Local-electrode Atom-probe Tomography Research Articles

Influence of carrier localization on photoluminescence emission from sub-monolayer quantum dot layers

We have investigated the origins of photoluminescence from quantum dot (QD) layers prepared by alternating depositions of sub-monolayers and a few monolayers of size-mismatched species, termed as sub-monolayer (SML) epitaxy, in comparison with their Stranski–Krastanov (SK) QD counterparts. Using measured nanostructure sizes and local In-compositions from local-electrode atom probe tomography as input into self-consistent Schrödinger–Poisson simulations, we compute the 3D confinement energies, probability densities, and photoluminescence (PL) spectra for both InAs/GaAs SML- and SK-QD layers. A comparison of the computed and measured PL spectra suggests one-dimensional electron confinement, with significant 3D hole localization in the SML-QD layers that contribute to their enhanced PL efficiency in comparison to their SK-QD counterparts.

Influence of non-stoichiometry and local atomic environments on carrier transport in GaAs1−x−yNxBiy alloys

We have investigated the influence of non-stoichiometry and local atomic environments on carrier transport in GaAs(N)Bi alloy films using local-electrode atom probe tomography (LEAP) in conjunction with time-resolved terahertz photoconductivity measurements. The local concentrations of N, Bi, and excess As, as well as Bi pair correlations, are quantified using LEAP. Using time-resolved THz photoconductivity measurements, we show that carrier transport is primarily limited by excess As, with the highest carrier mobilities for layers with yBi > 0.035.

Effects of organic additives on the impurity and grain structure of electrodeposited cobalt

Co films were electrodeposited with different additives including dimethylglyoxime, sodium chloride, and mercapto-propanesulfonate (MPS). A systematic study was conducted using scanning electron microscope, four-point probe, secondary ion mass spectrum, and X-ray diffraction to understand the effects of such additives on the morphology, resistivity, impurity incorporation, and grain structure of film, as well as how thermal annealing influences these attributes. The addition of MPS into the electrolyte was found to significantly increase the S incorporation level and decrease the grain size, both contributing to a higher sheet resistance of film. Transmission electron microscope with energy dispersive X-ray spectroscopy and local electrode atom probe tomography were used to determine the distribution of different impurity elements. A pronounced grain structure change from columnar grains to pseudo-spherical grains was observed upon the addition of MPS in electrolyte or the incorporation of S in film. In addition to the precipitation of sporadic S-rich clusters, S and C were also found to segregate at the grain boundaries in annealed Co films, both of which are believed to correlate to the grain growth and a significant resistance drop upon annealing.

Identification of core/shell structure in Cd‐Zn‐Se QDs inside silicate glasses using 3D elemental distribution analysis

AbstractCore‐shell structure in Cd‐Zn‐Se quantum dots (QDs) precipitated in the silicate glass matrix by continuous wave laser (λ = 532 nm) were demonstrated using transmission electron microscopy (TEM) and local electrode atom probe (LEAP) tomography. TEM images of the QDs in glasses after 60 min of irradiation at 9 W/cm2 show two portions with different contrasts, and diameters of 5.97 and 22.49 nm, respectively; 3D elemental distribution in these glasses from LEAP tomography shows areas of high Zn concentration near the CdSe QDs. Iso‐concentration images obtained using visualization software show that Cd‐rich clusters were not completely covered by the Zn‐rich shells in the glass after 15 min irradiation that resulted emission due to the surface defects of CdSe QDs. As the irradiation time increased to 60 min, the Zn‐rich cluster gradually surrounded the Cd‐rich core to form shells that eliminated the surface defects. During laser irradiation, CdSe QDs precipitated first due to the high degree of supersaturation of Cd and Se followed by the sequential incorporation of Zn on the Cd‐rich QDs forming shells.

Influence of quantum dot morphology on the optical properties of GaSb/GaAs multilayers

We examine the influence of quantum dot (QD) morphology on the optical properties of two-dimensional (2D) GaSb/GaAs multilayers, with and without three-dimensional nanostructures. Using nanostructure sizes from scanning transmission electron microscopy and local Sb compositions from local-electrode atom-probe tomography as input into self-consistent Schrödinger–Poisson simulations based on 8 × 8 k·p theory, we compute confinement energies for QDs, circular arrangements of smaller QDs, termed QD-rings, and 2D layers on GaAs substrates. The computed confinement energies and the measured photoluminescence emission energies increase from QDs to QD-rings to 2D layers, enabling direct association of nanostructure morphologies with the optical properties of the GaSb/GaAs multilayers. This work opens up opportunities for tailoring near to far infrared optoelectronic devices by varying the QD morphology.

Diffusion of calcium in forsterite and ultra-high resolution of experimental diffusion profiles in minerals using local electrode atom probe tomography

We report measurements of experimentally induced diffusion profiles in minerals using local electrode atom probe (LEAP) tomography, and demonstrate that this is a viable technique for use in experimental diffusion studies, with distinct advantages over current state-of-the-art techniques such as secondary ion mass spectrometry and Rutherford backscattering spectroscopy. For this purpose, we have investigated Ca diffusivity in synthetic forsterite, which we measured parallel to [1 0 0] from 750 to 1300 °C, at 1 atm. pressure in air, with silica activity buffered by forsterite-enstatite or forsterite-periclase assemblages. We observe no dependence of Ca diffusion on silica activity, and argue that this may be due the strong preference of Ca for the M2 site, while M-site vacancies in olivine are more favorably located in M1 sites. Taken together, our data yield the following Arrhenius relation for Ca diffusion parallel to [1 0 0] in synthetic forsterite at 1 atm. pressure: logDCa_[100]m2s-1=-8.42±0.36-250±9kJmol-12.303RT. Time series experiments at 750 °C reveal that the shortest profiles measured by secondary ion mass spectrometry depth profiling (using an oxygen primary ion beam) are compromised by ion-beam mixing, while LEAP tomography yields accurate diffusion coefficients from the same samples. Because LEAP tomography does not suffer from the type of analytical artefacts that complicate the measurement of extremely short profiles via secondary ion mass spectrometry depth profiling, and also typically produces an order of magnitude better sensitivity than Rutherford backscattering spectroscopy, this technique offers a valuable new tool for quantifying the diffusivities of slow-diffusing trace elements in crystalline materials.

Isothermal aging of Al-Ni-Sc alloy containing Al 3 Ni microfibers and Al 3 Sc nanoprecipitates

Binary Al-Sc alloys have excellent strength at room and elevated temperature up to 300°C due to coherent of Al 3 Sc nanoprecipitates. Binary Al-Ni alloys are an alternative alloy for high-temperature applications owing to high-thermal stability of Al 3 Ni microfibers. In this study, the hardness evolution at 300°C for up to 672 h for eutectic ternary Al-6Ni-0.4Sc (wt%) was studied and compared with binary Al-0.4Sc and Al-6Ni alloys. The Al-6Ni alloy maintains a constant hardness after aging at 300°C up to 672 h. Both Al-0.4Sc and Al-6Ni-0.4Sc alloys show strong precipitation strengthening response, with peak hardness reached after about 3 h aging, due to Al 3 Sc precipitates. These precipitates were studied in Al-6Ni-0.4Sc via local electrode atom probe tomography and contain small amounts of Ni (0.04 at%) which do not affect the kinetics of precipitation. The microhardness evolution for Al-6Ni-0.4Sc can be explained through superposition of binary Al-0.4Sc and Al-6Ni alloys, indicating that both Al 3 Sc nanoprecipitates and Al 3 Ni microfibers contribute to strength.

Improving coarsening resistance of dilute Al-Sc-Zr-Si alloys with Sr or Zn additions

We study the effect of a small addition of Zn (0.5 at.%) or a micro-addition of Sr (0.005 at.%) to a dilute Al-0.05Sc-0.07Zr-0.02Si (at.%) on precipitation of Al3(Sc,Zr) nanoprecipitates. Upon aging at 400 °C, both the Zn- and Sr-modified alloys retain peak-aged hardness (600 ± 10 MPa) for at least two weeks, while the Zn/Sr-free control alloy shows lower peak hardness (475 ± 8 MPa), which starts dropping after only 3 days. Similarly, peak hardness values achieved at 475 °C during isochronal aging, are much higher for the Zn/Sr-modified alloy (610 ± 10 and 590 ± 14 MPa) than for the Zn/Sr-free alloy (480 ± 5 MPa). Local electrode atom probe tomography shows that Sr and Zn atoms segregate to the Al3(Sc,Zr) nanoprecipitates: up to 0.1 at. % for Sr, and up to 1.1 at.% for Zn, with a peak of ∼5 at.% at the interface with the matrix. This may affect their coarsening rate by (i) reducing the diffusivity of Sc and Zr in the matrix and (ii) altering the precipitate/matrix interfacial energy. In the peak-aged condition, the Zn-modified alloy has the same creep resistance at 300 °C than the Zn-free control alloy, but its higher overaging resistance allows for longer creep operating times.

Prediction of Carbon Partitioning and Austenite Stability via Non-equilibrium Thermodynamics in Quench and Partition (Q&P) Steel

Thermodynamics-based predictive modeling for phase characteristics after the quench and partition (Q&P) process is key to the design of new alloys and processing cycles with the best combination of mechanical properties. The austenite carbon content influences its phase stability during mechanical deformation and thus determines the improvement to total elongation from transformation-induced plasticity. The current article describes a carbon partition model based on para-equilibrium simulations with the addition of a temperature-dependent effective stored energy model that predicts carbon enrichment in austenite after Q&P processing. A retained austenite stability model is also proposed that uses the predicted carbon in austenite to quantify the austenite stability in terms of the Msσ temperature. The developed models were calibrated and subsequently validated using measurements from advanced characterization techniques such as local electrode atom probe tomography, synchrotron-based x-ray diffraction and uniaxial tensile tests at varying test temperatures.

Correlated Chemical and Electrically Active Dopant Analysis in Catalyst-Free Si-Doped InAs Nanowires.

Direct correlations between dopant incorporation, distribution, and their electrical activity in semiconductor nanowires (NW) are difficult to access and require a combination of advanced nanometrology methods. Here, we present a comprehensive investigation of the chemical and electrically active dopant concentrations in n-type Si-doped InAs NW grown by catalyst-free molecular beam epitaxy using various complementary techniques. N-type carrier concentrations are determined by Seebeck effect measurements and four-terminal NW field-effect transistor characterization and compared with the Si dopant distribution analyzed by local electrode atom probe tomography. With increased dopant supply, a distinct saturation of the free carrier concentration is observed in the mid-1018 cm-3 range. This behavior coincides with the incorporated Si dopant concentrations in the bulk part of the NW, suggesting the absence of compensation effects. Importantly, excess Si dopants with very high concentrations (>1020 cm-3) segregate at the NW sidewall surfaces, which confirms recent first-principles calculations and results in modifications of the surface electronic properties that are sensitively probed by field-effect measurements. These findings are expected to be relevant also for doping studies of other noncatalytic III-V NW systems.

Multi-component Cu-Strengthened Steel Welding Simulations: Atom Probe Tomography and Synchrotron X-ray Diffraction Analyses

An experimental steel with the composition Fe-1.39Cu-2.70Ni-0.58Al-0.48Mn-0.48Si-0.065Nb-0.05C (wt pct) or alternatively Fe-1.43Cu-2.61Ni-1.21Al-0.48Mn-0.98Si-0.039Nb-0.23C (at. pct) has been developed at Northwestern University, which has both high toughness and high strength after quenching and aging treatments. Simulated heat-affected zone (HAZ) samples are utilized to analyze the microstructures typically obtained after gas metal arc welding (GMAW). Dissolution within the HAZ of cementite (Fe3C) and NbC (F.C.C.) is revealed using synchrotron X-ray diffraction, while dissolution of Cu precipitates is measured employing local electrode atom probe tomography. The results are compared to Thermo-Calc equilibrium calculations. Comparison of measured Cu precipitate radii, number density, and volume fraction with similar measurements from a GMAW sample suggests that the cooling rate in the simulations is faster than in the experimental GMAW sample, resulting in significantly less Cu precipitate nucleation and growth during the cooling part of the weld thermal cycle. The few Cu precipitates detected in the simulated samples are primarily located on grain boundaries resulting from heterogeneous nucleation. The dissolution of NbC precipitates and the resultant austenite coarsening in the highest-temperature sample, coupled with a rapid cooling rate, results in the growth of bainite, and an increase in the strength of the matrix in the absence of significant Cu precipitation.

Atom-Probe Tomographic Analyses of Hydrogen Interstitial Atoms in Ultrahigh Purity Niobium.

Atomic-scale characterization of hydrogen and formation of niobium hydrides, using ultraviolet (wavelength=355 nm) picosecond laser-assisted local-electrode atom-probe tomography, was performed for ultrahigh purity niobium utilizing different laser pulse energies, 10 or 50 pJ/pulse or voltage pulsing. At 50 pJ/pulse, hydrogen atoms migrate onto the 110 and 111 poles as a result of stimulated surface diffusion, whereas they are immobile for <10 pJ/pulse or for voltage pulsing. Accordingly, the highest concentrations of H and NbH were obtained at 50 pJ/pulse. This is attributed to the thermal energy of the laser pulses being transferred to pure niobium specimens. Therefore, we examined the effects of the laser pulse energy being increased systematically from 1 to 20 pJ/pulse and then decreasing it from 20 to 1 pJ/pulse. The concentrations of H, H2, and NbH and the atomic concentration ratios H2/H, NbH/Nb, and Nb3+/Nb2+ were calculated with respect to the systematically changing laser pulse energies. The atomic concentration ratios H2/H and NbH/Nb are greater when decreasing the laser pulse energy than when increasing it, because the higher residual thermal energy after decreasing the laser pulse energy increases the mobility of H atoms by supplying sufficient thermal energy to form H2 or NbH.

A model Ni-Al-Mo superalloy studied by ultraviolet pulsed-laser-assisted local-electrode atom-probe tomography.

This study investigates the effects of the charge-state ratio of evaporated ions on the accuracy of local-electrode atom-probe (LEAP) tomographic compositional and structural analyses, which employs a picosecond ultraviolet pulsed laser. Experimental results demonstrate that the charge-state ratio is a better indicator of the best atom-probe tomography (APT) experimental conditions compared with laser pulse energy. The thermal tails in the mass spectra decrease significantly, and the mass resolving power (m/Δm) increases by 87.5 and 185.7% at full-width half-maximum and full-width tenth-maximum, respectively, as the laser pulse energy is increased from 5 to 30 pJ/pulse. The measured composition of this alloy depends on the charge-state ratio of the evaporated ions, and the most accurate composition is obtained when Ni2+/Ni+ is in the range of 0.3-20. The γ(f.c.c.)/γ'(L12) interface is quantitatively more diffuse when determined from the measured concentration profiles for higher laser pulse energies. Conclusions of the APT compositional and structural analyses utilizing the same suitable charge-state ratio are more comparable than those collected with the same laser pulse energy.

Comparison between dislocation dynamics model predictions and experiments in precipitation-strengthened Al–Li–Sc alloys

Precipitate distributions were quantitatively measured by local electrode atom-probe (LEAP) tomography in several age-hardenable Al-based alloys to provide input to both analytical models and dislocation dynamics simulations of critical resolved shear strength, for comparison with experimentally measured Vickers microhardness values. The method of reconstructing precipitate-containing volumes from LEAP tomography, then importing these data to dislocation dynamics simulations, is explained in detail in the supplementary material available in the online version of this paper. Two alloys were studied: Al–2.9Li–0.11Sc at.% (Al–Li–Sc) and Al–6.3Li–0.07Sc–0.02Yb at.% (Al–Li–Sc–Yb). Heat treatment of these alloys produced nanometer-scale α′-Al3(Li,Sc,Yb)(L12) precipitates after isothermal aging at 325°C. In some cases δ′-Al3Li(L12) shells were formed on these precipitates after subsequent isothermal aging at 170°C. Dislocation dynamics results and experimental measurements were combined to define empirical strengthening superposition rules for the cases of contributions from: (i) Li in solid-solution plus α′-Al3(Li,Sc,Yb)(L12) precipitates; (ii) α′-Al3(Li,Sc,Yb)(L12) precipitates plus δ′-Al3Li(L12) shells in doubly aged Al–Li–Sc–Yb. Simulations of aged Al–Li–Sc overpredict the strength if a single dislocation is used, and underpredict the strength if instead a cooperative dislocation pair is considered. For simulations of dislocation pairs in Al–Li–Sc (single-phase precipitates), the precipitate bypass mechanism depends on the aging condition of the alloy. At peak age, precipitate shearing occurs mainly by pairs of closely spaced dislocations moving cooperatively. As overaging progresses, Orowan looping increasingly dominates and the distance between the leading and trailing dislocations increases. For dislocation pairs in doubly aged Al–Li–Sc–Yb with some core/shell precipitates, the measured and simulated strength values agree to within their uncertainties.

Cellular Precipitation at a 17-7 PH Stainless Steel Interphase Interface During Low-Temperature Nitridation

Cellular precipitation of Cr-rich nitrides was observed at an austenite–ferrite interface in 17-7 PH stainless steel after low-temperature nitridation. Fine-scale lamellar rocksalt-structured nitride (MN1−x , M: randomly distributed Fe, Cr, and Al) was identified at the interfaces between austenite and ferrite by local-electrode atom-probe tomography and transmission electron microscopy. The small size and spacing of the nitride lamellae reflect the low mobility of substitutional atoms under the conditions of low-temperature nitridation. Nitrides of the same structure were formed within the ferrite grain as extremely small particles. The face-centered cubic nitride precipitates in the Bain orientation relationship with the ferrite.

Mechanical alloying of lanthana-bearing nanostructured ferritic steels

A novel nanostructured ferritic steel powder with the nominal composition Fe–14Cr–1Ti–0.3Mo–0.5La2O3 (wt.%) was developed via high energy ball milling. La2O3 was added to this alloy instead of the traditionally used Y2O3. The effects of varying the ball milling parameters, such as milling time, steel ball size and ball to powder ratio, on the mechanical properties and microstructural characteristics of the as-milled powder were investigated. Nanocrystallites of a body-centered cubic ferritic solid solution matrix with a mean size of approximately 20nm were observed by transmission electron microscopy. Nanoscale characterization of the as-milled powder by local electrode atom probe tomography revealed the formation of Cr–Ti–La–O-enriched nanoclusters during mechanical alloying. The Cr:Ti:La:O ratio is considered “non-stoichiometric”. The average size (radius) of the nanoclusters was about 1nm, with number density of 3.7×1024m−3. The mechanism for formation of nanoclusters in the as-milled powder is discussed. La2O3 appears to be a promising alternative rare earth oxide for future nanostructured ferritic steels.

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (−84.4 °C, −120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (−84.4 °C, −120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M2C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.

Direct Atomic-Scale Imaging of Hydrogen and Oxygen Interstitials in Pure Niobium Using Atom-Probe Tomography and Aberration-Corrected Scanning Transmission Electron Microscopy

Imaging the three-dimensional atomic-scale structure of complex interfaces has been the goal of many recent studies, due to its importance to technologically relevant areas. Combining atom-probe tomography and aberration-corrected scanning transmission electron microscopy (STEM), we present an atomic-scale study of ultrathin (~5 nm) native oxide layers on niobium (Nb) and the formation of ordered niobium hydride phases near the oxide/Nb interface. Nb, an elemental type-II superconductor with the highest critical temperature (T(c) = 9.2 K), is the preferred material for superconducting radio frequency (SRF) cavities in next-generation particle accelerators. Nb exhibits high solubilities for oxygen and hydrogen, especially within the RF-field penetration depth, which is believed to result in SRF quality factor losses. STEM imaging and electron energy-loss spectroscopy followed by ultraviolet laser-assisted local-electrode atom-probe tomography on the same needle-like sample reveals the NbO(2), Nb(2)O(5), NbO, Nb stacking sequence; annular bright-field imaging is used to visualize directly hydrogen atoms in bulk β-NbH.

Spin Polarization Measurement of Homogeneously Doped Fe1–xCoxSi Nanowires by Andreev Reflection Spectroscopy

We report a general method for determining the spin polarization from nanowire materials using Andreev reflection spectroscopy implemented with a Nb superconducting contact and common electron-beam lithography device fabrication techniques. This method was applied to magnetic semiconducting Fe(1-x)Co(x)Si alloy nanowires with x̅ = 0.23, and the average spin polarization extracted from 6 nanowire devices is 28 ± 7% with a highest observed value of 35%. Local-electrode atom probe tomography (APT) confirms the homogeneous distribution of Co atoms in the FeSi host lattice, and X-ray magnetic circular dichroism (XMCD) establishes that the elemental origin of magnetism in this strongly correlated electron system is due to Co atoms.

Microstructural evolution and creep properties of precipitation-strengthened Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys

The aging behavior at 300 °C of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb (at.%) alloys is studied by local-electrode atom-probe tomography, transmission electron microscopy and microhardness measurements. The ternary alloys exhibit high number densities of coherent L1 2 precipitates ( N v ≅ 10 22 m - 3 ) at aging times up to 1536 h (64 days). In the Al–0.06Sc–0.02Gd alloy, the Al 3(Sc 1− x Gd x ) precipitates are always Sc-rich, displaying a small Gd concentration ( x < 0.12) in the precipitates. In the Al–0.06Sc–0.02Yb alloy, the precipitates are initially Yb-rich, Al 3(Yb 1− x Sc x ), with Sc diffusing subsequently to the precipitates, resulting in a core/shell structure and an overall Sc-rich composition, Al 3(Sc 1− x Yb x ). Gd and Yb, like other lanthanides but unlike the transition metals Zr and Ti, do not retard the coarsening kinetics compared with binary Al–Sc alloys. Additionally, the creep resistance of these alloys is greater than that of Al–Sc alloys. The coarsening kinetics and creep properties of Al–0.06Sc–0.02Gd and Al–0.06Sc–0.02Yb alloys are compared with other Al–Sc-based alloys and with coarsening models for ternary alloys.