Atom probe tomography using an extreme ultraviolet trigger pulse.

Atom probe tomography (APT) is a powerful materials characterization technique capable of measuring the isotopically resolved three-dimensional (3D) structure of nanoscale specimens with atomic resolution. Modern APT instrumentation most often uses an optical pulse to trigger field ion evaporation-most commonly, the second or third harmonic of a Nd laser is utilized (∼λ = 532 nm or λ = 355 nm). Herein, we describe an APT instrument that utilizes ultrafast extreme ultraviolet (EUV) optical pulses to trigger field ion emission. The EUV light is generated via a commercially available high harmonic generation system based on a noble-gas-filled capillary. The centroid of the EUV spectrum is tunable from around 25 eV (λ = 50 nm) to 45 eV (λ = 28 nm), dependent on the identity of the gas in the capillary (Xe, Kr, or Ar). EUV pulses are delivered to the APT analysis chamber via a vacuum beamline that was optimized to maximize photon flux at the APT specimen apex while minimizing complexity. We describe the design of the beamline in detail, including the various compromises involved. We characterize the spectrum of the EUV light and its evolution as it propagates through the various optical elements. The EUV focus spot size is measured at the APT specimen plane, and the effects of misalignment are simulated and discussed. The long-term stability of the EUV source has been demonstrated for more than a year. Finally, APT mass spectra are shown, demonstrating the instrument's ability to successfully trigger field ion emission from semiconductors (Si, GaN) and insulating materials (Al2O3).

Comparative Apex Electrostatics of Atom Probe Tomography Specimens.

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 Eapex has been approximated by the relation Eapex=SV/(kr), 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

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.

A Three-Dimensional Atom Probe Microscope Incorporating a Wavelength-Tuneable Femtosecond-Pulsed Coherent Extreme Ultraviolet Light Source.

Pulsed coherent extreme ultraviolet (EUV) radiation is a potential alternative to pulsed near-ultraviolet (NUV) wavelengths for atom probe tomography. EUV radiation has the benefit of high absorption within the first few nm of the sample surface for elements across the entire periodic table. In addition, EUV radiation may also offer athermal field ion emission pathways through direct photoionization or core-hole Auger decay processes, which are not possible with the (much lower) photon energies used in conventional NUV laser-pulsed atom probe. We report preliminary results from what we believe to be the world's first EUV radiation-pulsed atom probe microscope. The instrument consists of a femtosecond-pulsed, coherent EUV radiation source interfaced to a local electrode atom probe tomograph by means of a vacuum manifold beamline. EUV photon-assisted field ion emission (of substrate atoms) has been demonstrated on various insulating, semiconducting, and metallic specimens. Select examples are shown.

Atomically thin layers of B-N-C-O with tunable composition.

In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp(2)-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm(2) V(-1) s(-1), and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < E g < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Gold Nanoparticle Quantitation by Whole Cell Tomography.

Many proposed biomedical applications for engineered gold nanoparticles require their incorporation by mammalian cells in specific numbers and locations. Here, the number of gold nanoparticles inside of individual mammalian stem cells was characterized using fast focused ion beam-scanning electron microscopy based tomography. Enhanced optical microscopy was used to provide a multiscale map of the in vitro sample, which allows cells of interest to be identified within their local environment. Cells were then serially sectioned using a gallium ion beam and imaged using a scanning electron beam. To confirm the accuracy of single cross sections, nanoparticles in similar cross sections were imaged using transmission electron microscopy and scanning helium ion microscopy. Complete tomographic series were then used to count the nanoparticles inside of each cell and measure their spatial distribution. We investigated the influence of slice thickness on counting single particles and clusters as well as nanoparticle packing within clusters. For 60 nm citrate stabilized particles, the nanoparticle cluster packing volume is 2.15 ± 0.20 times the volume of the bare gold nanoparticles.

Morphological and Electrical Characterization of MWCNT Papers and Pellets.

Six types of commercially available multiwall carbon nanotube soot were obtained and prepared into buckypapers by pellet pressing and by filtration into a paper. These samples were evaluated with respect to thickness, compressibility and electrical conductivity. DC conductivity results by two-point and four-point (van der Pauw) measurement methods as a function of preparation parameters are presented. Topology was investigated qualitatively by way of scanning electron microscopy and helium ion microscopy and from this, some generalizations about the nanotube structural properties and manufacturing technique with respect to conductivity are given.

Citrate-stabilized gold nanoparticles as negative controls for measurements of neurite outgrowth.

Gold nanoparticles (AuNPs) are promising candidates for medical diagnostics and therapeutics, due to their chemical stability, optical properties, and ease of functionalization. Citrate-stabilized reference materials also have potential as negative controls in toxicology studies of other nanoparticles. Here we examine the impact of 30 nm particles on the in vitro development of rat-cortex neural progenitor cells (NPCs), which mimic aspects of the developing neurological environment. AuNPs dispersed in a low serum culture medium initially agglomerated, but then remained stable during a three day incubation period, and agglomerated only slightly during a ten day incubation period, as determined by dynamic light scattering. Transmission electron microscopy indicated the presence of individual nanoparticles at all time points examined. Fixed cells were cross-sectioned by ion milling and imaged by scanning electronmicroscopy and helium-ion microscopy to evaluate particle incorporation. Individual nanoparticles could be resolved inside cross-sectioned cells. AuNPs were incubated with developing NPCs for ten days at concentrations of 0.5 µg/mL Au, 0.1 µg/mL Au, or 0.05 µg/mL Au. Adenosine triphosphate levels, as determined by bioluminescence measurements sensitive to low cell numbers, were not affected by AuNPs and the particles did not interfere with the assay. Multiple endpoints of neurite outgrowth were not altered by AuNPs, in particular, total neurite outgrowth per cell, a sensitive measure of neuronal development. Slide-level comparisons demonstrated the consistent response of NPCs to gold nanoparticles and a positive control chemical, neuroactive lithium. These results indicate that 30 nm citrate-stabilized AuNPs could serve as negative-control reference materials for in vitro measurements of neurite outgrowth.

Statistical sampling of carbon nanotube populations by thermogravimetric analysis.

Carbon nanotubes are one of the most promising nanomaterials available with applications in electronics devices, sensing, batteries, composites and medicine. Strict control of the carbon nanotube chemistry and properties is necessary as the applications proceed into more specialized areas. Thermogravimetric analysis (TGA) is one analytical method currently utilized for the characterization of carbon nanotubes. Though TGA can provide quantitative measurements of the composition of a sample, many researchers do not ensure the variance of the sample is properly captured. This research demonstrates for four single-wall carbon nanotube (SWCNT) samples how to statistically evaluate the material with TGA to ensure that the variance within the material is represented. SEM results are used to help reach conclusions about purity of the material by providing a visual means for inspection. This data is used to select the SWCNT material with the lowest variability and highest quality, as evaluated by composition and reproducibility.

Homoepitaxial n-core: p-shell gallium nitride nanowires: HVPE overgrowth on MBE nanowires.

We present the homoepitaxial growth of p-type, magnesium doped gallium nitride shells by use of halide vapor phase epitaxy (HVPE) on n-type gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy (MBE). Scanning electron microscopy shows clear dopant contrast between the core and shell of the nanowire. The growth of magnesium doped nanowire shells shows little or no effect on the lattice parameters of the underlying nanowires, as measured by x-ray diffraction (XRD). Photoluminescence measurements of the nanowires show the appearance of sub-bandgap features in the blue and the ultraviolet, indicating the presence of acceptors. Finally, electrical measurements confirm the presence of electrically active holes in the nanowires.

Accelerated reliability testing of highly aligned single-walled carbon nanotube networks subjected to DC electrical stressing.

We investigate the electrical reliability of nanoscale lines of highly aligned, networked, metallic/semiconducting single-walled carbon nanotubes (SWCNTs) fabricated through a template-based fluidic assembly process. We find that these SWCNT networks can withstand DC current densities larger than 10 MA cm(-2) for several hours and, in some cases, several days. We develop test methods that show that the degradation rate, failure predictability and total device lifetime can be linked to the initial resistance. Scanning electron and transmission electron microscopy suggest that fabrication variability plays a critical role in the rate of degradation, and we offer an empirical method of quickly determining the long-term performance of a network. We find that well-fabricated lines subject to constant electrical stress show a linear accumulation of damage reminiscent of electromigration in metallic interconnects, and we explore the underlying physical mechanisms that could cause such behavior.