Imperial College London

Professor David W. McComb

Faculty of EngineeringDepartment of Materials

Adjunct Professor
 
 
 
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Contact

 

+44 (0)20 7594 6750d.mccomb Website

 
 
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Location

 

Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

293 results found

Klosowski M, Carzaniga R, Shefelbine S, Porter A, McComb Det al., 2018, Nanoanalytical electron microscopy of events predisposing to mineralisation of turkey tendon, Scientific Reports, Vol: 8, ISSN: 2045-2322

The macro- and micro-structures of mineralised tissues hierarchy are well described and understood. However, investigation of their nanostructure is limited due to the intrinsic complexity of biological systems. Preceding transmission electron microscopy studies investigating mineralising tissues have not resolved fully the initial stages of mineral nucleation and growth within the collagen fibrils. In this study, analytical scanning transmission electron microscopy and electron energy-loss spectroscopy were employed to characterise the morphology, crystallinity and chemistry of the mineral at different stages of mineralization using a turkey tendon model. In the poorly mineralised regions, calcium ions associated with the collagen fibrils and ellipsoidal granules and larger clusters composed of amorphous calcium phosphate were detected. In the fully mineralised regions, the mineral had transformed into crystalline apatite with a plate-like morphology. A change in the nitrogen K-edge was observed and related to modifications of the functional groups associated with the mineralisation process. This transformation seen in the nitrogen K-edge might be an important step in maturation and mineralisation of collagen and lend fundamental insight into how tendon mineralises.

Journal article

Amamou W, Pinchuk IV, Trout AH, Williams REA, Antolin N, Goad A, O'Hara DJ, Ahmed AS, Windl W, McComb DW, Kawakami RKet al., 2018, Magnetic proximity effect in Pt/CoFe2 O4 bilayers, Physical Review Materials, Vol: 2

© 2018 American Physical Society. We observe the magnetic proximity effect (MPE) in Pt/CoFe2O4 bilayers grown by molecular beam epitaxy. This is revealed through angle-dependent magnetoresistance measurements at 5 K, which isolate the contributions of induced ferromagnetism (i.e., anisotropic magnetoresistance) and the spin Hall effect (i.e., the spin Hall magnetoresistance) in the Pt layer. The strong evidence for induced ferromagnetism in Pt via the anisotropic magnetoresistance is supported further by density functional theory calculations and various control measurements including the insertion of a Cu spacer layer to suppress the induced ferromagnetism. In addition, anomalous Hall effect measurements show an out-of-plane magnetic hysteresis loop of the induced ferromagnetic phase with larger coercivity and larger remanence than the bulk CoFe2O4. By demonstrating the MPE in Pt/CoFe2O4, these results establish the spinel ferrite family as a promising material for the MPE and spin manipulation via proximity exchange fields.

Journal article

Marsh JM, Mamak M, Wireko F, Lebron A, Cambron T, Huber D, Boona I, Williams REA, McComb DWet al., 2018, Multimodal evidence of mesostructured calcium fatty acid deposits in human hair and their role on hair properties, ACS Applied Bio Materials, Vol: 1, Pages: 1174-1183

© 2018 American Chemical Society. We provide the first conclusive evidence for the presence of exogenous calcium fatty acid deposits, which not only form in-between the cuticle layers in the lipid-rich cell membrane complex, but also grow to dimensions large enough to cause the structure to bulge, thereby impacting the optical and mechanical properties of the hair fiber. The composition and phase of these deposits were probed using a multimodal analytical approach with spatially resolved techniques including synchrotron micro X-ray fluorescence coupled with X-ray scattering, focused ion beam (FIB)- scanning electron microscopy (SEM), scanning transmission electron microscopy, X-ray energy dispersive spectroscopy, and Fourier transform infrared and Raman imaging where the collective analysis is consistent with a meso-phase composed of calcium C16/C18 saturated fatty acids from natural sources such as sebum. X-ray microtomography and serial "slice and view" FIB/SEM both reveal the location and volumetric shape of the deposits.

Journal article

Chen Q, Svoboda C, Zheng Q, Sales BC, Mandrus DG, Zhou HD, Zhou JS, McComb D, Randeria M, Trivedi N, Yan JQet al., 2017, Magnetism out of antisite disorder in the J=0 compound Ba2YIrO6, Physical Review B, Vol: 96, ISSN: 2469-9950

© 2017 American Physical Society. We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this 5d4 compound. The experimental investigation involved comparison of the magnetic properties and atomic imaging of (1) a slow-cooled crystal, (2) a crystal quenched from 900C after growth, and (3) a crystal grown using a faster cooling rate during growth than the slow-cooled one. Atomic-scale imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900C introduces Ir-rich antiphase boundaries in the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. Compared to the slow-cooled crystals, Ba2YIrO6 crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment, while quenched crystals with Ir-rich antiphase boundary show a slightly suppressed moment. Our DFT and model magnetic Hamiltonian calculations suggest magnetic condensation is unlikely, as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local nonmagnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased orbital overlap and (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in Ba2YIrO6 and other J=0 systems.

Journal article

Alexander JA, Scheltens FJ, Drummy LF, Durstock MF, Hage FS, Ramasse QM, McComb DWet al., 2017, High-resolution monochromated electron energy-loss spectroscopy of organic photovoltaic materials., Ultramicroscopy, Vol: 180, Pages: 125-132

Advances in electron monochromator technology are providing opportunities for high energy resolution (10 - 200meV) electron energy-loss spectroscopy (EELS) to be performed in the scanning transmission electron microscope (STEM). The energy-loss near-edge structure in core-loss spectroscopy is often limited by core-hole lifetimes rather than the energy spread of the incident illumination. However, in the valence-loss region, the reduced width of the zero loss peak makes it possible to resolve clearly and unambiguously spectral features at very low energy-losses (<3eV). In this contribution, high-resolution EELS was used to investigate four materials commonly used in organic photovoltaics (OPVs): poly(3-hexlythiophene) (P3HT), [6,6] phenyl-C61 butyric acid methyl ester (PCBM), copper phthalocyanine (CuPc), and fullerene (C60). Data was collected on two different monochromated instruments - a Nion UltraSTEM 100 MC 'HERMES' and a FEI Titan3 60-300 Image-Corrected S/TEM - using energy resolutions (as defined by the zero loss peak full-width at half-maximum) of 35meV and 175meV, respectively. The data was acquired to allow deconvolution of plural scattering, and Kramers-Kronig analysis was utilized to extract the complex dielectric functions. The real and imaginary parts of the complex dielectric functions obtained from the two instruments were compared to evaluate if the enhanced resolution in the Nion provides new opto-electronic information for these organic materials. The differences between the spectra are discussed, and the implications for STEM-EELS studies of advanced materials are considered.

Journal article

Lee AJ, Brangham JT, Cheng Y, White SP, Ruane WT, Esser BD, McComb DW, Hammel PC, Yang Fet al., 2017, Metallic ferromagnetic films with magnetic damping under 1.4 × 10-3., Nat Commun, Vol: 8

Low-damping magnetic materials have been widely used in microwave and spintronic applications because of their low energy loss and high sensitivity. While the Gilbert damping constant can reach 10-4 to 10-5 in some insulating ferromagnets, metallic ferromagnets generally have larger damping due to magnon scattering by conduction electrons. Meanwhile, low-damping metallic ferromagnets are desired for charge-based spintronic devices. Here, we report the growth of Co25Fe75 epitaxial films with excellent crystalline quality evident by the clear Laue oscillations and exceptionally narrow rocking curve in the X-ray diffraction scans as well as from scanning transmission electron microscopy. Remarkably, the Co25Fe75 epitaxial films exhibit a damping constant <1.4 × 10-3, which is comparable to the values for some high-quality Y3Fe5O12 films. This record low damping for metallic ferromagnets offers new opportunities for charge-based applications such as spin-transfer-torque-induced switching and magnetic oscillations.Owing to their conductivity, low-damping metallic ferromagnets are preferred to insulating ferromagnets in charge-based spintronic devices, but are not yet well developed. Here the authors achieve low magnetic damping in CoFe epitaxial films which is comparable to conventional insulating ferromagnetic YIG films.

Journal article

Miller DR, Williams RE, Akbar SA, Morris PA, McComb DWet al., 2017, Measuring optical properties of individual SnO<inf>2</inf> nanowires via valence electron energy-loss spectroscopy, Journal of Materials Research, Vol: 32, Pages: 2479-2486, ISSN: 0884-2914

© Materials Research Society 2017. For the first time, valence electron energy-loss spectroscopy (VEELS) was applied to individual single-crystalline SnO2 nanowires to investigate the dielectric function, band gap, and optical absorption coefficient. The results are compared with data from optical techniques such as spectroscopic ellipsometry and UV-Vis, and theoretical calculations from variations of density functional theory. The data obtained agree well with the standard optical and theoretical techniques. The dielectric function and optical absorption coefficient are given up to 20 eV, which otherwise requires a synchrotron source and large single crystals via optical methods. The energy loss function is given up to 40 eV, which gives a useful comparison to previous theoretical studies in an energy range that cannot be achieved via optical measurements. The comparison gives confidence in the accuracy of this method for exploring spatially-resolved measurements in individual nanoparticles or more complex nanostructures that are otherwise difficult to measure accurately using optical techniques.

Journal article

Alexander JA, Scheltens FJ, Drummy LF, Durstock MF, Gilchrist JB, Heutz S, McComb DWet al., 2017, Measuring Optical Absorption in Organic Photovoltaics Using Monochromated Electron Energy-Loss Spectroscopy, IEEE 44th Photovoltaic Specialist Conference (PVSC), Publisher: IEEE, Pages: 966-969, ISSN: 0160-8371

Conference paper

Ahmed AS, Esser BD, Rowland J, McComb DW, Kawakami RKet al., 2017, Molecular beam epitaxy growth of [CrGe/MnGe/FeGe] superlattices: Toward artificial B20 skyrmion materials with tunable interactions, Journal of Crystal Growth, Vol: 467, Pages: 38-46, ISSN: 0022-0248

© 2017 Elsevier B.V. Skyrmions are localized magnetic spin textures whose stability has been shown theoretically to depend on material parameters including bulk Dresselhaus spin orbit coupling (SOC), interfacial Rashba SOC, and magnetic anisotropy. Here, we establish the growth of a new class of artificial skyrmion materials, namely B20 superlattices, where these parameters could be systematically tuned. Specifically, we report the successful growth of B20 superlattices comprised of single crystal thin films of FeGe, MnGe, and CrGe on Si(1 1 1) substrates. Thin films and superlattices are grown by molecular beam epitaxy and are characterized through a combination of reflection high energy electron diffraction, X-ray diffraction, and cross-sectional scanning transmission electron microscopy (STEM). X-ray energy dispersive spectroscopy (XEDS) distinguishes layers by elemental mapping and indicates good interface quality with relatively low levels of intermixing in the [CrGe/MnGe/FeGe] superlattice. This demonstration of epitaxial, single-crystalline B20 superlattices is a significant advance toward tunable skyrmion systems for fundamental scientific studies and applications in magnetic storage and logic.

Journal article

Skinner SJ, McComb DW, Harrington GF, Cavallaro A, Kilner JAet al., 2017, The effects of lattice strain, dislocations, and microstructure on the transport properties of YSZ films, Physical Chemistry Chemical Physics, Vol: 19, Pages: 14319-14336, ISSN: 1463-9084

Enhanced conductivity in YSZ films has been of substantial interest over the last decade. In this paper we examine the effects of substrate lattice mismatch and film thickness on the strain in YSZ films and the resultant effect on the conductivity. 8 mol% YSZ films have been grown on MgO, Al2O3, LAO and NGO substrates, thereby controlling the lattice mismatch at the film/substrate interface. The thickness of the films was varied to probe the interfacial contribution to the transport properties, as measured by impedance spectroscopy and tracer diffusion. No enhancement in the transport properties of any of the films was found over single crystal values, and instead the effects of lattice strain were found to be minimal. The interfaces of all films were more resistive due to a heterogeneous distribution of grain boundaries, and no evidence for enhanced transport down dislocations was found.

Journal article

Wu K-T, Tellez H, Druce J, Burriel M, Yang F, McComb DW, Ishihara T, Kilner JA, Skinner SJet al., 2017, Surface chemistry and restructuring in thin-film Lan+1NinO3n+1 (n=1, 2 and 3) Ruddlesden-Popper oxides, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 5, Pages: 9003-9013, ISSN: 2050-7488

Understanding the surface chemistry and oxygen surface exchange activity in mixed conducting perovskite and related perovskite oxides is of great relevance in developing electrochemical devices. Mixed conducting Ruddlesden–Popper Lan+1NinO3n+1 phases (n = 1, 2 and 3) have been considered as promising electrodes for electrochemical energy conversion cells due to their layered structure allowing non-stoichiometric defect structures. This study focuses on a systematic investigation of the chemical composition of the outermost atomic surfaces of as-deposited and annealed epitaxial films of Lan+1NinO3n+1 (n = 1, 2 and 3). For both as-deposited and annealed films, the analysis of the outermost surface using low energy ion scattering shows preferential LaO-termination. The results also provide evidence of an associated Ni-enrichment below the outermost surface. These findings suggest significant atomic rearrangement occurs during deposition and subsequent annealing. To investigate the thermal stability of these films during deposition, further microstructural analysis was carried out by means of high-resolution scanning transmission electron microscopy, showing significant re-orientation of LaO layers after a post-annealing heat treatment. In thin films of n = 2, 3 phases, surface restructuring reduces the epitaxy of the films and hence any potential beneficial anisotropy in transport properties will be lost. Care must therefore be exercised in processing these materials for electrode applications.

Journal article

Gilchrist JB, Heutz S, McComb DW, 2017, Revealing structure and electronic properties at organic interfaces using TEM, Current Opinion in Solid State and Materials Science, Vol: 21, Pages: 68-76, ISSN: 1359-0286

Molecules and atoms at material interfaces have properties that are distinct from those found in the bulk. Distinguishing the interfacial species from the bulk species is the inherent difficulty of interfacial analysis. For organic photovoltaic devices, the interface between the donor and acceptor materials is the location for exciton dissociation. Dissociation is thought to occur via a complex route effected by microstructure and the electronic energy levels. The scale of these devices and the soft nature of these materials create an additional level of difficulty for identification and analysis at these interfaces. The transmission electron microscope (TEM) and the spectroscopic techniques it incorporates can allow the properties of the donor-acceptor interfaces to be revealed. Cross-sectional sample preparation, using modern focused ion beam instruments, enables these buried interfaces to be uncovered with minimal damage for high resolution analysis. This powerful instrument combination has the ability to draw conclusions about interface morphology, structure and electronic properties of organic donor-acceptor interfaces at the molecular scale. Recent publications have demonstrated these abilities, and this article aims to summarise some of that work and provide scope for the future.

Journal article

Miller DR, Williams RE, Akbar SA, Morris PA, McComb DWet al., 2017, STEM-Cathodoluminescence of SnO<inf>2</inf> nanowires and powders, Sensors and Actuators, B: Chemical, Vol: 240, Pages: 193-203, ISSN: 0925-4005

© 2016 Elsevier B.V. For the first time, ultra-high spatial resolution STEM-cathodoluminescence has been applied to SnO2 nanowires and nanoparticles in order to study the spatial distribution of point defects that form deep levels in the band gap and often dictate their gas-sensor performance. SnO2 nanowires consistently had emission signals centered at 1.76, 1.99, and 2.45 eV. Emission at 1.99 eV was found to be more intense with the STEM probe at the edges of nanowires and in higher surface-area nanoparticles compared to emission most intense at 2.45 eV with the probe in the center of a nanowire or particle. This result is contrary to recent studies proposing that both emission energies are associated with two different surface oxygen vacancy types. The ability to spatially map the luminescence of a single nanostructure also facilitated much less-ambiguous spectral deconvolution consistent across many particles which gives confidence in properly assigning the energies of each defect state. Individually-probed SnO2 nanowires were found to have much more consistent and reproducible defect states compared to commercial nanoparticles which has implications in stability as gas sensors and efforts to model interactions between analyte gases and the oxide surface.

Journal article

Cheah WL, McComb DW, Finnis MW, 2017, Structure and ionic diffusivity in an yttria-stabilised zirconia/strontium titanate multilayer., Acta Materialia, Vol: 129, Pages: 388-397, ISSN: 1359-6454

Enhanced ionic conductivity observed in a heteroepitaxial multilayer of yttria-stabilised zirconia and (YSZ) and strontium titanate (STO) has variously been attributed to lattice dilation or a disordered oxygen sublattice, leading to high interfacial mobility of anions, as compared to those of the constituent bulk oxides. We seek to understand the mechanism of ionic motion in such heterostructures by first simulating the atomic structure assuming coherent interfaces. After investigating possible low-energy interface structures using a genetic algorithm, we perform molecular dynamics simulations on these structures to examine the anionic diffusivity in the system. We find that the extreme biaxial tensile strain in the YSZ layer, as imposed between layers of STO, induces phases that differ from fluorite. The lowest energy structure is an unknown phase, which we refer to as quasi-cubic and whose cation sublattice resembles an extension of the perovskite; this structure does not lead to enhanced ionic conductivity, in contradiction to some reports in the literature.

Journal article

Luo X, Li B, Zhang X, Zhao W, Bratasz A, Deng B, McComb DW, Dong Yet al., 2017, Dual-functional lipid-like nanoparticles for delivery of mRNA and MRI contrast agents., Nanoscale, Vol: 9, Pages: 1575-1579

Multi-functional nanomaterials possess unique properties, facilitating both therapeutic and diagnostic applications among others. Herein, we developed dual-functional lipid-like nanoparticles for simultaneous delivery of mRNA and magnetic resonance imaging (MRI) contrast agents in order to express functional proteins and provide real-time visualization. TT3-Gd18 LLNs were identified as a lead formulation, which was able to encapsulate 91% of mRNA and 74% of Gd. This formulation showed a comparable or a slightly higher delivery efficiency of mRNA compared to the initial TT3 LLNs. Moreover, a strong MRI signal was observed in the cell pellets treated with TT3-Gd18 LLNs. More importantly, TT3-Gd18 LLNs demonstrated an efficient delivery of mRNA and Gd contrast agents in vivo.

Journal article

Gallagher JC, Meng KY, Brangham JT, Wang HL, Esser BD, McComb DW, Yang FYet al., 2017, Robust Zero-Field Skyrmion Formation in FeGe Epitaxial Thin Films., Phys Rev Lett, Vol: 118

B20 phase magnetic materials have been of significant interest because they enable magnetic Skyrmions. One major effort in this emerging field is the stabilization of Skyrmions at room temperature and zero magnetic field. We grow phase-pure, high crystalline quality FeGe epitaxial films on Si(111). Hall effect measurements reveal a strong topological Hall effect after subtracting the ordinary and anomalous Hall effects, demonstrating the formation of high density Skyrmions in FeGe films between 5 and 275 K. In particular, a substantial topological Hall effect was observed at a zero magnetic field, showing a robust Skyrmion phase without the need of an external magnetic field.

Journal article

Gonzalez Arellano DL, Bhamrah Harley J, Yang J, Gilchrist JB, McComb, Ryan MP, Heutz SEMet al., 2017, Room temperature routes towards the creation of zinc oxide films from molecular precursors, ACS Omega, Vol: 2, Pages: 98-104, ISSN: 2470-1343

The advent of “flexible” electronics on plastic substrates with low melting points requires the development of thin film deposition techniques that operate at low temperatures. This is easily achieved with vacuum or solution-processed molecular or polymeric semiconductors, but oxide materials remain a significant challenge. Here we show that zinc oxide (ZnO) can be prepared using only room-temperature processes, using the molecular thin film precursor zinc phthalocyanine (ZnPc), followed by vacuum ultra-violet light treatment to elicit degradation of the organic components and transformation of the deposited film to oxide material. The degradation mechanism was assessed by studying the influence of the atmosphere during the reaction: it was particularly sensitive to oxygen pressure in the chamber and optimal degradation conditions were established as 3 mbar with 40% oxygen in nitrogen. The morphology of the film was relatively unchanged during the reaction, but detailed analysis of itscomposition using both scanning transmission electron microscopy (STEM) and secondary ion mass spectrometry (SIMS) revealed that a 40 nm thick layer containingZnO results from the 100 nm thick precursor after complete reaction. Our methodology represents a simple route for the fabrication of oxides and multilayer structuresthat can be easily integrated into current molecular thin film growth setups,without the need for a high temperature step.

Journal article

Deitz JI, McComb DW, Grassman TJ, 2017, Probing the electronic structure at the heterovalent GaP/Si interface using electron energy-loss spectroscopy, Pages: 1-4

© 2017 IEEE. Accurate determination, with nanometer scale spatial resolution, of the semiconductor electronic structure information via valence electron energy-loss spectroscopy (VEELS) is discussed. Specifically, the use of reduced accelerating voltages, higher collection angles, and thinner specimens is investigated. This discussion is applied to the heteroepitaxial GaP/Si materials system to analyze potential electronic shifts near and away the heterovalent, lattice-mismatched interface. Results to date suggest that residual misfit strain in the GaP epilayer yields changes within in the material's dielectric function, while such changes are not observed in the bottom Si substrate. While this effort is still ongoing, such results imply a greater importance of issues like strain, even in the absence of dislocation defects, may still have an impact on heterostructured device design and performance, and indicate that the VEELS technique could ultimately provide unprecedented detail with respect to important electronic structure details.

Conference paper

Boona SR, Vandaele K, Boona IN, McComb DW, Heremans JPet al., 2016, Observation of spin Seebeck contribution to the transverse thermopower in Ni-Pt and MnBi-Au bulk nanocomposites, Nature Communications, Vol: 7

© 2016 The Author(s). Transverse thermoelectric devices produce electric fields perpendicular to an incident heat flux. Classically, this process is driven by the Nernst effect in bulk solids, wherein a magnetic field generates a Lorentz force on thermally excited electrons. The spin Seebeck effect also produces magnetization-dependent transverse electric fields. It is traditionally observed in thin metallic films deposited on electrically insulating ferromagnets, but the films' high resistance limits thermoelectric conversion efficiency. Combining Nernst and spin Seebeck effect in bulk materials would enable devices with simultaneously large transverse thermopower and low electrical resistance. Here we demonstrate experimentally that this is possible in composites of conducting ferromagnets (Ni or MnBi) containing metallic nanoparticles with strong spin-orbit interactions (Pt or Au). These materials display positive shifts in transverse thermopower attributable to inverse spin Hall electric fields in the nanoparticles. This more than doubles the power output of the Ni-Pt materials, establishing proof of principle that the spin Seebeck effect persists in bulk nanocomposites.

Journal article

Smith TM, Esser BD, Antolin N, Carlsson A, Williams REA, Wessman A, Hanlon T, Fraser HL, Windl W, McComb DW, Mills MJet al., 2016, Phase transformation strengthening of high-temperature superalloys, Nature Communications, Vol: 7

© 2016 The Author(s). Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new 'phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ to η phase along stacking faults in γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.

Journal article

Deitz JI, McComb DW, Grassman TJ, 2016, Probing the electronic structure at the heterovalent GaP/Si interface using electron energy-loss spectroscopy, Pages: 1545-1548, ISSN: 0160-8371

© 2016 IEEE. Accurate determination, with nanometer scale spatial resolution, of the semiconductor electronic structure information via valence electron energy-loss spectroscopy (VEELS) is discussed. Specifically, the use of reduced accelerating voltages, higher collection angles, and thinner specimens is investigated. This discussion is applied to the heteroepitaxial GaP/Si materials system to analyze potential electronic shifts near and away the heterovalent, lattice-mismatched interface. Results to date suggest that residual misfit strain in the GaP epilayer yields changes within in the material's dielectric function, while such changes are not observed in the bottom Si substrate. While this effort is still ongoing, such results imply a greater importance of issues like strain, even in the absence of dislocation defects, may still have an impact on heterostructured device design and performance, and indicate that the VEELS technique could ultimately provide unprecedented detail with respect to important electronic structure details.

Conference paper

Terock M, Konrad CH, Popp R, Völkl R, Yang F, McComb DW, Mills MJ, Glatzel Uet al., 2016, Tailored platinum-nickel nanostructures on zirconia developed by metal casting, internal oxidation and dealloying, Corrosion Science, Vol: 112, Pages: 246-254, ISSN: 0010-938X

© 2016 Elsevier Ltd Noble-metal nanostructures are crucial, highly active materials in applications such as fuel cells, sensors, electrodes and catalysts. The present work shows a new and efficient method for the in-situ synthesis of platinum-nickel nanostructures on porous zirconia. Ni-Zr-Y metallic alloys with defined Zr/Y-ratios and small amounts of platinum are internally oxidized. An interpenetrating network of yttria-stabilized zirconia (YSZ) in pure nickel is formed. Dealloying of the nickel matrix generates a multimodal porous ceramic support structure with platinum-nickel nanostructures on the surface of the zirconia-ceramic with a broad spectrum for tailored functionality, for example in after-treatment of exhaust gases.

Journal article

Arguilla MQ, Katoch J, Krymowski K, Cultrara ND, Xu J, Xi X, Hanks A, Jiang S, Ross RD, Koch RJ, Ulstrup S, Bostwick A, Jozwiak C, McComb DW, Rotenberg E, Shan J, Windl W, Kawakami RK, Goldberger JEet al., 2016, NaSn<inf>2</inf>As<inf>2</inf>: An Exfoliatable Layered van der Waals Zintl Phase, ACS Nano, Vol: 10, Pages: 9500-9508, ISSN: 1936-0851

© 2016 American Chemical Society. The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties enables the realization of unique physical phenomena in these few-atom-thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = group 1 or 2 element, Tt = group 14 tetrel element, and Pn = group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman-active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant toward oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy, density functional theory, and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 ω·m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics.

Journal article

Esser BD, Hauser AJ, Williams REA, Allen LJ, Woodward PM, Yang FY, McComb DWet al., 2016, Quantitative STEM Imaging of Order-Disorder Phenomena in Double Perovskite Thin Films, Physical Review Letters, Vol: 117, ISSN: 0031-9007

© 2016 American Physical Society. Using aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), we investigate ordering phenomena in epitaxial thin films of the double perovskite Sr2CrReO6. Experimental and simulated imaging and diffraction are used to identify antiphase domains in the films. Image simulation provides insight into the effects of atomic-scale ordering along the beam direction on HAADF-STEM intensity. We show that probe channeling results in ±20% variation in intensity for a given composition, allowing 3D ordering information to be probed using quantitative STEM.

Journal article

Kłosowski MM, Carzaniga R, Abellan P, Ramasse Q, McComb DW, Porter AE, Shefelbine SJet al., 2016, Electron microscopy reveals structural and chemical changes at the nanometer scale in the osteogenesis imperfecta murine pathology, ACS biomaterials science & engineering, Vol: 3, Pages: 2788-2797, ISSN: 2373-9878

Alternations of collagen and mineral at the molecular level may have a significant impact on the strength and toughness of bone. In this study, scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) were employed to study structural and compositional changes in bone pathology at nanometer spatial resolution. Tail tendon and femoral bone of osteogenesis imperfecta murine (oim, brittle bone disease) and wild type (WT) mice were compared to reveal defects in the architecture and chemistry of the collagen and collagen-mineral composite in the oim tissue at the molecular level. There were marked differences in the substructure and organization of the collagen fibrils in the oim tail tendon; some regions have clear fibril banding and organization, while in other regions fibrils are disorganized. Malformed collagen fibrils were loosely packed, often bent and devoid of banding pattern. In bone, differences were detected in the chemical composition of mineral in oim and WT. While mineral present in WT and oim bone exhibited the major characteristics of apatite, examination in EELS of the fine structure of the carbon K ionization edge revealed a significant variation in the presence of carbonate in different regions of bone. Variations have been also observed in the fine structure and peak intensities of the nitrogen K-edge. These alterations are suggestive of differences in the maturation of collagen nucleation sites or cross-links. Future studies will aim to establish the scale and impact of the modifications observed in oim tissues. The compositional and structural alterations at the molecular level cause deficiencies at larger length scales. Understanding the effect of molecular alterations to pathologic bone is critical to the design of effective therapeutics.

Journal article

Alexander JA, Scheltens FJ, Drummy LF, Durstock MF, Gilchrist JB, Heutz S, McComb DWet al., 2016, Measurement of optical properties in organic photovoltaic materials using monochromated electron energy-loss spectroscopy, JOURNAL OF MATERIALS CHEMISTRY A, Vol: 4, Pages: 13636-13645, ISSN: 2050-7488

Journal article

Podhorska L, Delcassian D, Goode AE, Agyei M, McComb DW, Ryan MP, Dunlop IEet al., 2016, Mechanisms of polymer-templated nanoparticle synthesis: contrasting ZnS and Au, Langmuir, Vol: 32, Pages: 9216-9222, ISSN: 0743-7463

We combine solution small-angle X-ray scattering (SAXS) and high-resolution analytical transmission electron microscopy (ATEM) to gain a full mechanistic understanding of substructure formation in nanoparticles templated by block copolymer reverse micelles, specifically poly(styrene)-block-poly(2-vinyl pyridine). We report a novel substructure for micelle-templated ZnS nanoparticles, in which small crystallites (~4 nm) exist within a larger (~20 nm) amorphous organic-inorganic hybrid matrix. The formation of this complex structure is explained via SAXS measurements that characterize in situ for the first time the intermediate state of the metal-loaded micelle core: Zn2+ ions are distributed throughout the micelle core, which solidifies as a unit on sulfidation. The nanoparticle size is thus determined by the radius of the metal-loaded core, rather than the quantity of available metal ions. This mechanism leads to particle size counter-intuitively decreasing with increasing metal content, based on the modified interactions of the metal-complexed monomers in direct contrast to gold nanoparticles templated by the same polymer.

Journal article

Brangham JT, Meng KY, Yang AS, Gallagher JC, Esser BD, White SP, Yu S, McComb DW, Hammel PC, Yang Fet al., 2016, Thickness dependence of spin Hall angle of Au grown on Y3 F e5 O12 epitaxial films, Physical Review B, Vol: 94, ISSN: 2469-9950

© 2016 American Physical Society. We measure the spin Hall angle in Au layers of 5-100 nm thicknesses by spin pumping from Y3Fe5O12 epitaxial films grown by ultrahigh vacuum, off-axis sputtering. We observe a striking increase in the spin Hall angle for Au layers thinner than the measured spin diffusion length of 12.6 nm. In particular, the 5 nm Au layer shows a large spin Hall angle of 0.087, compared to those of 0.016 and 0.017 for the 50 and 100 nm Au layers, respectively, suggesting that the top surface plays a dominant role in spin Hall physics when the spin current is able to reach it. Other spin pumping related parameters, including Gilbert damping enhancement, interfacial spin mixing conductance, and spin current are also determined for Au layers of various thicknesses. Given the pervasive role of ultrathin films in electrical and spin transport applications, this result emphasizes the importance of considering the impact of the top surface and reveals the possibility of tuning critical spin parameters by film thickness.

Journal article

Chater RJ, Cavallaro A, Bayliss RD, Cook SN, Esser BD, McComb DW, Kilner JAet al., 2016, Fast grain boundary oxygen ion diffusion in the α-phase of Bi2O3, Solid State Ionics, Vol: 299, Pages: 89-92, ISSN: 0167-2738

The low temperature stable α-phase of pure Bi2O3 is known to have an oxygen ion diffusivity that is over 7 orders of magnitude lower than the high temperature fluorite-structured δ-phase. Tracer oxygen-18 diffusion studies of polycrystalline α-Bi2O3 at 600 °C using an ion microscope with lateral resolutions of ~ 50 nm for the surface distributions of the oxygen isotopes have resolved the secondary fast migration pathways as well as the normal bulk diffusion profile in the grains. A grain boundary pathway for the oxygen migration is distinguished from other extended defects, some of which are also active in the overall oxygen diffusivity. This experimental study highlights the potential manipulation of the micro-structure of this material for enhanced oxygen ion conduction in intermediate temperature solid oxide fuel cells as has been shown for perovskite MIEC electrode materials.

Journal article

Gallagher JC, Yang AS, Brangham JT, Esser BD, White SP, Page MR, Meng KY, Yu S, Adur R, Ruane W, Dunsiger SR, McComb DW, Yang F, Hammel PCet al., 2016, Exceptionally high magnetization of stoichiometric Y<inf>3</inf>Fe<inf>5</inf>O<inf>12</inf> epitaxial films grown on Gd<inf>3</inf>Ga<inf>5</inf>O<inf>12</inf>, Applied Physics Letters, Vol: 109, ISSN: 0003-6951

© 2016 Author(s). The saturation magnetization of Y3Fe5O12 (YIG) epitaxial films 4 to 250 nm in thickness has been determined by complementary measurements including the angular and frequency dependencies of the ferromagnetic resonance fields as well as magnetometry measurements. The YIG films exhibit state-of-the-art crystalline quality, proper stoichiometry, and pure Fe3+ valence state. The values of YIG magnetization obtained from all the techniques significantly exceed previously reported values for single crystal YIG and the theoretical maximum. This enhancement of magnetization, not attributable to off-stoichiometry or other defects in YIG, opens opportunities for tuning magnetic properties in epitaxial films of magnetic insulators.

Journal article

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