72 Search Results

Titanium-based porous transport layers (PTLs) and iridium-based catalyst layers (CLs) are two main components of proton exchange membrane water electrolyzers (PEMWEs). PTLs are typically coated with platinum to minimize interfacial losses and to support long-term operation. Optimizing coatings and the PTL-CL interface requires comprehensive characterization. This study establishes time-of-flight secondary ion mass spectrometry (ToF-SIMS) as a valuable technique for PTL characterization, addressing capabilities and limitations related to PTL morphology. A methodology was developed that uses a Cs⁺ sputter beam for dynamic depth profiling, with data collected in both positive-ion (MCs⁺) and negative-ion modes to generate depth profiles, 2D ion maps,more » and 3D ion reconstructions. ToF-SIMS detected relative differences in platinum-layer thickness between samples; these trends were validated by cross-sectional scanning transmission electron microscope (STEM) measurements and flat-titanium substrate controls. Interfacial oxide layers are identified in both ion modes, with enhanced oxide sensitivity in negative mode. The technique’s high sensitivity enables detection of nanometer-scale coatings and trace impurities within the bulk PTL structure. These results provide a methodological framework for analyzing Pt-coated PTLs, with the potential to extend to other components in PEMWEs and other electrolyzer systems.« less

Driven by environmental and health concerns related to per- and polyfluoroalkyl substances (PFAS), there has been growing interest in developing fluorine-free proton (H⁺) exchange membrane (PEM) materials for fuel cells and water electrolyzers. In this study, we present a side-by-side comparison of the key transport properties of submicron thick, PFAS-free amorphous silicon dioxide (SiO₂) membranes to Nafion, a fluorinated polymer electrolyte membrane that represents the industry standard for PEM fuel cells and electrolyzers. Here, measurements of proton (H⁺) conductivity (σ_H⁺), hydrogen (H₂) permeability (P_H2), and electrical resistivity (ρ_e^–) were conducted using model thin films comprised of SiO₂ membranes deposited bymore » atomic layer deposition (ALD). Although the H⁺ conductivity of the SiO₂ membranes is 2–3 orders of magnitude lower than Nafion, the addition of phosphorus dopants (PO_x) improves H⁺ conductivity such that the area specific membrane resistance of thin (<50 nm) PO_x-doped SiO₂ membranes is more than an order of magnitude lower than Nafion-117. Importantly, the safe operation of such nanoscale membranes within a PEM electrolyzer is feasible thanks to the low H₂ permeability of dense SiO₂-based membranes, which are predicted to limit H₂ crossover rates to acceptable levels for pressures up to ≈ 100 bar. As a proof-of-principle demonstration, a chip-scale water electrolyzer based on 100 nm thick PO_x-SiO₂ membrane is shown to achieve a current density of 2 A cm^–2 at a potential of 2.5 V. If this technology can be successfully scaled up, H⁺ conducting oxide membranes offer an attractive PFAS-free alternative to Nafion for efficient and durable water electrolysis and fuel cell technologies.« less

Leveraging the reciprocal-space proximity effect between superconducting bulk and topological surface states (TSSs) offers a promising way to topological superconductivity. However, elucidating the mutual influence of bulk and TSSs on topological superconductivity remains a challenge. Here, we report pioneering transport evidence of a thickness-dependent transition from conventional to unconventional superconductivity in 2M-phase WS₂ (2M-WS₂). As the sample thickness reduces, we see clear changes in key superconducting metrics, including critical temperature, critical current, and carrier density. Notably, while thick 2M-WS₂ samples show conventional superconductivity, with an in-plane (IP) upper critical field constrained by the Pauli limit, samples under 20 nm exhibitmore » a pronounced IP critical field enhancement, inversely correlated with 2D carrier density. This marks a distinct crossover to unconventional superconductivity with strong spin-orbit-parity coupling. Furthermore, our findings underscore the crucial role of sample thickness in accessing topological states in 2D topological superconductors, offering pivotal insights into future studies of topological superconductivity.« less

Spin-helical Dirac Fermions at a doped topological insulator’s boundaries can support Majorana quasiparticles when coupled with s-wave superconductors, but in n-doped systems, the requisite induced Cooper pairing in topological states is often buried at heterointerfaces or complicated by degenerate coupling with bulk conduction carriers. Rarely probed are p-doped topological structures with nondegenerate Dirac and bulk valence bands at the Fermi level, which may foster long-range superconductivity without sacrificing Majorana physics. Using ultrahigh-resolution photoemission, we report proximity pairing with a large decay length in p-doped topological Sb₂Te₃ on superconducting Nb. Despite no momentum-space degeneracy, the topological and bulk states of Sb₂Te₃/Nbmore » exhibit the same isotropic superconducting gaps at low temperatures. Furthermore, our results unify principles for realizing accessible pairing in Dirac Fermions relevant to topological superconductivity.« less

We detail scientific and engineering advances which enable the controlled spalling and layer transfer of single crystal 4H silicon carbide (4H-SiC) from bulk substrates. 4HSiC’s properties, including high thermal conductivity and a wide bandgap, make it an ideal semiconductor for power electronics. Moreover, 4H-SiC is an excellent host of solid-state atomic defect qubits for quantum computing and quantum networking. Because 4H-SiC substrates are expensive (due to long growth times and limited yield), techniques for removal and transfer of bulk-quality films are desirable for substrate reuse and integration of the separated films. In this work, we utilize updated approaches for stressormore » layer thickness control and spalling crack initiation to demonstrate controlled spalling of 4H-SiC, the highest fracture toughness crystal spalled to date. We achieve coherent spin control of neutral divacancy (VV⁰) qubit ensembles and measure a quasi-bulk spin T₂ of 79.7 μs in the spalled films.« less

The diversification of electronic materials in devices provides a strong incentive for methods to rapidly correlate device performance with fabrication decisions. In this work, we present a low-cost automated test station for gated electronic transport measurements of field-effect transistors. Utilizing open-source PyMeasure libraries for transparent instrument control, the “ATLAS-MAP” system serves as a customizable interface between sourcemeters and samples under test and is programmed to conduct transfer curve and van der Pauw methods with static and sweeping gate voltages. Zinc oxide transistors of variable thickness (5, 10, and 20 nm) and channel size (50 μm to 3 mm, of equalmore » length and width) were fabricated to validate the design. Standardization of testing procedures and raw data formatting enabled automated data analysis. A detailed list of parts and code files for the system are provided.« less

Monolayer two-dimensional (2D) materials have been intensively studied while research on multilayers is still in its infancy. Here, we induce defects inside bulk MoS₂ through thermal annealing and near the surface of multilayer MoS₂ using 375 nm laser irradiation, and investigate their photoluminescence (PL) and fluorescence lifetime imaging (FLIM). Enhanced emission is limited within a certain MoS₂ thickness. The observed enhanced emission is evidenced by a threshold behavior in super-linear PL intensity increase, strong polarization effects, and increased lifetime of defect peak. The laser power threshold for enhanced emission is much smaller in defects near the surface than that insidemore » the bulk of multilayer MoS₂. The mechanical strain from a wrinkle of the sample further lowers the laser power threshold for enhanced emission. By exciting with a 639 nm laser that is close to the fundamental gap between the conduction band minimum and the valence band maximum, the lifetime of defect enhanced emission increased by 5 times. Furthermore, one of the competing indirect bandgap emissions disappears, and the defect emission peak dominates the PL spectrum in the wrinkle area with a strain. Furthermore, the discovered principle can be applied to future studies on the integration of enhanced emission and single photon emission involving selectively depopulating the conduction band of the host crystal to defect levels for quantum emitters.« less

Blending block copolymers (BCP) with additives is a useful approach for controlling BCP morphology and properties. In athermal systems, blends of BCPs with polymer additives having very high molecular (M_n) mass generally result in macrophase separation. Bottlebrush polymers, which consist of a linear backbone and grafted side chains, present an interesting alternative where the overall M_n of the system can be very large but the low M_n side chains may drive miscibility with the BCP. Here, in this study, a bottlebrush with a polynorbornene backbone and polystyrene (PS) side chains is blended with PS-b-poly(methyl methacrylate) (PS-b-PMMA) of varying M_n, andmore » the resulting morphologies are examined in both the bulk and thin films. Two different M_n of PS-b-PMMA were used in the bulk study, and the analysis of small-angle X-ray scattering data shows that the blends were miscible and lamellar at all concentrations. This deviates from reference series of both low and high M_n linear polymer additives, which either showed morphological transitions from lamellae to cylinders (low M_n) or were immiscible at all mass fractions studied (high M_n). The relative molecular mass of the side chain (N_SC) and the corresponding component in the BCP (N_A) dictate the distribution of the bottlebrush throughout the BCP, analogous to BCP/linear blends or grafted nanoparticles in a homopolymer matrix. The studies on thin films show a thickness dependence for bottlebrush mass fractions at or above 0.17, a behavior which may be driven by conformational changes of the bottlebrush upon confinement.« less

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D–2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate–2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe₂ and WSe₂ monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHsmore » were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Finally, our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.« less