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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

An Automated Site-Specific Tip Preparation Method for Atom Probe Tomography Using Script-Controlled Focused Ion Beam/Scanning Electron Microscopy.

The automation of the atom probe tomography (APT) tip preparation using a focused ion beam (FIB) with a scanning electron microscopy (SEM) dual-beam system will certainly contribute to systematic APT research with higher throughput and reliability. While our previous work established a method to prepare tips with a specified tip curvature and taper angle automatically, by using script-controlled FIB/SEM, the technique has been expanded to automated "site-specific" tip preparation in the current work. The improved procedure can automatically detect not only the tip shape but also the interface position in the tip; thus, the new function allows for control of the tip apex position. In other words, automated "site-specific" tip preparations are possible. The details of the automation procedure and some experimental demonstrations, that is, a Pt cap on Si, InGaN-based MQWs, and a p-n junction of GaAs, are presented.

Interfacial Segregation of Sn during the Continuous Annealing and Selective Oxidation of Fe-Mn-Sn Alloys.

The effect of Mn on interfacial Sn segregation during the selective oxidation of Fe-(0-10)Mn-0.03Sn (at.%) alloys was determined for annealing conditions compatible with continuous galvanizing. Significant Sn enrichment was observed at the substrate free surface and metal/oxide interface for all annealing conditions and Mn levels. Sn enrichment at the free surface was insensitive to the Mn alloy concentration, which was partially attributed to the opposing effects of Mn on segregation thermodynamics and kinetics: Mn increases the driving force for Sn segregation via reducing Sn solubility in Fe but also reduces the effective Sn diffusivity by increasing the austenite volume fraction. This insensitivity was exacerbated by the depletion of solute Mn near the surface due to the selective oxidation of Mn. Thus, Sn segregation occurred in regions with a local Mn concentration lower than the nominal bulk composition of the alloys suggested. Sn enrichment at the metal/external oxide interface was reduced compared to the free surface and decreased with increasing bulk Mn content, which was attributed to changes in the external oxide morphology and metal/internal oxide interfaces acting as Sn sinks.

Influence of the Emitter Shape on the Field-of-View in Atom Probe Tomography.

Atom probe tomography (APT) is a unique analytical technique that offers three-dimensional elemental mapping with a spatial resolution down to the sub-nanometer. When APT is applied on complex heterogenous systems and/or under certain experimental conditions, that is, laser illumination, the specimen shape can deviate from an ideal hemisphere. Insufficient consideration of this aspect can introduce artifacts in the reconstructed dataset, ultimately degrading its spatial accuracy. So far, there has been limited investigation into the detailed evolution of emitter shape and its impact on the field-of-view (FOV). In this study, we numerically and experimentally investigated the FOV for asymmetric emitters and its evolution throughout the analysis depth. Our analysis revealed that, for asymmetric emitters, the ions evaporated from the topmost region of the specimen (summit) project approximately to the detector center. Furthermore, we demonstrated the implications of this finding on the FOV location for asymmetric emitters. Based on our findings, the location of the center of the FOV can deviate from the specimen central axis with an evolution depending on the evolution of the emitter shape. This study highlights the importance of accounting for the specimen shape when developing advanced data reconstruction schemes to enhance spatial resolution and accuracy.

Silica-embedded Gold Nanoparticles Analyzed by Atom Probe Tomography.

Nanoparticles are utilized in a multitude of applications due to their unique properties. Consequently, characterization of nanoparticles is crucial, and various methods have been employed in these pursuits. One such method is Atom Probe Tomography (APT). However, existing sample preparation techniques for APT generally involve embedding of the nanoparticles in a matrix different from their environment in solutions or at solid-liquid interfaces. In this work, we demonstrate a methodology based on silica embedding and explore how it can be utilized to form a matrix for nanoparticles suitable for APT analysis. Through chemisorption to a surface, gold nanoparticles were densely packed, ensuring a high probability of encountering at least one particle in the APT analyses. The nanoparticle-covered surface was embedded in a silica film, replacing the water and thus making this method suitable for studying nanoparticles in their hydrated state. The nanoparticle's silver content and its distribution, originating from the nanoparticle synthesis, could be identified in the APT analysis. Sodium clusters, possibly originating from the sodium citrate used to stabilize the particles in solution, were observed on the nanoparticle surfaces. This indicates the potential for silica embedding to be used for studying ligands on nanoparticles in their hydrated state.

In Situ Metallic Coating of Atom Probe Specimen for Enhanced Yield, Performance, and Increased Field-of-View.

Atom probe tomography requires needle-shaped specimens with a diameter typically below 100 nm, making them both very fragile and reactive, and defects (notches at grain boundaries or precipitates) are known to affect the yield and data quality. The use of a conformal coating directly on the sharpened specimen has been proposed to increase yield and reduce background. However, to date, these coatings have been applied ex situ and mostly are not uniform. Here, we report on the controlled focused-ion beam in situ deposition of a thin metal film on specimens immediately after specimen preparation. Different metallic targets e.g. Cr were attached to a micromanipulator via a conventional lift-out method and sputtered using Ga or Xe ions. We showcase the many advantages of coating specimens from metallic to nonmetallic materials. We have identified an increase in data quality and yield, an improvement of the mass resolution, as well as an increase in the effective field-of-view. This wider field-of-view enables visualization of the entire original specimen, allowing to detect the complete surface oxide layer around the specimen. The ease of implementation of the approach makes it very attractive for generalizing its use across a very wide range of atom probe analyses.

Study of Precipitates in Oxide Dispersion-Strengthened Steels by SANS, TEM, and APT.

In this work, the nanostructure of oxide dispersion-strengthened steels was studied by small-angle neutron scattering (SANS), transmission electron microscopy (TEM), and atom probe tomography (APT). The steels under study have different alloying systems differing in their contents of Cr, V, Ti, Al, and Zr. The methods of local analysis of TEM and APT revealed a significant number of nanosized oxide particles and clusters. Their sizes, number densities, and compositions were determined. A calculation of hardness from SANS data collected without an external magnetic field, or under a 1.1 T field, showed good agreement with the microhardness of the materials. The importance of taking into account two types of inclusions (oxides and clusters) and both nuclear and magnetic scattering was shown by the analysis of the scattering data.

Significant Oxygen Underestimation When Quantifying Barium-Doped SrTiO Layers by Atom Probe Tomography.

In this paper, the capability for quantifying the composition of Ba-doped SrTiO layers from an atom probe measurement was explored. Rutherford backscattering spectrometry and time-of-flight/energy elastic recoil detection were used to benchmark the composition where the amount of titanium was intentionally varied between samples. The atom probe results showed a significant divergence from the benchmarked composition. The cause was shown to be a significant oxygen underestimation (≳14 at%). The ratio between oxygen and titanium for the samples varied between 2.6 and 12.7, while those measured by atom probe tomography were lower and covered a narrower range between 1.4 and 1.7. This difference was found to be associated with the oxygen and titanium predominantly field evaporating together as a molecular ion. The evaporation fields and bonding chemistries determined showed inconsistencies for explaining the oxygen underestimation and ion species measured. The measured ion charge state was in excellent agreement with that predicted by the Kingham postionization theory. Only by considering the measured ion species, their evaporation fields, the coordination chemistry, the analysis conditions, and some recently reported density functional theory modeling for oxide field emission were we able to postulate a field emission and oxygen neutral desorption process that may explain our results.

Near-Atomic-Scale Perspective on the Oxidation of Ti3 C2 Tx MXenes: Insights from Atom Probe Tomography.

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties, bearing great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography (APT) analysis of as-synthesized Ti3 C2 Tx MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalis are enriched in TiO2 nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of APT to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

Preparation of atom probe tips from (nano)particles in dispersion using (di)electrophoresis and electroplating.

The behavior of catalytic particles depends on their chemical structure and morphology. To reveal this information, the characterization with atom probe tomography has huge potential. Despite progresses and papers proposing various approaches towards the incorporation of particles inside atom probe tips, no single approach has been broadly applicable to date. In this paper, we introduce a workflow that allowed us to prepare atom probe specimens from Ga particles in suspension in the size range of 50 nm up to 2 µm. By combining dielectrophoresis and electrodeposition in a suitable way, we achieve a near-tip shape geometry, without a time-consuming FIB lift-out. This workflow is a simple and quick method to prepare atom probe tips and allows for a high preparation throughput. Also, not using a lift-out allowed us to use a cryo-stage, avoiding melting of the Ga particles, while ensuring a mechanical stable atom probe tip. The specimen prepared by this workflow enable a stable measurement and low fracture rates. RESEARCH HIGHLIGHTS: Enabling cryo-preparation of (nano)particles for the atom probe. Characterization of surface and bulk elemental distribution of GaPt model SCALMS.