surface in thin sections so that concentrations are too low for bubbling or for spectroscopic detection. Significantly, this finding indicates that higher electron doses can be used during the imaging of radiation-sensitive hydrated soft materials and, consequently, higher spatial resolution can be achieved, if sufficiently thin specimens are used in order to avoid the evolution of hydrogen-based artefacts.

The resolution of a conventional transmission electron microscope (TEM) deteriorates as the specimen thickness increases, because chromatic aberration of the objective lens is caused by the energy loss of electrons). In the case of a scanning electron microscope (SEM), chromatic aberration does not exist as the restrictive factor for the resolution of the transmitted electron image, for the SEM has no imageforming lens. It is not sure, however, that the equal resolution to the probe diameter can be obtained in the case of a thick specimen. To study the relation between the specimen thickness and the resolution of the trans-mitted electron image obtained by the SEM, the following experiment was carried out.

Transmission electron microscopy (TEM) and related characterization techniques are widely used to inspect semiconductor devices. Conventional TEM (CTEM) and scanning TEM (STEM) methods mostly keep the incident electron beam at a fixed angle, aligned with the optical axis of the electron microscope. While effective in many scenarios, this invariance could limit the potential for more complex applications. We investigate a technique that allows us to acquire multidimensional data from a series of beam tilt angles. The acquired data can be further processed to provide more information and to reduce the possible errors associated with single data sets. In this paper, we demonstrate the advantages of this technique through several semiconductor use cases. These benefits include reducing unwanted background information in electric field mapping on a SiC film, enhancing strain mapping quality on a Si-SiGe multilayer structure and improving grain mapping on a 3D-NAND device. This beam-tilt technique can be executed via an automated script once the experimental conditions are optimized, paving the way for large-volume investigation through automated TEM.

Janus nanoparticles are nanoparticles (NPs) displaying two sides of different chemical nature which makes them of high interest. The formation of the Janus shell provides the NPs an amphiphilic character, which can therefore self‐assemble in a tunable fashion by varying different experimental parameters such as size, polymer ratio and temperature. Here, Au NPs coated by polyethylene glycol (PEG) and both polystyrene (PS) and poly(N‐isopropylacrylamide) (PNIPAM) were investigated. [1] The true Janus character of the NPs was determined by electron tomography. Characterization of polymer coated particles by electron microscopy is challenging due to the low contrast of the organic materials. The goal was to distinguish the two different polymers in two types of particles (PEG+PNIPAM and PEG+PS coated Au NPs). Different approaches were used to improve the contrast between the two polymers. The PEG+PNIPAM particles were characterized by specific staining of the hemispheres while the character of the PEG+PS particles was investigated by the growth of silica. First, the PEG+PNIPAM coated NPs were stained with 3 mM CuSO 4 .5H 2 O solution to generate sufficient contrast in the Transmission Electron Microscope (TEM). PNIPAM‐coated NPs showed damage of the polymer upon electron beam irradiation. For PEG‐coated NPs a stable shell was observed. In an attempt to distinguish PNIPAM and PEG, PEG+PNIPAM coated particles were stained and investigated with TEM (Fig. 1). Since conventional TEM images are 2D projections from a 3D object, electron tomography was performed. It must be noted that the conventional technique for tomography in materials science, high angle annular dark field scanning TEM (HAADF‐STEM), could not be applied due to the large difference in atomic number Z for Au and the elements in the polymer shell. Instead, Bright‐Field TEM tomography was applied. Images were acquired over the largest possible range (‐76° to +76°) every 2°, aligned and reconstructed by the SIRT (Simultaneous Iterative Reconstruction Technique) algorithm in the Astra Toolbox. [2] The obtained 3D reconstruction displayed rather low contrast between the shell and the background. Manual segmentation based on the orthoslices (xy‐, yx‐ and zx‐direction) could be carried out. This method provided a 3D reconstruction that clearly confirms the formation of stained half‐shells (Fig. 3). In the second case of PEG+PS coated Au NPs, an attempt of staining with calcium phosphate was carried out. In TEM images hemispheres could be observed, but selective staining could not be confirmed. Due to the high affinity of PEG by silica, PEG is able to act as a primer to promote silica condensation. Since PS is hydrophobic and does not allow silica nucleation, the silica shell was expected to occur only at the parts of the NPs that are coated by PEG (Fig. 2). Either staining or silica shell growth over PEG‐coated areas resulted in the observation of semi‐shells (Janus character) in Au NPs by TEM and electron tomography.

The microstructure of p‐type Al0.69Ga0.31N with high Mg‐doping was studied by conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Spike‐like domains were observed with an average diameter of ∼10 nm, height of ∼400 nm and density of 2.2 × 1011/cm2. These long and narrow domains nucleated about 200 nm above the Mg‐doping boundary and resembled ordinary inversion domains induced by Mg‐doping. However, the energy dispersive X‐ray spectroscopy line‐scan results provided unambiguous evidence that the domains were Ga‐rich. The lateral composition segregation between the domains and surrounded matrix was further confirmed by high angular dark field images in STEM. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Whole-cell imaging of the budding yeast Saccharomyces cerevisiae by high-voltage scanning transmission electron tomography

The enigmatic Bøggild intergrowth in iridescent labradorite crystals was revisited in light of recent work on the incommensurately modulated structures in the intermediated plagioclase. Five igneous samples and one metamorphic labradorite sample with various compositions and lamellar thicknesses were studied in this paper. The lamellar textures were characterized with conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The compositions of individual lamellae were analyzed with high-resolution energy-dispersive X-ray spectroscopy (EDS) mapping and atom probe tomography (APT). The average structure states of the studied samples were also compared with single-crystal X-ray diffraction data (SC-XRD). The Na-rich lamellae have a composition of An44–48, and the Ca-rich lamellae range from An56 to An63. Significant differences between the lamellar compositions of different samples were observed. The compositions of the Bøggild intergrowth do not only depend on the bulk compositions, but also on the thermal history of the host rock. The implications on the subsolidus phase relationships of the plagioclase feldspar solid solution are discussed. The results cannot be explained by a regular symmetrical solvus such as the Bøggild gap, but they support an inclined two-phase region that closes at low temperature.

This paper discusses methods of characterizing pore size distributions in low dielectric constant (low‐k) films by transmission electron microscopy (TEM), comparing both conventional TEM and high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM). Methods of sample preparation were tailored to protect pore structure during cross‐sectional thinning allowing techniques similar to conventional mechanical polishing and low‐angle ion milling. Knowledge of TEM sample thickness allows quantification of pore density distributions. TEM sample thickness and quantitative image analysis methods are discussed.

Scanning and transmission electron microscopy (TEM) was applied to study the structure of oxide dispersion strengthened (ODS) martensitic 9Cr steel before and after mechanical deformation at elevated temperatures. The tensile tests were performed in the temperature range of 25°C to 800°C at a nominal strain rate of 10 ‐3 s ‐1 . High angle annular dark field (HAADF) scanning TEM (STEM) with energy‐dispersive X‐ray (EDX) spectroscopy for the determination of the composition and conventional TEM with selected area diffraction for the crystallographic investigations were applied. Figure 1 shows an overview HAADF STEM image (a) with corresponded schematic (b) of the investigated steel in tempered conditions. Owning to the composition and heat treatments, the structure of the steel typically consists of tempered martensite with a high density of hierarchically organized internal interfaces. During quench, the prior austenite grains are sub‐structured by martensitic laths and packets. This transformation causes a large local deformation of the matrix, resulting in a high dislocation density. The following tempering process allows the recovery of the martensitic structure with transformation into a ferritic structure, precipitation of solute atoms and recovery of the dislocation cell structure into subgrain structure. Therefore, the final microstructure consists of some portions that are composed of typical laths (with several µm in length and 0.25 ± 0.06 µm in width) and others portions that are filled with subgrains (with size of 0.29 ± 0.13 µm). The non‐regular shaped M 23 C 6 carbides precipitate are arranged preferentially at prior austenite grain boundaries, packet and lath boundaries. HRTEM analysis of M 23 C 6 (here not shown) reveals that many carbides have fcc structure. The relation between the carbide and ferrite is: M 23 C 6 || Fe. The detail EDX analysis shows that carbide consist of Fe and Cr. The fine ODS particles are mostly revealed inside the boundaries of laths and subgrains, yet on them as well. Figure 2 shows HAADF STEM image of the ODS particle (a) with linescan EDX profiles across the particle (b). The detail EDX analysis of many ODS particles shows that they have composition of Y 2 Ti 2‐x O 7‐2x . For investigation on the deformed state, TEM samples were prepared from the gauge section region near the fracture surface of the tempered martensitic state specimens. At room temperature, the deformed microstructure shows similar characteristics as those typically observed for an as‐received state, but the dislocations are mostly pinned to the ODS particles. At elevated temperatures, structural evolution becomes prominent with the large decrease in dislocation density and the appearance of polygonal subgrains that replace the original lath structure. In addition, coarsening of M 23 C 6 carbides was observed with increase of testing temperatures. Figure 3 presents TEM bright‐field (a) and (110) dark‐field (b) images from the same place of sample obtained after the tensile test at 800°C. The equiaxed α‐Fe has high dislocation density at the grain boundaries (area 1) and dislocations that are pinned to the ODS particles at various locations (marked by arrows). The partially illuminated grain in Figure 3b confirms the presence of low‐angle grain boundary LGB that appears to be undergoing recovery process.

Theory of the spatial resolution of (scanning) transmission electron microscopy in liquid water or ice layers

Determining the extent of oxidation in batches of metal nanoparticles is essential to predict the behaviour of the material. Using aberration corrected transmission electron microscopy (TEM) it was possible to detect the formation of an oxide shell, of thickness 3 nm, on the surface of copper nanoparticles. Further analysis showed that this shell actually consists of two layers, both of which were polycrystalline in nature with domains in the size range of 1–2 nm, and having a thickness of 1.5 nm each. Energy dispersive x-ray spectroscopy confirms that the layers arise due to the formation of oxides, but it was not possible to determine their exact nature. Analysis of the intensity variation within images obtained via probe corrected scanning TEM combined with a high angle annular dark field detector indicates that the shell consists of an inner layer of cuprous oxide (Cu2O) and an outer layer of cupric oxide (CuO). This work was complemented by conventional TEM which provided size distribution and revealed that the majority of particles have a core consisting of a single crystal of copper. This demonstrates the ability of TEM to help to determine the oxidation state of nanoparticles and its potential to be applied to a wide range of homogenous and heterogeneous nanoparticles.

Ultrathin cross sections of the tracheid wall in black spruce and silver fir have been examined at high resolution by scanning transmission electron microscopy (STEM) and by conventional transmission electron microscopy (TEM). For both softwoods, lamellation of the S2 layer was evident but the lamellae were seen more clearly in the STEM photomicrographs. The interlamellar distance was 7.1 nm in the case of spruce and 8.4 nm for silver fir.