The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy ...technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions.
Rigorous electrostatic modeling of the specimen electrode environment is required to better understand the fundamental processes of atom probe tomography (APT) and guide the analysis of APT data. We ...have developed a simulation tool that self-consistently solves the nonlinear electrostatic Poisson equation along with the mobile charge carrier concentrations and provides a detailed picture of the electrostatic environment of APT specimen tips. We consider cases of metals, semiconductors, and dielectrics. Traditionally in APT, and regardless of specimen composition, the apex electric field
E
apex
has been approximated by the relation
E
apex
=
S
V
/
(
k
r
)
, which was originally derived for sharp, metallic conductors; we refer to this equation as the “K-factor approximation”. Here,
SV
is tip-electrode bias,
r
is the radius of curvature of the tip apex, and
k
is a dimensionless fitting parameter with
1.5
<
k
<
8.5
. As expected, our Poisson solver agrees well with the k-factor approximation for metal tips; it also agrees remarkably well for semiconductor tips-regardless of the semiconductor doping level. We ascribe this finding to the fact that even if a semiconductor tip is fully depleted of majority carriers under the typical
SV
conditions used in APT, an inversion layer will appear at the apex surface. The inversion forms a thin, conducting layer that screens the interior of the tip, thus mimicking metallic behavior at the apex surface. By contrast, we find that the k-factor approximation yields a very poor representation of the electrostatics of a purely dielectric tip. We put our numerical results into further context with a brief discussion of our own separate work and the results of other publications.
Advances in genetic manipulation of the biopolymers that compose plant cell walls will facilitate more efficient production of biofuels and chemicals from biomass and lead to specialized biomaterials ...with tailored properties. Here we investigate several genetic variants of Arabidopsis: the wild type, which makes a lignin polymer of primarily guaiacyl (G) and syringyl (S) monomeric units, the fah1 mutant, which makes lignin from almost exclusively G subunits, and a ferulate 5-hydroxylase (F5H) overexpressing line (C4H:F5H) that makes lignin from S subunits. We employ multiscale, multimodal imaging techniques that reveal the biomass of the C4H:F5H transgenic to be more susceptible to deconstruction by maleic acid treatment than the other variants. Enzymatic saccharification assays of the treated materials show that C4H:F5H transgenic tissue is significantly more digestible than the wild type, while the fah1 mutant is clearly the least digestible of these materials. Finally, we show by contact resonance force microscopy, an atomic force microscopy technique, that F5H overexpression in C4H:F5H transgenic plants significantly reduces the stiffness of the cell walls in the region of the compound middle lamella relative to wild type and fah1.
•Electrostatic effects degrade the quality of mass spectra for electrically insulating samples in straight-flight-path atom probe microscopes.•A parameter-free ‘systematic energy deficit’ correction ...was developed to correct for the electrostatic effects.•The ‘systematic energy deficit’ correction significantly improves the quality of CeO2 and SiO2 mass spectra.
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.
Shape, in addition to size and material composition, has a strong effect on the optical scattering characteristics of a particle. In this work, we computationally study how the measured 3-D shapes of ...individual lunar regolith particles affect their optical scattering characteristics. The shapes of 25 lunar regolith particles collected during the Apollo 11 mission were measured using X-ray nano computed tomography (XCT). The scattering cross sections (CSs) of particles that ranged in size between 400 and 1000 nm were computed using the Discrete Dipole Scattering (DDSCAT) software in the visible to the infrared frequency range. The orientation-averaged scattering CS of the lunar regolith particles was then compared with that of spheres of the same volume. The comparison shows that the shape of lunar regolith particles decreases the resonance wavelength by 20%, on average, compared with the spheres of the same volume. On the other hand, the peak scattering CS is found to be nearly independent of shape for the suite of particles studied. Also, lunar regoliths are found to generate a slightly lower single scattering albedo and have a higher phase function asymmetry parameter in comparison to spherical particles of the same volume. These differences highlight the importance of accounting for 3-D shape variations in understanding the optical response of lunar regolith particles.
Laser-pulsed atom probe tomography (LAPT) is a materials characterization technique that has been widely applied in the study and characterization of III-nitride semiconductors. To date, most of ...these studies have used light sources ranging from the visible to the near-ultraviolet region of the spectrum. In this manuscript, we demonstrate that a recently developed extreme ultraviolet (EUV) radiation pulsed atom probe tomograph can trigger controlled field ion evaporation from III-nitride samples. Experiments indicate that EUV radiation can reliably trigger field ion evaporation from undoped and Mg-doped p-GaN, as well as from Al x Ga1–x N and In x Ga1–x N alloys using extremely low EUV fluence pulses. While measurements of the chemical composition for GaN using conventional LAPT are known to be highly sensitive to the experimental parameters, the EUV radiation-triggered APT (EUV APT) produces no significant variation in the measured composition over the range of experimentally attainable gallium charge-state ratios. Additionally, the Mg doping concentration values from EUV APT agreed with those obtained by other characterization techniques. The low EUV photon fluence used in these measurements does not appear capable of generating ions via the commonly accepted bulk thermal model, suggesting a different evaporation mechanism.
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.
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