Correlative Atom Probe Tomography Research Articles

Chemistry and microstructure of C-supported Ru catalyst nanoparticles: A correlative study

Ru@Pt core shell nanoparticles possess optimal catalytic properties that facilitate the anodic oxidation reaction of H2 with decreased Pt loading in hydrogen fuel cells. Moreover, since they preferentially oxidize CO, Pt poisoning is considerably reduced, which significantly improves the stability of the cell. The Ru cores used in this system are usually synthesized by dissolving a RuCl3*H2O precursor in an ethylene glycol-carbon black-NaOH mixture. However, the possibility that remnant Cl and Na from the synthesis process are present in the Ru nanoparticles has not been extensively studied.Therefore, due to the challenges in detecting impurities with traditional characterization methods, here correlative atom probe tomography (APT) with scanning transmission electron microscopy ((S)TEM) techniques were implemented. The capabilities of APT to obtain chemical information with high sensitivity at the nanoscale, in combination with the high spatial resolving power of (S)TEM, provide the necessary resolution to fully characterize the structure and chemical makeup of Ru nanoparticles.

Correlative atom probe tomography and optical spectroscopy: An original gateway to materials science and nanoscale physics

Atom probe tomography (APT) correlated with optical spectroscopy has yielded original results in the domain of semiconductor nanoscale heterostructures. Statistical correlation (i.e., microscopy correlated with spectroscopy performed on macroscopic samples) has opened the way to a deeper understanding of carrier localization and recombination mechanisms in quantum-well systems. However, photoluminescence spectroscopy (PL) can be performed even on APT samples fabricated by focused ion beam, making it possible to perform sequential correlations on a single nanoscale object, which allows for a higher precision and accuracy. Finally, the laser pulse used for triggering the evaporation in laser-assisted APT can also generate a photoluminescence signal: this opportunity is exploited in the photonic atom probe. This instrument not only represents an original and effective way to perform in situ correlative microscopy, but also opens the way to study nanoscale physical phenomena driven by field, stress, or sample shape via the interpretation of the dynamically acquired APT and PL information.Graphical abstract

Optimal Specimen Preparation for Correlative Atom Probe Tomography and Electron Microscopy of Environmentally Sensitive Materials

Optimal Specimen Preparation for Correlative Atom Probe Tomography and Electron Microscopy of Environmentally Sensitive Materials

Effect of Cd diffusion on the electrical properties of the Cu(In,Ga)Se2 thin-film solar cell

Cu(In,Ga)Se2 (CIGSe)-based solar cells are promising candidates for efficient sunlight harvesting. However, their complex composition and microstructure can change under operation conditions, for instance heating from sun light illumination can lead to a degradation in performance. Here, we investigate the thermally-induced degradation processes in a set of CIGSe-based solar cells that were annealed at temperatures between 150 °C and 300 °C. Using correlative atom probe tomography (APT)/transmission electron microscope (TEM), we found that the buffer/absorber interface is not sharp but consists of an interfacial zone (2–6.5 nm wide) where a gradient of constituent elements belonging to the CdS buffer and CIGSe absorber appears. An enhanced short-range Cd in-diffusion inside the CIGSe was observed whenever a low Ga/(Ga + In) ratio (≤ 0.15) occurred at the interface. This might indicate the presence of Ga vacancies as a channeling defect for Cd in-diffusion inside the CIGSe layer leading to a buried p/n-homojunction. We evidence that a considerable amount of Cd is found inside the CIGSe layer at annealing temperatures higher than 150 °C. Further investigations of the elemental redistribution inside the CIGSe layer combined with C–V measurements support the formation of CdCu+ donor like defects deep inside the p-type CIGSe which lead to a strong compensation of the CIGSe layer and hence to strong deterioration of cell efficiency at annealing temperatures higher than 200 °C. Hence, understanding the degradation processes in Cu(In,Ga)Se2 (CIGSe)-based solar cells opens new opportunities for further improvement of the long-term device performance.

Correlative atom probe tomography and scanning transmission electron microscopy reveal growth sequence of LPSO phase in Mg alloy containing Al and Gd

Atom probe tomography (APT) and transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) have been used correlatively to explore atomic-scale local structure and chemistry of the exactly same area in the vicinity of growth front of a long-period stacking ordered (LPSO) phase in a ternary Mg–Al–Gd alloy. It is proved for the first time that enrichment of Gd atoms in four consecutive (0001) atomic layers precedes enrichment of Al atoms so that the formation of Al6Gd8 clusters occurs only after sufficient Al atoms to form Al6Gd8 clusters diffuse into the relevant portions. Lateral growth of the LPSO phase is found to occur by ‘ledge’ mechanism with the growth habit plane either {1overline{1}00} or {11overline{2}0} planes. The motion of ledges that give rise to lateral growth of the LPSO phase is considered to be controlled by diffusion of Al atoms.

Quantitative evaluation of spinodal decomposition in thermally aged binary Fe-35 at.% Cr alloys by correlative atom probe tomography and small angle neutron scattering analyses

In this work, spinodal decomposition (SD) in Fe-35 at.% Cr alloy is quantified by correlatively combining two complimentary techniques namely, Atom Probe Tomography (APT) and Small Angle Neutron Scattering (SANS). In order to overcome the uncertainty often encountered in quantification of the characteristic SD parameters (wavelength λSD, amplitude A and volume fraction of Cr-rich α/-phase Φ), we adopt the following: (i) utilize the unique specific attribute of a technique suitable for evaluating any particular SD parameter; (ii) incorporate such reliably extracted parameter as input for evaluating other parameters wherever applicable. This novel correlative approach builds on each other's strength and provides improved accuracy. λSD is determined independently from APT and SANS analyses. For evaluation of A and Φ, APT analysis utilizes the value of λSD determined from SANS, whereas the phase compositions evaluated by APT are utilized in SANS analysis. With progressive phase separation, α/α/ compositional difference increases continually (from 19.8% after 10 h to 48.2% after 500 h) along with concomitant contraction of the interface (from 1.92 nm to 1.55 nm), underscoring the distinctive characteristics of SD process. Signature of rarely observed early stage of phase separation in the form of a dense population (9.78 × 1024/m3) of extremely small (0.67 nm radius) Cr-rich nanoclusters is registered after aging only for 1 h. In addition, temporal evolution of λSD clearly distinguishes early stage of SD (time exponent of 0.15) from the coarsening stage (time exponent of 0.29, 120 h onwards).

Correlative chemical and structural investigations of accelerated phase evolution in a nanocrystalline high entropy alloy

Based on our recently-developed combinatorial processing platforms for accelerated investigations of phase evolution in multinary alloys, a novel correlative atom probe tomography and transmission electron microscopy approach is proposed to study phase stability in a nanocrystalline CrMnFeCoNi alloy. We observed that the material can decompose at 250 °C for 5 h or 300 °C for 1 h, having the same decomposed products as in its coarse-grained counterpart after annealing at 500 °C for 500 days. A low apparent activation energy for the diffusion of Ni in the nanocrystalline alloy is derived and explains the fast kinetics of phase decomposition in nanocrystalline alloys.

Atom Probe Tomography Study of Gettering in High-Performance Multicrystalline Silicon

During the production of high-performance multicrystalline silicon (HPMC-Si) solar cells, gettering occurs inherently during the formation of an emitter. The material benefits with an increase in minority carrier lifetime from the external gettering of impurities into the diffused layer. However, depending on the thermal budget and parameters of the emitter diffusion, as well as the specific material properties, the process can also be detrimental in terms of increased recombination activity of specific crystallographic defects. Thus, it is important to understand the root causes behind the change in recombination activity of defects following gettering. Here, we present a correlative atom probe tomography study of grain boundaries in both p- and n-type HPMC-Si before and after gettering. The presence of nitrogen was found to directly correlate with the increase in recombination activity at grain boundaries. Additionally, an estimation of the atom probe tomography detection limit for transition metals in silicon is made and found to be greater than known impurity levels in commercial HPMC-Si.

Characterization of carbides in Q&P steels using a combination of high-resolution methods

Current strategies for the 3rd generation of advanced high strength steels focus on the creation of a multiphase microstructure containing substantial amounts of retained austenite to enhance the mechanical properties. In the case of quenching and partitioning (Q&P) steels, the stabilization of austenite is achieved by carbon diffusion from the supersaturated martensite into the austenite, but carbon partitioning is often jeopardized by competing reactions such as carbide formation. In the present study, in-situ high-energy X-ray diffraction (HEXRD) was used to study the transformation kinetics during Q&P processing, and especially carbide formation at higher partitioning temperatures.It was found that this carbide precipitation resulted mainly from martensite tempering and partially also from a decomposition of the austenite. The detection of minor diffraction peaks that appeared during the partitioning step was assigned to θ- and χ-carbide formation, which was supported by correlative atom probe tomography (APT) and transmission electron microscopy (TEM). Additionally, NbC was detected by APT, but the amount was obviously too low to be detected by HEXRD. The applied methods are finally compared with regard to their applicability for carbide identification in Q&P steels.

Three-dimensional measurement of Mg dopant distribution and electrical activity in GaN by correlative atom probe tomography and off-axis electron holography

The distribution and electrical activity of p-type doping (Mg) in gallium nitride (GaN) grown by metal organic chemical vapor deposition was investigated by correlating atom probe tomography (APT) and off-axis electron holography. APT results revealed that high Mg concentrations promote the formation of Mg-rich clusters. This is associated with the formation of pyramidal inversion domains (PIDs). The direct measurement of the doping concentration outside the clusters provided by APT suggests a saturation in the p-type electrical activity for Mg concentrations above 7 × 1019 cm−3. Maps of the electrostatic potential provided by off-axis electron holography confirm that the highest carrier concentration was achieved in the regions with the highest dopant concentration of 2 × 1020 cm−3, despite the presence of a high density of Mg-rich clusters revealed by APT. The correlation of these techniques suggests that PIDs are not the major cause of the reduction in electrostatic potential.

Elemental re-distribution inside shear bands revealed by correlative atom-probe tomography and electron microscopy in a deformed metallic glass

A density variation in shear bands visible by electron microscopy is correlated with compositionally altered locations measured by atom-probe tomography in plastically-deformed Al85.6Y7.5Fe5.8 metallic-glass ribbons. Two compositionally distinct regions are identified along shear bands, one is Al-rich (~92 at.%), the other is Al-depleted (~82.5 at.%) and both regions show marginal concentration fluctuations of Y and Fe. The elemental re-distribution is observed within shear bands only, and no chemical exchange with the surrounding glassy matrix is observed.

Correlative Atom Probe Tomography and Transmission Electron Microscopy Analysis of Grain Boundaries in Thermally Grown Alumina Scale.

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine. We also demonstrate a cost-efficient, robust, and easy-handling setup for correlative analysis based solely on commercially available components, which can be used with all conventional TEM tools without the need to modify the specimen holder assembly.

Atom Probe Tomography: A Review of Correlative Analysis of Interfaces and Precipitates in Metals and Alloys

This article provides a brief review of the history and recent developments of direct correlative atom probe tomography (APT) as related to characterization of internal interfaces and precipitates in metals and alloys. Correlative APT is defined as APT combined with a correlative electron microscopy technique, analytical technique, or computational method for the purpose of gaining greater understanding of phase transformations or structure–property relationships. In the first part of this article, the early history of correlative APT is reviewed. Additionally, recent advances in specimen preparation hardware and methods that facilitate performing direct correlative APT are discussed in the second section. In the third part of the article, several examples of direct correlative APT are presented.

A new method for mapping the three-dimensional atomic distribution within nanoparticles by atom probe tomography (APT)

We present a new method of preparing needle-shaped specimens for atom probe tomography from freestanding Pd and C-supported Pt nanoparticles. The method consists of two steps, namely electrophoresis of nanoparticles on a flat Cu substrate followed by electrodeposition of a Ni film acting as an embedding matrix for the nanoparticles. Atom probe specimen preparation can be subsequently carried out by means of focused-ion-beam milling. Using this approach, we have been able to perform correlative atom probe tomography and transmission electron microscopy analyses on both nanoparticle systems. Reliable mass spectra and three-dimensional atom maps could be obtained for Pd nanoparticle specimens. In contrast, atom probe samples prepared from C-supported Pt nanoparticles showed uneven field evaporation and hence artifacts in the reconstructed atom maps. Our developed method is a viable means of mapping the three-dimensional atomic distribution within nanoparticles and is expected to contribute to an improved understanding of the structure-composition-property relationships of various nanoparticle systems.

Correlative atomic scale characterisation of secondary carbides in M50 bearing steel

Correlative atom probe tomography (APT) and transmission electron microscopy (TEM) are used to characterise the microstructure and chemistry of carbide precipitation in M50 bearing steel. This is a prerequisite in establishing relationships between the microstructure and hydrogen embrittlement (HE) resistance. Secondary carbides are the focus of this study, as they play a major role in improving HE-resistance. Secondary carbides are observed in APT, with compositions close to M4C3, M2C and M3C. Correlative TEM measured orientation relationships between the martensite matrix and carbides, enabling the confirmation of M3C cementite precipitates in the corresponding APT reconstruction. Additionally, other precipitates observed in TEM were correlated to the M2C carbides in APT data. The M4C3 carbides are found to have a significantly lower volume fraction than the M2C carbides.

Precipitation of the ordered α2 phase in a near-α titanium alloy

Precipitate evolution in a near-α alloy was studied using transmission electron microscopy (TEM) and correlative atom probe tomography (APT) after ageing at 550–700 for times up to 28days. It is found that precipitation occurs much faster and is more prolific in samples heat treated at higher temperatures. Particles were spherical after ageing at 550°C, while after ageing at 700°C they become ellipsoids with the major axis lying close to the [0001] direction. At longer ageing times, the α2 precipitates were found to contain greater amounts of Sn+Si, indicating that Sn and Si are stronger Ti3(Al,Sn,Si) formers than Al.

Shear-induced mixing governs codeformation of crystalline-amorphous nanolaminates.

Deformation of ductile crystalline-amorphous nanolaminates is not well understood due to the complex interplay of interface mechanics, shear banding, and deformation-driven chemical mixing. Here we present indentation experiments on 10 nm nanocrystalline Cu-100 nm amorphous CuZr model multilayers to study these mechanisms down to the atomic scale. By using correlative atom probe tomography and transmission electron microscopy we find that crystallographic slip bands in the Cu layers coincide with noncrystallographic shear bands in the amorphous CuZr layers. Dislocations from the crystalline layers drag Cu atoms across the interface into the CuZr layers. Also, crystalline Cu blocks are sheared into the CuZr layers. In these sheared and thus Cu enriched zones the initially amorphous CuZr layer is rendered into an amorphous plus crystalline nanocomposite.

Correlative atom-probe tomography and transmission electron microscope study of a chemical transition in a spinel on an oxidized nickel-based superalloy

Complex thermally grown oxides (TGOs) are formed on a single-crystal Ni-based superalloy due to a large fraction of alloying elements. Combining atom-probe tomography and transmission electron microscopy analyses, the three-dimensional elemental distributions of metal oxides of the spinel and refractory metal-oxide regions are presented. A spinel-type oxide, Ni(Cr,Al)2O4, is formed with a crystallographic orientation relationship with the alloy substrate, which decomposes into other oxides. The sequence of oxides observed in the TGO are NiO → Cr2O3 → Ta2O5 → NiTaO4, NiTa2O6 → Ni(Cr,Al)2O4 → Al2O3.