Atom probe tomography spatial reconstruction: Status and directions

In this review we present an overview of the current atom probe tomography spatial data reconstruction paradigm, and explore some potential routes to improve the current methodology in order to yield a more accurate representation of nanoscale microstructure. Many of these potential improvement methods are directly tied to extensive application of advanced numerical methods, which are also very briefly reviewed. We have described effects resulting from the application of the standard model and then introduced several potential improvements, first in the far field, and, second, in the near field. The issues encountered in both cases are quite different but ultimately they combine to determine the spatial resolution of the technique.     

On the current role of atom probe tomography in materials characterization and materials science

Atom probe tomography has without any doubt become a routine technique to analyze the detailed three-dimensional chemistry of materials at the nanoscale. This article provides a general overview of what APT can reliably do today and what it might do tomorrow in terms of material characterization. The recent achievements in the analysis of new materials and new materials structures are first presented allowing some speculation on future possible developments. The ability to provide unique quantitative chemical information to link processing to device performance is then reviewed in the context of the recent nanowire and gate structures analyses. Finally examples of the systematic use of atom probe tomography to explore material behaviors and kinetic processes controlling microstructure evolution are presented.     

Laser pulsing of field evaporation in atom probe tomography

The processes by which field evaporation in an atom probe is momentarily stimulated by impingement of a laser beam on a specimen are considered. For metals, the dominant and perhaps only sensible mechanism is energy absorption leading to thermal pulsing, which has been well established. The energy of a laser beam is absorbed in a thin optical skin depth on the surface of the specimen. For materials with a band gap such as semiconductors and dielectrics, it is found that energy absorption in a thin surface layer dominates the process as well and leads to similar thermal pulsing. The relative amount of surface absorption versus volume absorption can strongly influence the heat flow and therefore the mass spectrum of the specimen. Thus it appears for very different reasons that all materials behave similarly in response to laser pulsing in atom probe tomography.     

A study of nanoscale TiB2 precipitation during titanium silicidation using atom probe tomography

Atom Probe Tomography (APT) was applied to analyze the silicidation reaction between a titanium metal film, capped by a TiN layer, and a boron-implanted silicon substrate. The concentration depth profile observed by APT, depicts low concentrations of B in titanium silicide itself and the B accumulation at the interface between the TiSi₂ and the TiN capping layer. Moreover the three dimensional atomic reconstruction from APT revealed a laterally inhomogeneous B distribution along the interface as well as B precipitation. APT enables the stoichiometric identification of TiB₂ precipitates smaller than 7 nm in diameter.     

Opening previously impossible avenues for phase transformation in innovative steels by atom probe tomography

Abstract

After decades of debate on the mechanism for the formation of bainite, it is accepted that bainite grows via a displacive mechanism, i.e. as plate shaped transformation products exhibiting an invariant plane strain surface relief effect. But there is still much discussion on the diffusion or diffusionless nature of bainite. The purpose of this atom probe tomography study was to track the atom distributions during the bainite reaction in steels with different carbon and silicon contents. The steels were transformed over a wide range of temperatures (200–525°C) to elucidate the role of reaction rate and diffusion in the formation of bainite with and without cementite precipitation.     

The rise of computational techniques in atom probe microscopy

Much effort has been devoted to the development of computational techniques in atom probe microscopy over the past decade. There have been several drivers for this effort. Firstly, there has been effort devoted to addressing the challenges of discerning information from the increasingly large size of the data, and capturing the opportunities that this large data presents. Secondly, there has been significant new effort devoted to the simulation of atom probe data so that pristine datasets that contain microstructural features of increasing complexity can be generated in-silico, and subjected to complex data-mining algorithms. This has enabled the benchmarking of various algorithms, guided the setting of parameters for particular analyses, and exposed the effects of instrumentation parameters such as detector efficiency and aberrations in ionic flight path. The authors are especially interested in the prospects of converging atomic-scale microscopy with atomic-scale materials modelling via first principles approaches. This involves excising parts of the APM data and using these as super-cell inputs to calculations of materials properties via density functional theory. It is our opinion that this represents a major advance for materials science because it enables microscopy to advance microstructure–property relationships to the direct mapping of such relationships based on many-body interactions. As such, this approach has great potential for materials design and development.The final part of this paper focuses on how cloud-based computing represents an exciting frontier of the computational aspects of atom probe microscopy. We discuss the opportunities and the barriers for conducting new materials science through the analysis and visualisation of atom probe data via new generation tools that are cloud-based, and which are managed, curated and governed with significant user-community input and integrated with contemporary electronic laboratory notebook technology.     

Atomic-scale analysis of light alloys using atom probe tomography

The present paper reviews recent progress in atomic-scale characterisation of composition and nanostructure of light alloy materials using the technique of atom probe tomography. In particular, the present review will highlight atom-by-atom analysis of solid solution architecture, including solute clustering and short-range order, with reference to current limitations of spatial resolution and detector efficiency of atom probe tomography and methods to address these limitations. This leads to discussion of prediction of mechanical properties by simulation and modelling of the strengthening effect exerted by solute clusters and the role of experimental atom probe data to assist in this process. The unique contribution of atom probe tomography to the study of corrosion and hydrogen embrittlement of light alloys will also be discussed as well as a brief insight into its potential application for the investigation of solute strengthening of twinning in Mg alloys.     

Spinodal decomposition of Ti0.33Al0.67N thin films studied by atom probe tomography

Details of the phase decomposition in NaCl-structure Ti_0.33Al_0.67N thin films deposited by cathodic arc evaporation are studied by atom probe tomography. We demonstrate that as-deposited films are in the earliest stage of decomposition for which electron microscopy and x-ray diffraction indicate a single-phase solid solution. Annealing at 900 °C further activates spinodal decomposition of the films, although pockets of undecomposed material remain after 2 h. N preferentially segregates to the AlN and TiN domains, causing the TiAlN matrix to be understoichiometric, by the energetics of N vacancies in TiAlN. The corresponding modulation in stoichiometry implies a Kirkendall effect, caused by different Al and Ti diffusivities.     

Synthesis and characterization of shaped ZnS nanocrystals in water in oil microemulsions

Shaped zinc sulfide nanocrystals were synthesized in W/O microemulsions by using cyclohexane/Triton X-100/n-pentanol/water system. Under different synthetic conditions appearance of two distinct morphologies of ZnS nanocrystals, either cubes or nanowires, was proven by transmission electron microscopy (TEM). The ZnS cubes have an average size of about 25 nm, while the ZnS nanowires have 25 Å diameter and length ranging from several hundred nanometers up to a few microns. The X-ray diffraction analysis (XRD) revealed formation of ZnS with cubic zinc blende crystal structure. Due to two dimensional confinement the exciton of ZnS nanowires is blue shifted compared to the bulk material. Four well-resolved photoluminescence bands in visible spectral region were observed upon excitation of cubic ZnS particles, while in the case of ZnS nanowires emission band was observed at 421 nm. The origin of photoluminescence bands was discussed in details.     

Insights into microstructural interfaces in aerospace alloys characterised by atom probe tomography

Atom probe tomography (APT) is becoming increasingly applied to understand the relationship between the structure and composition of new alloys at the micro- and nanoscale and their physical properties. Here, we use APT datasets from two modern aerospace alloys to highlight the detailed information available from APT analysis, along with potential pitfalls that can affect data interpretation. The interface between two phases in a Ti–6Al–4V alloy is used to illustrate the importance of parameter choice when using proximity histograms or concentration profiles to characterise interfacial chemistry. The higher number density of precipitates and large number of constituent elements in a maraging steel (F1E) present additional challenges such as peak overlaps that vary across the dataset, along with inhomogeneous interface chemistries.     

Emerging methods and opportunities in nanoscale materials characterization

The emergence of powerful nanomaterials characterization techniques promises to underpin a new range of advances in materials research. There have been significant developments in the characterization of the phase, structure, composition, and dynamics of materials at the nanoscale. Articles in this issue report recent advances in three areas: atom probe tomography, x-ray nanobeam scattering and diffraction, and x-ray photon correlation spectroscopy. Each of these provides three-dimensional insight into hard materials in ways that have been previously unavailable. Taken together, these emerging methods have the potential to provide new tests for materials theory and computation and to extend significantly the range of questions that can be answered in materials research.     

Characterization of Arsenic segregation at Si/SiO2 interface by 3D atom probe tomography

Dopant loss due to the segregation and dopant pile up at the Si/SiO₂ interface are crucial phenomena in the scaling trend of MOSFET devices for the 22-nm technology node. Arsenic segregation and pile-up at the Si/SiO₂ interface have been studied by the atom probe tomography (APT) technique which allows the 3D observation and the chemical analyses of dopant distribution with the atomic scale resolution. Arsenic (10¹⁶ at/cm², 32 keV) was implanted in mono-crystalline silicon and then annealed at 900 °C for 6 h after a cleaning step and an oxide growing. The thickness of the segregation layer was determined at 2.3 nm containing 9.36 × 10¹⁴ at/cm² dose of segregated arsenic. Finally, the obtained arsenic segregated dose has been compared to the resistivity profile performed by spreading resistance profiling technique.     

Molecular dynamic simulations of uniaxial tension at nanoscale of semiconductor materials for micro-electro-mechanical systems (MEMS) applications

Molecular dynamic (MD) simulations of uniaxial tension at nanoscale were conducted on two semiconductor materials, namely, silicon (Si) and germanium (Ge) to determine their mechanical properties and investigate the nature of deformation under applied load at nanolevel. A general form of Tersoff-type, three-body potential was used for the interaction between the Si atoms and between the Ge atoms in the simulations. Both, Si and Ge were found to exhibit a linear elastic behavior followed by a nonlinear increase in stress in the plastic region up to the ultimate tensile stress (instead of catastrophic brittle fracture soon after the elastic limit, which is typical of most nominally brittle materials at macrolevel). Further loading beyond the ultimate tensile stress resulted in catastrophic failure of these materials by a ductile fracture mode, namely, slip at 45° to the loading direction. The strain at failure was found to be much higher than the corresponding values at macroscale possibly due to the higher loading rates used. Based on the simulation results, the Young's modulii of Si and Ge in the [100] direction were determined to be 130 and 103 GPa, respectively, and the ultimate strengths, 25 and 20 GPa, respectively, at 500 m s⁻¹. These results are in reasonable agreement with the experimental and simulation results reported in the literature. The effect of strain rate via the rate of loading (10–500 m s⁻¹, where 1 m s⁻¹ corresponds to 10⁻² Å ps⁻¹) on the nature of deformation and the measured properties were also investigated. As the rate of loading (or the strain rate) decreases, the stress–strain curves more or less overlap up to the ultimate strength with a slight decrease in the ultimate tensile stress but a significant decrease in the value of strain at failure or strain at ultimate tensile stress.     

Sol-gel processing of materials for electronic and related applications

Various techniques of sol-gel processing for the preparation of electronic and related materials are described and reviewed. Typical examples are chosen from thin films and coatings of gels, crystalline materials and glasses as also bulk glasses to illustrate the variations in processing parameters and material properties.     

Study of the decomposition behavior of retained austenite and the partitioning of alloying elements during tempering in CMnSiAl TRIP steels

Tempering approach is designed for better understanding the effects of heat treatment induced by production process when manufacturing on the transformation-induced plasticity steels containing 1.0 wt.% Al. Specific attention is placed on the roles of tempering temperature and the holding time on the decomposition of retained austenite and the redistribution of alloying elements. Using transmission electron microscopy, we found the retained austenite was decomposed into ε-carbide and ferrite in the steels tempered at 300 °C for 9 h. An increase in the temperature of 400 °C and the holding time over 3 h accelerate the nucleation kinetics of cementite formation, leading to the deteriorated thermal stability of austenite. In addition, atom probe tomography studies confirmed the partitioning tendency of alloying elements across the ferrite/cementite interfaces as well as the compositional spikes of Mn at the interfaces during tempering over 400 °C for 9 h.     

Atom Probe Tomography for 3D Structural and Chemical Analysis of Individual Proteins

Determination of the 3D structure of proteins and other biomolecules is a major goal in structural biology, to provide insights to their biological function. Such structures are historically unveiled experimentally by X‐ray crystallography or NMR spectroscopy, and in recent years using cryo‐electron microscopy. Here, a method for structural analysis of individual proteins on the sub‐nanometer scale using atom probe tomography is described. This technique offers a combination of high‐resolution analysis of biomolecules in 3D, and the chemical sensitivity of mass spectrometry. As a model protein, the well‐characterized antibody IgG is used. IgG is encapsulated in an amorphous solid silica matrix via a sol–gel process to provide the requisite support for atom probe analysis. The silica synthesis is tuned to resemble physiological conditions. The 3D reconstructions show good agreement with the protein databank IgG crystal structure. This suggests that the silica‐embedding strategy can open the field of atom probe tomography to the analysis of biological molecules. In addition to high‐resolution structural information, the technique may potentially provide chemical information on the atomic scale using isotopic labeling. It is envisaged that this method may constitute a useful complement to existing tools in structural biology, particularly for the examination of proteins with low propensity for crystallization.     

Limits of detectability for clusters and solute segregation to grain boundaries

The abilities to detect and characterize the sizes and distribution of solute clusters, embryos, precipitates and solute atoms in the matrix by friends-of-friends methods in atom probe tomography are shown to improve with single atom position-sensitive detectors with high detection efficiency. In low solute matrices, clusters with as few as 5 atoms can be detected. A characterization method is presented that enables complete characterizations of grain boundaries and triple junctions by atom probe tomography in terms of the orientation relationships of the adjacent grains, as well as the local variations of the habit plane, surface curvature, and the solute excesses over the surface of a grain boundary with up to a 1 nm by 1 nm spatial resolution.     

Multi-scale investigation of electronic transport and electromechanical behavior in carbon nanotube materials

Using home-built experimental setups, electrical properties and electromechanical characterization of two systems based on multiwalled carbon nanotubes (MWNTs) were investigated at room temperature. The first system is formed by carbon nanotubes (CNTs) either isolated or in small groups on a gold substrate, while the second one concerns a macroscale three-dimensional entanglement of CNTs in powder form. The local electrical resistance on systems of the first type was measured using an atomic force microscopy with a conductive tip and showed a narrow distribution of resistance values as well for isolated CNTs as for small groups of them. However, in this latter case the average resistance value has been found to be one order of magnitude higher than that of individual CNTs, which was attributed to the contact resistance between CNTs. This parameter was then studied from a statistical viewpoint through electromechanical tests performed at a macroscopic scale. They consisted in applying an external compression to CNTs powder samples and measuring the evolution of the electrical resistance across the pressed material. These tests demonstrated an outstanding decrease of the electrical resistance resulting from the increasing number of random connections between CNTs under compression, and the experimental curves were fitted with an analytical model. Furthermore, it was deduced from this model that the elementary contact resistance between CNTs decreases under compression. The stability of this electrical contact was verified over several durations and under different constant applied loads.     

Tunable structural color in organisms and photonic materials for design of bioinspired materials

In this paper, the key topics of tunable structural color in biology and material science are overviewed. Color in biology is considered for selected groups of tropical fish, octopus, squid and beetle. It is caused by nanoplates in iridophores and varies with their spacing, tilting angle and refractive index. These examples may provide valuable hints for the bioinspired design of photonic materials. 1D multilayer films and 3D colloidal crystals with tunable structural color are overviewed from the viewpoint of advanced materials. The tunability of structural color by swelling and strain is demonstrated on an example of opal composites.