Laser assisted field evaporation of oxides in atom probe analysis

Atom Probe Tomography of Encapsulated Hydroxyapatite Nanoparticles

Hydroxyapatite nanoparticles (HAP NPs) are important for medicine, bioengineering, catalysis, and water treatment. However, current understanding of the nanoscale phenomena that confer HAP NPs their many useful properties is limited by a lack of information about the distribution of the atoms within the particles. Atom probe tomography (APT) has the spatial resolution and chemical sensitivity for HAP NP characterization, but difficulties in preparing the required needle-shaped samples make the design of these experiments challenging. Herein, two techniques are developed to encapsulate HAP NPs and prepare them into APT tips. By sputter-coating gold or the atomic layer deposition of alumina for encapsulation, partially fluoridated HAP NPs are successfully characterized by voltage- or laser-pulsing APT, respectively. Analyses reveal that significant tradeoffs exist between encapsulant methods/materials for HAP characterization and that selection of a more robust approach will require additional technique development. This work serves as an essential starting point for advancing knowledge about the nanoscale spatiochemistry of HAP NPs.

An algorithm for correcting systematic energy deficits in the atom probe mass spectra of insulating samples

Improvements in the mass resolution of a mass spectrometer directly correlate to improvements in peak identification and quantification. Here, we describe a post-processing technique developed to increase the quality of mass spectra of strongly insulating samples in laser-pulsed atom probe microscopy. The technique leverages the self-similarity of atom probe mass spectra collected at different times during an experimental run to correct for electrostatic artifacts that present as systematic energy deficits. We demonstrate the method on fused silica (SiO₂) and neodymium-doped ceria (CeO₂) samples which highlight the improvements that can be made to the mass spectrum of strongly insulating samples.

Field Ion Emission in an Atom Probe Microscope Triggered by Femtosecond-Pulsed Coherent Extreme Ultraviolet Light

This paper describes initial experimental results from an extreme ultraviolet (EUV) radiation-pulsed atom probe microscope. Femtosecond-pulsed coherent EUV radiation of 29.6 nm wavelength (41.85 eV photon energy), obtained through high harmonic generation in an Ar-filled hollow capillary waveguide, successfully triggered controlled field ion emission from the apex of amorphous SiO2 specimens. The calculated composition is stoichiometric within the error of the measurement and effectively invariant of the specimen base temperature in the range of 25 K to 150 K. Photon energies available in the EUV band are significantly higher than those currently used in the state-of-the-art near-ultraviolet laser-pulsed atom probe, which enables the possibility of additional ionization and desorption pathways. Pulsed coherent EUV light is a new and potential alternative to near-ultraviolet radiation for atom probe tomography.

Laser-assisted field evaporation of metal oxides: A time-dependent density functional theory study

To understand laser-assisted field evaporation of semiconductors and insulators at the microscopic level, we study the time evolution of the electronic and atomic structure of a MgO cluster in high electrostatic fields subjected to strong laser pulses. We find that the critical laser intensity for evaporation decreases linearly as the electrostatic field strength increases. The optical absorption enhancement in high electrostatic field is confirmed by the redshift of the optical absorption spectra, the reduction of the energy gap, and the increase of the absorption cross section.

Role of the resistivity of insulating field emitters on the energy of field-ionised and field-evaporated atoms

In order to improve the accuracy of laser atom probe analyses, it is important to understand all the physical processes induced by the combination of the high electrical field and the femtosecond laser beam during field evaporation. New information can be accessed from the energy of evaporated surface atoms or field-ionised atoms of an imaging gas. In order to study the ions energy, we combine La-APT and FIM analyses in a new experimental setup equipped with electrostatic lenses. We report measurements for semiconductors and oxides and we study the influence of the illumination conditions (laser power and wavelength), the evaporation rate, the sample geometry and the tip preparation processes. The results are discussed taking into account the resistive properties of non-metallic samples and the photo-stimulated conductivity. This work clarifies the role of the laser and DC field in the energy deficit of field evaporated ions.

Effects of laser energy and wavelength on the analysis of LiFePO₄ using laser assisted atom probe tomography

The effects of laser wavelength (355 nm and 532 nm) and laser pulse energy on the quantitative analysis of LiFePO₄ by atom probe tomography are considered. A systematic investigation of ultraviolet (UV, 355 nm) and green (532 nm) laser assisted field evaporation has revealed distinctly different behaviors. With the use of a UV laser, the major issue was identified as the preferential loss of oxygen (up to 10 at%) while other elements (Li, Fe and P) were observed to be close to nominal ratios. Lowering the laser energy per pulse to 1 pJ/pulse from 50 pJ/pulse increased the observed oxygen concentration to nearer its correct stoichiometry, which was also well correlated with systematically higher concentrations of (16)O₂(+) ions. Green laser assisted field evaporation led to the selective loss of Li (~33% deficiency) and a relatively minor O deficiency. The loss of Li is likely a result of selective dc evaporation of Li between or after laser pulses. Comparison of the UV and green laser data suggests that the green wavelength energy was absorbed less efficiently than the UV wavelength because of differences in absorption at 355 and 532 nm for LiFePO₄. Plotting of multihit events on Saxey plots also revealed a strong neutral O2 loss from molecular dissociation, but quantification of this loss was insufficient to account for the observed oxygen deficiency.

Correlation of microphotoluminescence spectroscopy, scanning transmission electron microscopy, and atom probe tomography on a single nano-object containing an InGaN/GaN multiquantum well system

A single nanoscale object containing a set of InGaN/GaN nonpolar multiple-quantum wells has been analyzed by microphotoluminescence spectroscopy (μPL), high-resolution scanning transmission electron microscopy (HR-STEM) and atom probe tomography (APT). The correlated measurements constitute a rich and coherent set of data supporting the interpretation that the observed μPL narrow emission lines, polarized perpendicularly to the crystal c-axis and with energies in the interval 2.9-3.3 eV, are related to exciton states localized in potential minima induced by the irregular 3D In distribution within the quantum well (QW) planes. This novel method opens up interesting perspectives, as it will be possible to apply it on a wide class of quantum confining emitters and nano-objects.

Role of Photoexcitation and Field Ionization in the Measurement of Accurate Oxide Stoichiometry by Laser-Assisted Atom Probe Tomography

The addition of pulsed lasers to atom probe tomography (APT) extends its high spatial and mass resolution capability to nonconducting materials, such as oxides. For a prototypical metal oxide, MgO, the measured stoichiometry depends strongly on the laser pulse energy and applied voltage. Very low laser energies (0.02 pJ) and high electric fields yield optimal stoichiometric accuracy. Correlated APT and aberration-corrected transmission electron microscopy (TEM) are used to establish the high density of corner and terrace sites on MgO sample surfaces before and after APT. For MgO, long-lifetime photoexcited holes localized at oxygen corner sites can assist in the creation of oxygen neutrals that may spontaneously desorb either as atomic O or as molecular O2. The observed trends are best explained by the relative field-dependent ionization of photodesorbed O or O2 neutrals. These results emphasize the importance of considering electronic excitations in APT analysis of oxide materials.

Atom probe tomography of apatites and bone-type mineralized tissues

Nanocrystalline biological apatites constitute the mineral phase of vertebrate bone and teeth. Beyond their central importance to the mechanical function of our skeleton, their extraordinarily large surface acts as the most important ion exchanger for essential and toxic ions in our body. However, the nanoscale structural and chemical complexity of apatite-based mineralized tissues is a formidable challenge to quantitative imaging. For example, even energy-filtered electron microscopy is not suitable for detection of small quantities of low atomic number elements typical for biological materials. Herein we show that laser-pulsed atom probe tomography, a technique that combines subnanometer spatial resolution with unbiased chemical sensitivity, is uniquely suited to the task. Common apatite end members share a number of features, but can clearly be distinguished by their spectrometric fingerprint. This fingerprint and the formation of molecular ions during field evaporation can be explained based on the chemistry of the apatite channel ion. Using end members for reference, we are able to interpret the spectra of bone and dentin samples, and generate the first three-dimensional reconstruction of 1.2 × 10(7) atoms in a dentin sample. The fibrous nature of the collagenous organic matrix in dentin is clearly recognizable in the reconstruction. Surprisingly, some fibers show selectivity in binding for sodium ions over magnesium ions, implying that an additional, chemical level of hierarchy is necessary to describe dentin structure. Furthermore, segregation of inorganic ions or small organic molecules to homophase interfaces (grain boundaries) is not apparent. This has implications for the platelet model for apatite biominerals.

Atom-probe tomography of nickel-based superalloys with green or ultraviolet lasers: a comparative study

Recent developments in the technology of laser-pulsed local-electrode atom-probe (LEAP) tomography include a picosecond ultraviolet (UV) laser system having a 355 nm wavelength and both external and in-vacuum optics. This approach ensures focusing of the laser beam to a smaller spot diameter than has heretofore been obtained using a green (532 nm wavelength) picosecond laser. We compare the mass spectra acquired, using either green or UV laser pulsing, from nickel-based superalloy specimens prepared either electrochemically or by lifting-out from bulk material using ion-beam milling in a dual-beam focused ion beam microscope. The utilization of picosecond UV laser pulsing yields improved mass spectra, which manifests itself in higher signal-to-noise ratios and mass-resolving power (m/Δm) in comparison to green laser pulsing. We employ LEAP tomography to investigate the formation of misoriented defects in nickel-based superalloys and demonstrate that UV laser pulsing yields better accuracy in compositional quantification than does green laser pulsing. Furthermore, we show that using a green laser the quality of mass spectra collected from specimens that were lifted-out by ion milling is usually poorer than for electrochemically-sharpened specimens. Employing UV laser pulsing yields, however, improved mass spectra in comparison to green laser pulsing even for ion-milled microtips.