Dr Pierre Couture

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 transition-metal chalcogenides include some of the most important and ubiquitous families of 2D materials. They host an exceptional variety of electronic and collective states, which can in principle be readily tuned by combining different compounds in van der Waals heterostructures. Achieving this, however, presents a significant materials challenge. The highest quality heterostructures are usually fabricated by stacking layers exfoliated from bulk crystals, which - while producing excellent prototype devices - is time consuming, cannot be easily scaled, and can lead to significant complications for materials stability and contamination. Growth via the ultra-high vacuum deposition technique of molecular-beam epitaxy (MBE) should be a premier route for 2D heterostructure fabrication, but efforts to achieve this are complicated by non-uniform layer coverage, unfavorable growth morphologies, and the presence of significant rotational disorder of the grown epilayer. This work demonstrates a dramatic enhancement in the quality of MBE grown 2D materials by exploiting simultaneous deposition of a sacrificial species from an electron-beam evaporator during the growth. This approach dramatically enhances the nucleation of the desired epi-layer, in turn enabling the synthesis of large-area, uniform monolayers with enhanced quasiparticle lifetimes, and facilitating the growth of epitaxial van der Waals heterostructures. Molecular-beam epitaxy of 2D chalcogenides typically yields small, disconnected islands, with premature onset of multilayer formation. This work reports how utilizing excited ions of a sacrificial species during the growth can dramatically enhance nucleation of the epitaxial layer, enabling growth of large-area monolayers with enhanced carrier lifetimes and facilitating the fabrication of all-epitaxial van der Waals heterostructures. image

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.

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.

•Analysis of 3D micron scale devices is demonstrated using ensemble RBS.•High sensitivity is achieved by probing many 3D devices simultaneously.•Ensemble RBS is applied to the study of microfluidic devices.•Enhanced capabilities of ensemble RBS relative to microbeam RBS are demonstrated. Rutherford backscattering spectrometry (RBS) is an analytical method able to provide quantitatively elemental information with high accuracy in the near surface region of samples. However, the technique conventionally lacks the required (sub)micron spatial resolution for many semiconductor applications. Firstly, the ion beam current of a highly focused beam is very small, limiting the analytical sensitivity of the measurement. Secondly, the exposure of a sample to a highly focused ion beam readily leads to sample damage, surface sputtering, and accordingly to a measurement error. As a solution to these problems, ensemble RBS is presented whereby multiple devices are measured simultaneously using a broad beam. A judicious choice of the scattering conditions and related data interpretation nevertheless leads to the ability to analyse 3D-devices of micrometre sizes. We demonstrate the potential of this approach through the analysis of atomic species present on the different surfaces of 3D-microfluidic devices. The performance of the technique is demonstrated by the analysis of microfluidic devices after Pt deposition at an oblique angle, and the analysis of the same microfluidic devices after a site-selective deposition of a sub-monolayer of Hf. Further, the performance of ensemble RBS on these structures is compared to the one of microbeam RBS. [Display omitted]

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.

BiFeO3 and BiCrO3 films were made by room temperature sputtering followed by thermal annealing in a partial oxygen atmosphere. The annealed films were found to be nanocrystalline, with an average particle size of 11 nm for BiFeO3 and 8 nm for BiCrO3. The saturation moment per formula unit is 0.39 µB for BiFeO3 which is significantly greater than that found in bulk BiFeO3 (0.02 µB). A similar enhancement was also found in previous studies of BiFeO3 nanoparticles where the nanoparticle size was small. However, no large enhancement of the saturation moment per formula unit was identified for the annealed BiCrO3 films. The annealed BiFeO3 films displayed superparamagnetic behaviour and the particle size estimated from the blocking temperature is comparable to that estimated from the X-ray diffraction data. Our results show that sputtering and oxygen annealing is a method that can be used to make nanocrystalline BiFeO3 and BiCrO3 films.

Starbursts are found through the whole Universe and represent an important phase in the evolution of galaxies. Distant starbursts may be easily observed and study in the ultraviolet light because of their massive stars. I will present magnitude and color simulations of young stellar populations in order to characterize their age, metallicity, initial mass function, and star formation rate and history. These simulations take into account the filters available with the Ultraviolet Imaging Telescope.

Multiferroic nanocrystalline BiFeO3films have been successfully made by room temperature sputtering and thermal annealing in oxygen at 500C. Nanocrystalline Bi was seen before annealing as well asb-Bi2O3 and the iron oxide phases, magnetite, maghemite, and FeO. Superparamagnetism was observed that can be attributed to magnetite and maghemite nanoparticles. The thermally annealed film contained BiFeO3 nanoparticles and magnetite, maghemite, and hematite as well as unidentified BiFexOyphases. Superparamagnetism was also seen after annealing and the magnetic properties are predominately due to magnetite and maghemite nanoparticles rather than from multiferroic BiFeO3. The saturation magnetic moment was 60% lower after annealing, which was due to some of the Fe in the iron oxide nanoparticles being incorporated into the BiFeO3nanoparticles. An exchange bias was observed before and after annealing that cannot be attributed to a structure that includes BiFeO3. It is likely to arise from magnetite and maghemite cores with spin-disordered shells.