Atomic-scale age resolution of planetary events.

Resolving the timing of crustal processes and meteorite impact events is central to understanding the formation, evolution and habitability of planetary bodies. However, identifying multi-stage events from complex planetary materials is highly challenging at the length scales of current isotopic techniques. Here we show that accurate U-Pb isotopic analysis of nanoscale domains of baddeleyite can be achieved by atom probe tomography. Within individual crystals of highly shocked baddeleyite from the Sudbury impact structure, three discrete nanostructural domains have been isolated yielding average 206Pb/238U ages of 2,436±94 Ma (protolith crystallization) from homogenous-Fe domains, 1,852±45 Ma (impact) from clustered-Fe domains and 1,412±56 Ma (tectonic metamorphism) from planar and subgrain boundary structures. Baddeleyite is a common phase in terrestrial, Martian, Lunar and asteroidal materials, meaning this atomic-scale approach holds great potential in establishing a more accurate chronology of the formation and evolution of planetary crusts.

Encapsulation method for atom probe tomography analysis of nanoparticles.

Open-space nanomaterials are a widespread class of technologically important materials that are generally incompatible with analysis by atom probe tomography (APT) due to issues with specimen preparation, field evaporation and data reconstruction. The feasibility of encapsulating such non-compact matter in a matrix to enable APT measurements is investigated using nanoparticles as an example. Simulations of field evaporation of a void, and the resulting artifacts in ion trajectory, underpin the requirement that no voids remain after encapsulation. The approach is demonstrated by encapsulating Pt nanoparticles in an ZnO:Al matrix created by atomic layer deposition, a growth technique which offers very high surface coverage and conformality. APT measurements of the Pt nanoparticles are correlated with transmission electron microscopy images and numerical simulations in order to evaluate the accuracy of the APT reconstruction.

Analysis of implanted silicon dopant profiles.

Atom probe tomography implant dose measurements are reported for National Institute of Standards and Technology Standard Reference Material 2134 (arsenic implant). Efforts were taken to manufacture specimens with limited variation in size and shape to minimize variation in physical reconstruction parameters. A tip profile reconstruction was utilized where measurements of tip profile, post-analysis specimen radius and sphere-to-cone radius ratio were required as inputs into the reconstruction process. A variation of 4% is observed in the dose measurement under these conditions. Various considerations necessary to narrow the observed variation in measured dose, toward the limit imposed by counting statistics, are discussed.

Focused ion beam fabrication of solidified ferritin into nanoscale volumes for compositional analysis using atom probe tomography.

The purpose of this investigation was to conduct atom probe tomography (APT) analyses on ferritin specimens prepared with focused ion beam (FIB) to assess whether this approach can be used to effectively characterize biomaterials. Soft matter is particularly sensitive to ion beam exposure which can induce physical and chemical changes. We employ protective metal films and low-energy ion fluence to mitigate potential problems that may be introduced by FIB. This study had two major objectives: (1) to qualitatively assess the viability of the specimens when subjected to the unique physical conditions of APT analysis, namely ultrahigh vacuum, high electric field, and thermal pulsing using a laser and (2) to quantitatively assess the data from such specimens under various experimental parameters and compare the results with appropriate control specimens. For the first objective, a range of experimental parameters were determined that met the basic criteria necessary to validate that ferritin-based specimens prepared with FIB can retain structural integrity during APT analysis. Initial field evaporation time-of-flight mass spectrometry (TOF-MS) data show that the specimens fabricated with FIB are capable of emitting ions under various laser pulsing conditions with a high electric field applied. For the second objective, the experimental parameter space was narrowed to a range that yielded data quality sufficient to produce meaningful comparison between the ferritin-based specimens and the salt-only controls.

Three-dimensional chemical imaging of embedded nanoparticles using atom probe tomography.

Analysis of nanoparticles is often challenging especially when they are embedded in a matrix. Hence, we have used laser-assisted atom probe tomography (APT) to analyze the Au nanoclusters synthesized in situ using ion-beam implantation in a single crystal MgO matrix. APT analysis along with scanning transmission electron microscopy and energy dispersive spectroscopy (STEM-EDX) indicated that the nanoparticles have an average size ~8-12 nm. While it is difficult to analyze the composition of individual nanoparticles using STEM, APT analysis can give three-dimensional compositions of the same. It was shown that the maximum Au concentration in the nanoparticles increases with increasing particle size, with a maximum Au concentration of up to 50%.

Effect of analysis direction on the measurement of interfacial mixing in thin metal layers with atom probe tomography.

The accuracy and precision of thin-film interfacial mixing as measured with atom probe tomography (APT) are assessed by considering experimental and simulated field-evaporation of a Co/Cu/Co multilayer structure. Reconstructions were performed using constant shank angle and Z-scale reordering algorithms. Reconstruction of simulated data (zero intermixing) results in a 10-90% intermixing width of ~0.2 nm while experiential intermixing (measured from multiple runs) was 0.47 ± 0.19 and 0.49 ± 0.10nm for Co-on-Cu and Cu-on-Co interfaces, respectively. The experimental data were collected in analysis orientations both parallel and anti-parallel to film growth direction and the impact of this on the interfacial mixing measurements is discussed. It is proposed that the resolution of such APT measurements is limited by the combination of specimen shape and reconstruction algorithms rather than by an inherent instrumentation limit.

Improvements in planar feature reconstructions in atom probe tomography.

Standard atom probe tomography spatial reconstruction techniques have been reasonably successful in reproducing single crystal datasets. However, artefacts persist in the reconstructions that can be attributed to the incorrect assumption of a spherical evaporation surface. Using simulated and experimental field evaporation, we examine the expected shape of the evaporating surface and propose the use of a variable point projection position to mitigate to some degree these reconstruction artefacts. We show initial results from an implementation of a variable projection position, illustrating the effect on simulated and experimental data, while still maintaining a spherical projection surface. Specimen shapes during evaporation of model structures with interfaces between regions of low- and high-evaporation-field material are presented. Use of two-and three-dimensional projection-point maps in the reconstruction of more complicated datasets is discussed.

Evolution of tip shape during field evaporation of complex multilayer structures.

Atom-probe tomography analysis of complex multilayer structures is a promising avenue for studying interfacial properties. However, significant artefacts in the three-dimensional reconstructed data arise due to the field evaporation process. To clarify the origin and impact of these artefacts for a FeCoB/FeCo/MgO/FeCo/IrMn multilayer, tip shapes were observed by transmission electron microscopy and compared to those obtained by finite difference modelling of electric fields and evaporation processes. It was found that the emitter shape is not spherical and its surface morphology evolves during successive evaporation of the different layers. This evolving morphology contributes to the artefacts generally observed in the reconstructed atom-probe data for multilayer structures because algorithms for three-dimensional reconstruction are based on the assumption that the shape of the emitter during field evaporation is spherical. Some proposed improvements to data reconstruction are proposed.

Characterization of dilute species within CVD-grown silicon nanowires doped using trimethylboron: protected lift-out specimen preparation for atom probe tomography.

Three-dimensional quantitative compositional analysis of nanowires is a challenge for standard techniques such as secondary ion mass spectrometry because of specimen size and geometry considerations; however, it is precisely the size and geometry of nanowires that makes them attractive candidates for analysis via atom probe tomography. The resulting boron composition of various trimethylboron vapour-liquid-solid grown silicon nanowires were measured both with time-of-flight secondary ion mass spectrometry and pulsed-laser atom probe tomography. Both characterization techniques yielded similar results for relative composition. Specialized specimen preparation for pulsed-laser atom probe tomography was utilized and is described in detail whereby individual silicon nanowires are first protected, then lifted out, trimmed, and finally wet etched to remove the protective layer for subsequent three-dimensional analysis.

Atom probe tomography analysis of poly(3-alkylthiophene)s.

Pulsed-laser atom-probe tomography is used to compare the field-evaporation mass spectrum and spatial distribution of molecular fragments from various poly(3-alkylthiophene) films deposited on sharpened aluminium specimen carriers using two different deposition methods. Films deposited via a modified solution-cast methodology yield small fragments with a uniform structural morphology whereas films deposited via an electrospray ionization methodology yield a wide range of fragments with a very non-uniform structural morphology. The main field-evaporated chemical species identified for both deposition types were, in order of typical relative abundance, C(2)H(5) (+), CH(3) (+), C(2)H(4) (+), followed by C(3)H(7,8) (+)/SC(+) and SCH(+). Thick electrospray depositions allowed investigation of the influence of laser-pulse energy on the analysis. Evidence is presented supporting the presence of a critical laser-pulse energy whereby changes in film morphology are signalled by the appearance of a new mass fragment at 190 Da.