In the landscape of proton exchange membrane fuel cells (PEMFCs), there is a strong need for durable, low hydrogen crossover membranes that retain high current output and proton conductivity during operation. This study presents the use of UV-Ozone induced defects in graphene to eliminate the proton conductivity penalty commonly associated with traditional crossover mitigation strategies. We report a defect engineered graphene material that demonstrates an increase in hydrogen/proton selectivity of 27%, a decrease in H2 crossover of 24%, with limited to no impact on current output. Furthermore, we demonstrate a membrane that is 39% more durable than state of the art GORE Select membranes and shows no loss in performance after a 100 h accelerated stress test (AST). This study illustrates the viability of 2D material membranes to sieve between H2 and H3O+ in industrial testing conditions and serve as highly scalable and durable fuel cell membranes that represent a significant upgrade over current state of the art membranes for hydrogen fuel cell vehicles and clean energy generation.

The ability to scale two-dimensional (2D) material thickness down to a single monolayer presents a promising opportunity to realize high-speed energy-efficient memristors. Here, we report an ultra-fast memristor fabricated using atomically thin sheets of 2D hexagonal Boron Nitride, exhibiting the shortest observed switching speed (120 ps) among 2D memristors and low switching energy (2pJ). Furthermore, we study the switching dynamics of these memristors using ultra-short (120ps-3ns) voltage pulses, a frequency range that is highly relevant in the context of modern complementary metal oxide semiconductor (CMOS) circuits. We employ statistical analysis of transient characteristics to gain insights into the memristor switching mechanism. Cycling endurance data confirms the ultra-fast switching capability of these memristors, making them attractive for next generation computing, storage, and Radio-Frequency (RF) circuit applications.

Low-temperature large-area growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) is critical for their integration with silicon chips. Especially, if the growth temperatures can be lowered below the back-end-of-line (BEOL) processing temperatures, the Si transistors can interface with 2D devices (in the back end) to enable high-density heterogeneous circuits. Such configurations are particularly useful for neuromorphic computing applications where a dense network of neurons interacts to compute the output. In this work, we present low-temperature synthesis (400 degrees C) of 2D tungsten diselenide (WSe2) via the selenization of the W film under ultrahigh vacuum (UHV) conditions. This simple yet effective process yields large-area, homogeneous films of 2D TMDs, as confirmed by several characterization techniques, including reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and different spectroscopy methods. Memristors fabricated using the grown WSe2 film are leveraged to realize a novel compact neuron circuit that can be reconfigured to enable homeostasis.

Atomically thin 2D materials present the potential for advancing membrane separations via a combination of high selectivity (from molecular sieving) and high permeance (due to atomic thinness). However, the creation of a high density of precise nanopores (narrow-size-distribution) over large areas in 2D materials remains challenging, and nonselective leakage from nanopore heterogeneity adversely impacts performance. Here, we demonstrate protein-enabled size-selective defect sealing (PDS) for atomically thin graphene membranes over centimeter scale areas by leveraging the size and reactivity of permeating proteins to preferentially seal larger nanopores (>= 4 nm) while preserving a significant amount of smaller nanopores (via steric hindrance). Our defect-sealed nanoporous atomically thin membranes (NATMs) show stability up to similar to 35 days during size-selective diffusive separations with a model dialysis biomolecule fluorescein isothiocyanate (FITC)-Ficoll 70 in phosphate buffer saline (PBS) solution as well as outperform state-of-the-art commercially available dialysis membranes (molecular-weight-cutoff similar to 3.5-5 kDa and similar to 8-10 kDa) with significantly higher permeance for smaller solutes KCl (similar to 0.66 nm) similar to 5.1-6 x 10-5 ms-1 and vitamin B12 (B12, similar to 1.5 nm) similar to 2.8-4 x 10-6 ms-1 compared to small protein lysozyme (Lz, similar to 4 nm) similar to 4-6.4 x 10-8 m s-1, thereby allowing unprecedented selectivity for B12/Lz similar to 70 and KCl/Lz similar to 1280. Our work introduces proteins as nanoscale tools for size-selective defect sealing in atomically thin membranes to overcome persistent issues and advance separations for dialysis, protein desalting, small molecule separations/purification, and other bioprocesses.

Threshold switches based on conductive metal bridge devices are useful as selectors to block sneak leakage paths in memristor arrays used in neuromorphic computing and emerging nonvolatile memory. We demonstrate that control of Ag-cation concentration in Al2O3 electrolyte and Ag filament size and density play an important role in the high on/off ratio and self-compliance of metal-ion-based volatile threshold switching devices. To control Ag-cation diffusion, we inserted an engineered defective graphene monolayer between the Ag electrode and the Al2O3 electrolyte. The Ag-cation migration and the Ag filament size and density are limited by the pores in the defective graphene monolayer. This leads to quantized conductance in the Ag filaments and self-compliance resulting from the formation and dissolution of the Ag conductive filament.

Defects in crystalline lattices cause modulation of theatomicdensity, and this leads to variations in the associated electrostaticsat the nanoscale. Mapping these spatially varying charge fluctuationsusing transmission electron microscopy has typically been challengingdue to complicated contrast transfer inherent to conventional phasecontrast imaging. To overcome this, we used four-dimensional scanningtransmission electron microscopy (4D-STEM) to measure electrostaticfields near point dislocations in a monolayer. The asymmetry of theatomic density in a (1,0) edge dislocation core in graphene yieldsa local enhancement of the electric field in part of the dislocationcore. Through experiment and simulation, the increased electric fieldmagnitude is shown to arise from "long-range" interactionsfrom beyond the nearest atomic neighbor. These results provide insightsinto the use of 4D-STEM to quantify electrostatics in thin materialsand map out the lateral potential variations that are important formolecular and atomic bonding through Coulombic interactions.

Moiré superlattices in graphene arise from rotational twists in stacked 2D layers, leading to specific band structures, charge density and interlayer electron and excitonic interactions. The periodicities in bilayer graphene moiré lattices are given by a simple moiré basis vector that describes periodic oscillations in atomic density. The addition of a third layer to form trilayer graphene generates a moiré lattice comprised of multiple harmonics that do not occur in bilayer systems, leading to nontrivial crystal symmetries. Here, we use atomic resolution 4D-scanning transmission electron microscopy to study atomic structure in bilayer and trilayer graphene moiré superlattices and use 4D-STEM to map the electric fields to show subtle variations in the long-range moiré patterns. We show that monolayer graphene folded into an S-bend graphene pleat produces trilayer moiré superlattices with both small (<2°) and larger twist angles (7-30°). Annular in-plane electric field concentrations are detected in high angle bilayers due to overlapping rotated graphene hexagons in each layer. The presence of a third low angle twisted layer in S-bend trilayer graphene, introduces a long-range modulation of the atomic structure so that no real space unit cell is detected. By directly imaging trilayer moiré harmonics that span from picoscale to nanoscale using 4D-STEM, we gain insights into the complex spatial distributions of atomic density and electric fields in trilayer twisted layered materials.

Two-dimensional (2D) materials form heterostructures in both the lateral and vertical directions when two different materials are interfaced, but with totally different bonding mechanisms of covalent in-plane to van der Waal's layered interactions. Understanding how the competition between lateral and vertical forces influences the epitaxial growth is important for future materials development of complex mixed layered heterostructures. Here, we use atomic-resolution annular dark-field scanning transmission electron microscopy to study the detailed atomic arrangements at mixed 2D heterostructure interfaces composed of two semiconductors with distinctly different crystal symmetry and elemental composition, Pd2Se3:MoS2, in order to understand the role of different chemical bonds on the resultant epitaxy. Pd2Se3 is grown off the step edge in bilayer MoS2, and the vertical and lateral epitaxial relationships of the Pd2Se3-MoS2 heterostructures are investigated. We find that the similarity of geometry at the interface with one metal (Pd or Mo) atoms bonded with two chalcogens (S or Se) are the crucial factors to make the atomically stitched lateral junction of 2D heterostructures. In addition, the vertical van der Waal interactions that are normally dominant in layered materials can be overcome by in-plane forces if the interfacial atomic stitching is high in quality and low in defect density. This knowledge should help guide the approaches for improving the epitaxy in mixed 2D heterostructures and seamless stitching of in-plane 2D heterostructures with various complex monolayer structures.

Palladium selenides, relatively unexplored platinum group metal chalcogenides, have been attracting significant interest due to their versatility in different applications and their numerous stable phases with different crystal structures. Most vapor-based bottom-up growth approaches using furnaces have resulted in the layered PdSe2 phase due to the reactions occurring in a saturated chalcogen vapor environment, in which PdSe2 is the most stable phase. Here, we show that precise control over the Pd/Se stoichiometry can be achieved through tuning the chalcogen flux, both incoming and outgoing, resulting in control over the local Se concentration on the growth substrate. Increasing temperatures are used to increase the outgoing flux of Se, leading to a phase transition from PdSe2 to Pd17Se15. Increasing the incoming flux of Se, through high mass loading, is then used to tune the reaction back to the growth of the PdSe2 phase. Detailed material characterization is performed to prove the different crystal structures and show the ability to tailor the Pd/Se family across two stable phases, with one being layered (PdSe2) and the other being three-dimensional (3D) bulk (Pd17Se15). Finally, we demonstrate the lifecycle of phase tuning by transforming PdSe2 into Pd17Se15 and then back again, through control of temperature and Se mass loading. These results open pathways for exploring the solid-state properties of this family of Pd-Se materials in optoelectronics and catalysis, which have been limited to date for vapor-based furnace growth approaches on substrates.

Four-dimensional (4D) scanning transmission electron microscopy is used to study the electric fields at the edges of 2D semiconducting monolayer MoS2. Sub-nanometer 1D features in the 2D electric field maps are observed at the outermost region along zigzag edges and also along nanowire MoS-terminated MoS2 edges. Atomic-scale oscillations are detected in the magnitude of the 1D electromagnetic edge state, with spatial variations that depend on the specific periodic edge reconstructions. Electric field reconstructions, along with integrated differential phase contrast reconstructions, reveal the presence of low Z number atoms terminating many of the uniform edges, which are difficult to detect by annular dark field scanning transmission electron microscopy due to its limited dynamic range. Density functional theory calculations support the formation of periodic 1D edge states and also show that enhancement of the electric field magnitude can occur for some edge terminations. The experimentally observed electric fields at the edges are attributed to the absence of an opposing electric field from a nearest neighbor atom when the electron beam propagates through the 2D monolayer and interacts. These results show the potential of 4D-STEM to map the atomic scale structure and fluctuations of electric fields around edge atoms with different bonding states than bulk atoms in 2D materials, beyond conventional imaging.

Artificially introduced small twist angles at the interfaces of vertical layered heterostructures (VLHs) have allowed deterministic tuning of electronic and optical properties such as strongly correlated electronic phases and Moiré excitons. But creating a Moiré twist in van der Waals (vdWs) systems by manual stacking is challenging in reproducibility, uniformity, and accuracy of the twist angle, which hinders future studies. Here, it is demonstrated that contrary to the commonly believed 0°-orientation in vdWs epitaxy, these VLHs show small twist angles controlled by the low-order commensurate phase with low energy and local atomic relaxation. A commensurate multilevel map is proposed to predict possible orientations. Remarkably, high-mismatch VLHs show discrete and sometimes non-zero twist angles dependent on their natural mismatch value. Such framework is experimentally confirmed in five epitaxially grown VLHs under high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and can provide significant insights for large-scale engineering of twist angle in VLHs.

Resistive switching in 2D materials such as hexagonal boron nitride (hBN) and Transition Metal Dichalcogenides (TMDs) have been demonstrated recently [1]–[3]. These memory devices with an ultra-thin switching layer have the potential to achieve low operating voltages, low variability and are also suitable for flexible electronic applications [4]. Here we report the first experimental observation of sub-nanosecond switching of 2D hBN based resistive random access memory (RRAM) devices. This is the fastest switching speed in 2D RRAMs, surpassing the previously reported 5ns switching [5]. Devices also exhibit consistent repeatable switching between high-low memory states with ultra-short pulses (pulse-width ~ 2.7ns).