Atom Probe Tomography Reconstruction Research Articles

Experimental Tip Shape Imaging Routes to Assist Data Reconstruction in Atom Probe Tomography

Experimental Tip Shape Imaging Routes to Assist Data Reconstruction in Atom Probe Tomography

Field-dependent abundances of hydride molecular ions in atom probe tomography of III-N semiconductors.

We investigate the microscopic behaviour of hydrogen-containing species formed on the surface of III-N semiconductor samples by the residual hydrogen in the analysis chamber in laser-assisted atom probe tomography (APT). We analysed AlGaN/GaN heterostructures containing alternate layers with a thickness of about 20nm. The formation of H-containing species occurs at field strengths between 22 and 26V/nm and is independent of the analysed samples. The 3D APT reconstruction makes it possible to map the evolution of the surface behaviour of these species issued by chemical reactions. The results highlight the strong dependence of the relative abundances of hydrides on the surface field during evaporation. The relative abundances of the hydrides decrease when the surface field increases due to the evolution of the tip shape or the different evaporation behaviour of the different layers.

A study of the carbon distribution in bainitic ferrite

The carbon distribution in bainitic ferrite (BF) of two carbide-free bainite alloys isothermally transformed at three different temperatures was studied using a unique combination of in-situ high energy X-ray diffraction (HEXRD), atom probe tomography (APT) and electron energy loss spectroscopy (EELS). Results from all three techniques are consistent and unambiguously confirm the presence of significant excess carbon levels in BF, even in these low nominal carbon (0.22 wt.%) steels. The carbon concentration in BF decreases as the transformation temperature increases, but remains significant up to 430 °C. Further, statistical analysis indicates that the excess carbon content does not follow a normal distribution. APT reconstructions suggest that much (but not all) of the excess carbon is segregated to defects or clusters. No clear evidence for bainite tetragonality was observed. Surprisingly, the addition of a strong carbide forming element (vanadium) in solid solution did not influence the carbon distribution in BF.

Standardizing Spatial Reconstruction Parameters for the Atom Probe Analysis of Common Minerals

AbstractWell-defined reconstruction parameters are essential to quantify the size, shape, and distribution of nanoscale features in atom probe tomography (APT) datasets. However, the reconstruction parameters of many minerals are difficult to estimate because intrinsic spatial markers, such as crystallographic planes, are not usually present within the datasets themselves. Using transmission and/or scanning electron microscopy imaging of needle-shaped specimens before and after atom probe analysis, we test various approaches to provide best-fit reconstruction parameters for voltage-based APT reconstructions. The results demonstrate that the length measurement of evaporated material, constrained by overlaying pre- and post-analysis images, yields more consistent reconstruction parameters than the measurement of final tip radius. Using this approach, we provide standardized parameters that may be used in APT reconstructions of 11 minerals. The adoption of standardized reconstruction parameters by the geoscience APT community will alleviate potential problems in the measurement of nanoscale features (e.g., clusters and interfaces) caused by the use of inappropriate parameters.

Probing the Surface Chemistry of Nanoporous Gold via Electrochemical Characterization and Atom Probe Tomography.

Surface chemistry information is crucial in understanding catalytic and sensing mechanisms. However, resolving the outermost monolayer composition of metallic nanoporous materials is challenging due to the high tortuosity of their morphology. In this study, we first elaborate on the capabilities and limitations of atom probe tomography (APT) in resolving interfaces. Subsequently, an electrochemical approach is designed to characterize the surface composition of nanoporous gold (NPG), developed from dealloying an inexpensive precursor (95 at. % Ag, 5 at. % Au), by the means of aqueous electrochemical measurements of the selective electrosorption of sulfide ions, which react strongly with Ag, but to a significantly lesser extent with Au. Accordingly, cyclic voltammetry was performed at various scan rates on NPG in alkaline aqueous solutions (0.2 M NaOH; pH 13) in the presence and absence of 1 mM Na2S. Calibrations via similar voltammetric measurements on pure polycrystalline Ag and Au surfaces allowed for a quantitative estimation for the Ag surface coverage of NPG. The sensitivity threshold for the detection of the adsorbate–Ag interaction was assessed to be approximately 2% Ag surface coverage. As curves measured on NPG only showed featureless capacitive currents, no faradaic charge density associated with sulfide electrosorption could be detected. This study opens a new avenue to gain further insight into the monolayer surface coverage of metallic nanoporous materials and assists in enhancement of the interpretation of APT reconstructions.

Quantification of hydrogen in nanostructured hydrogenated passivating contacts for silicon photovoltaics combining SIMS-APT-TEM: A multiscale correlative approach

Multiscale characterization of the hydrogenation process of silicon solar cell contacts based on c-Si/SiOx/nc-SiCx(p) has been performed by combining dynamic secondary ion mass-spectrometry (D-SIMS), atom probe tomography (APT), and transmission electron microscopy (TEM). These contacts are formed by high-temperature firing, which triggers the crystallization of SiCx, followed by a hydrogenation process to passivate remaining interfacial defects. Due to the difficulty of characterizing hydrogen at the nm-scale, the exact hydrogenation mechanisms have remained elusive. Using a correlative TEM-SIMS-APT analysis, we are able to locate hydrogen trap sites and quantify the hydrogen content. Deuterium (D), a heavier isotope of hydrogen, is used to distinguish hydrogen introduced during hydrogenation from its background signal. D-SIMS is used, due to its high sensitivity, to get an accurate deuterium-to-hydrogen ratio, which is then used to correct deuterium profiles extracted from APT reconstructions. This new methodology to quantify the concentration of trapped hydrogen in nm-scale structures sheds new insights on hydrogen distribution in technologically important photovoltaic materials.

A Crystallography-Mediated Reconstruction (CMR) Approach for Atom Probe Tomography: Solution for a Singleton Pole

Spatially accurate atom probe tomography reconstructions are vitally important when quantitative spatial measurements such as distances, volumes and morphologies etc. of nanostructural features are required information for the researcher. It is well known that the crystallographic information contained within the atom probe data of crystalline materials can be used to calibrate the tomographic reconstruction. Specifically, the crystallographic information projected into the field evaporation images is used. This offers a powerful and accurate enhancement of the atom probe technique. However, this is often difficult to do in practice. In previously reported approaches, it was necessary to index at least two poles to compute the image compression factor ‘ξ’ and observe crystallographic planes in at least one of the pole regions to obtain a measure of the field factor ‘kf’ while also manually accounting for a change in reconstruction parameters throughout the dataset. Not only is this error-prone and time consuming, it does not work for materials that exhibit limited crystallographic information in their field evaporation image. Here, we extend the applicability of the crystallographic calibration of atom probe data by proposing a reconstruction methodology where only one pole with observable lattice planes is required in the projected detector image. Our proposal also accounts for dynamic variations in the reconstruction parameters throughout the 3D dataset. The method is simpler and significantly faster to implement and is applicable to more atom probe situations than previously approaches. Our single-pole crystallography mediated reconstruction (SP-CMR) utilizes the Hawkes-Kasper projection model (equivalent to the equidistant-azimuthal projection model) and the direct fourier (DF) fit algorithm to determine the precise reconstruction parameters required to produce flat atomic planes. It is applied to experimental Al and highly Sb-doped Si data. The discrepancies between the spatial dimensions of the SP-CMR reconstructions compared to uncalibrated reconstructions are visually apparent. Consistent plane spacings and angles between crystallographic directions matching the theoretically known values for each crystal structure are demonstrated.

Compensating Local Magnifications in Atom Probe Tomography for Accurate Analysis of Nano-Sized Precipitates.

Local magnification artifacts in atom probe tomography (APT) caused by multiphase materials with heterogeneous evaporation behavior are a well-known problem. In particular, the analysis of the exact size, shape, and composition of small precipitates is, therefore, not trivial. We performed numerical simulations of APT measurements to predict the reconstructed morphology of precipitates with contrasting evaporation thresholds. Based on a statistical approach that avoids coarse graining, the simulated data are evaluated to develop a model for the calculation of the original size of the precipitates. The model is tested on experimental APT data of precipitates with a higher and lower evaporation field in a ferritic alloy. Accurate sizes, proven by a complementary investigation by transmission electron microscopy, are obtained. We show further, how the size information can be used to obtain compositional information of the smallest precipitates and present a new methodology to determine a correct in-depth scaling of the APT reconstruction in case no complementary geometric information about the specimen exists or if no lattice planes are visible in the reconstruction.

On the formation of nano-sized precipitates during cooling of NiAl- strengthened ferritic alloys

We present an experimental study on NiAl- type precipitates in ferritic superalloys that aim for application as structural materials at high temperature. The focus of this study is on the characterization of hyperfine precipitates that form during sample cooling after the aging treatment and which have a strong influence on the alloys' room temperature hardness. Size and crystallographic information of the precipitates were obtained by electron microscopy. The chemical nature of different phases was analyzed by atom probe tomography (APT). In order to identify the hyperfine precipitates in APT reconstructions, a modified version of the maximum separation method for cluster selection is proposed. Instead of a separate erosion step, the proposed method makes use of a Delaunay tessellation which does not require any user defined input parameters and further, gives a direct access to the morphology of the clusters. The modified algorithm was tested on simulated datasets and then successfully applied to experimental datasets containing precipitates with radii down to one nanometer. The result of the chemical analysis shows that the formation of precipitates during sample cooling is a kinetically controlled process which is dominated by two different mechanisms in dependence on the temperature.

Towards model-driven reconstruction in atom probe tomography

Reconstructions in atom probe tomography (APT) are plagued by image distortions arising from changes in the specimen geometry throughout the experiment. The simplistic and inaccurate geometrical assumptions that underpin the conventional reconstruction approach account for much of this distortion. Here we extend our previous work of modelling APT experiments using level set methods to three dimensions (3D). This model is used to generate and subsequently reconstruct synthetic APT datasets from electron tomography (ET) of an multiphase specimen. Finally, we apply our model to the reconstruction of an experimental field-effect transistor (finFET) dataset. This model-driven reconstruction successfully reduces density distortions compared to conventional methods. By combining prior knowledge about the specimen geometry from sources such as ET, such an approach promises new distortion correcting APT reconstruction applicable to complex specimen geometries.

(Invited) Advanced Multi-Technique Characterization of Nanoporous Materials

Nanoporous materials are widely used across renewable energy technologies as catalysts, supports, and storage materials. However, they are intrinsically challenging to characterize since their bulk and surface properties are difficult to identify using traditional characterization methods. While it is common to focus on structural and morphological properties like porosity and crystallinity, determined using physisorption and x-ray diffraction respectively, a combination of multiple techniques is necessary to assess chemical composition of these materials in order to determine structure-property-performance correlations.In order to better understand these materials at a fundamental level, multi-technique characterization must be utilized strategically to identify chemical and elemental differences in the surface, sub-surface, and bulk. The most common techniques include x-ray photoelectron spectroscopy (XPS) for surface chemical information, transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) for identification of morphological differences, and energy dispersive x-ray spectroscopy (EDS) for nanoscale elemental distributions. Atom probe tomography (APT) can be used as a stepping stone to understanding these materials at an atomic-scale and to bridge the gap between the compositions in the surface, sub-surface and bulk of materials. APT is a technique that has traditionally been used to reconstruct metallic and ceramic samples in 3D with sub-nanometer-resolution to enhance the understanding of specific grain-boundaries and precipitates. Expanding this technique to nanoporous materials requires significant optimization of existing procedures and considerations. The ability to elucidate dopant locations, nano-scale contaminations, and other important features of nanoporous materials at sub-nm scale in 3D can provide unprecedented opportunities towards optimization of these materials.Several classes of porous materials will be discussed in this talk. Nitrogen-doped carbon nanospheres are used as a model, carbon-based catalyst support material in order to show the advantages of the proposed characterization approach and demonstrate feasibility of the APT analysis of porous nanomaterials. Understanding surface chemistry of these materials is essential for elucidation of effects of nitrogen dopants on activity and stability of metal catalyst nanoparticles. Initial progress in characterization will be presented along with challenges and possible solutions. Further analysis of these porous carbons modified with both nitrogen and transition metal sites as examples of model catalysts with atomically dispersed active sites will also be presented. These efforts aim to further improve 3D APT reconstructions of carbon-based porous materials. And finally, efforts towards characterization of porous hydrogen storage/carrier materials based on functionalized magnesium borohydride (MBH) materials will be discussed. While these materials present additional challenges in characterization due to their complex chemistries and reactivity, multi-technique characterization with sub-nm resolution could significantly advance understanding of hydrogen desorption/adsorption mechanisms. The integration of 2D and 3D data at multiple scales provides a pathway for understanding these three classes of porous materials and a wide variety of other complex nanoporous materials used in electrochemical applications.

Interpreting nanovoids in atom probe tomography data for accurate local compositional measurements

Quantifying chemical compositions around nanovoids is a fundamental task for research and development of various materials. Atom probe tomography (APT) and scanning transmission electron microscopy (STEM) are currently the most suitable tools because of their ability to probe materials at the nanoscale. Both techniques have limitations, particularly APT, because of insufficient understanding of void imaging. Here, we employ a correlative APT and STEM approach to investigate the APT imaging process and reveal that voids can lead to either an increase or a decrease in local atomic densities in the APT reconstruction. Simulated APT experiments demonstrate the local density variations near voids are controlled by the unique ring structures as voids open and the different evaporation fields of the surrounding atoms. We provide a general approach for quantifying chemical segregations near voids within an APT dataset, in which the composition can be directly determined with a higher accuracy than STEM-based techniques.

Atom probe analysis of Ni–Nb–Zr metallic glasses

Atomic short arrangements of two Ni–Nb–Zr glassy alloys (doped and undoped) were examined through the 3D atomic reconstruction technique, atom probe tomography (APT). The chemical short configurations predicted according to two distinct theoretical models were compared with APT reconstructions, and inconsistencies were found. Additionally, diffraction experiments confirmed that the alloys under investigation were amorphous in nature.

Fast modelling of field evaporation in atom probe tomography using level set methods

Current imaging algorithms in atom probe tomography (APT) assume a simple geometric model for the trajectories of evaporated ions between the emitter and detector. Such point-projection approaches fail to incorporate changes in sample geometry due to different phase evaporation behaviour, leading to severe distortions in reconstructed APT data. Here we propose a new approach to APT reconstruction where a continuum model is used to account for changes in sample geometry. A 2D model is demonstrated, readily extendable to 3D, capable of simulating the complete evaporation of samples within minutes, while accounting for dielectric and crystallographic behaviour. Ion trajectories are calculated from the sample surface onto a simulated detector, allowing for the accurate mapping of detector positions onto the sample surface throughout time. Such an approach could drive a new reconstruction algorithm applicable to full size experimental datasets that can correct for the distortions currently present within APT reconstructions.

Origins of meteoritic nanodiamonds investigated by coordinated atom‐probe tomography and transmission electron microscopy studies

AbstractTo constrain the origins of meteoritic nanodiamonds, the abundance ratios of stable C isotopes in acid residues from the carbonaceous chondritic meteorite Allende CV3 were measured using coordinated atom‐probe tomography (APT) and transmission electron microscopy (TEM). We combined our data with previously published APT data. A statistical analysis of this combined data set suggests an upper bound of 1 in 102 on the subpopulation that could have a large isotopic enrichment in 13C relative to 12C, consistent with the possible detection by secondary ion mass spectrometry of a similar enrichment in a 1 in 105 fraction, abundant enough to account for the Xe‐HL anomalous isotopic component carried by the acid residues. Supernovae are believed to be the source of Xe‐HL, leading to the mystery of why all other supernova minerals do not carry Xe‐HL. The lack of Xe‐HL in low‐density disordered supernova graphite suggests that the isotopically anomalous component is the nanodiamonds, but the disordered C in the residue is not ruled out. We discuss possible origins of the disordered C and implications of our results for proposed formation scenarios for nanodiamonds. At least 99% of the meteoritic acid residue exhibits no unambiguous evidence of presolar formation, although production with solar isotope ratios in asymptotic giant branch stars is not ruled out. Comparison of TEM and APT results indicates that a minority of the APT reconstructions may preferentially sample disordered C rather than nanodiamonds. If this is the case, a presolar origin for a larger fraction of the nanodiamonds remains possible.

Nanometer scale structural and compositional inhomogeneities of half-Heusler CoTi1-xFexSb thin films

The correlative use of X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and atom probe tomography (APT) allows the structure of substitutionally alloyed half-Heusler compound CoTi1-xFexSb to be characterized at the nanometer scale. For x = 0.2, 0.3, and 0.5, XRD patterns indicate an epitaxial cube-on-cube growth with no detectable secondary phases and are suggestive of high crystalline quality for all three of the films. This is supported using atomic resolution STEM images, which revealed the absence of secondary phases or polycrystalline regions, thus confirming the half-Heusler structure of the CoTi1-xFexSb films. For all three samples, the APT reconstructions were optimized to resolve the atomic planes in the [001] growth direction allowing the exploitation of reliable datasets. The presence of Fe-rich phases in the samples with Fe atoms sitting on Ti crystallographic sites is revealed. A strong phase separation is observed for x = 0.2 and x = 0.3 and is almost no longer observed for x = 0.5 in good agreement with the magnetic properties of the films.

Assessing the Spatial Accuracy of the Reconstruction in Atom Probe Tomography and a New Calibratable Adaptive Reconstruction.

We define a measure for the accuracy of tomographic reconstruction in atom probe tomography, named here the spatial error index. We demonstrate that this index can be used to compare rigorously the spatial accuracy of various different approaches to the calculation of tomographic reconstruction. This is useful, for example, to evaluate the performance of alternate tomographic reconstruction approaches, and ensures that the comparisons are independent of individual data quality or other instrumental parameters. We then introduce a new "adaptive reconstruction" formalism that uses a progression of reconstruction parameters based on a per-atom correction from the cube root of the inverse of the voltage, along with linear correction factors linked to the evaporation sequence. We apply the measure for spatial accuracy to this new reconstruction protocol.

Calibration of Atom Probe Tomography Reconstructions Through Correlation with Electron Micrographs.

Although atom probe tomography (APT) reconstructions do not directly influence the local elemental analysis, any structural inferences from APT volumes demand a reliable reconstruction of the point cloud. Accurate estimation of the reconstruction parameters is crucial to obtain reliable spatial scaling. In the current work, a new automated approach of calibrating atom probe reconstructions is developed using only one correlative projection electron microscopy (EM) image. We employed an algorithm that implements a 2D cross-correlation of microstructural features observed in both the APT reconstructions and the corresponding EM image. We apply this protocol to calibrate reconstructions in a Cu(In,Ga)Se2-based semiconductor and in a Co-based superalloy. This work enables us to couple chemical precision to structural information with relative ease.

Introducing a Crystallography-Mediated Reconstruction (CMR) Approach to Atom Probe Tomography.

Current approaches to reconstruction in atom probe tomography produce results that exhibit substantial distortions throughout the analysis depth. This is largely because of the need to apply a multitude of assumptions when estimating the evolution of the tip shape, and other pseudo-empirical reconstruction factors, which vary both across the face of the tip and throughout the analysis depth. We introduce a new crystallography-mediated reconstruction to improve the spatial accuracy and dramatically reduce these in-depth variations. To achieve this, we developed a barycentric transform to directly relate atomic positions in detector space to real space. This is mediated by novel crystallographic analysis techniques, including: (1) calculating the orientation of a crystal directly from the field evaporation map, (2) tracking pole locations throughout the evaporation sequence, and (3) accounting for the evolving tip radius in a manner that removes the dependence on the geometric field factor. By improving the in-depth spatial accuracy of the atom probe reconstruction, a greater accuracy of the atomic neighborhood relationships is available. This is critical in modern materials science and engineering, where an understanding of the solid solution architecture, precipitate dispersions, and descriptions of the interfaces between phases or grains are key inputs to microstructure-property relationships.

Correlative Atom Probe Tomography and Transmission Electron Microscopy Analysis of Grain Boundaries in Thermally Grown Alumina Scale.

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine. We also demonstrate a cost-efficient, robust, and easy-handling setup for correlative analysis based solely on commercially available components, which can be used with all conventional TEM tools without the need to modify the specimen holder assembly.