Microsc. Microanal. 23, 255–268, 2017 doi:10.1017/S1431927617000277
© MICROSCOPY SOCIETY OF AMERICA 2017
Automated Atom-By-Atom Three-Dimensional (3D) Reconstruction of Field Ion Microscopy Data
Michal Dagan,1 Baptiste Gault,1,2 George D. W. Smith,1 Paul A. J. Bagot,1 and Michael P. Moody1,*
1Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH UK 2Max Planck Institut für Eisenforschung GmbH, Max-Planck Straße 1, 40237 Düsseldorf, Germany
Abstract: 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.
Key words: field ion microscopy, 3D reconstruction, atomic resolution, crystal defects
INTRODUCTION Materials performance is intimately linked tomicrostructural architecture at the atomic scale. The continued development of models to explain mechanical properties, and ultimately design new and improved materials is therefore critical for an increasing number of applications. However, direct imaging in three dimensions (3D), not only all constituent atoms within a material, but also vacancies and other types of defects, has proven to be beyond the resolution limit of most current conventional microscopy techniques, and remains a formidable challenge. For example, such atomic-scale char- acterization capabilities are essential in radiation damage studies. A remaining hurdle in the development of nuclear fusion power generation is the lack of materials that can withstand the extreme operating conditions inside the reactor (Bolt et al., 2002; Mansur et al., 2004). A thorough under- standing of radiation-induced degradation mechanisms of the internal microstructure under such conditions is therefore of paramount importance. Recent efforts in electron microscopy have proposed
complex solutions to atomic-scale tomographic imaging of materials (Li et al., 2008; Azubel et al., 2014; Xu et al., 2015). Field ion microscopy (FIM) was the first microscopy tech- nique to image individual atoms (Müller & Bahadur, 1956), lattice defects in metals including single vacancies (Speicher et al., 1966) and interstitials and extended dislocations (Fortes et al., 1968; Smith et al., 1968). In contrast to other
*Corresponding author.
michael.moody@
materials.ox.ac.uk Received July 8, 2016; accepted January 24, 2017
high-resolution microscopy techniques, FIM relies on the ionization of inert gas atoms from prominent positions on the specimen surface, exploiting an intense electric field. The electric field is generated at the apex of the very sharp needle- shaped specimen (Gault et al., 2012)maintained at cryogenic
temperature and biased to a few kV. The divergent electric field drives the ions away from the specimen and onto a phosphor screen, creating a highly magnified, quasi- stereographic, 2D projection of the atomic arrangement at the surface of the specimen. A schematic illustration of the FIM set-up is shown in Figure 1a. Voltage pulses super- imposed on the DC voltage enable a highly controlled removal of the atoms constituting the specimen via field evaporation. Although each FIM image is intrinsically 2D, this procedure results in a series of images, each representing a “snapshot” of the surface over the course of the field eva- poration process. As the depth of the specimen is probed by the removal of the constituent atoms, the images progres- sively provide 3D atomic information on the specimen. Ground-breaking research in the 1960s and 1970s
demonstrated the unique insights FIM can bring into the study of fine-scale radiation damage. In a series of experi- ments, Seidman et al. directly imaged the early stages of radiation damage, manifesting as very small, atomic-scale defects in the crystal lattice by developing a novel 3D FIM approach (Seidman, 1978; Wei & Seidman, 1979; Seidman et al., 1981; Wei et al., 1981). In these experiments, the nature and spatial distribution of single vacancies and larger vacancy clusters were measured across 3D volumes of the specimens. As these experiments predated digital data collection, FIM imageswererecordedon film and analyzed manually.
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