Characterization of Carbide Precipitation in Low-Carbon Martensitic Steels Using an Ultrawide Field-of-View 3D Atom Probe.

It is important to understand the carbide distribution around high-energy sites such as dislocations and grain boundaries in martensitic steels as they have a major influence on the alloy performance. The aim of this study is to characterize fine ε carbides precipitated in low-carbon lath martensitic steel using the ultrawide field-of-view (FoV) CAMECA Invizo 6000 atom probe. We demonstrate the advantages of the wide FoV and determine the optimum conditions for analysis, by comparing the results such as the background noise and the C++/C+ charge state ratio (CSR) between voltage-pulsed and laser-pulsed modes. Increasing the laser pulse energy decreased the background noise and the CSR, where 70 pJ laser pulse energy produced a comparable mass-to-charge ratio spectrum to that recorded in voltage-pulsed mode, with the bulk compositions of C, Si, and Mn closest to that measured using voltage-pulsed mode. Increasing laser pulse energies to above 300 pJ decreased the bulk carbon content, with a more diffuse distribution of carbon around the carbides. This paper outlines some of the important experimental considerations when performing quantitative study of carbide precipitation in low-carbon martensitic steels using the Invizo 6000, considerations that can also be applied to other ferrous and non-ferrous alloy systems.

Towards Establishing Best Practice in the Analysis of Hydrogen and Deuterium by Atom Probe Tomography.

As hydrogen is touted as a key player in the decarbonization of modern society, it is critical to enable quantitative hydrogen (H) analysis at high spatial resolution and, if possible, at the atomic scale. H has a known deleterious impact on the mechanical properties (strength, ductility, toughness) of most materials that can hinder their use as part of the infrastructure of a hydrogen-based economy. Enabling H mapping including local hydrogen concentration analyses at specific microstructural features is essential for understanding the multiple ways that H affect the properties of materials including embrittlement mechanisms and their synergies. In addition, spatial mapping and quantification of hydrogen isotopes is essential to accurately predict tritium inventory of future fusion power plants thus ensuring their safe and efficient operation. Atom probe tomography (APT) has the intrinsic capability to detect H and deuterium (D), and in principle the capacity for performing quantitative mapping of H within a material's microstructure. Yet, the accuracy and precision of H analysis by APT remain affected by complex field evaporation behavior and the influence of residual hydrogen from the ultrahigh vacuum chamber that can obscure the signal of H from within the material. The present article reports a summary of discussions at a focused workshop held at the Max-Planck Institute for Sustainable Materials in April 2024. The workshop was organized to pave the way to establishing best practices in reporting APT data for the analysis of H. We first summarize the key aspects of the intricacies of H analysis by APT and then propose a path for better reporting of the relevant data to support interpretation of APT-based H analysis in materials.

Atomistic Compositional Details and Their Importance for Spin Qubits in Isotope-Purified Silicon Quantum Wells.

Understanding crystal characteristics down to the atomistic level increasingly emerges as a crucial insight for creating solid state platforms for qubits with reproducible and homogeneous properties. Here, isotope concentration depth profiles in a SiGe/28Si/SiGe heterostructure are analyzed with atom probe tomography (APT) and time-of-flight secondary-ion mass spectrometry down to their respective limits of isotope concentrations and depth resolution. Spin-echo dephasing times T 2 echo = 128 µ s $T_2^\mathbf {echo}=128 \,\umu\mathrm{s}$ and valley energy splittings EVS around 200 µ e V $200 \,\umu\mathrm{e\mathrm{V}}$ have been observed for single spin qubits in this quantum well (QW) heterostructure, pointing toward the suppression of qubit decoherence through hyperfine interaction with crystal host nuclear spins or via scattering between valley states. The concentration of nuclear spin-carrying 29Si is 50 ± 20ppm in the 28Si QW. The resolution limits of APT allow to uncover that both the SiGe/28Si and the 28Si/SiGe interfaces of the QW are shaped by epitaxial growth front segregation signatures on a few monolayer scale. A subsequent thermal treatment, representative of the thermal budget experienced by the heterostructure during qubit device processing, broadens the top SiGe/28Si QW interface by about two monolayers, while the width of the bottom 28Si/SiGe interface remains unchanged. Using a tight-binding model including SiGe alloy disorder, these experimental results suggest that the combination of the slightly thermally broadened top interface and of a minimal Ge concentration of 0.3 $0.3$ % in the QW, resulting from segregation, is instrumental for the observed large E VS = 200 µ e V $E_\mathrm{VS}=200 \,\umu\mathrm{e\mathrm{V}}$ . Minimal Ge additions <1%, which get more likely in thin QWs, will hence support high EVS without compromising coherence times. At the same time, taking thermal treatments during device processing as well as the occurrence of crystal growth characteristics into account seems important for the design of reproducible qubit properties.

On the Resolution Limit of Electrohydrodynamic Redox 3D Printing.

Additive manufacturing (AM) will empower the next breakthroughs in nanotechnology by combining unmatched geometrical freedom with nanometric resolution. Despite recent advances, no micro-AM technique has been able to synthesize functional nanostructures with excellent metal quality and sub-100 nm resolution. Here, significant breakthroughs in electrohydrodynamic redox 3D printing (EHD-RP) are reported by directly fabricating high-purity Cu (>98 at.%) with adjustable voxel size from >6µm down to 50 nm. This unique tunability of the feature size is achieved by managing in-flight solvent evaporation of the ion-loaded droplet to either trigger or prevent the Coulomb explosion. In the first case, the landing of confined droplets on the substrate allows the fabrication of high-aspect-ratio 50 nm-wide nanopillars, while in the second, droplet disintegration leads to large-area spray deposition. It is discussed that the reported pillar width corresponds to the ultimate resolution achievable by EHD printing. The unrivaled feature size and growth rate (>100 voxel s-1) enable the direct manufacturing of 30 µm-tall atom probe tomography (APT) tips that unveil the pristine microstructure and chemistry of the deposit. This method opens up prospects for the development of novel materials for 3D nano-printing.

The impact of electric field strength on the accuracy of boron dopant quantification in silicon using atom probe tomography.

This study investigates the impact of the surface electric field on the quantification accuracy of boron (B) implanted silicon (Si) using atom probe tomography (APT). The Si Charge-State Ratio (CSR(Si) = Si2+/Si+) was used as an indirect measure of the average apex electric field during analysis. For a range of electric fields, the accuracy of the total implanted dose and the depth profile shape determined by APT was evaluated against the National Institute of Standards and Technology Standard Reference Material 2137. The radial (non-)uniformity of the detected B was also examined. At a higher surface electric field (i.e., a greater CSR(Si)), the determined B dose converges on the certified dose. Additionally, the depth profile shape tends towards that derived by secondary ion mass spectrometry. This improvement coincides with a more uniform radial B distribution, evidenced by desorption maps. In contrast, for lower surface electric fields (i.e., a lower CSR(Si)), the B dose is significantly underestimated, and the depth profile is artificially stretched. The desorption maps also indicate a highly inhomogeneous B emission localized around the center of the detector, which is believed to be an artifact of B surface migration on the tip of the sample. For the purposes of routine investigations of semiconductor devices using APT, these results illustrate the potential origin of quantification artifacts and their severity at different operating conditions, thus providing pathways towards best practices for accurate and repeatable measurements.

Testing Outlier Detection Algorithms for Identifying Early Stage Solute Clusters in Atom Probe Tomography.

Atom probe tomography (APT) is commonly used to study solute clustering and precipitation in materials. However, standard techniques used to identify and characterize clusters within atom probe data, such as the density-based spatial clustering applications with noise (DBSCAN), often underperform with respect to small clusters. This is a limitation of density-based cluster identification algorithms, due to their dependence on the parameter Nmin, an arbitrary lower limit placed on detectable cluster sizes. Therefore, this article attempts to consider the characterization of clustering in atom probe data as an outlier detection problem of which k-nearest neighbors local outlier factor and learnable unified neighborhood-based anomaly ranking algorithms were tested against a simulated dataset and compared to the standard method. The decision score output of the algorithms was then auto thresholded by the Karcher mean to remove human bias. Each of the major models tested outperforms DBSCAN for cluster sizes of <25 atoms but underperforms for sizes >30 atoms using simulated data. However, the new combined k-nearest neighbors (k-NN) and DBSCAN method presented was able to perform well at all cluster sizes. The combined k-NN and seven methods are presented as a new approach to identifying clusters in APT.

Optimization of Parameters for Atom Probe Tomography Analysis of ß-Tricalcium Phosphates.

The biocompatibility and resorption characteristics of ß-tricalcium phosphate (ß-TCP, Ca3(PO4)2) have made it a coveted alternative for bone grafts. However, the underlying mechanisms governing the biological interactions between ß-tricalcium phosphate and osteoclasts remain elusive. It has been speculated that the composition at grain boundaries might vary and affect ß-TCP resorption properties. Atom probe tomography (APT) offers a quantitative approach to assess the composition of the grain boundaries, and thus advance our comprehension of the biological responses within the microstructure and chemical composition at the nanoscale. The precise quantitative analysis of chemical composition remains a notable challenge in APT, primarily due to the influence of measurement conditions on compositional accuracy. In this study, we investigated the impact of laser pulse energy on the composition of ß-TCP using APT, aiming for the most precise Ca:P ratio and consistent results across multiple analyses performed with different sets of analysis conditions and on two different instruments.

Revisiting Compositional Accuracy of Carbides Using a Decreased Detector Efficiency in a LEAP 6000 XR Atom Probe Instrument.

The accuracy of carbon composition measurement of carbide precipitates in steel or other alloys is limited by the evaporation characteristics of carbon and the performance of current detector systems. Carbon evaporates in a higher fraction as clustered ions leading to detector pile-up during so-called multiple hits. To achieve higher accuracy, a grid was positioned behind the local electrode, reducing the detection efficiency from 52 to 7% and thereby reducing the fraction of multi-hit events. This work confirms the preferential loss of carbon due to detector pile-up. Furthermore, we demonstrate that the newer generation of commercial atom probe instruments displays somewhat higher discrepancy of carbon composition than previous generations. The reason for this might be different laser-matter interaction leading to less metal ions in multi-hit events.

Investigation of thermal effects of laser micromachining for APT and TEM specimen preparation: A modeling and experimental study.

Laser micromachining can serve as a coarse machining step during sample preparation for high-resolution characterization methods leading to swift sample preparation. However, selecting the right laser parameters is crucial to minimize the heat-affected zone, which can potentially compromise the microstructure of the specimen. This study focuses on evaluating the size of heat-affected zone in laser annular milling, aiming to ascertain a minimal scan diameter that safeguards the inner region of micropillars against thermal damage. A computational model based on the finite element method was utilized to simulate the laser heating process. To validate the simulation results, a picosecond pulsed laser is then used to machine the micropillars of Al and Si. The laser-machined samples were subjected to surface and microstructural analysis using Scanning Electron Microscope (SEM) and Electron Backscatter Diffraction (EBSD) scans. The length of heat affected zone obtained from simulations was approximately 6 µm for silicon and 12 µm for aluminum. The diameter of micropillars formed with laser machining was 10 µm for silicon 26 µm for aluminum. The core of the pillars was preserved with less than one degree of microstructural misorientations making it suitable for further processing for preparing specimens for techniques like APT and TEM. For silicon micropillars, the preserved central region has a diameter of 6 µm and for aluminum its around 20-24 µm. Additionally, the study determines the minimum scan diameter that can be achieved using the given laser machining setup across a range of common materials.

Effects of Co on Mechanical Properties and Precipitates in a Novel Secondary-Hardening Steel with Duplex Strengthening of M2C and ß-NiAl.

Synergistic strengthening of nano-scaled M2C and ß-NiAl has become a new route to develop ultra-high secondary-hardening steel. At present, the effect of Co on the synergistic precipitation behavior of duplex phases of M2C and ß-NiAl has been rarely reported. This paper revealed the effects of Co on the mechanical properties and duplex precipitates of M2C and ß-NiAl in a novel 2.5 GPa ultra-high strength secondary-hardening steel. The tensile tests indicated that a 10% Co-alloy steel achieved a much stronger secondary-hardening effects compared to a Co-free steel during aging process, especially in the early-aging state. Needle-shaped M2C and spherical ß-NiAl particles were observed in both Co-alloy and Co-free steels. However, the number density, and volume fraction of M2C were significantly enhanced in the 10% Co-alloy steel. The Mo contents in M2C carbide and α-Fe after aging treatment were both analyzed through experimental determination and thermodynamic calculation, and the results indicated that Co decreased the solubility of Mo in α-Fe, thus promoting the precipitation of Mo-rich carbides.

Analysis of Water Ice in Nanoporous Copper Needles Using Cryo Atom Probe Tomography.

The application of atom probe tomography (APT) to frozen liquids is limited by difficulties in specimen preparation. Here, we report on the use of nanoporous Cu needles as a physical framework to hold water ice for investigation using APT. Nanoporous Cu needles are prepared by electropolishing and dealloying Cu-Mn matchstick precursors. Cryogenic scanning electron microscopy and focused ion beam milling reveal a hierarchical, dendritic, highly wettable microstructure. The atom probe mass spectrum is dominated by peaks of Cu+ and H(H2O)n+ up to n ≤ 3, and the reconstructed volume shows the protrusion of a Cu ligament into an ice-filled pore. The continuous Cu ligament network electrically connects the apex to the cryostage, leading to an enhanced electric field at the apex and increased cooling, both of which simplify the mass spectrum compared to previous reports.

A MATLAB Toolbox for Findable, Accessible, Interoperable, and Reusable Atom Probe Data Science.

Atom probe tomography (APT) data analytics have traditionally been based on manual analytics by researchers. As newer atom probes together with focused ion beam-based specimen preparation have opened APT to many more materials, yielding much more complex mass spectra, building up a systematic understanding of the pathway from raw data to final interpretation has increasingly become important. This demands a system in which the data and treatment can be traced, ideally by any interested party. Such an approach of findable, accessible, interoperable, and reusable (FAIR) data and analysis policies is becoming increasingly important, not just in APT. In this paper, we present a toolbox, written in MATLAB, which allows the user to store the raw and processed data in a standardized FAIR format (hierarchical data format 5) and process the data in a largely scriptable environment to minimize manual user input. This allows for the experiment data to be interchanged without owner explanations and the analysis to be reproduced. We have devised a metadata scheme that is extensible to other experiments in the materials science domain. With this toolbox, collective knowledge can be built up, and a large number of data sets can be analyzed in a fully automated fashion.

Nanoporous Gold Thin Films as Substrates to Analyze Liquids by Cryo-atom Probe Tomography.

Cryogenic atom probe tomography (cryo-APT) is being developed to enable nanoscale compositional analyses of frozen liquids. Yet, the availability of readily available substrates that allow for the fixation of liquids while providing sufficient strength to their interface is still an issue. Here, we propose the use of 1-2-µm-thick binary alloy film of gold-silver sputtered onto flat silicon, with sufficient adhesion without an additional layer. Through chemical dealloying, we successfully fabricate a nanoporous substrate, with an open-pore structure, which is mounted on a microarray of Si posts by lift-out in the focused-ion beam system, allowing for cryogenic fixation of liquids. We present cryo-APT results obtained after cryogenic sharpening, vacuum cryo-transfer, and analysis of pure water on the top and inside the nanoporous film. We demonstrate that this new substrate has the requisite characteristics for facilitating cryo-APT of frozen liquids, with a relatively lower volume of precious metals. This complete workflow represents an improved approach for frozen liquid analysis, from preparation of the films to the successful fixation of the liquid in the porous network, to cryo-APT.

Compensating Image Distortions in a Commercial Reflectron-Type Atom Probe.

In atom probe tomography, the spatial resolution and accuracy of the data critically depend on the 3D reconstruction of the 2D detector data. Atom probes with a reflectron have an improved mass resolving power and must include a model of the imaging properties of the reflectron. However, for modern wide-angle reflectron instruments, these imaging properties are not trivial and need to be determined for the reflectron used. This is typically done by the instrument manufacturer, and due to the proprietary nature of the instrument design, the imaging properties are opaque to the user. In this paper, we provide a method to determine the imaging properties of a reflectron that can easily be carried out on commercial instrumentation. This method is used to provide the user with a transformation function from the provided detector data, which can already contain some corrections applied, to a virtual detector placed before the reflectron. From there on, 3D reconstructions can be carried out analogous to straight flight path instruments. Correction algorithms and reference data for Imago/CAMECA LEAP 3000, 4000, 5000, and 6000 series instruments are also provided.

Nanoscale Analysis of Frozen Water by Atom Probe Tomography Using Graphene Encapsulation and Cryo-Workflows.

There has been an increasing interest in atom probe tomography (APT) to characterize hydrated and biological materials. A major benefit of APT compared to microscopy techniques more commonly used in biology is its combination of outstanding three-dimensional (3D) spatial resolution and mass sensitivity. APT has already been successfully used to characterize biominerals, revealing key structural information at the atomic scale, however there are many challenges inherent to the analysis of soft hydrated materials. New preparation protocols, often involving specimen preparation and transfer at cryogenic temperature, enable APT analysis of hydrated materials and have the potential to enable 3D atomic scale characterization of biological materials in the near-native hydrated state. In this study, samples of pure water at the tips of tungsten needle specimens were prepared at room temperature by graphene encapsulation. A comparative study was conducted where specimens were transferred at either room temperature or cryo-temperature and analyzed by APT by varying the flight path and pulsing mode. The differences between the analysis workflows are presented along with recommendations for future studies, and the compatibility between graphene coating and cryogenic workflows is demonstrated.

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.

Facilitating Atom Probe Tomography of 2D MXene Films by In Situ Sputtering.

2D materials are emerging as promising nanomaterials for applications in energy storage and catalysis. In the wet chemical synthesis of MXenes, these 2D transition metal carbides and nitrides are terminated with a variety of functional groups, and cations such as Li+ are often used to intercalate into the structure to obtain exfoliated nanosheets. Given the various elements involved in their synthesis, it is crucial to determine the detailed chemical composition of the final product, in order to better assess and understand the relationships between composition and properties of these materials. To facilitate atom probe tomography analysis of these materials, a revised specimen preparation method is presented in this study. A colloidal Ti3C2Tz MXene solution was processed into an additive-free free-standing film and specimens were prepared using a dual beam scanning electron microscope/focused ion beam. To mechanically stabilize the fragile specimens, they were coated using an in situ sputtering technique. As various 2D material inks can be processed into such free-standing films, the presented approach is pivotal for enabling atom probe analysis of other 2D materials.

Direct Observation of Trace Elements in Barium Titanate of Multilayer Ceramic Capacitors Using Atom Probe Tomography.

Accurately controlling trace additives in dielectric barium titanate (BaTiO3) layers is important for optimizing the performance of multilayer ceramic capacitors (MLCCs). However, characterizing the spatial distribution and local concentration of the additives, which strongly influence the MLCC performance, poses a significant challenge. Atom probe tomography (APT) is an ideal technique for obtaining this information, but the extremely low electrical conductivity and piezoelectricity of BaTiO3 render its analysis with existing sample preparation approaches difficult. In this study, we developed a new APT sample preparation method involving W coating and heat treatment to investigate the trace additives in the BaTiO3 layer of MLCCs. This method enables determination of the local concentration and distribution of all trace elements in the BaTiO3 layer, including additives and undesired impurities. The developed method is expected to pave the way for the further optimization and advancement of MLCC technology.

Miniaturized gas exposure devices for atom probe experiments.

To detect hydrogen in materials at the atomic scale, atom probe tomography is now regularly used. In order to avoid cumbersome cryo-preparation to suppress diffusion, often hydrogen is charged only into the finished specimen. Here, the use of hydrogen gas over electrochemical hydrogen has the advantage that the specimen is not contaminated with an electrolyte. So far, this "charging" has been done in large, expensive systems. Here, we introduce small devices that enable the exposure of atom probe specimens to hydrogen and potentially other gases, using only very small gas volumes. This enables the operation in regular laboratory environments without additional safety measures. These devices can be used to expose the specimen to hydrogen up to 10 bar/90°C. Higher temperatures may be attained with small changes. Validation of the success of charging with these setups is demonstrated through experiments employing deuterium charging of palladium atom probe tips. RESEARCH HIGHLIGHTS: Enabling the exposure of atom probe specimens to hydrogen and potentially other gases, using only very small gas volumes. Built setups can be assembled with little money and from freely available parts, making specimen charging much easier. Validation of the success of charging with these setups is demonstrated through experiments employing deuterium charging of palladium atom probe tips.

Quasi-"In Situ" Analysis of the Reactive Liquid-Solid Interface during Magnesium Corrosion Using Cryo-Atom Probe Tomography.

The early stages of corrosion occurring at liquid-solid interfaces control the evolution of the material's degradation process, yet due to their transient state, their analysis remains a formidable challenge. Here corrosion tests are performed on a MgCa alloy, a candidate material for biodegradable implants using pure water as a model system. The corrosion reaction is suspended by plunge freezing into liquid nitrogen. The evolution of the early-stage corrosion process on the nanoscale by correlating cryo-atom probe tomography (APT) with transmission-electron microscopy (TEM) and spectroscopy, is studied. The outward growth of Mg hydroxide Mg(OH)2 and the inward growth of an intermediate corrosion layer consisting of hydrloxides of different compositions, mostly monohydroxide Mg(OH) instead of the expected MgO layer, are observed. In addition, Ca partitions to these newly formed hydroxides and oxides. Density-functional theory calculations suggest a domain of stability for this previously experimental unreported Mg(OH) phase. This new approach and these new findings advance the understanding of the early stages of magnesium corrosion, and in general reactions and processes at liquid-solid interfaces, which can further facilitate the development of corrosion-resistant materials or better control of the biodegradation rate of future implants.