In Situ Pulsed Hydrogen Implantation in Atom Probe Tomography.

The investigation of hydrogen in atom probe tomography appears as a relevant challenge due to its low mass, high diffusion coefficient, and presence as a residual gas in vacuum chambers, resulting in multiple complications for atom probe studies. Different solutions were proposed in the literature like ex situ charging coupled with cryotransfer or H charging at high temperature in a separate chamber. Nevertheless, these solutions often faced challenges due to the complex control of specimen temperature during hydrogen charging and subsequent analysis. In this paper, we propose an alternative route for in situ H charging in atom probe derived from a method developed in field ion microscopy. By applying negative voltage nanosecond pulse on the specimen in an atom probe chamber under a low pressure of H2, it is demonstrated that a high dose of H can be implanted in the range 2-20 nm beneath the specimen surface. An atom probe chamber was modified to enable direct negative pulse application with controlled gas pressure, pulse repetition rate, and pulse amplitude. Through electrodynamical simulations, we show that the implantation energy falls within the range 100-1,000 eV and a theoretical depth of implantation was predicted and compared to experiments.

Improving Spatial and Elemental Associations in Analytical Field Ion Microscopy.

Chemically resolved atomic resolution imaging can give fundamental information about material properties. However, even today, a technique capable of such achievement is still only an ambition. Here, we take further steps in developing the analytical field ion microscopy (aFIM), which combines the atomic spatial resolution of field ion microscopy (FIM) with the time-of-flight spectrometry of atom probe tomography (APT). To improve the performance of aFIM that are limited in part by a high level of background, we implement bespoke flight path time-of-flight corrections normalized by the ion flight distances traversed in electrostatic simulations modeled explicitly for an atom probe chamber. We demonstrate effective filtering in the field evaporation events upon spatially and temporally correlated multiples, increasing the mass spectrum's signal-to-background. In an analysis of pure tungsten, mass peaks pertaining to individual W isotopes can be distinguished and identified, with the signal-to-background improving by three orders of magnitude over the raw data. We also use these algorithms for the analysis of a CoTaB amorphous film to demonstrate application of aFIM beyond pure metals and binary alloys. These approaches facilitate elemental identification of the FIM-imaged surface atoms, making analytical FIM more precise and reliable.

Introducing a Dynamic Reconstruction Methodology for Multilayered Structures in Atom Probe Tomography.

Atom probe tomography (APT) is a powerful three-dimensional nanoanalyzing microscopy technique considered key in modern materials science. However, progress in the spatial reconstruction of APT data has been rather limited since the first implementation of the protocol proposed by Bas et al. in 1995. This paper proposes a simple semianalytical approach to reconstruct multilayered structures, i.e., two or more different compounds stacked perpendicular to the analysis direction. Using a field evaporation model, the general dynamic evolution of parameters involved in the reconstruction of this type of structure is estimated. Some experimental reconstructions of different structures through the implementation of this method that dynamically accommodates variations in the tomographic reconstruction parameters are presented. It is shown both experimentally and theoretically that the depth accuracy of reconstructed APT images is improved using this method. The method requires few parameters in order to be easily usable and substantially improves atom probe tomographic reconstructions of multilayered structures.

Reflections on the Spatial Performance of Atom Probe Tomography in the Analysis of Atomic Neighborhoods.

Atom probe tomography (APT) is often introduced as providing "atomic-scale" mapping of the composition of materials and as such is often exploited to analyze atomic neighborhoods within a material. Yet quantifying the actual spatial performance of the technique in a general case remains challenging, as it depends on the material system being investigated as well as on the specimen's geometry. Here, by using comparisons with field-ion microscopy experiments, field-ion imaging and field evaporation simulations, we provide the basis for a critical reflection on the spatial performance of APT in the analysis of pure metals, low alloyed systems and concentrated solid solutions (i.e., akin to high-entropy alloys). The spatial resolution imposes strong limitations on the possible interpretation of measured atomic neighborhoods, and directional neighborhood analyses restricted to the depth are expected to be more robust. We hope this work gets the community to reflect on its practices, in the same way, it got us to reflect on our work.

Development of an Energy-Sensitive Detector for the Atom Probe Tomography.

A position and energy-sensitive detector has been developed for atom probe tomography (APT) instruments in order to deal with some mass peak overlap issues encountered in APT experiments. Through this new type of detector, quantitative and qualitative improvements could be considered for critical materials with mass peak overlaps, such as nitrogen and silicon in TiSiN systems, or titanium and carbon in cemented carbide materials. This new detector is based on a thin carbon foil positioned on the front panel of a conventional MCP-DLD detector. According to several studies, it has been demonstrated that the impact of ions on thin carbon foils has the effect of generating a number of transmitted and reflected secondary electrons. The number generated mainly depends on both the kinetic energy and the mass of incident particles. Despite the fact that this phenomenon is well known and has been widely discussed for decades, no studies have been performed to date for using it as a means to discriminate particles energy. Therefore, this study introduces the first experiments on a potential new generation of APT detectors that would be able to resolve mass peak overlaps through the energy-sensitivity of thin carbon foils.

Analytical Three-Dimensional Field Ion Microscopy of an Amorphous Glass FeBSi.

Three-dimensional field ion microscopy is a powerful technique to analyze material at a truly atomic scale. Most previous studies have been made on pure, crystalline materials such as tungsten or iron. In this article, we study more complex materials, and we present the first images of an amorphous sample, showing the capability to visualize the compositional fluctuations compatible with theoretical medium order in a metallic glass (FeBSi), which is extremely challenging to observe directly using other microscopy techniques. The intensity of the spots of the atoms at the moment of field evaporation in a field ion micrograph can be used as a proxy for identifying the elemental identity of the imaged atoms. By exploiting the elemental identification and positioning information from field ion images, we show the capability of this technique to provide imaging of recrystallized phases in the annealed sample with a superior spatial resolution compared with atom probe tomography.

Development of Wide Field of View Three-Dimensional Field Ion Microscopy and High-Fidelity Reconstruction Algorithms to the Study of Defects in Nuclear Materials.

This article presents a fast and highly efficient algorithm developed to reconstruct a three-dimensional (3D) volume with a high spatial precision from a set of field ion microscopy (FIM) images, and specific tools developed to characterize crystallographic lattice and defects. A set of FIM digital images and image processing algorithms allow the construction of a 3D reconstruction of the sample at the atomic scale. The capability of the 3D FIM to resolve the crystallographic lattice and the finest defects in metals opens a new way to analyze materials. This spatial precision was quantified on tungsten, analyzed at different analyzing conditions. A specific data mining tool, based on Fourier transforms, was also developed to characterize lattice distortions in the reconstructed volumes. This tool has been used in simulated and experimental volumes to successfully locate and characterize defects such as dislocations and grain boundaries.

Influence of field conditions on quantitative analysis of single crystal thorium dioxide by atom probe tomography.

Atom probe tomography (APT), a 3D microscopy technique, has great potential to reveal atomic scale compositional variations, such as those associated with irradiation damage. However, obtaining accurate compositional quantification by APT for high bandgap materials is a longstanding challenge, given the sensitivity to field evaporation parameters and inconsistent behaviors across different oxides. This study investigates the influence of APT laser energy and specimen base temperature on compositional accuracy in single crystal thoria (ThO2). ThO2 has a broad range of applications, including advanced nuclear fuels, sensors, lasers and scintillators, electrodes, catalysis, and photonics and optoelectronics. The expected stoichiometry of ThO2 is achieved at APT base temperature of 24 K and laser energy of 100 pJ. To overcome mass resolution limitations associated with significant thermal tails, Bayesian methods are applied to deconvolute ion identity within the mass spectra. This approach affirms that the parameters chosen are appropriate for APT analysis of ThO2.

Dynamic Effects in Voltage Pulsed Atom Probe.

Atom probe tomography (APT) is particularly suited for the analysis of nanoscale microstructural features in metallic alloys. APT has become important in the quantitative assessment at high spatial resolution of light elements, which are notoriously difficult to analyze by electron- or X-ray-based techniques. These control the physical properties of high-strength materials and semiconductors. However, the mass spectrometer of state-of-the-art commercial atom probes with the highest spatial precision and detection efficiency are optimized for elements with mass-to-charge ratios corresponding to Fe and neighboring elements. Little is known on the theoretical performances for light elements. Here, we discuss the theoretical instrumental performance of one such instrument using accurate three-dimensional transient electrostatic simulations in a time-varying field approach. We compare the simulations to experimental measurements obtained on an FeBSi bulk-metallic glass. Dynamics effects during the ion's flight are revealed when examining multi-hit mass-to-charge correlations, and we demonstrate their influence on the mass resolution. The model reveals significant differences in ion projection as a function of the mass. We discuss how these chromatic aberrations affect the spatial precision. This approach shows that by tuning the shape of the voltage pulses used to trigger field evaporation, minimizing the influence of these detrimental dynamic effects is possible.

Super-resolution Optical Spectroscopy of Nanoscale Emitters within a Photonic Atom Probe.

Atom Probe Tomography (APT) is a microscopy technique allowing for the 3D reconstruction of the chemical composition of a nanoscale needle-shaped sample with a precision close to the atomic scale. The photonic atom probe (PAP) is an evolution of APT featuring in situ and operando detection of the photoluminescence signal. The optical signatures of the light-emitting centers can be correlated with the structural and chemical information obtained by the analysis of the evaporated ions. It becomes thus possible to discriminate and interpret the spectral signatures of different light emitters as close as 20 nm, well beyond the resolution limit set by the exciting laser wavelength. This technique opens up new perspectives for the study of the physics of low dimensional systems, defects and optoelectronic devices.

Preferential Evaporation in Atom Probe Tomography: An Analytical Approach.

Atom probe tomography (APT) analysis conditions play a major role in the composition measurement accuracy. Preferential evaporation (PE), which significantly biases the apparent composition, more than other well-known phenomena in APT, is strongly connected to those analysis conditions. One way to optimize them, in order to have the most accurate measurement, is therefore to be able to predict and then to estimate their influence on the apparent composition. An analytical model is proposed to quantify the PE. This model is applied to three different alloys such as NiCu, FeCrNi, and FeCu. The model explains not only the analysis temperature dependence, as in an already existing model, but also the dependence to the pulse fraction and the pulse frequency. Moreover, the model can also provide an energetic constant directly linked to the energy barrier required to field evaporate atom from the sample surface.

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.

Enhancing Element Identification by Expectation-Maximization Method in Atom Probe Tomography.

This paper describes an alternative way to assign elemental identity to atoms collected by atom probe tomography (APT). This method is based on Bayesian assignation of label through the expectation-maximization method (well known in data analysis). Assuming the correct shape of mass over charge peaks in mass spectra, the probability of each atom to be labeled as a given element is determined, and is used to enhance data visualization and composition mapping in APT analyses. The method is particularly efficient for small count experiments with a low signal to noise ratio, and can be used on small subsets of analyzed volumes, and is complementary to single-ion decomposition methods. Based on the selected model and experimental examples, it is shown that the method enhances our ability to observe and extract information from the raw dataset. The experimental case of the superimposition of the Si peak and N peak in a steel is presented.

An Automated Computational Approach for Complete In-Plane Compositional Interface Analysis by Atom Probe Tomography.

We introduce an efficient, automated computational approach for analyzing interfaces within atom probe tomography datasets, enabling quantitative mapping of their thickness, composition, as well as the Gibbsian interfacial excess of each solute. Detailed evaluation of an experimental dataset indicates that compared with the composition map, the interfacial excess map is more robust and exhibits a relatively higher resolution to reveal compositional variations. By field evaporation simulations with a predefined emitter mimicking the experimental dataset, the impact of trajectory aberrations on the measurement of the thickness, composition, and interfacial excess of the decorated interface are systematically analyzed and discussed.

Unraveling the Metastability of C n2+ ( n = 2-4) Clusters.

Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of C n2+ ( n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to C n-1+ + C+. During C22+ → C+ + C+, significant kinetic energy release (∼5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic 12C22+ dications are either in weakly bound 3Πu and 3Σg- states, quickly dissociating under the intense electric field, or in a deeply bound electronic 5Σu- state with lifetimes >180 ps.

Enhanced dynamic reconstruction for atom probe tomography.

Many studies have focussed on the optimization of atom probe tomography data reconstruction. In this article, an enhanced dynamic reconstruction algorithm based on the approach initially developed by Gault et al. in 2011 is proposed. The dynamic reconstruction takes into account the evolution of the reconstruction parameters during the field evaporation of the sample, contrary to the standard reconstruction protocol, for example proposed by Bas et al. in 1995. Here, the evolution of reconstruction parameters is retrieved by field evaporation simulation. This approach allows a complete tabulation of the parameters evolution as a function of the analyzed sample morphology and its initial microstructure. The only inputs for experimental data are the voltage curve and some simple morphological parameters of the sample. This will be demonstrated by applying this algorithm to some experimental cases. Drastic optimization of the spatial accuracy in reconstructed datasets is experimentally and theoretically demonstrated.

On the detection of multiple events in atom probe tomography.

In atom probe tomography (APT), multiple events can arise as a consequence of e.g. correlated field evaporation and molecular ion dissociation. They represent challenging cases for single-particle detectors and can cause compositional as well as spatial inaccuracies. Here, two state-of-the-art atom probe microscopes (Cameca LEAP 5000 XS and 5000 XR) were used to investigate cemented tungsten carbide, which exhibits high amounts of multiple events. By advanced data analysis methods, the natural character of the multiple events, as well as the performance of the APT detectors, are assessed. Accordingly, possible signal loss mechanisms are discussed.

Carrier Localization in GaN/AlN Quantum Dots As Revealed by Three-Dimensional Multimicroscopy.

The localization of carrier states in GaN/AlN self-assembled quantum dots (QDs) is studied by correlative multimicroscopy relying on microphotoluminescence, electron tomography, and atom probe tomography (APT). Optically active field emission tip specimens were prepared by focused ion beam from an epitaxial film containing a stack of quantum dot layers and analyzed with different techniques applied subsequently on the same tip. The transition energies of single QDs were calculated in the framework of a 6-bands k.p model on the basis of APT and scanning transmission electron microscopy characterization showing that a good agreement between experimental and calculated energies can be obtained, overcoming the limitations of both techniques. The results indicate that holes effectively localize at interface fluctuations at the bottom of the QD, decreasing the extent of the wave function and the band-to-band transition energy. They also represent an important step toward the correlation of the three-dimensional atomic scale structural information with the optical properties of single light emitters based on quantum confinement.