David Sprouster

Graphite's resilience to high temperatures and neutron damage makes it vital for nuclear reactors, yet irradiation alters its microstructure, degrading key properties. We used small‐ and wide‐angle X‐ray scattering to study neutron‐irradiated fine‐grain nuclear graphite (Grade G347A) across varied temperatures and fluences. Results show significant shifts in internal strain and porosity, correlating with radiation‐induced volume changes. Notably, porosity volume distribution (fractal dimensions) follows non‐monotonic volume changes, suggesting a link to the Weibull distribution of fracture stress.

Molten fluoride salts such as Li₂BeF₄ (FLiBe) are used in molten salt reactors, fluoride-salt-cooled high-temperature reactors and fusion reactors as a fuel solvent, coolant and/or tritium breeding medium. In engineered systems that use molten salt, solid-state material will be present during melting and freezing scenarios, and therefore the temperature-dependent properties of the solid and solid/liquid phase transition merit investigation. To observe the behavior of the solid state of Li₂BeF₄ from room temperature to melting, this work used neutron and X-ray diffraction to measure the changes in the lattice parameters and volume of the crystalline unit cell and compared the results with prior low-temperature data for solid Li₂BeF₄. From neutron diffraction data it is also possible to identify anisotropy: centimetre-scaled crystals align preferentially with the a axes parallel to the direction of freezing front propagation, and the c axes expand 54% more than the a axes. This work provides the lattice constants as a function of temperature, quantifies the thermal expansion, and determines the equation describing the change in density for solid Li₂BeF₄ from room temperature to 459°C to be ρ_solid (kg m⁻³) = 2182 (3) − 0.115 (2) T (°C) and the volume expansion upon melting to be less than 5%. This density changes depending on molecular weight and enrichment.

The chemical interactions in Fe–HfH2 metal matrix composites (MMCs) are studied across multiple length scales to elucidate the decomposition of the parent phases and corresponding reaction zone physics during direct current sintering. Fe–HfH2 composites were synthesized with increasing as-mixed hydride contents of Fe–25% HfH2, Fe–40% HfH2, Fe–55% HfH2, and Fe–70% HfH2 (all in vol. %) to demonstrate the ability to achieve sintered MMCs with target hydride contents. Samples were probed across multiple length scales through a multi-modal workflow employing x-ray diffraction, scanning electron microscopy and segmentation analysis, and synchrotron techniques including hard x-ray fluorescence mapping and nanoprobe x-ray absorption near-edge structure measurements. Under the selected sintering temperature and pressure conditions, hydrogen evolution is seen to evolve through parallel paths: thermal decomposition from during the transformation of HfH2 to HfHx<2 and through subsequent reaction with the Fe matrix leading to intermetallic phase formation. Specifically, HfFe and HfFe2 intermetallic formation accelerates the release of hydrogen with a subsequent HfO2 phase forming at grain boundaries. For this MMC, the consumption or loss of hydrogen can be considerable in compacts with initial hydride loading of 25%–40% HfH2 approaching 83% hydrogen loss for the lower volume fraction composites. Increasing the volume fraction of HfH2 to 70% enhanced the retained hydrogen content to 53% and attributed to the reduced interfacial area intrinsic to the increased HfH2 loading in this MMC.

We reported cellulose filter paper, treated with weak acids, and reinforced with resorcinol bis(diphenyl phosphate) (RDP), for incorporation into the membrane electrolyte assembly (MEA) of proton exchange membrane fuel cells (PEMFCs).

With significant improvement in High Temperature Superconductors (HTS), several projects are adopting HTS technology for fusion power systems. Compact HTS tokamaks offer potential advantages including lower plant costs, enhanced plasma control, and ultimately lower cost of electricity. However, as compact reactors have a reduced radial build to accommodate shielding, HTS degradation due to radiation damage or heating is a significant and potentially design limiting issue. Shielding must mitigate threats to the superconducting coils: neutron cascade damage, heat deposition and potentially organic insulator damage due x-rays. Unfortunately, there are currently no hi-performance shielding materials to enable the potential performance enhancement offered by HTS. In this work, we present a manufacturing method to fabricate a new class of composite shields that are high performance, high operating temperature, and simultaneously neutron absorbing and neutron moderating. The composite design consists of an entrained metal-hydride phase within a radiation stable MgO ceramic host matrix. We discuss the fabrication, characterization, and thermophysical performance data for a series of down-selected composite materials inspired by future fusion core designs and their operational performance metrics. To our knowledge these materials represent the first ceramic composite shield materials containing significant metal hydrides.

The use of molten salts as coolants, fuels, and tritium breeding blankets in the next generation of fission and fusion nuclear reactors benefits from furthering the characterization of the molecular structure of molten halide salts, paving the way to predictive capability of the chemical and thermophysical properties of molten salts. Due to its neutronic, chemical, and thermochemical properties, 2LiF-BeF2 is a candidate molten salt for several fusion- and fission-reactor designs. We performed neutron and x-ray total-scattering measurements to determine the atomic structure of liquid 2LiF-BeF2. We also performed and neural-network molecular-dynamics simulations to predict the structure obtained by neutron- and x-ray-diffraction experiments. The use of machine learning provides improvements to the efficiency in predicting the structure at a longer length scales than is achievable with simulations at significantly lower computational expense while retaining near accuracy. We found that the NNMD simulations accurately predicted the BeF42− oligomer formations seen in the experimental first-structure-factor peak. Our combination of high-resolution measurements with large-scale molecular dynamics provided an avenue to explore and experimentally verify the intermediate-range ordering beyond the first-nearest neighbor that has posed too many experimental and computational challenges in previous works. With a deeper understanding of the salt structure and ion ordering, the evolution of salt chemistry over the lifetime of a reactor can be better predicted, which is crucial to the licensing and operation of advanced fission and fusion reactors that employ molten salts. To this end, this work will serve as a reference for future studies of salt structure and macroscopic properties with and without the addition of solutes.

Despite the recent progress in increasing the power generation of Anion‐exchange membrane fuel cells (AEMFCs), their durability is still far lower than that of Proton exchange membrane fuel cells (PEMFCs). Using the complementary techniques of X‐ray micro‐computed tomography (CT), Scanning Electron Microscopy (SEM) and Energy Dispersive X‐ray (EDX) spectroscopy, we have identified Pt ion migration as an important factor to explain the decay in performance of AEMFCs. In alkaline media Pt⁺² ions are easily formed which then either undergo dissolution into the carbon support or migrate to the membrane. In contrast to PEMFCs, where hydrogen cross over reduces the ions forming a vertical “Pt line” within the membrane, the ions in the AEM are trapped by charged groups within the membrane, leading to disintegration of the membrane and failure. Diffusion of the metal components is still observed when the Pt/C of the cathode is substituted with a FeCo−N−C catalyst, but in this case the Fe and Co ions are not trapped within the membrane, but rather migrate into the anode, thereby increasing the stability of the membrane.

This work underscores the relationship between the mechanical properties of anion exchange membranes and pH, which together can have a profound effect on the power output of the fuel cells.

The use of molten salts as coolants, fuels, and tritium breeding blankets in the next generation of fission and fusion nuclear reactors benefits from furthering the characterization of the molecular structure of molten halide salts, paving the way to predictive capability of chemical and thermo-physical properties of molten salts. Due to its neutronic, chemical, and thermo-chemical properties, 2LiF−BeF2 is a candidate molten salt for several fusion and fission reactor designs. We perform neutron and X-ray total scattering measurements to determine the atomic structure of 2LiF−BeF2. We also perform ab-initio and neural network molecular dynamics simulations to predict the structure obtained by neutron and X-ray diffraction experiments. The use of machine learning provides improvements to the efficiency in predicting the structure at a longer length scales than is achievable with ab-initio simulations at significantly lower computational expense while retaining near ab-initio accuracy. The comparison among experimental and modeling results at a higher resolution and efficiency than previous measurements provides the opportunity to explore the structural determination of 2LiF−BeF2 beyond the first-nearest neighbor analysis that had been previously achieved with X-ray diffraction measurements of a FLiBe melt. This work may serve as a reference for future studies of salt structure and macroscopic properties with and without the addition of solutes.

The atomic structures of the lanthanide tantalates, Ln₃TaO₇, series (Ln = Pr, Tb, Dy, Ho, Tm, Yb) were systematically investigated using total scattering techniques.

The atomic structures of the lanthanide tantalates, Ln₃TaO₇, series (Ln = Pr, Tb, Dy, Ho, Tm, Yb) were systematically investigated using total scattering techniques.

Effects of corrosion products on molten salt properties are a focus of many studies motivated by the needs of molten-salt nuclear reactors. We demonstrate in-situ measurements of corrosion products from NiCr (80%-20%) foil in molten FLiNaK. We used X-ray absorption spectroscopy (XAS) and a combination of XAS with electrochemical spectroscopy. Using XAS, we measured both the space and time distributions of Ni and Cr ions leaching from the foil and characterized their valence state and the local structure. We also correlated the XAS results with simultaneous electrochemical measurements. We also demonstrated a novel sample environment that we developed that enables simultaneous XAS and electrochemical measurements of molten fluoride salts. The next step of this program is to measure the corrosion on structural alloys such as stainless steel in molten FLiBe.

Exploiting grain boundary engineering in the design of alloys for extreme environments provides a promising pathway for enhancing performance relative to coarse-grained counterparts. Due to its attractive properties as a plasma facing material for fusion devices, tungsten presents an opportunity to exploit this approach in addressing the significant materials challenges imposed by the fusion environment. Here, we employ a ternary alloy design approach for stabilizing W against recrystallization and grain growth while simultaneously enhancing its manufacturability through powder metallurgical processing. Mechanical alloying and grain refinement in W-10 at.% Ti-(10,20) at.% Cr alloys are accomplished through high-energy ball milling with transitions in the microstructure mapped as a function of milling time. We demonstrate the multi-modal nature of the resulting nanocrystalline grain structure and its stability up to 1300 °C with the coarser grain size population correlated to transitions in crystallographic texture that result from the preferred slip systems in BCC W. Field-assisted sintering is employed to consolidate the alloy powders into bulk samples, which, due to the deliberately designed compositional features, are shown to retain ultrafine grain structures despite the presence of minor carbides formed during sintering due to carbon impurities in the ball-milled powders.

Silicon carbide (SiC) formed through pyrolysis of preceramic polymers loaded with SiC particles has gained significant attention for applications such as coatings, composite matrix modifications, and most importantly additive manufacturing. This work presents combined synchrotron XRD, Raman spectroscopy, scanning electron microscopy, nano-indentation, and Vickers indentation of pyrolysis bonded SiC to shed light on the changes of composition and mechanical properties of these materials. Characterization was performed on samples that were heat treated ranging from the synthesis 850 °C up to 1500 °C. Pre-treatments of the powders prior to pellet synthesis, such as heat treatment and etching using a hydrofluoric acid (HF), were investigated. It is shown that the degradation of mechanical properties when exposed to higher temperatures is due to the burnout of amorphous carbon clusters remnant of the pyrolysis process of the preceramic polymer. Furthermore, prior HF etching and removal of the native oxide layer of the powders showed improved density and hardness values in the final pellets. The average Vickers hardness of the control samples were 4.59 GPa and later 3.74 GPa when exposed to 1500 °C, while the samples synthesized using powders that were etched with HF had an average hardness value of 9.37 GPa and later 6.86 GPa when exposed to 1500 °C.

Understanding and controlling the physical and chemical processes at molten salt‐alloy interfaces is vital for molten‐salt nuclear reactors. Corrosion processes in molten salts are highly dependent on the redox potential of the solution that changes with the addition of fission and corrosion products. Therefore, reactor designers develop online electrochemical methods of salt monitoring. But electrochemical spectroscopy relies on the deconvolution of broad peaks, a process that may be imprecise in the presence of multiple species in the solution. Here, we describe our developments towards monitoring the concentration and the chemical state of corrosion products in the melt by a combination of electrochemistry and X-ray absorption spectroscopy. We placed NiCr foil in molten FLiNaK and found the presence of both Ni2+ ions and metallic Ni in the melt, which we attribute to the disintegration of the corroding foil due to Cr dealloying. Although extremely challenging, spectroelectrochemical measurements add a promising rich new data stream for online salt monitoring.

We demonstrate effects of Cs ions on the melting transition and molecular structure of molten FLiNaK (a eutectic mixture of LiF-NaF-KF). FLiNaK is a commonly studied multi-component model system, which represents the physical and chemical behavior of fluoride salts for nuclear energy applications. Dissolution of nuclear fuels leads to the formation of fission products directly in the molten salt. Cs is one of the most important fission products, due to its relative abundance, long half life, and potential environmental and health effects. Here, we determine the molecular structure and phase equilibria of dissolved Cs in FLiNaK by a combination of X-ray diffraction, X-ray total scattering, ab initio molecular dynamics calculations, and computational thermodynamics. Although Cs ions have a relatively large size, we did not find significant evidence that they disrupt the existing molecular structure of the liquid. We found good agreement between our simulated and measured structure factors, and calculated that the coordination number of Cs is close to 10. X-ray diffraction in combination with computational thermodynamics demonstrates that upon freezing Cs ions are captured into a CsLiF2 compound, with a lower melting temperature than the FLiNaK mixture and much higher than that predicted for CsLiF2 by computational thermodynamics. We also demonstrated a novel sample environment that we developed to X-ray measurements of molten fluoride or fuel salts.

Combined neutron and X-ray total scattering with calorimetric measurements of the solid solution series Ho₂Ti_2−xZr_xO₇ reveals a complex order–disorder transition across short, intermediate, and long length scales induced by chemical substitution.

Combined neutron and X-ray total scattering with calorimetric measurements of the solid solution series Ho₂Ti_2−xZr_xO₇ reveals a complex order–disorder transition across short, intermediate, and long length scales induced by chemical substitution.

Combined neutron and X-ray total scattering with calorimetric measurements of the solid solution series Ho₂Ti_2−xZr_xO₇ reveals a complex order–disorder transition across short, intermediate, and long length scales induced by chemical substitution.

The resurgence of the Molten-Salt Nuclear Reactors (MSR) creates interesting problems in molten-salt chemistry. As MSRs operate, the composition and physical properties of salts change because of fission and corrosion. Since Cr is the principal corrosion product and NaCl is a common constituent, we studied the atomic structure of molten NaCl-CrCl3. We found networks of CrCl3− 6 octahedra and an intermediate-range order with nonmonotonic temperature behavior in a remarkable agreement between measurements and ab initio simulations. Even though the corrosion results in minute quantities of dissolved Cr, the speciation of Cr could lead to changes in molten-salt properties in nuclear and solar salts. In particular, we found a much lower than expected melting temperature and a broad metastable liquid-solid coexistence phase. The availability of Cr isotopes with very different neutron-scattering properties makes Cr an ideal model multi-valent ion for experimental validation of new atomistic models such as neural network interatomic potentials.

The resurgence of the Molten-Salt Nuclear Reactors (MSR) creates interesting problems in molten-salt chemistry. As MSRs operate, the composition and physical properties of salts change because of fission and corrosion. Since Cr is the principal corrosion product and NaCl is a common constituent, we studied the atomic structure of molten NaCl-CrCl3. We found networks of CrCl3− 6 octahedra and an intermediate-range order with nonmonotonic temperature behavior in a remarkable agreement between measurements and ab initio simulations. Even though the corrosion results in minute quantities of dissolved Cr, the speciation of Cr could lead to changes in molten-salt properties in nuclear and solar salts. In particular, we found a much lower than expected melting temperature and a broad metastable liquid-solid coexistence phase. The availability of Cr isotopes with very different neutron-scattering properties makes Cr an ideal model multi-valent ion for experimental validation of new atomistic models such as neural network interatomic potentials.

The implantation of noble gas atoms into metals at high gas concentrations can lead to the self-organization of nanobubbles into superlattices with symmetry similar to the metal host matrix. Here, we examine the influence of implantation parameters on the formation and structure of helium gas bubble superlattices within a tungsten host matrix to uncover mechanistic insight into the formation process. The determination of the size and symmetry of the gas bubbles was performed using a combination of small angle x-ray scattering and transmission electron microscopy. The former was demonstrated to be particularly useful in determining size and structure of the gas bubble superlattice as a function of irradiation conditions. Prior to the formation of a superlattice, we observe a persistent substructure characterized by inter-bubble spacings similar to those observable when the gas bubble superlattice has formed with very large ordering parameters. As the implantation fluence increases, the inter-bubble ordering parameter decreases, indicating improved ordering, until a superlattice is formed. Multiple implantation-specific differences were observed, including a temperature-dependent superlattice parameter that increases with increasing temperature and a flux-dependent superlattice parameter that decreases with increasing flux. The trends quantified here are in excellent agreement with our recent theoretical predictions for gas bubble superlattice formation and highlight that superlattice formation is strongly dependent on the diffusion of vacancy and implanted He atoms.

SiGe nanoparticles were formed in an amorphous Si3N4 matrix by Ge+ ion implantation and thermal annealing. The size of the nanoparticles was determined by transmission electron microscopy and their atomic structure by x-ray absorption spectroscopy. Nanoparticles were observed for excess Ge concentrations in the range from 9 to 12 at. % after annealing at temperatures in the range from 700 to 900 °C. The average nanoparticle size increased with excess Ge concentration and annealing temperature and varied from an average diameter of 1.8 ± 0.2 nm for the lowest concentration and annealing temperature to 3.2 ± 0.5 nm for the highest concentration and annealing temperature. Our study demonstrates that the structural properties of embedded SiGe nanoparticles in amorphous Si3N4 are sensitive to the implantation and post implantation conditions. Furthermore, we demonstrate that ion implantation is a novel pathway to fabricate and control the SiGe nanoparticle structure and potentially useful for future optoelectronic device applications.

SiGe nanoparticles were formed in an amorphous Si3N4 matrix by Ge+ ion implantation and thermal annealing. The size of the nanoparticles was determined by transmission electron microscopy and their atomic structure by x-ray absorption spectroscopy. Nanoparticles were observed for excess Ge concentrations in the range from 9 to 12 at. % after annealing at temperatures in the range from 700 to 900 °C. The average nanoparticle size increased with excess Ge concentration and annealing temperature and varied from an average diameter of 1.8 ± 0.2 nm for the lowest concentration and annealing temperature to 3.2 ± 0.5 nm for the highest concentration and annealing temperature. Our study demonstrates that the structural properties of embedded SiGe nanoparticles in amorphous Si3N4 are sensitive to the implantation and post implantation conditions. Furthermore, we demonstrate that ion implantation is a novel pathway to fabricate and control the SiGe nanoparticle structure and potentially useful for future optoelectronic device applications.

We report on the effects of dopant concentration and substrate stoichiometry on the electrical and structural properties of In-implanted Si1−xGex alloys. Correlating the fraction of electrically active In atoms from Hall Effect measurements with the In atomic environment determined by X-ray absorption spectroscopy, we observed the transition from electrically active, substitutional In at low In concentration to electrically inactive metallic In at high In concentration. The In solid-solubility limit has been quantified and was dependent on the Si1−xGex alloy stoichiometry; the solid-solubility limit increased as the Ge fraction increased. This result was consistent with density functional theory calculations of two In atoms in a Si1−xGex supercell that demonstrated that In–In pairing was energetically favorable for x ≲ 0.7 and energetically unfavorable for x ≳ 0.7. Transmission electron microscopy imaging further complemented the results described earlier with the In concentration and Si1−xGex alloy stoichiometry dependencies readily visible. We have demonstrated that low resistivity values can be achieved with In implantation in Si1−xGex alloys, and this combination of dopant and substrate represents an effective doping protocol.

We report on the effects of dopant concentration and substrate stoichiometry on the electrical and structural properties of In-implanted Si1−xGex alloys. Correlating the fraction of electrically active In atoms from Hall Effect measurements with the In atomic environment determined by X-ray absorption spectroscopy, we observed the transition from electrically active, substitutional In at low In concentration to electrically inactive metallic In at high In concentration. The In solid-solubility limit has been quantified and was dependent on the Si1−xGex alloy stoichiometry; the solid-solubility limit increased as the Ge fraction increased. This result was consistent with density functional theory calculations of two In atoms in a Si1−xGex supercell that demonstrated that In–In pairing was energetically favorable for x ≲ 0.7 and energetically unfavorable for x ≳ 0.7. Transmission electron microscopy imaging further complemented the results described earlier with the In concentration and Si1−xGex alloy stoichiometry dependencies readily visible. We have demonstrated that low resistivity values can be achieved with In implantation in Si1−xGex alloys, and this combination of dopant and substrate represents an effective doping protocol.

At high dopant concentrations in Ge, electrically activating all implanted dopants is a major obstacle in the fulfillment of high-performance Ge-channel complementary metal oxide semiconductor devices. In this letter, we demonstrate a significant increase in the electrically-active dopant fraction in In-implanted Ge by co-doping with the isovalent element C. Electrical measurements have been correlated with x-ray absorption spectroscopy and transmission electron microscopy results in addition to density functional theory simulations. With C + In co-doping, the electrically active fraction was doubled and tripled at In concentrations of 0.2 and 0.7 at. %, respectively. This marked improvement was the result of C-In pair formation such that In-induced strain in the Ge lattice was reduced while the precipitation of In and the formation of In-V clusters were both suppressed.

Germanium nanoparticles embedded within dielectric matrices hold much promise for applications in optoelectronic and electronic devices. Here we investigate the formation of Ge nanoparticles in amorphous SiO1.67N0.14 as a function of implanted atom concentration and thermal annealing temperature. Using x-ray absorption spectroscopy and other complementary techniques, we show Ge nanoparticles exhibit significant finite-size effects such that the coordination number decreases and structural disorder increases as the nanoparticle size decreases. While the composition of SiO1.67N0.14 is close to that of SiO2, we demonstrate that the addition of this small fraction of N yields a much reduced nanoparticle size relative to those formed in SiO2 under comparable implantation and annealing conditions. We attribute this difference to an increase in an atomic density and a much reduced diffusivity of Ge in the oxynitride matrix. These results demonstrate the potential for tailoring Ge nanoparticle sizes and structural properties in the SiOxNy matrices by controlling the oxynitride stoichiometry.

Germanium nanoparticles embedded within dielectric matrices hold much promise for applications in optoelectronic and electronic devices. Here we investigate the formation of Ge nanoparticles in amorphous SiO1.67N0.14 as a function of implanted atom concentration and thermal annealing temperature. Using x-ray absorption spectroscopy and other complementary techniques, we show Ge nanoparticles exhibit significant finite-size effects such that the coordination number decreases and structural disorder increases as the nanoparticle size decreases. While the composition of SiO1.67N0.14 is close to that of SiO2, we demonstrate that the addition of this small fraction of N yields a much reduced nanoparticle size relative to those formed in SiO2 under comparable implantation and annealing conditions. We attribute this difference to an increase in an atomic density and a much reduced diffusivity of Ge in the oxynitride matrix. These results demonstrate the potential for tailoring Ge nanoparticle sizes and structural properties in the SiOxNy matrices by controlling the oxynitride stoichiometry.

We report on the effects of dopant concentration on the structural and electrical properties of In-implanted Ge. For In concentrations of ≤ 0.2 at. %, extended x-ray absorption fine structure and x-ray absorption near-edge structure measurements demonstrate that all In atoms occupy a substitutional lattice site while metallic In precipitates are apparent in transmission electron micrographs for In concentrations ≥0.6 at. %. Evidence of the formation of In-vacancy complexes deduced from extended x-ray absorption fine structure measurements is complimented by density functional theory simulations. Hall effect measurements of the conductivity, carrier density, and carrier mobility are then correlated with the substitutional In fraction.

In this work, we outline the development of an automated, high-throughput robotic system designed for the structural characterization of radioactive samples at the X-ray Powder Diffraction beamline of the National Synchrotron Light Source-II (NSLS II).

Aging in super glassy polymers such as poly(trimethylsilylpropyne) (PTMSP), poly(4‐methyl‐2‐pentyne) (PMP), and polymers with intrinsic microporosity (PIM‐1) reduces gas permeabilities and limits their application as gas‐separation membranes. While super glassy polymers are initially very porous, and ultra‐permeable, they quickly pack into a denser phase becoming less porous and permeable. This age‐old problem has been solved by adding an ultraporous additive that maintains the low density, porous, initial stage of super glassy polymers through absorbing a portion of the polymer chains within its pores thereby holding the chains in their open position. This result is the first time that aging in super glassy polymers is inhibited whilst maintaining enhanced CO₂ permeability for one year and improving CO₂/N₂ selectivity. This approach could allow super glassy polymers to be revisited for commercial application in gas separations.

Lift-off protocols for thin films for improved extended X-ray absorption fine structure (EXAFS) measurements are presented. Using wet chemical etching of the substrate or the interlayer between the thin film and the substrate, stand-alone high-quality micrometer-thin films are obtained. Protocols for the single-crystalline semiconductors GeSi, InGaAs, InGaP, InP and GaAs, the amorphous semiconductors GaAs, GeSi and InP and the dielectric materials SiO₂and Si₃N₄are presented. The removal of the substrate and the ability to stack the thin films yield benefits for EXAFS experiments in transmission as well as in fluorescence mode. Several cases are presented where this improved sample preparation procedure results in higher-quality EXAFS data compared with conventional sample preparation methods. This lift-off procedure can also be advantageous for other experimental techniques (e.g.small-angle X-ray scattering) that benefit from removing undesired contributions from the substrate.

The pressure-induced phase transformations of a form of amorphous silicon (a-Si) with well characterized impurity levels and structure are examined at pressures up to 40 GPa usingin situsynchrotron X-ray radiation. At ∼12 GPa crystallization commences, but it is not completed until ∼16 GPa. At higher pressures, not all the crystalline phases observed for crystalline silicon (c-Si) appear. On pressure release, none of the metastable crystalline phases observed for c-Si nucleate. Instead an amorphous phase is re-formed. This is in contrast to all previous diamond-anvil studies on a-Si. If full pressure-induced crystallization occurred, the material remained crystalline on unloading. The formation of a-Si upon unloading was only observed when a high-density amorphous phase was reported on loading. The fully characterized nature of the a-Si used in this current study allows for the interpretation of this significant diversity in terms of impurity content of the a-Si used. Namely, this suggests that `ideal' (pure, voidless, structurally relaxed) a-Si will follow the same transition pathway as observed for c-Si, while crystallization of a-Si forms with a high impurity content is retarded or even inhibited. The a-Si used here straddles both regimes and thus, although full crystallization occurs, the more complex crystalline structures fail to nucleate.

This article reviews the size-dependent structural properties of ion beam synthesized Co nanoparticles (NPs) and the influence of ion irradiation on the size, shape, phase and structure. The evolution of the aforementioned properties were determined using complementary laboratory- and advanced synchrotron-based techniques, including cross-sectional transmission electron microscopy, small-angle X-ray scattering and X-ray absorption spectroscopy. Combining such techniques reveals a rich array of transformations particular to Co NPs. This methodology highlights the effectiveness of ion implantation and ion irradiation procedures as a means of fine tuning NP properties to best suit specific technological applications. Furthermore, our results facilitate a better understanding and aid in identifying the underlying physics particular to this potentially technologically important class of nanomaterials.

Crystalline copper nanoparticles (NPs) were formed in silica by multi-energy MeV ion implantations and then transformed to amorphous NPs by irradiation with 5 MeV Sn3+ ions. Optical absorption spectra of both the phases were evaluated in the ultra-violet to near-infrared regions. Compared with corresponding crystalline NPs of the same mean diameter, the amorphous NPs showed a low-energy shift of the surface plasmon resonance around 2.2 eV and less prominent absorption structure around 4 eV. These differences are explained by a strongly reduced electron mean-free-path in the amorphous NPs due to the loss of lattice periodicity.

Face-centered cubic Ni nanoparticles were formed in SiO2 by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of states.

Face-centered cubic Ni nanoparticles were formed in SiO2 by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of states.

We report on the effects of swift heavy ion irradiation of embedded Pt nanocrystals (NCs), which change from spheres to prolate spheroids to rods upon irradiation. Using a broad range of ion irradiation energies and NC mean sizes we demonstrate that the elongation and dissolution processes are energy and size dependent, attaining comparable levels of shape transformation and dissolution upon a given energy density deposited in the matrix. The NC shape transformation remains operative despite discontinuous ion tracks in the matrix and exhibits a constant threshold size for elongation. In contrast, for ion irradiations in which the ion tracks are continuous, the threshold size for elongation is clearly energy dependent.

The structural properties and H desorption from embedded Pt nanocrystals (NCs) following irradiation with swift heavy ions were investigated as a function of energy and fluence. From x-ray absorption near-edge spectroscopy analysis, Pt–H bonding was identified in NCs annealed in a forming gas (95% N₂ + 5% H₂) ambient. The H content decreased upon irradiation and the desorption process was NC-size dependent such that larger NCs required a higher fluence to achieve a H-free state. Pt–H bonding and NC dissolution both perturbed the NC structural parameters (coordination number, bond-length and mean-square relative displacement) as determined with extended x-ray absorption fine structure measurements.

The shape and structural evolution of Co nanoparticles embedded in SiO2 and subjected to swift heavy-ion irradiation have been investigated over a wide energy and fluence range. Modifications of the nanoparticle size and shape were characterized with transmission electron microscopy and small-angle x-ray scattering. Nanoparticles below a threshold diameter remained spherical in shape and progressively decreased in size under irradiation due to dissolution. Nanoparticles above the threshold diameter transformed into nanorods with their major dimension parallel to the incident ion direction. Modifications of the atomic-scale structure of the Co nanoparticles were identified with x-ray absorption spectroscopy. Analysis of the x-ray absorption near-edge spectra showed that prior to irradiation all Co atoms were in a metallic state, while after irradiation Co atoms were in both oxidized and metallic environments, the former consistent with dissolution. The evolution of the nanoparticle short-range order was determined from extended x-ray absorption fine structure spectroscopy. Structural changes in the Co nanoparticles as a function of ion fluence included an increase in disorder and asymmetric deviation from a Gaussian interatomic distance distribution coupled with a decrease in bondlength. Such changes resulted from the irradiation-induced decrease in nanoparticle size and subsequent dissolution.

We report on the structural and vibrational properties of Co nanoparticles formed by ion implantation and thermal annealing in amorphous silica. The evolution of the nanoparticle size, phase, and structural parameters were determined as a function of the formation conditions using transmission electron microscopy, small-angle x-ray scattering, and x-ray absorption spectroscopy. The implantation fluence and annealing temperature governed the spherical nanoparticle size and phase. To determine the latter, x-ray absorption near-edge structure analysis was used to quantify the hexagonal close packed, face-centered cubic and oxide fractions. The structural properties were characterized by extended x-ray absorption fine structure spectroscopy (EXAFS) and finite-size effects were readily apparent. With a decrease in nanoparticle size, an increase in structural disorder and a decrease in both coordination number and bondlength were observed as consistent with the non-negligible surface-area-to-volume ratio characteristic of nanoparticles. The surface tension of Co nanoparticles calculated using a liquid drop model was more than twice that of bulk material. The size-dependent vibrational properties were probed with temperature-dependent EXAFS measurements. Using a correlated anharmonic Einstein model and thermodynamic perturbation theory, Einstein temperatures for both nanoparticles and bulk material were determined. Compared to bulk Co, the mean vibrational frequency of the smallest nanoparticles was reduced as attributed to a greater influence of loosely bonded, undercoordinated surface atoms relative to the effect of capillary pressure generated by surface curvature.

The transformation of Au nanoparticles (NPs) embedded in SiO2 from spherical to rod-like shapes induced by swift heavy ion irradiation has been studied. Irradiation was performed with A197u ions at energies between 54 and 185 MeV. Transmission electron microscopy and small angle x-ray scattering measurements reveal an energy dependent saturation width of the NP rods as well as a minimum size required for the NPs to elongate. The NP saturation width is correlated with the ion track diameter in the SiO2. NP melting and in-plane strain in the irradiated SiO2 are discussed as potential mechanisms for the observed deformation.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

The growth and structure of Pt nanocrystals (NCs) formed by ion implantation in a-SiO2 has been investigated as a function of the annealing conditions. Transmission electron microscopy and small-angle x-ray scattering measurements demonstrate that the annealing ambient has a significant influence on NC size. Samples annealed in either Ar, O2, or forming gas (95% N2: 5% H2) at temperatures ranging from 500 °C–1300 °C form spherical NCs with mean diameters ranging from 1–14 nm. For a given temperature, annealing in Ar yields the smallest NCs. O2 and forming gas ambients produce NCs of comparable size though the latter induces H chemisorption at 1100 °C and above, as verified with x-ray absorption spectroscopy. This H intake is accompanied by a bond-length expansion and increased structural disorder in NCs of diameter >3 nm.