55 results found
Kapuria N, Conroy M, Lebedev VA, et al., 2022, Subsuming the Metal Seed to Transform Binary Metal Chalcogenide Nanocrystals into Multinary Compositions, ACS NANO, Vol: 16, Pages: 8917-8927, ISSN: 1936-0851
Moore K, O'Connell EN, Griffin SM, et al., 2022, Charged domain wall and polar vortex topologies in a room-temperature magnetoelectric multiferroic thin film, ACS Applied Materials and Interfaces, Vol: 14, Pages: 5525-5536, ISSN: 1944-8244
Multiferroic topologies are an emerging solution for future low-power magnetic nanoelectronics due to their combined tuneable functionality and mobility. Here, we show that in addition to being magnetoelectric multiferroic at room temperature, thin-film Aurivillius phase Bi6TixFeyMnzO18 is an ideal material platform for both domain wall and vortex topology-based nanoelectronic devices. Utilizing atomic-resolution electron microscopy, we reveal the presence and structure of 180°-type charged head-to-head and tail-to-tail domain walls passing throughout the thin film. Theoretical calculations confirm the subunit cell cation site preference and charged domain wall energetics for Bi6TixFeyMnzO18. Finally, we show that polar vortex-type topologies also form at out-of-phase boundaries of stacking faults when internal strain and electrostatic energy gradients are altered. This study could pave the way for controlled polar vortex topology formation via strain engineering in other multiferroic thin films. Moreover, these results confirm that the subunit cell topological features play an important role in controlling the charge and spin state of Aurivillius phase films and other multiferroic heterostructures.
Garcia-Gil A, Biswas S, Roy A, et al., 2022, Growth and analysis of the tetragonal (ST12) germanium nanowires, NANOSCALE, Vol: 14, Pages: 2030-2040, ISSN: 2040-3364
OConnell E, Moore K, McFall E, et al., 2021, TopoTEM: a python package for quantifying and visualising scanning transmission electron microscopy data of polar topologies, Publisher: arXiv
The exotic internal structure of polar topologies in multi-ferroic materialsoffers a rich landscape for materials science research. As the spatial scale ofthese entities are often sub-atomic in nature, aberration correctedtransmission electron microscopy (TEM) is the ideal characterisation technique.Software to quantify and visualise the slight shifts in atomic placement withinunit cells is of paramount importance due to the now routine acquisition ofimages at such resolution. In the previous ~decade since the commercialisationof aberration corrected TEM, many research groups have written their own codeto visualise these polar entities. More recently, open access Python packageshave been developed for the purpose of TEM atomic position quantification.Building on these packages, we introduce the TEMUL Toolkit: a Python packagefor analysis and visualisation of atomic resolution images. Here, we focusspecifically on the TopoTEM module of the toolkit where we show an easy tofollow, streamlined version of calculating the atomic displacements relative tothe surrounding lattice and thus polarisation plotting. We hope this toolkitwill benefit the rapidly expanding field of topology based nano-electronic andquantum materials research, and we invite the electron microscopy community tocontribute to this open access project.
Conroy M, Jennifer C, Ursel B, 2021, High Resolution Analytical Electron Microscopy of Ceramics and Glasses, Encyclopedia of Materials: Technical Ceramics and Glasses
Guy JGM, Cochard C, Aguado-Puente P, et al., 2021, Anomalous motion of charged domain walls and associated negative capacitance in copper-chlorine boracite., Advanced Materials, Vol: 33, Pages: 1-10, ISSN: 0935-9648
During switching, the microstructure of a ferroelectric normally adapts to align internal dipoles with external electric fields. Favorably oriented dipolar regions (domains) grow at the expense of those in unfavorable orientations and this is manifested in a predictable field-induced motion of the walls that separate one domain from the next. Here, the discovery that specific charged 90°domain walls in copper-chlorine boracite move in the opposite direction to that expected, increasing the size of the domain in which polarization is anti-aligned with the applied field, is reported. Polarization-field (P-E) hysteresis loops, inferred from optical imaging, show negative gradients and non-transient negative capacitance, throughout the P-E cycle. Switching currents (generated by the relative motion between domain walls and sensing electrodes) confirm this, insofar as their signs are opposite to those expected conventionally. For any given bias, the integrated switching charge due to this anomalous wall motion is directly proportional to time, indicating that the magnitude of the negative capacitance component should be inversely related to frequency. This passes Jonscher's test for the misinterpretation of positive inductance and gives confidence that field-induced motion of these specific charged domain walls generates a measurable negative capacitance contribution to the overall dielectric response.
Kilian S, McCarthy K, Stokes K, et al., 2021, Direct Growth of Si, Ge, and Si-Ge Heterostructure Nanowires Using Electroplated Zn: An Inexpensive Seeding Technique for Li-Ion Alloying Anodes., Small, Vol: 17
A scalable and cost-effective process is used to electroplate metallic Zn seeds on stainless steel substrates. Si and Ge nanowires (NWs) are subsequently grown by placing the electroplated substrates in the solution phase of a refluxing organic solvent at temperatures >430 °C and injecting the respective liquid precursors. The native oxide layer formed on reactive metals such as Zn can obstruct NW growth and is removed in situ by injecting the reducing agent LiBH4 . The findings show that the use of Zn as a catalyst produces defect-rich Si NWs that can be extended to the synthesis of Si-Ge axial heterostructure NWs with an atomically abrupt Si-Ge interface. As an anode material, the as grown Zn seeded Si NWs yield an initial discharge capacity of 1772 mAh g-1 and a high capacity retention of 85% after 100 cycles with the active participation of both Si and Zn during cycling. Notably, the Zn seeds actively participate in the Li-cycling activities by incorporating into the Si NWs body via a Li-assisted welding process, resulting in restructuring the NWs into a highly porous network structure that maintains a stable cycling performance.
Biswas S, Doherty J, Galluccio E, et al., 2021, Stretching the Equilibrium Limit of Sn in Ge<inf>1- x</inf>Sn<inf>x</inf>Nanowires: Implications for Field Effect Transistors, ACS Applied Nano Materials, Vol: 4, Pages: 1048-1056
Ge1-xSnx nanowires incorporating a large amount of Sn would be useful for mobility enhancement in nanoelectronic devices, a definitive transition to a direct bandgap for application in optoelectronic devices and to increase the efficiency of the GeSn-based photonic devices. Here we report the catalytic bottom-up fabrication of Ge1-xSnx nanowires with very high Sn incorporation (x > 0.3). These nanowires are grown in supercritical toluene under high pressure (21 MPa). The introduction of high pressure in the vapor-liquid-solid (VLS) like growth regime resulted in a substantial increase of Sn incorporation in the nanowires, with a Sn content ranging between 10 and 35 atom %. The incorporation of Sn in the nanowires was found to be inversely related to nanowire diameter; a high Sn content of 35 atom % was achieved in very thin Ge1-xSnx nanowires with diameters close to 20 nm. Sn was found to be homogeneously distributed throughout the body of the nanowires, without apparent clustering or segregation. The large inclusion of Sn in the nanowires could be attributed to the nanowire growth kinetics and small nanowire diameters, resulting in increased solubility of Sn in Ge at the metastable liquid-solid interface under high pressure. Electrical investigation of the Ge1-xSnx (x = 0.10) nanowires synthesized by the supercritical fluid approach revealed their potential in nanoelectronics and sensor-based applications.
Moore K, Bangert U, Conroy M, 2021, Aberration corrected STEM techniques to investigate polarization in ferroelectric domain walls and vortices, APL MATERIALS, Vol: 9, ISSN: 2166-532X
Hadjimichael M, Li Y, Zatterin E, et al., 2021, Metal-ferroelectric supercrystals with periodically curved metallic layers, NATURE MATERIALS, Vol: 20, Pages: 495-+, ISSN: 1476-1122
Subedi RC, Min JW, Mitra S, et al., 2021, Highly efficient transverse-electric-dominant ultraviolet-C emitters employing GaN multiple quantum disks in AlN nanowire matrix, ISSN: 0277-786X
Heavy reliance on extensively studied AlGaN based light emitting diodes (LEDs) to replace environmentally hazardous mercury based ultraviolet (UV) lamps is inevitable. However, external quantum efficiency (EQE) for AlGaN based deep UV emitters remains poor. Dislocation induced nonradiative recombination centers and poor electron-hole wavefunction overlap due to the large polarization field induced quantum confined stark effect (QCSE) in "Al"rich AlGaN are some of the key factors responsible for poor EQE. In addition, the transverse electric polarized light is extremely suppressed in "Al"-rich AlGaN quantum wells (QWs) because of the undesired crossing over among the light hole (LH), heavy hole (HH) and crystal-field split-off (SH) states. Here, optical and structural integrities of dislocation-free ultrathin GaN quantum disk (QDisk) (∼ 1.2 nm) embedded in AlN barrier (∼ 3 nm) grown employing plasma-assisted molecular beam epitaxy (PAMBE) are investigated considering it as a novel nanostructure to realize highly efficient TE polarized deep UV emitters. The structural and chemical integrities of thus grown QDisks are investigated by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We, particularly, emphasize the polarization dependent photoluminescence (PL) study of the GaN Disks to accomplish almost purely TE polarized UV (∼ 260 nm) light. In addition, we observed significantly high internal quantum efficiency (IQE) of ∼ 80 %, which is attributed to the enhanced overlap of the electron-hole wavefunction in extremely quantum confined ultrathin GaN QDisks, thereby presenting GaN QDisks embedded in AlN nanowires as a practical pathway towards the efficient deep UV emitters.
Galluccio E, Mirabelli G, Harvey A, et al., 2021, Cell formation in stanogermanides using pulsed laser thermal anneal on Ge<inf>0.91</inf>Sn<inf>0.09</inf>, Materials Science in Semiconductor Processing, Vol: 121, ISSN: 1369-8001
Pulsed laser thermal annealing (LTA) has been thoroughly investigated for the formation of low-resistance stanogermanide contacts on Ge0.91Sn0.09 substrates. Three different metals (Ni, Pt, and Ti) were characterized using a wide laser energy density range (100–500 mJ/cm2). Electrical performance, surface quality, cross-sectional crystallographic, and elemental analysis have been systematically examined in order to identify the ideal process window. Electrical characterization showed that the samples processed by LTA had lower resistance variability compared with the rapid thermal anneal (RTA) counterpart. Among the three metals used, Ni and Pt were the most promising candidates for future sub-nm applications based on the low resistance values. The stanogermanide alloys suffered a high degeneration as the LTA thermal budget increased. Cross-sectional elemental analysis showed a highly unusual Sn segregation effect, particularly for high LTA energy densities, where vertical columns of Sn-rich alloy were formed, also known as cell formation, similar to that seen for Sb hyperdoping of Si when using LTA. This effect is linked to solid solubility and distribution coefficient of Sn in Ge, as well as the velocity of the liquid-solid interface during crystallization as the samples cool.
Moore K, Conroy M, O'Connell EN, et al., 2020, Highly charged 180 degree head-to-head domain walls in lead titanate, COMMUNICATIONS PHYSICS, Vol: 3, ISSN: 2399-3650
Subedi RC, Min J-W, Mitra S, et al., 2020, Quantifying the Transverse-Electric-Dominant 260 nm Emission from Molecular Beam Epitaxy-Grown GaN-Quantum-Disks Embedded in AlN Nanowires: A Comprehensive Optical and Morphological Characterization., ACS Appl Mater Interfaces, Vol: 12, Pages: 41649-41658
There has been a relentless pursuit of transverse electric (TE)-dominant deep ultraviolet (UV) optoelectronic devices for efficient surface emitters to replace the environmentally unfriendly mercury lamps. To date, the use of the ternary AlGaN alloy inevitably has led to transverse magnetic (TM)-dominant emission, an approach that is facing a roadblock. Here, we take an entirely different approach of utilizing a binary GaN compound semiconductor in conjunction with ultrathin quantum disks (QDisks) embedded in AlN nanowires (NWs). The growth of GaN QDisks is realized on a scalable and low-cost Si substrate using plasma-assisted molecular beam epitaxy as a highly controllable monolayer growth platform. We estimated an internal quantum efficiency of ∼81% in a wavelength regime of ∼260 nm for these nanostructures. Additionally, strain mapping obtained by high-angle annular dark-field scanning transmission electron microscopy is studied in conjunction with the TE and TM modes of the carrier recombination. Moreover, for the first time, we quantify the TE and TM modes of the PL emitted by GaN QDisks for deep-UV emitters. We observed nearly pure TE-polarized photoluminescence emission at a polarization angle of ∼5°. This work proposes highly quantum-confined ultrathin GaN QDisks as a promising candidate for deep-UV vertical emitters.
Kessler SH, Lach TG, Garrett KE, et al., 2020, Direct observations of Pd–Te compound formation within noble metal inclusions in spent nuclear fuel, Journal of Nuclear Materials, Vol: 538, ISSN: 0022-3115
Although the existence of a five-metal (Mo-Tc-Ru-Rh-Pd) phase – as nanoparticles observed in irradiated nuclear fuel – has been known for more than half a century, the chemical and physical mechanisms controlling the formation and behavior of such particles remain stubbornly elusive. We present in this work new evidence for the presence of a separate nonmetallic phase associated with the metallic particles and containing a significant fraction of Te in addition to Pd. While this new phase potentially complicates the thermodynamic picture of a mixed alloy in equilibrium with the surrounding fuel environment, it also provides new clues in the search for a chemical mechanism for Pd migration through the uranium dioxide matrix and the nucleation behavior of the particles. Fractionation between phases may subsequently affect the mechanical performance of fuels during irradiation and their interactions with the surrounding environment during long-term waste storage.
A domain wall-enabled memristor is created, in thin film lithium niobate capacitors, which shows up to twelve orders of magnitude variation in resistance. Such dramatic changes are caused by the injection of strongly inclined conducting ferroelectric domain walls, which provide conduits for current flow between electrodes. Varying the magnitude of the applied electric-field pulse, used to induce switching, alters the extent to which polarization reversal occurs; this systematically changes the density of the injected conducting domain walls in the ferroelectric layer and hence the resistivity of the capacitor structure as a whole. Hundreds of distinct conductance states can be produced, with current maxima achieved around the coercive voltage, where domain wall density is greatest, and minima associated with the almost fully switched ferroelectric (few domain walls). Significantly, this “domain wall memristor” demonstrates a plasticity effect: when a succession of voltage pulses of constant magnitude is applied, the resistance changes. Resistance plasticity opens the way for the domain wall memristor to be considered for artificial synapse applications in neuromorphic circuits.
Courtney E, Conroy M, Bangert U, 2020, Metal configurations on 2D materials investigated via atomic resolution HAADF stem, JOURNAL OF MICROSCOPY, Vol: 279, Pages: 274-281, ISSN: 0022-2720
Rodríguez L, Del Corro E, Conroy M, et al., 2020, Self-Pixelation through Fracture in VO<inf>2</inf>Thin Films, ACS Applied Electronic Materials, Vol: 2, Pages: 1433-1439
Vanadium dioxide (VO2) is an archetypal Mott material with a metal-insulator transition (MIT) at near room temperature. In thin films, this transition is affected by substrate-induced strain but as film thickness increases, the strain is gradually relaxed and the bulk properties are recovered. Epitaxial films of VO2 on (001)-oriented rutile titanium dioxide (TiO2) relax substrate strain by forming a network of fracture lines that crisscross the film along well-defined crystallographic directions. This work shows that the electronic properties associated with these lines result in a pattern that resembles a "street map"of fully strained metallic VO2 blocks separated by insulating VO2 stripes. Each block of VO2 is thus electronically self-insulated from its neighbors, and its MIT can be locally induced optically with a laser, or electronically via the tip of a scanning probe microscope so that the films behave functionally as self-patterned pixel arrays.
Doherty J, McNulty D, Biswas S, et al., 2020, Germanium tin alloy nanowires as anode materials for high performance Li-ion batteries., Nanotechnology, Vol: 31
The combination of two active Li-ion materials (Ge and Sn) can result in improved conduction paths and higher capacity retention. Here we report for the first time, the implementation of Ge1-x Sn x alloy nanowires as anode materials for Li-ion batteries. Ge1-x Sn x alloy nanowires have been successfully grown via vapor-liquid-solid technique directly on stainless steel current collectors. Ge1-x Sn x (x = 0.048) nanowires were predominantly seeded from the Au0.80Ag0.20 catalysts with negligible amount of growth was also directly catalyzed from stainless steel substrate. The electrochemical performance of the the Ge1-x Sn x nanowires as an anode material for Li-ion batteries was investigated via galvanostatic cycling and detailed analysis of differential capacity plots (DCPs). The nanowire electrodes demonstrated an exceptional capacity retention of 93.4% from the 2nd to the 100th charge at a C/5 rate, while maintaining a specific capacity value of ∼921 mAh g-1 after 100 cycles. Voltage profiles and DCPs revealed that the Ge1-x Sn x nanowires behave as an alloying mode anode material, as reduction/oxidation peaks for both Ge and Sn were observed, however it is clear that the reversible lithiation of Ge is responsible for the majority of the charge stored.
Clark RA, Conroy MA, Lach TG, et al., 2020, Distribution of metallic fission-product particles in the cladding liner of spent nuclear fuel (vol 17, pg 861, 2020), NPJ MATERIALS DEGRADATION, Vol: 4
Moore K, Conroy M, Bangert U, 2020, Rapid polarization mapping in ferroelectrics using Fourier masking, JOURNAL OF MICROSCOPY, Vol: 279, Pages: 222-228, ISSN: 0022-2720
Prabaswara A, Kim H, Min J-W, et al., 2020, Titanium Carbide MXene Nucleation Layer for Epitaxial Growth of High-Quality GaN Nanowires on Amorphous Substrates, ACS NANO, Vol: 14, Pages: 2202-2211, ISSN: 1936-0851
Clark RA, Conroy MA, Lach TG, et al., 2020, Distribution of metallic fission-product particles in the cladding liner of spent nuclear fuel, NPJ MATERIALS DEGRADATION, Vol: 4
Conroy M, Moore K, O'Connell E, et al., 2020, Probing the Dynamics of Topologically Protected Charged Ferroelectric Domain Walls with the Electron Beam at the Atomic Scale, ISSN: 1431-9276
Moore K, Keeney L, Downing C, et al., 2020, Charge Carriers in Dynamic Ferroelectric Domain Walls, ISSN: 1431-9276
Schwantes JM, Conroy M, Lach TG, et al., 2019, Changing the rules of the game: used fuel studies outside of a remote handling facility, Journal of Radioanalytical and Nuclear Chemistry, Vol: 322, Pages: 1267-1272, ISSN: 0236-5731
Pacific Northwest National Laboratory (PNNL) has leveraged focused ion beam capability at their Category II Nuclear Facility to facilitate nuclear materials analysis and experimentation at the micron scale. For this particular study, micron-size specimens of un-irradiated UO2 fuel pellets of various enrichments were prepared and irradiated to a burnup equivalent of 8–3700 MWd/MTU. This represents first of its kind study of used fuel investigations outside of a hot cell facility, dramatically minimizing resource requirements through reduction in scale. Results of this study provide insight into the initial production of noble metal phase particles in used nuclear fuel at extremely low burnup levels.
Lu H, Tan Y, McConville JPV, et al., 2019, Electrical Tunability of Domain Wall Conductivity in LiNbO3 Thin Films., Adv Mater, Vol: 31
Domain wall nanoelectronics is a rapidly evolving field, which explores the diverse electronic properties of the ferroelectric domain walls for application in low-dimensional electronic systems. One of the most prominent features of the ferroelectric domain walls is their electrical conductivity. Here, using a combination of scanning probe and scanning transmission electron microscopy, the mechanism of the tunable conducting behavior of the domain walls in the sub-micrometer thick films of the technologically important ferroelectric LiNbO3 is explored. It is found that the electric bias generates stable domains with strongly inclined domain boundaries with the inclination angle reaching 20° with respect to the polar axis. The head-to-head domain boundaries exhibit high conductance, which can be modulated by application of the sub-coercive voltage. Electron microscopy visualization of the electrically written domains and piezoresponse force microscopy imaging of the very same domains reveals that the gradual and reversible transition between the conducting and insulating states of the domain walls results from the electrically induced wall bending near the sample surface. The observed modulation of the wall conductance is corroborated by the phase-field modeling. The results open a possibility for exploiting the conducting domain walls as the electrically controllable functional elements in the multilevel logic nanoelectronics devices.
Devanathan R, Jiang W, Kruska K, et al., 2019, Hexagonal close-packed high-entropy alloy formation under extreme processing conditions, Journal of Materials Research, Vol: 34, Pages: 709-719, ISSN: 0884-2914
We assess the validity of criteria based on size mismatch and thermodynamics in predicting the stability of the rare class of high-entropy alloys (HEAs) that form in the hexagonal close-packed crystal structure. We focus on nanocrystalline HEA particles composed predominantly of Mo, Tc, Ru, Rh, and Pd along with Ag, Cd, and Te, which are produced in uranium dioxide fuel under the extreme conditions of nuclear reactor operation. The constituent elements are fission products that aggregate under the combined effects of irradiation and elevated temperature as high as 1200 °C. We present the recent results on alloy nanoparticle formation in irradiated ceria, which was selected as a surrogate for uranium dioxide, to show that radiation-enhanced diffusion plays an important role in the process. This work sheds light on the initial stages of alloy nanoparticle formation from a uniform dispersion of individual metals. The remarkable chemical durability of such multiple principal element alloys presents a solution, namely, an alloy waste form, to the challenge of immobilizing Tc.
Jiang W, Conroy MA, Kruska K, et al., 2019, In Situ Study of Particle Precipitation in Metal-Doped CeO<inf>2</inf> during Thermal Treatment and Ion Irradiation for Emulation of Irradiating Fuels, Journal of Physical Chemistry C, Vol: 123, Pages: 2591-2601, ISSN: 1932-7447
Metallic particles formed in oxide fuels (e.g., UO2) during neutron irradiation have an adverse impact on fuel performance. A fundamental investigation of particle precipitation is needed to predict the fuel performance and potentially improve fuel designs and operations. This study reports on the precipitation of Mo-dominant β-phase particles in polycrystalline CeO2 (surrogate for UO2) films doped with Mo, Pd, Rh, Ru, and Re (surrogate for Tc). In situ heating scanning transmission electron microscopy indicates that particle precipitation starts at ∼1073 K with a limited particle growth to ∼10 nm. While particle concentration increases with increasing temperature, particle size remains largely unchanged up to 1273 K. There is a dramatic change in the microstructure following vacuum annealing at 1373 K, probably due to phase transition of reduced cerium oxide. At the high temperature, particles grow up to 75 nm or larger with distinctive facets. The particles are predominantly composed of Mo with a body-centered cubic structure (β phase). An oxide layer was observed after storage at ambient conditions. In situ heating X-ray photoelectron spectroscopy reveals an increasing reduction of Ce charge state from 4+ to 3+ in the doped CeO2 film at temperatures from 673 to 1273 K. In situ ion irradiation transmission electron microscopy with 2 MeV Al2+ ions up to a dose of ∼20 displacements per atom at nominally room temperature does not lead to precipitation of visible particles. However, irradiation with 1.7 MeV Au3+ ions to ∼10 dpa at 973 K produces ∼2 nm sized pure Pd particles; Au3+ irradiation at 1173 K appears to result in precipitates of ∼6 nm in size. Some of the defects produced by ion irradiation could be nucleation sites for precipitation, leading to generation of smaller particles with a higher concentration. ©
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