Quantification of small, convex particles by TEM

It is shown how size distributions of arbitrarily oriented, convex, non-overlapping particles extracted from conventional transmission electron microscopy (TEM) images may be determined by a variation of the Schwartz-Saltykov method. In TEM, particles cut at the surfaces have diminished projections, which alter the observed size distribution. We represent this distribution as a vector and multiply it with the inverse of a matrix comprising thickness-dependent Scheil or Schwartz-Saltykov terms. The result is a corrected size distribution of the projections of uncut particles. It is shown how the real (3D) distribution may be estimated when particle shape is considered. Computer code to generate the matrix is given. A log-normal distribution of spheres and a real distribution of pill-box-shaped dispersoids in an Al-Mg-Si alloy are given as examples. The errors are discussed in detail.     

Zirconium oxidation on the atomic scale

Zirconium alloys are used in the nuclear industry as fuel rod cladding. They are chosen for this role because of their good mechanical properties and low thermal neutron absorption. Oxidation of these alloys by coolant is one of the chief limiting factors of the fuel burn-up efficiency. The aim of the present study is to understand these oxidation mechanisms. As a first step, a fundamental study of the oxidation of commercially pure zirconium has been conducted using the 3D atom probe (3DAP). The current generation of 3DAPs allows both voltage and laser pulsing, providing data sets of many millions of ions.     

A study of the anomalous field evaporation of Sm(Co0.68 Fe0.20 Cu0.10 Zr0.02)7.5 alloy by 3D atom probe

3D atom probe analysis of the composition of a Sm(Co0.68 Fe0.20 Cu0.10 Zr0.02)7.5 alloy was conducted by varying the probing temperature from 10.6 to 65 K and pulse fraction from 10% to 20%. It was found that the preferential evaporation of Sm occurred at 65 K, due to the very low evaporation field of Sm, 15.2 V/nm calculated by using the charge exchange model. With decreasing the specimen temperature, preferential evaporation of Sm was alleviated. The optimum analysis conditions which give reasonably good measurement of the composition were: the specimen temperature of 20 K and a pulse fraction 15%. The effects of the specimen temperature and pulse fraction on the measured composition of the alloy are discussed, based on the charge exchange model.     

A model of secondary electron imaging in the helium ion scanning microscope

A combination of the ‘semi-empirical’ model for secondary electron production and the TRIM routines which describe ion stopping power, scattering, and transport, has been used to construct a Monte Carlo simulation (IONiSE) that can quantitatively interpret the generation of secondary electrons (SE) from materials by fast helium ions. This approach requires that the parameters of the semi-empirical model be determined by fitting to experimental yield data but has the merit that, unlike more fundamental models, it can be applied with equal ease to both pure elements and complex compounds. The application of the model to predict the topographic yield variation of helium generated SE as a function of energy and material, and to investigate the ratio between SE generated by incident and backscattered ions, is demonstrated.     

Electron irradiation-induced destruction of carbon nanotubes in electron microscopes

Observations of carbon nanotubes under exposure to electron beam irradiation in standard transmission electron microscope (TEM) and scanning electron microscope (SEM) systems show that such treatment in some cases can cause severe damage of the nanotube structure, even at electron energies far below the approximate 100 keV threshold for knock-on damage displacing carbon atoms in the graphene structure. We find that the damage we observe in one TEM can be avoided by use of a cold finger. This and the morphology of the damage imply that water vapour, which is present as a background gas in many vacuum chambers, can damage the nanotube structure through electron beam-induced chemical reactions. Though, the dependence on the background gas makes these observations specific for the presently used systems, the results demonstrate the importance of careful assessment of the level of subtle structural damage that the individual electron microscope system can do to nanostructures during standard use.     

Imaging of radiation-sensitive samples in transmission electron microscopes equipped with Zernike phase plates

We have optimized a bright-field transmission electron microscope for imaging of high-resolution radiation-sensitive materials by calculating the imaging dose n(0) needed to obtain a signal-to-noise ratio (SNR)=5. Installing a Zernike phase plate (ZP) decreases the dose needed to detect single atoms by as much as a factor of two at 300 kV. For imaging larger objects, such as Gaussian objects with full-width at half-maximum larger than 0.15 nm, ZP appears more efficient in reducing the imaging dose than correcting for spherical aberration. The imaging dose n(0) does not decrease with extending of chromatic resolution limit by reducing chromatic aberration, using high accelerating potential (U(0)=300 kV), because the image contrast increases slower than the reciprocal of detection radius. However, reducing chromatic aberration would allow accelerating potential to be reduced leading to imaging doses below 10 e(-)/A(2) for a single iodine atom when a CS-corrector and a ZP are used together. Our simulations indicate that, in addition to microscope hardware, optimization is heavily dependent on the nature of the specimen under investigation.     

In situ TEM studies of local transport and structure in nanoscale multilayer films

This paper describes a novel technique for studying structure-transport correlations in nanoscale multilayer thin films. Here, local current-voltage characteristics from simplified magnetic tunnel junctions are measured in situ on cross-sectional transmission electron microscopy (TEM) samples and correlated directly with TEM images of the microstructure at the tunneling site. It is found that local variations in barrier properties can be detected by a point probe method, and that the tunneling barrier height and width can be extracted.     

On the detection of MX-precipitates in microalloyed steels using energy-filtering TEM

The properties of microalloyed steels and HSLA steels depend predominantly on the size and composition of nanometre-sized MX precipitates consisting of Nb, Ti, V, C and N. In this work we present a rapid and powerful method for visualizing these small precipitates directly in the steel matrix. This can be achieved by energy-filtering TEM in a 200-kV microscope. We have recorded elemental maps (three-window method) and jump-ratio images (two-window method) using the inner-shell ionization edges; e.g. Fe-M₂₃, Fe-L₂₃, V-M₂₃, V-L₂₃, Nb-N₂₃, Nb-M₄₅, C-K and N-K edges. We have found that precipitates in a steel matrix can be best visualized by recording the jump-ratio image of the matrix element. If the specimens are thinner than 30 nm and high resolution in the images and low detection limits are essential, low energy-loss edges should be used preferentially; in the case of steels the Fe-M₂₃ edge should be used instead of the Fe-L₂₃ edge. If the jump-ratio images are recorded with the Fe-M₂₃ edge, precipitates about 2 nm in diameter may be detected. With thicker samples and if high resolution is not as essential, higher energy-loss edges are preferable, because diffraction effects are less possible. The chemical composition of the larger precipitates (10 nm) could be determined by recording EDX spectra using an STEM probe. We found Nb and V with an atomic ratio of about 3, but we could not detect carbon or nitrogen in the spectra. The Nb and V distribution could be best visualized with a jump-ratio image including both the Nb-N₂₃ and the V-M₂₃ edges. However, it was not possible to image the carbon or nitrogen content of the precipitates unequivocally due to overlapping of the C-K and N-K edges with the Nb-M₄₅ edge.     

A comparison of some methods for measuring the size distribution of two-dimensionally clustered latex particles in TEM images using mathematical morphology

The particle size distribution of polymer latexes of, for example, polybutadiene can be measured on transmission electron micrographs using image analysis methods. By taking suitable measures in preparing the sample it can be ensured that the image only consists of projections of two-dimensional clusters of adhering latex spheres. For the deagglomeration of these clusters a number of methods based on mathematical morphology are discussed. Speed and accuracy of the methods were tested on a personal-computer-based image analysis system and compared to manual measurements. Repeated openings or erosions applied to a binary image are relatively fast global methods with a low accuracy due to the fact that particle diameters are measured in whole numbers of pixels; only the horizontal diameters of rectangular pixels are measured. With an iterative method the individual components of the clusters are found by repeated conditional thickening of the erosion kernels. The method is accurate but very slow due to the large number of processing steps. The watershed method as applied to the original grey-scale transmission electron micrograph makes use of the sharp local maxima which arise at the interface of the projection of two adhering particles. This method is as accurate as the iterative method, but about eight times as fast.     

Physical and chemical changes in polystyrene during electron irradiation using EELS in the TEM: contribution of the dielectric function

We have performed electron energy-loss spectroscopy analysis of polystyrene in the analytical transmission electron microscope in order to evaluate the possibility of obtaining both chemical and dielectric information on the polymer at the submicrometre scale. Irradiation has also been studied, particularly the influence of the specimen temperature on the kinetics of degradation.
The main results show that polystyrene is resistant to electron beam with a critical dose of about 10⁴ C m⁻² at 127 K. Spectra could be acquired with doses less than this critical dose. We were thus able to propose an identification of the different chemical bonds of carbon (including the C–H bond), in agreement with previous X-ray absorption near-edge structure experiments. The chemical changes in polystyrene due to severe irradiation damage are also visible in the carbon K-edge near-edge structure.
At the same time, we calculated the dielectric function from the low-loss part of the spectra. Interestingly, this part of the spectrum is the most sensitive to irradiation, since great changes can be seen during exposure.
Lastly, we propose a degradation process, in agreement with all these results.