Dr Matthew Sharpe

The discovery of Mn-Ca complex in photosystem II stimulates research of manganese-based catalysts for oxygen evolution reaction (OER). However, conventional chemical strategies face challenges in regulating the four electron-proton processes of OER. Herein, we investigate alpha-manganese dioxide (α-MnO2) with typical MnIV-O-MnIII-HxO motifs as a model for adjusting proton coupling. We reveal that pre-equilibrium proton-coupled redox transition provides an adjustable energy profile for OER, paving the way for in-situ enhancing proton coupling through a new “reagent”— external electric field. Based on the α-MnO2 single-nanowire device, gate voltage induces a 4-fold increase in OER current density at 1.7 V versus reversible hydrogen electrode. Moreover, the proof-of-principle external electric field-assisted flow cell for water splitting demonstrates a 34% increase in current density and a 44.7 mW/cm² increase in net output power. These findings indicate an in-depth understanding of the role of proton-incorporated redox transition and develop practical approach for high-efficiency electrocatalysis.

The unambiguous detection of hydrogen in solar cell contact structures is critical to understanding passivation and degradation phenomena. Deuterium is often used to depict the distribution of hydrogen more clearly. However, experimental noise and artifacts can hinder the clear identification of species. This work provides a report of time-of-flight elastic recoil detection (ToF-ERD) analysis to identify H/D contents in a thin poly-Si/SiOx passivating contact. The structure contained a 1.3 nm interfacial SiOx and an n+ doped poly-Si layer with a partly deuterated SiNx coating. The samples were annealed to release H/D, and ToF-ERD was used to detect H/D in monatomic, singly charged forms, without the detection artifacts associated with conventional secondary ion mass spectroscopy. Chlorine ions were used to recoil surface species, which were analyzed to clearly and unambiguously resolve H and D. Depth profiles for the recoiled Si, N, O, D, and H atoms were calculated from the energy and velocity information registered after scattering events, which enabled the analysis of the structure of the multilayer stack. Even though the surface roughness and experimental limitations cause visible broadening of the profiles, which can hinder clear detection at the interfacial oxide, the ability to resolve hydrogen-related species makes ToF-ERD a significant and promising tool for studying the role of hydrogen in the performance and degradation of solar cell passivating contacts.

The distribution of surfactants in waterborne colloidal polymer films is of significant interest for scientific understanding and defining surface properties in applications including pressure-sensitive adhesives and coatings. Because of negative effects on appearance, wetting, and adhesion, it is desirable to prevent surfactant accumulation at film surfaces. The effect of particle deformation on surfactant migration during film formation was previously investigated by Gromer et al. through simulations, but experimental investigations are lacking. Here, we study deuterium-labeled sodium dodecyl sulfate surfactant in a poly(butyl acrylate) latex model system. The particle deformability was varied via cross-linking of the intraparticle polymer chains by differing extents. The cross-linker concentration varied from 0 to 35 mol % in the copolymer, leading to a transition from viscoelastic to elastic. Ion beam analysis was used to probe the dry films and provide information on the near-surface depth distribution of surfactant. Films of nondeformable particles, containing the highest concentration of cross-linker, show no surfactant accumulation at the top surface. Films from particles partially deformed by capillary action show a distinct surfactant surface layer (ca. 150 nm thick). Films of coalesced particles, containing little or no cross-linker, show a very small amount of surfactant on the surface (ca. 20 nm thick). The observed results are explained by considering the effect of cross-linking on rubber elasticity and applying the viscous particle deformation model by Gromer et al. to elastically deformed particles. We find that partially deformed particles allow surfactant transport to the surface during film formation, whereas there is far less transport when skin formation acts as a barrier. With elastic particles, the surfactant is carried in the water phase as it falls beneath the surface of packed particles. The ability to exert control over surfactant distribution in waterborne colloidal films will aid in the design of new high-performance adhesives and coatings.

Previous research on transistor gate oxides reveals a clear link between hydrogen content and oxide breakdown. This has implications for redox-based resistive random access memory (ReRAM) devices, which exploit soft, reversible, dielectric breakdown, as hydrogen is often not considered in modeling or measured experimentally. Here quantitative measurements, corroborated across multiple techniques are reported, that reveal ReRAM devices, whether manufactured in a university setting or research foundry, contain concentrations of hydrogen at levels likely to impact resistance switching behavior. To the knowledge this is the first empirical measurement depth profiling hydrogen concentration through a ReRAM device. Applying a recently-developed Secondary Ion Mass Spectrometry analysis technique enables to measure hydrogen diffusion across the interfaces of SiO x ReRAM devices as a result of operation. These techniques can be applied to a broad range of devices to further understand ReRAM operation. Careful control of temperatures, precursors, and exposure to ambient during fabrication should limit hydrogen concentration. Additionally, using thin oxynitride or TiO 2 capping layers should prevent diffusion of hydrogen and other contaminants into devices during operation. Applying these principles to ReRAM devices will enable considerable, informed, improvements in performance.

The effects of room temperature 200 keV Ne + ion irradiation on Ag nanoparticles (NPs) embedded in a silicon nitride film have been studied via Medium Energy Ion Scattering (MEIS), Scanning Transmission Electron Microscopy (STEM), and the results are discussed in comparison with Monte Carlo TRI3DYN simulations. The experiment was designed to test the mechanisms controlling the room temperature irradiation -induced microstructure evolution of individual NPs and the overall system. We present an innovative MEIS approach capable of quantitatively determining Ag's content and spatial distribution in solution. The STEM observations demonstrate the nucleation of smaller Ag NPs distributed within the solute field. The TRI3DYN simulations suggest that a fraction of the solute content is reincorporated within the NPs. These results are discussed considering that room -temperature thermal diffusion processes are significantly retarded in silicon nitride substrates, which leads to an interpretation primarily based on the irradiation -induced atomic displacements that render a microstructure evolution driven by the minimization of chemical potential as an elemental thermodynamic force. Our findings are of applied interest concerning the application of ion beams to manipulate NP ensembles.

Aluminium-doped ZnO (AZO) thin films were deposited by remote plasma sputtering of a ZnO:Al2O3 98:2 wt.% ceramic target in a pulsed DC configuration. The target power was kept constant at 445 W and the RF plasma power was varied between 0.5 and 2.5 kW. The as-deposited AZO thin films exhibited an optimum resistivity of 6.35 x 10-4 .cm and optical transmittance of 92 % at a RF plasma power 1.5 kW. The thin film microstructure, chemical composition, and residual stress were investigated using SEM, RBS, XPS and XRD. Accurate determination of the chemical composition and correct interpretation of GIXRD data for AZO thin films are a particular focus of this work. The AZO layer thickness was 500 - 700 nm and Al content in the range of 2.3 - 3.0 at.%, determined by RBS. The AZO thin films exhibited a strong (002) preferential orientation and grain sizes between 70 and 110 nm. The (103) peak intensity enhancement in GIXRD is proven to be a result of the strong (002) preferential orientation and GIXRD geometrical configuration rather than a change in the crystallite orientation at the surface. XPS depth profiles show preferential sputtering of O and Al using a 500 eV Ar+ beam, which can be reduced, but not eradicated using an 8 keV Ar150+ beam. The preferential sputtering can be successfully modelled using the simulation software TRIDYN. A plasma power of 1.5 kW corresponds to a highly ionised plasma and various microstructural and compositional factors have all contributed to the optimum low resistivity occurring at this plasma power. The grain size exhibits a maximum in the 1.25 - 1.5 kW range and there is improved (002) orientation, minimising grain boundary scattering. The highest carrier concentration and mobility was observed at the plasma power of 1.5 kW which may be associated with the maximum in the aluminium doping concentration (3.0 at.%). The lowest residual stress is also observed at 1.5 kW.

LEIS depth profiles, obtained by low energy (0.5 keV) Ar+ sputtering, have been analysed for the mixed conducting oxide material La0.6Sr0.4Co0.2Fe0.8O3-δ. Samples have been examined after differing thermal treatments to examine the sub-surface reorganisation of the cation species. The profiles have shown considerable changes, but these are not strongly correlated with the thermal treatments. The similarity between the profiles suggests that preferential sputtering effects can dominate the sub-surface region (~1–3 nm) where sputtering has not reached equilibrium. Preferential sputtering of oxygen in oxide materials is well known, but here we provide evidence of the preferential sputtering of the cationic species in a complex multicomponent oxide. Of note is strong enrichment (~30%) of the sputtered surface with the heaviest of the elements, La. Simulations using the code TRIDYN have confirmed the observations, in particular, La surface enrichment and the fluence needed to achieve steady state sputtering of > 3 × 1016cm−2.

In this work, we used a combination of photoluminescence (PL), high resolution X-ray diffraction (XRD), and Rutherford backscattering spectrometry (RBS) techniques to investigate material quality and structural properties of MBE-grown InGaAsBi samples (with and without an InGaAs cap layer) with targeted bismuth composition in the 3%–4% range. XRD data showed that the InGaAsBi layers are more homogeneous in the uncapped samples. For the capped samples, the growth of the InGaAs capped layer at higher temperature affects the quality of the InGaAsBi layer and bismuth distribution in the growth direction. Low-temperature PL exhibited multiple emission peaks; the peak energies, widths, and relative intensities were used for comparative analysis of the data in line with the XRD and RBS results. RBS data at a random orientation together with channeled measurements allowed both an estimation of the bismuth composition and analysis of the structural properties. The RBS channeling showed evidence of higher strain due to possible antisite defects in the capped samples grown at a higher temperature. It is also suggested that the growth of the capped layer at high temperature causes deterioration of the bismuth-layer quality. The RBS analysis demonstrated evidence of a reduction of homogeneity of uncapped InGaAsBi layers with increasing bismuth concentration. The uncapped higher bismuth concentration sample showed less defined channeling dips suggesting poorer crystal quality and clustering of bismuth on the sample surface.

The uses of pressure-sensitive adhesives (PSAs) are wide ranging, with applications including labels, tapes, and graphics. To achieve good adhesion, a PSA must exhibit a balance of viscous and elastic properties. Previous research has found that a thin, elastic surface layer on top of a softer, dissipative layer resulted in greater tack adhesion compared with the single layers. Superior properties were achieved through a bilayer obtained via successive depositions, which consume energy and time. To achieve a multilayered structure via a single deposition process, we have stratified mixtures of waterborne colloidal polymer particles with two different sizes: large poly(acrylate) adhesive particles (ca. 660 nm in diameter) and small poly(butyl acrylate) (pBA) particles (ca. 100 nm). We used two types of pBA within the particles: either viscoelastic pBA without an added cross-linker or elastic pBA with a fully cross-linked network. Stratified surface layers of deuterium-labeled pBA particles with thicknesses of at least 1 μm were found via elastic recoil detection and qualitatively verified via the analysis of surface topography. The extent of stratification increased with the evaporation rate; films that were dried slowest exhibited no stratification. This result is consistent with a model of diffusiophoresis. When the elastic, cross-linked pBA particles were stratified at the surface, the tack adhesion properties made a transition from brittle failure to tacky. For pBA without an added cross-linker, all adhesives showed fibrillation during debonding, but the extent of fibrillation increased when the films were stratified. These results demonstrate that the PSA structure can be controlled through the processing conditions to achieve enhanced properties. This research will aid the future development of layered or graded single-deposition PSAs with designed adhesive properties.