Callum D McAleese

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.

An accurate description of the distribution of hydrogen at solar cell interfaces is critical for understanding both passivation and degradation phenomena. Time-of-flight elastic recoil detection analysis (ToF-ERDA) has recently been employed to study this hydrogen distribution by providing a one-dimensional (1D) depth profile. In this work, ToF-ERDA was used to investigate the hydrogen profile in a SiOX / SiNX passivating stack. The ability to resolve the interface with the c-Si interface was studied by using polished wafers and thin (20 nm) passiv-ating stacks. This approach, coupled with Monte Carlo ERD (MCERD) modelling, showed that the identification of the interfacial oxide was much more clearly defined compared with previous reports using ToF-ERDA. Annealing the SiOX / SiNX at 450 °C for 5 minutes substantially increased the effective lifetime. However, no noticeable change in the H distribution measured with ToF-ERD was observed. We comment on the difficulty of correlating physical hydrogen measurements with the surface recombination properties.

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.