Field ion microscopy allows for direct imaging of surfaces with true atomic resolution. The high charge density distribution on the surface generates an intense electric field that can induce ionization of gas atoms. We investigate the dynamic nature of the charge and the consequent electrostatic field redistribution following the departure of atoms initially constituting the surface in the form of an ion, a process known as field evaporation. We report on a new algorithm for image processing and tracking of individual atoms on the specimen surface enabling quantitative assessment of shifts in the imaged atomic positions. By combining experimental investigations with molecular dynamics simulations, which include the full electric charge, we confirm that change is directly associated with the rearrangement of the electrostatic field that modifies the imaging gas ionization zone. We derive important considerations for future developments of data reconstruction in 3D field ion microscopy, in particular for precise quantification of lattice strains and characterization of crystalline defects at the atomic scale.

Journal Article Atomistic Simulations of Surface Effects Under High Electric Fields Get access Stefan Parviainen, Stefan Parviainen GPM UMR 6634 CNRS, Universite et INSA de Rouen, Rouen, FRANCE Search for other works by this author on: Oxford Academic Google Scholar Michal Dagan, Michal Dagan Department of Materials, University of Oxford, UK Search for other works by this author on: Oxford Academic Google Scholar Shyam Katnagallu, Shyam Katnagallu Max-Planck Institut fur Eisenforschung, Dusseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Baptiste Gault, Baptiste Gault Max-Planck Institut fur Eisenforschung, Dusseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Michael Moody, Michael Moody Department of Materials, University of Oxford, UK Search for other works by this author on: Oxford Academic Google Scholar Francois Vurpillot Francois Vurpillot GPM UMR 6634 CNRS, Universite et INSA de Rouen, Rouen, FRANCE Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 644–645, https://doi.org/10.1017/S1431927617003889 Published: 04 August 2017

Journal Article High Fidelity Reconstruction of Experimental Field Ion Microscopy Data by Atomic Relaxation Simulations Get access Shyam Katnagallu, Shyam Katnagallu Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Ali Nematollahi, Ali Nematollahi Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Michal Dagan, Michal Dagan Department of Materials, University of Oxford, Parks Road, Oxford, UK Search for other works by this author on: Oxford Academic Google Scholar Michael Moody, Michael Moody Department of Materials, University of Oxford, Parks Road, Oxford, UK Search for other works by this author on: Oxford Academic Google Scholar Blazej Grabowski, Blazej Grabowski Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Baptiste Gault, Baptiste Gault Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Dierk Raabe, Dierk Raabe Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Jörg Neugebauer Jörg Neugebauer Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 642–643, https://doi.org/10.1017/S1431927617003877 Published: 04 August 2017

An automated procedure has been developed for the reconstruction of field ion microscopy (FIM) data that maintains its atomistic nature. FIM characterizes individual atoms on the specimen’s surface, evolving subject to field evaporation, in a series of two-dimensional (2D) images. Its unique spatial resolution enables direct imaging of crystal defects as small as single vacancies. To fully exploit FIM’s potential, automated analysis tools are required. The reconstruction algorithm developed here relies on minimal assumptions and is sensitive to atomic coordinates of all imaged atoms. It tracks the atoms across a sequence of images, allocating each to its respective crystallographic plane. The result is a highly accurate 3D lattice-resolved reconstruction. The procedure is applied to over 2000 tungsten atoms, including ion-implanted planes. The approach is further adapted to analyze carbides in a steel matrix, demonstrating its applicability to a range of materials. A vast amount of information is collected during the experiment that can underpin advanced analyses such as automated detection of “out of sequence” events, subangstrom surface displacements and defects effects on neighboring atoms. These analyses have the potential to reveal new insights into the field evaporation process and contribute to improving accuracy and scope of 3D FIM and atom probe characterization.

This article reviews recent advances utilizing field-ion microscopy (FIM) to extract atomic-scale three-dimensional images of materials. This capability is not new, as the first atomic-scale reconstructions of features utilizing FIM were demonstrated decades ago. The rise of atom probe tomography, and the application of this latter technique in place of FIM has unfortunately severely limited further FIM development. Currently, the ubiquitous availability of extensive computing power makes it possible to treat and reconstruct FIM data digitally and this development allows the image sequences obtained utilizing FIM to be extremely valuable for many material science and engineering applications. This article demonstrates different applications of these capabilities, focusing on its use in physical metallurgy and semiconductor science and technology.

An automated procedure has been developed for the 3D reconstruction of field ion microscopy (FIM) data that maintains the atomistic nature of the technique. FIM characterises individual atoms on the surface of a specimen, that is evolving subject to the process of field evaporation, in a series of 2D time-ordered images. It’s unique spatial resolution enables the direct imaging of crystal defects, as small as single vacancies. In order to exploit the full potential of FIM, automated analysis tools are required. The reconstruction algorithm developed in this work relies on minimal assumptions and is sensitive to the atomic coordinates of all imaged atoms. It tracks the atoms across a sequence of FIM images and allocates each one to its respective crystallographic plane. The result is a highly accurate 3D lattice-resolved reconstruction. The procedure is applied here to over 2000 tungsten atoms, including several ionimplanted planes of the sample. The approach is also adapted for the analysis of carbides in a bearing steel matrix, to demonstrate its potential applicability to the study of a broader range of materials. A vast amount of additional information is collected during the experiment. This information can underpin advanced analyses such as automated detection of ‘out of sequence’ events, sub-angstrom displacements of atoms and defects effects on neighbouring atoms. These analyses have the potential to reveal new insights into the field evaporation process and contribute to improving accuracy and scope of 3DFIM and atom probe characterisation.

In this work, new reconstruction and analysis methods were developed for 3D field ion microscopy (FIM) data, motivated by the goal of atomic scale characterisation of radiation damage for fusion applications. A comparative FIM/ atom probe tomography (APT) study of radiation damage in self-implanted tungsten revealed FIM advantages in atomistic crystallographic characterisation, able to identify dislocations, large vacancy clusters, and single vacancies. While the latter is beyond the detection capabilities of APT, larger damage features were observed indirectly in APT data via trajectory aberrations and solute segregation. An automated 3DFIM reconstruction approach was developed to maintain reliable, atomistic, 3D insights into the atomic arrangements and vacancies distribution in ion-implanted tungsten. The new method was utilized for the automated ‘atom-by-atom’ reconstruction of thousands of tungsten atoms yielding highly accurate reconstructions of atomically resolved poles but also applied to larger microstructural features such as carbides and a grain boundary, extending across larger portions of the sample. Additional tools were developed to overcome reconstruction challenges arising from the presence of crystal defects and the intrinsic distortion of FIM data. Those were employed for the automated 3D mapping of vacancies in ion-implanted tungsten, analysing their distribution in a volume extending across 50nm into the depth of the sample. The new FIM reconstruction also opened the door for more advanced analyses on FIM data. It was applied to the preliminary studies of the distortion of the reconstructed planes, found to depend on crystallographic orientation, with an increased variance in atomic positions measured in a radial direction to the centre of the poles. Additional analyses followed the subtle displacements in atomic coordinates on consecutive FIM images, to find them affected by the evaporation of atoms from the same plane. The displacements were found to increase with size as the distance to the evaporated atom decreased, and are likely to be the result of a convolution between image gas effects, surface atoms relaxation, and charge re-distribution. These measurements show potential to probe the dynamic nature of the FIM experiment and possibly resolve contributions from the different processes effecting the final image. Finally, APT characterisation was performed on bulk and pre-sharpened needles to determine the effect of sample’s geometry on the resulting implantation profiles, and the extent to which pre-sharpened needles could be employed in radiation damage studies. While the ions depth profiles in needles were not found within a good match to SRIM simulations, the damage profiles exhibited closer agreement. Further, the concentration of implanted ions in bulk samples was found significantly higher than in the respective needle implanted samples, with excessive loss found for the light ion implantation.

Radiation damage in tungsten and a tungsten–tantalum alloy, both of relevance to nuclear fusion research, has been characterized using a combination of field ion microscopy (FIM) imaging and atom probe tomography (APT). While APT provides 3D analytical imaging with sub-nanometer resolution, FIM is capable of imaging the arrangements of single atoms on a crystal lattice and has the potential to provide insights into radiation induced crystal damage, all the way down to its smallest manifestation – a single vacancy. This paper demonstrates the strength of combining these characterization techniques. In ion implanted tungsten, it was found that atomic scale lattice damage is best imaged using FIM. In certain cases, APT reveals an identifiable imprint in the data via the segregation of solute and impurities and trajectory aberrations. In a W–5at%Ta alloy, a combined APT–FIM study was able to determine the atomic distribution of tantalum inside the tungsten matrix. An indirect method was implemented to identify tantalum atoms inside the tungsten matrix in FIM images. By tracing irregularities in the evaporation sequence of atoms imaged with FIM, this method enables the benefit of FIM's atomic resolution in chemical distinction between the two species.

The refractory metal tungsten is a promising candidate material for plasma facing components (PFCs) in future fusion reactors due in part to its low sputter yield, good thermal conductivity and low activation under transmutation. However, tungsten suffers from a very high brittle-to-ductile transition temperature (BDTT) of 400-500 °C [1]. This inherent brittleness is only exacerbated under irradiation due to the irradiation-induced defects and the formation of second phase precipitates containing the transmutation products Re and Os. [2,3] Here, we discuss the use of novel field ion microscopy (FIM) and atom probe tomography (APT) techniques to investigate radiation effects in irradiated tungsten.

In the leading approach for the design of a fusion reactor, the plasma is confined magnetically inside a device called ‘Tokamak’. Prospective materials to construct this device must withstand extremely high temperature and radiation dose conditions. The divertor plates within the Tokamak remove heat and impurities and are a particularly challenging component to design. Tungsten, the element with the highest melting point, is a leading candidate for this role. Hence, an extensive, atomic scale investigation of tungsten behavior under extreme and cyclic conditions is critical to determine its suitability as a material for plasma facing components and their operational lifetime. Especially challenging, even for advanced microscopy techniques, is the detection of nanoscale damage to the crystal lattice. The formation of dislocations, nano-voids, clustering effects, self-interstitials and transmutation damage requires atomic-scale characterization and correlation to changes in mechanical properties of the irradiated material.