David R. Diercks

Atom probe tomography evaporation behavior of C-axis GaN nanowires: Crystallographic, stoichiometric, and detection efficiency aspects

The field evaporation behavior of c-axis GaN nanowires was explored in two different laser-pulsed atom probe tomography (APT) instruments. Transmission electron microscopy imaging before and after atom probe tomography analysis was used to assist in reconstructing the data and assess the observed evaporation behavior. It was found that the ionic species exhibited preferential locations for evaporation related to the underlying crystal structure of the GaN and that the species which evaporated from these locations was dependent on the pulsed laser energy. Additionally, the overall stoichiometry measured by APT was significantly correlated with the energy of the laser pulses. At the lowest laser energies, the apparent composition was nitrogen-rich, while higher laser energies resulted in measurements of predominantly gallium compositions. The percent of ions detected (detection efficiency) for these specimens was found to be considerably below that shown for other materials, even for laser energies which pr...

Alternating Magnetic Field Controlled, Multifunctional Nano-Reservoirs: Intracellular Uptake and Improved Biocompatibility

Biocompatible magnetic nanoparticles hold great therapeutic potential, but conventional particles can be toxic. Here, we report the synthesis and alternating magnetic field dependent actuation of a remotely controllable, multifunctional nano-scale system and its marked biocompatibility with mammalian cells. Monodisperse, magnetic nanospheres based on thermo-sensitive polymer network poly(ethylene glycol) ethyl ether methacrylate-co-poly(ethylene glycol) methyl ether methacrylate were synthesized using free radical polymerization. Synthesized nanospheres have oscillating magnetic field induced thermo-reversible behavior; exhibiting desirable characteristics comparable to the widely used poly-N-isopropylacrylamide-based systems in shrinkage plus a broader volumetric transition range. Remote heating and model drug release were characterized for different field strengths. Nanospheres containing nanoparticles up to an iron concentration of 6 mM were readily taken up by neuron-like PC12 pheochromocytoma cells and had reduced toxicity compared to other surface modified magnetic nanocarriers. Furthermore, nanosphere exposure did not inhibit the extension of cellular processes (neurite outgrowth) even at high iron concentrations (6 mM), indicating minimal negative effects in cellular systems. Excellent intracellular uptake and enhanced biocompatibility coupled with the lack of deleterious effects on neurite outgrowth and prior Food and Drug Administration (FDA) approval of PEG-based carriers suggest increased therapeutic potential of this system for manipulating axon regeneration following nervous system injury.

Role of engineered nanocarriers for axon regeneration and guidance: current status and future trends.

There are approximately 1.5 million people who experience traumatic injuries to the brain and 265,000 who experience traumatic injuries to the spinal cord each year in the United States. Currently, there are few effective treatments for central nervous system (CNS) injuries because the CNS is refractory to axonal regeneration and relatively inaccessible to many pharmacological treatments. Smart, remotely tunable, multifunctional micro- and nanocarriers hold promise for delivering treatments to the CNS and targeting specific neurons to enhance axon regeneration and synaptogenesis. Furthermore, assessing the efficacy of treatments could be enhanced by biocompatible nanovectors designed for imaging in vivo. Recent developments in nanoengineering offer promising alternatives for designing biocompatible micro- and nanovectors, including magnetic nanostructures, carbon nanotubes, and quantum dot-based systems for controlled release of therapeutic and diagnostic agents to targeted CNS cells. This review highlights recent achievements in the development of smart nanostructures to overcome the existing challenges for treating CNS injuries.

Techniques for Consecutive TEM and Atom Probe Tomography Analysis of Nanowires

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Three-dimensional nanoscale characterisation of materials by atom probe tomography

ABSTRACT The development of three-dimensional (3-D), characterisation techniques with high spatial and mass resolution is crucial for understanding and developing advanced materials for many engineering applications as well as for understanding natural materials. In recent decades, atom probe tomography (APT), which combines a point projection microscope and time-of-flight mass spectrometer, has evolved to be an excellent characterisation technique capable of providing 3-D nanoscale characterisation of materials with sub-nanometer scale spatial resolution, with equal sensitivity for all elements. This review discusses the current state, as of APT instrumentation, new developments in sample preparation methods, experimental procedures for different material classes, reconstruction of APT results, the current status of correlative microscopy, and application of APT for microstructural characterisation in established scientific areas like structural materials as well as new applications in semiconducting nanowires, semiconductor devices, battery materials, catalyst materials, geological materials, and biological materials. Finally, a brief perspective is given regarding the future of APT.

Atomic Layer Deposition of Tungsten Disulphide Solid Lubricant Nanocomposite Coatings on Rolling Element Bearings

Atomic layer deposition (ALD) has the potential to provide highly conformal coatings with precise control of thickness. This article describes the application of ALD nanocomposite containing ZnF2 in WS2 matrix solid lubricant coatings on fully assembled rolling element bearings. The torque behavior of the coated bearings was studied during oscillatory contacts and after exposure to vibration. The coatings exhibited a hexagonal layered structure with predominant preferentially orientated (002) basal planes. These basal planes when sheared imparted very low running torque values of ∼ 0.5 mN· m in dry nitrogen. The outer race, inner race, and ball surfaces showed WS2 transfer film protection on the native coating necessary to achieve low torque in dry nitrogen. Structural (re)ordering of the basal and prismatic planes with multiple random and branched orientations was observed through the thickness of the transfer films. There was no evidence of uniformly aligned c-axis perpendicular-orientated basal planes on the transfer film surface. The unique advantages of ALD to apply solid lubricant coatings on rolling elements of fully assembled miniature bearings are compared with conventional solid lubrication techniques.

Excellent biocompatibility of semiconductor quantum dots encased in multifunctional poly(N-isopropylacrylamide) nanoreservoirs and nuclear specific labeling of growing neurons

Quantum dots (QDs) have received attention for labeling biomolecules; however, toxicity of these nanostructures in the intracellular environment has prevented a biomedical breakthrough. Here we report biocompatibility of a QD based multifunctional system on neuronal cells. Moreover, the designed nanostructures bind with high affinity in the cell nucleus. Nucleus specific binding and enhanced biocompatibility, coupled with no deleterious effects on neurite outgrowth, even at high dosages (500 μg/ml sphere conc.) suggest increased therapeutic potential of this system for specific targeting followed by controlled release of drugs in treating neurodegenerative disorders.

Three-dimensional quantification of composition and electrostatic potential at individual grain boundaries in doped ceria

Despite typically comprising a small volume fraction, grain boundaries often limit many properties of ceramics for energy technologies. The localized charges present at grain boundaries affect local structure and composition which thus impact the ionic conductivities along with thermal, electrical, optical, magnetic, and mechanical properties. While theory regarding grain boundary effects has progressed, direct knowledge of the local chemistry and corresponding potential around individual boundaries remains elusive. Some model bicrystal systems have been well-characterized experimentally; however, the complexities of random grain boundary structures in bulk-prepared polycrystalline ceramics have prevented quantified analysis of the most commonly occurring types. Here, the three-dimensional quantification of oxygen and cation compositions around arbitrarily selected high-angle grain boundaries in a polycrystalline material as measured by atom probe tomography are used to extract nm-scale, quantitative values of the three-dimensional space-charge potentials around grain boundaries and are related to the observed macro-scale conductivities. We focus specifically on Nd-doped ceria, a well-known ion conducting oxide with significant energy applications. However, the techniques employed here are directly applicable to other technologically-relevant polycrystalline ceramics and create opportunities for correlating nano-scale composition with macro-scale properties for optimizing materials design, expanding progress in ionic chemistry theory, and refining simulations for “real-world” polycrystalline materials.

Comparison of convergent beam electron diffraction and geometric phase analysis for strain measurement in a strained silicon device

Convergent beam electron diffraction and geometric phase analysis were used to measure strain in the gate channel of a p‐type strained silicon metal–oxide–semiconductor field‐effect transistor. These measurements were made on exactly the same transmission electron microscopy specimen allowing for direct comparison of the relative advantages of each technique. The trends in the strain values show good agreement in both the [] and [001] directions, but the absolute strain values are offset from each other. This difference in the absolute strain measured using the two techniques is attributed to the way the reference strain is defined for each.

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.