Khalid Mikhiel Hattar

Spectral- and Pulse-Shape Discrimination in Triplet-Harvesting Plastic Scintillators

In this work, we describe a method to control the relative proportion of prompt and delayed luminosity of organic-based scintillators via direct and exponential emission from an extrinsic triplet state. This approach involves the incorporation of triplet-harvesting heavy metal complexes in plastic scintillator matrices to convert intrinsically non-luminescent host states to highly emissive guest states. Measurements on these plastic scintillators indicate improved light yields over the undoped polymers and the ability to perform neutron/gamma particle-discrimination. A similar extent of molecular-level control is not possible in traditional organic materials due to complex decay kinetics and the absence of spectral information for the delayed triplet-derived emission. The materials described here address these limitations through efficient host-guest triplet harvesting, which enables particle discrimination according to conventional pulse-shape discrimination (PSD) and a previously unreported spectral-shape discrimination (SSD) scheme.

In-situ transmission electron microscopy of liposomes in an aqueous environment.

The characterization of liposomes was undertaken using in-situ microfluidic transmission electron microscopy. Liposomes were imaged without contrast enhancement staining or cryogenic treatment, allowing for the observation of functional liposomes in an aqueous environment. The stability and quality of the liposome structures observed were found to be highly dependent on the surface and liposome chemistries within the liquid cell. The successful imaging of liposomes suggests the potential for the extension of in-situ microfluidic TEM to a wide variety of other biological and soft matter systems and processes.

Synthesis of mesoporous palladium with tunable porosity and demonstration of its thermal stability by in situ heating and environmental transmission electron microscopy

Palladium and its alloys have high-value applications as materials for high-performance hydrogen storage, chromatographic separation of hydrogen isotopes, electrocatalysis and catalysis. These materials can be formed by chemical or electrochemical reduction in a lyotropic liquid crystalline template that constrains their growth on the nanometer scale. This approach works for a variety of metals, but Pd presents special challenges due to the autocatalytic nature of its growth, which can disrupt the template structure, resulting in disordered pores. Presented herein is a scaleable synthesis that overcomes these challenges, yielding mesoporous Pd powder having pore diameters of 7 or 13 nm. Pore size control is effected by varying the size of the molecular template, polystyrene-block-polyethylene oxide. We have used heated-stage TEM for in situ observation of the materials in vacuum and in the presence of H2 gas, demonstrating that both pore diameter and the chemical state of the surface play important roles in determining thermal stability. Improved stability compared to previously reported examples facilitates preparation of scalable quantities of regularly mesoporous Pd that retains porosity at the elevated temperatures required for applications in hydrogen charge/discharge and catalysis.

Reduction in thermal boundary conductance due to proton implantation in silicon and sapphire

We measure the thermal boundary conductance across Al/Si and Al/Al2O3 interfaces that are subjected to varying doses of proton ion implantation with time domain thermoreflectance. The proton irradiation creates a major reduction in the thermal boundary conductance that is much greater than the corresponding decrease in the thermal conductivities of both the Si and Al2O3 substrates into which the ions were implanted. Specifically, the thermal boundary conductances decrease by over an order of magnitude, indicating that proton irradiation presents a unique method to systematically decrease the thermal boundary conductance at solid interfaces.

High Cycle Fatigue in the Transmission Electron Microscope

One of the most common causes of structural failure in metals is fatigue induced by cyclic loading. Historically, microstructure-level analysis of fatigue cracks has primarily been performed post mortem. However, such investigations do not directly reveal the internal structural processes at work near micro- and nanoscale fatigue cracks and thus do not provide direct evidence of active microstructural mechanisms. In this study, the tension-tension fatigue behavior of nanocrystalline Cu was monitored in real time at the nanoscale by utilizing a new capability for quantitative cyclic mechanical loading performed in situ in a transmission electron microscope (TEM). Controllable loads were applied at frequencies from one to several hundred hertz, enabling accumulations of 10(6) cycles within 1 h. The nanometer-scale spatial resolution of the TEM allows quantitative fatigue crack growth studies at very slow crack growth rates, measured here at ∼10(-12) m·cycle(-1). This represents an incipient threshold regime that is well below the tensile yield stress and near the minimum conditions for fatigue crack growth. Evidence of localized deformation and grain growth within 150 nm of the crack tip was observed by both standard imaging and precession electron diffraction orientation mapping. These observations begin to reveal with unprecedented detail the local microstructural processes that govern damage accumulation, crack nucleation, and crack propagation during fatigue loading in nanocrystalline Cu.

Direct Observation of Sink-Dependent Defect Evolution in Nanocrystalline Iron under Irradiation

Crystal defects generated during irradiation can result in severe changes in morphology and an overall degradation of mechanical properties in a given material. Nanomaterials have been proposed as radiation damage tolerant materials, due to the hypothesis that defect density decreases with grain size refinement due to the increase in grain boundary surface area. The lower defect density should arise from grain boundary-point defect absorption and enhancement of interstitial-vacancy annihilation. In this study, low energy helium ion irradiation on free-standing iron thin films were performed at 573 K. Interstitial loops of a0/2 [111] Burgers vector were directly observed as a result of the displacement damage. Loop density trends with grain size demonstrated an increase in the nanocrystalline (<100 nm) regime, but scattered behavior in the transition from the nanocrystalline to the ultra-fine regime (100–500 nm). To examine the validity of such trends, loop density and area for different grains at various irradiation doses were compared and revealed efficient defect absorption in the nanocrystalline grain size regime, but loop coalescence in the ultra-fine grain size regime. A relationship between the denuded zone formation, a measure of grain boundary absorption efficiency, grain size, grain boundary type and misorientation angle is determined.

Compressive Properties of ⟨110⟩ Cu Micro-Pillars after High-Dose Self-Ion Irradiation

Single-crystal Cu micro-pillars were self-ion irradiated up to 190 displacements per atom, a level commensurate with damage expected after long exposure in a reactor environment. Compression experiments performed along the ⟨ 110 ⟩ to 10% strain were compared against un-irradiated Cu. Two specimen configurations were explored: large 10 μm tall and small 4 μm tall pillars. Compared to un-irradiated Cu, the small irradiated pillars exhibited a flow stress increase of more than 500 MPa and were able to attain peak stresses approaching 1 GPa. These results are discussed in the context of an end of range effect, a damage gradient effect, and size effects.

Ion beam modification of topological insulator bismuth selenide

We demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.

Room Temperature Deformation Mechanisms of Alumina Particles Observed from In Situ Micro-compression and Atomistic Simulations

Aerosol deposition (AD) is a solid-state deposition technology that has been developed to fabricate ceramic coatings nominally at room temperature. Sub-micron ceramic particles accelerated by pressurized gas impact, deform, and consolidate on substrates under vacuum. Ceramic particle consolidation in AD coatings is highly dependent on particle deformation and bonding; these behaviors are not well understood. In this work, atomistic simulations and in situ micro-compressions in the scanning electron microscope, and the transmission electron microscope (TEM) were utilized to investigate fundamental mechanisms responsible for plastic deformation/fracture of particles under applied compression. Results showed that highly defective micron-sized alumina particles, initially containing numerous dislocations or a grain boundary, exhibited no observable shape change before fracture/fragmentation. Simulations and experimental results indicated that particles containing a grain boundary only accommodate low strain energy per unit volume before crack nucleation and propagation. In contrast, nearly defect-free, sub-micron, single crystal alumina particles exhibited plastic deformation and fracture without fragmentation. Dislocation nucleation/motion, significant plastic deformation, and shape change were observed. Simulation and TEM in situ micro-compression results indicated that nearly defect-free particles accommodate high strain energy per unit volume associated with dislocation plasticity before fracture. The identified deformation mechanisms provide insight into feedstock design for AD.

Unraveling irradiation induced grain growth with in situ transmission electron microscopy and coordinated modeling

Nanostructuring has been proposed as a method to enhance radiation tolerance, but many metallic systems are rejected due to significant concerns regarding long term grain boundary and interface stability. This work utilized recent advancements in transmission electron microscopy (TEM) to quantitatively characterize the grain size, texture, and individual grain boundary character in a nanocrystalline gold model system before and after in situ TEM ion irradiation with 10 MeV Si. The initial experimental measurements were fed into a mesoscale phase field model, which incorporates the role of irradiation-induced thermal events on boundary properties, to directly compare the observed and simulated grain growth with varied parameters. The observed microstructure evolution deviated subtly from previously reported normal grain growth in which some boundaries remained essentially static. In broader terms, the combined experimental and modeling techniques presented herein provide future avenues to enhance quantification and prediction of the thermal, mechanical, or radiation stability of grain boundaries in nanostructured crystalline systems.