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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

A Two-Dimensional Manganese Gallium Nitride Surface Structure Showing Ferromagnetism at Room Temperature

Practical applications of semiconductor spintronic devices necessitate ferromagnetic behavior at or above room temperature. In this paper we demonstrate a two-dimensional manganese gallium nitride surface structure (MnGaN-2D) which is atomically thin and shows ferromagnetic domain structure at room temperature as measured by spin-resolved scanning tunneling microscopy and spectroscopy. Application of small magnetic fields proves that the observed magnetic domains follow a hysteretic behavior. Two initially oppositely-oriented MnGaN-2D domains are rotated into alignment with only 120 mT and remain mostly in alignment at remanence. Additionally, the measurements are further supported by first-principles theoretical calculations which reveal highly spin-polarized and spin-split surface states, with spin polarization of up to 95% for manganese partial density of states.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Aberration-corrected STEM imaging of 2D materials: Artifacts and practical applications of threefold astigmatism

High-resolution scanning transmission electron microscopy (HR-STEM) with spherical aberration correction enables researchers to peer into two-dimensional (2D) materials and correlate the material properties with those of single atoms. The maximum intensity of corrected electron beam is confined in the area having sub-angstrom size. Meanwhile, the residual threefold astigmatism of the electron probe implies a triangular shape distribution of the intensity, whereas its tails overlap and thus interact with several atomic species simultaneously. The result is the resonant modulation of contrast that interferes the determination of phase transition of 2D materials. Here, we theoretically reveal and experimentally determine the origin of resonant modulation of contrast and its unintended impact on violating the power-law dependence of contrast on coordination modes between transition metal and chalcogenide atoms. The finding illuminates the correlation between atomic contrast, spatially inequivalent chalcogenide orientation, and residual threefold astigmatism on determining the atomic structure of emerging 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Engineering 2D Square Lattice Hubbard Models in 90° Twisted GeX/SnX (X =S, Se) Moiré Superlattices

Because of the large-period superlattices emerging in moiré two-dimensional (2D) materials, electronic states in such systems exhibit low energy flat bands that can be used to simulate strongly correlated physics in a highly tunable setup. While many investigations have thus far focused on moiré flat bands and emergent correlated electron physics in triangular, honeycomb, and quasi-one-dimensional lattices, tunable moiré realizations of square lattices subject to strong correlations remain elusive. Here, in this work, we propose a feasible scheme to construct moiré square lattice systems by twisting two or more layers of 2D materials in a rectangular lattice by 90°. We demonstrate the concept with twisted GeX/SnX (X =S, Se) moiré superlattices and calculate their electronic structures from first principles. We show that the lowest conduction flat band in these systems can be described by a square lattice Hubbard model with parameters which can be controlled by varying the choice of host materials, number of layers, and external electric fields. In particular, twisted double bilayer GeSe realizes a square lattice Hubbard model with strong frustration due to the next-nearest-neighbor hopping that could host unconventional superconductivity, in close analogy to the Hubbard model for copper-oxygen planes of cuprate high-temperature superconductors. The presented scheme uses 90° twisted 2D materials with rectangular unit cells as a promising platform for realizing the physical phenomena of square lattice Hubbard models, establishing a new route for studying its rich phase diagram of magnetism, charge order, and unconventional superconductivity in a highly tunable setting.

2-dimensional systems↗
DOE OSTI2015

Monolayer PtSe 2 , a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt

For single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. We found that a combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Selected Area Manipulation of MoS 2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing

Here we demonstrate direct-write patterning of single and multilayer MoS 2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF 2 precursor. MoS 2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS 2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source–drain current at constant gate and source–drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS 2 devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

van der Waals Epitaxy Growth of Bi 2 Se 3 on a Freestanding Monolayer Graphene Membrane: Implications for Layered Materials and Heterostructures

Research on two-dimensional and layered materials has expanded over the past 2 decades because of their unique properties and application potential. The key hurdles in realizing this potential are the challenges in controlling their atomic structure and their incompatibility with existing semiconductor nanofabrication techniques. Here we report on high-quality van der Waals epitaxial growth and characterization of a layered topological insulator on freestanding monolayer graphene transferred to different mechanical supports. This “templated” synthesis approach enables direct interrogation of the interfacial atomic structure of the as-grown materials and opens a route toward creating device structures with more traditional semiconductor nanofabrication techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Growth of Nanometer-Thick γ-InSe on Si(111) 7 × 7 by Molecular Beam Epitaxy for Field-Effect Transistors and Optoelectronic Devices

γ-InSe is a semiconductor that holds promising potential in high-performance field-effect transistors and optoelectronic devices. Large-scale, single-phase γ-InSe deposition has proven challenging because of the difficulty in precise control of stoichiometry and the coexistence of different indium selenide phases. In this study, we demonstrate the wafer-scale combinatorial approach to map out the growth window as functions of the Se/In ratio and growth temperature for γ-InSe on the Si(111) 7 × 7 substrate in molecular beam epitaxy. X-ray diffraction (XRD) was used to identify the indium selenide phases, while atomic force microscopy revealed four distinct surface morphologies of γ-InSe, enabling a discussion of the growth mechanisms associated with each morphology. Cross-sectional atomic resolution scanning transmission electron microscopy confirmed that the film was of high crystalline quality and had nearly single-phase γ-InSe. In conclusion, our comprehensive study elucidates the In–Se phase map for thin film growth parameters, providing invaluable landmarks for the reproducible synthesis of high-quality γ-InSe layers.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Atomically Thin Metal Sulfides

We developed a method to colloidally synthesize atomically thin metal sulfides (ATMS). Unlike conventional 2D systems such as MoS 2 and graphene, none of the systems developed here are inherently layered compounds nor have known layered polymorphs in their bulk forms. The synthesis proceeds via a cation-exchange reaction starting from single- and multi-layer Ag 2 S and going to various metal sulfides. The synthesized ATMS retain their size and shape during the cation-exchange reaction and are either single-layer or a few-layer, depending on the starting Ag 2 S samples. They have lateral dimensions on the order of 5-10 nm and are colloidally stabilized by Z- and L-type ligands. Here, we demonstrate the synthesis of single-layer and a few-layer ZnS, CdS, CoS 2 , and PbS. We find that the optical properties of these ATMS are quite distinct from the platelet or quantum-dot versions of the same metal sulfides.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Optical manipulation of valley pseudospin

The coherent manipulation of spin and pseudospin underlies existing and emerging quantum technologies, including quantum communication and quantum computation. Valley polarization, associated with the occupancy of degenerate, but quantum mechanically distinct valleys in momentum space, closely resembles spin polarization and has been proposed as a pseudospin carrier for the future quantum electronics. Valley exciton polarization has been created in the transition metal dichalcogenide monolayers using excitation by circularly polarized light and has been detected both optically and electrically. In addition, the existence of coherence in the valley pseudospin has been identified experimentally. The manipulation of such valley coherence has, however, remained out of reach. In this paper, we demonstrate all-optical control of the valley coherence by means of the pseudomagnetic field associated with the optical Stark effect. Using below-bandgap circularly polarized light, we rotate the valley exciton pseudospin in monolayer WSe 2 on the femtosecond timescale. Both the direction and speed of the rotation can be manipulated optically by tuning the dynamic phase of excitons in opposite valleys. Finally, this study unveils the possibility of generation, manipulation, and detection of the valley pseudospin by coupling to photons.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Nanoscale Turing patterns in a bismuth monolayer

Turing’s reaction–diffusion theory of morphogenesis has been very successful for understanding macroscopic patterns within complex objects ranging from biological systems to sand dunes. However, Turing patterns on microscopic length scales are extremely rare. Here we show that a strained atomic bismuth monolayer assembled on the surface of NbSe 2 —and subject to interatomic interactions and kinetics—displays Turing patterns. Our reaction–diffusion model produces stripe patterns with a period of five atoms (approximately 2 nm) and domain walls with Y-shaped junctions that bear a striking resemblance to what has been experimentally observed. Our work establishes that Turing patterns can occur at the atomic scale in a hard condensed-matter setting.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Negative Differential Conductance & Hot-Carrier Avalanching in Monolayer WS2 FETs

The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS 2 field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS 2 due to the narrow width of its energy bands. The various results presented here suggest that WS 2 exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.

36 MATERIALS SCIENCE↗
DOE OSTI2023

MXene/graphene oxide nanocomposites for friction and wear reduction of rough steel surfaces

Development of solid lubricant materials that render reliable performance in ambient conditions, are amenable to industrial size and design complexities, and work on engineered surfaces is reported. These coatings are composed of Ti 3 C 2 T x -Graphene Oxide blends, spray-coated onto bearing steel surfaces. The tribological assessment was carried out in ambient environmental conditions and high contact pressures in a ball-on-disc experimental set-up. The evaluation yielded that the use of Ti 3 C 2 T x -Graphene-Oxide coatings led to substantial reduction in friction down to 0.065 (at 1 GPa contact pressure and 100 mm/s) in comparison to the uncoated of single-component-coated surfaces, surpassing the state-of-the-art. The coatings also provided excellent protection against wear loss of the substrate and counter-face. The results were explained based on the observations from Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and nanoindentation measurements. In operando formation of a dense, hard and stiff, dangling-bond-saturated tribolayer was observed to be the reason for the sustained lubricity even at high test loads and sliding speeds. This report presents the holistic exploration and correlation of structure-property-processing pertaining to the advancement of solid lubrication science.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Effect of chemical substitution and external strain on phase stability and ferroelectricity in two dimensional M 2 CT 2 MXenes

Two dimensional ferroelectric materials are gaining increasing attention for use in ultrathin electronic devices owing to the presence of a spontaneous polarization down to one or two monolayers. However, such materials are difficult to identify, especially those with out-of-plane electric polarizations. Previous work predicted that a metastable ferroelectric phase exists in the 2D MXene Sc 2 CO 2 , while further studies have predicted that this phase exists in other MXene chemistries. However, questions remain about the origin of ferroelectricity, the stability of this phase relative to other competing phases, and the effect of external stimuli in these materials. In this work, we use density functional theory calculations to investigate 12 M 2 CT 2 MXenes (M = transition metal, T = surface terminating group) and determine which have the ferroelectric phase as their ground state. We compute these materials’ polarizations, densities of states, phonon band structures, Bader charges, and Born effective charges in the ferroelectric phase to elucidate the reasons for its stabilization. We demonstrate that this ferroelectric phase can be preferentially stabilized in non-ferroelectric MXenes through full chemical substitution of Sc or O, alloying of the Sc sites, or application of epitaxial strain. Finally, we show that these materials have excellent piezoelectric properties as well. This work provides a detailed understanding of ferroelectric MXenes and show how the number of 2D ferroelectric materials can be increased through chemical substitution or application of external stimuli.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomically sharp jagged edges of chemical vapor deposition-grown WS 2 for electrocatalysis

Chemical vapor deposition (CVD)-grown 2D transition metal dichalcogenides can adopt faceted edges. To investigate how sharp these sites can be, we utilize aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) to resolve the atomic structure of two-dimensional (2D) WS 2 domains that show jagged edges. Nanoscale triangular edge structures with S zigzag terminations are observed. Both the peak and valley regions exhibit near-atomic sharpness. The peaks are as sharp as two atoms in width. Highly ordered valley sites display a minimum width of three atoms, and prospective single-atom valleys appear as two-atom-wide sites in the ADF-STEM contrast. Regarding the kinetics, density-functional theory (DFT) calculations indicate that the 2-W-atom peak would not evolve into a single-W-atom peak even though the latter configuration is also stable with a high enough binding energy under the growth conditions. These results help deepen our understanding of the possible structuring at the nanoscale and the atomic-scale limits of peaks and valleys formed via intersection of two zigzag edges. The enriched edge sites lead to higher catalytic activities for the hydrogen evolution reaction (HER).

36 MATERIALS SCIENCE↗
DOE OSTI2020

Multiscale computational understanding and growth of 2D materials: a review

Abstract The successful discovery and isolation of graphene in 2004, and the subsequent synthesis of layered semiconductors and heterostructures beyond graphene have led to the exploding field of two-dimensional (2D) materials that explore their growth, new atomic-scale physics, and potential device applications. This review aims to provide an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and discuss how they can be utilized to assist/guide the design and synthesis of 2D materials beyond graphene. We focus on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations including density functional theory (DFT) calculations and molecular dynamics simulations employing empirical and reactive interatomic potentials; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. We discuss how machine learning can be combined with computation and experiments to understand the correlations between structures and properties of 2D materials, and to guide the discovery of new 2D materials. We will also provide an outlook for the applications of computational approaches to 2D materials synthesis and growth in general.

Momeni, Kasra↗
DOE OSTI2018

Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts

Oxygen evolution reaction (OER) electrocatalysts with high activity, high stability, and low costs are needed for proton-exchange membrane (PEM) electrolyzers. Based on the high cost and limited supply of iridium, approaches that result in iridium-based OER catalysts with increased catalytic activity are of significant interest. Here, we report a carbon-free, self-supported hydrous iridium-nickel oxide two-dimensional nanoframe structure synthesized by thermal treatment of iridium-decorated nickel oxide nanosheets under reducing conditions and subsequent chemical leaching in acid. The catalyst nanoarchitecture contains an interconnected network of metallic iridium-nickel alloy domains with hydrous iridium oxide and nickel oxide located in the surface region. The electrochemical oxidation step maintains the three-dimensional nanoarchitecture and results in a thin (~5 A) oxide/hydroxide surface layer. The temperature used for thermal reduction was found to strongly affect the catalyst surface structure and OER activity. Utilizing a lower thermal reduction temperature of 200 degrees C was determined to provide a higher relative surface concentration of hydroxides and nickel oxide and result in higher OER activities compared with materials treated at 300 degrees C. The 200 degrees C-treated hydrous iridium-nickel oxide electrocatalyst showed 15 times higher initial OER mass activity than commercial IrO 2 , and the activity remained 10 times higher than IrO 2 after accelerated durability testing. Density functional theory (DFT) calculations and analysis of the experimental Tafel slopes support that the second electron transfer step is the rate-limiting step for the reaction. The DFT calculations demonstrate that Ni substitution on the IrO 2 surface lowers the activation energy for adsorbed intermediates of the second electron transfer step of the OER reaction. This work establishes that noble metal-decorated metal oxide nanosheets can be transformed into high surface area, carbon-free electrocatalyst nanostructures with high catalytic activities and molecular accessibility and reveals the importance of using controlled thermal reduction temperatures to alter the surface structure and OER activity.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetism in Moiré Materials

This project focused on the theory of the electronic properties of magnetic moire materials. Moire materials can be formed by overlaying two or more two-dimensional crystals that are either semiconductors or semimetals and establishing a moire pattern. Low-energy electronic properties are then described by a continuum model with crystalline periodicity. Moire materials are therefore artificial two-dimensional crys- tals whose periodicity is at the tens of nanometers scale, increasing the unit cell area by a factor of about 10,000 compared to real crystals. They are important because the main tuning knob of chemistry, the number of electrons per effective atom or molecule (the analog of the number of electrons per unit cell in real crystals), can be shifted by more than one simply by using electrical gates. The research covered two diffeerent classes of moire materials, graphene multilayers twisted to a rotation angle at which correlations are strong, and transition metal dichalcogenide semiconductors. The proposal identifed one project in each area both related very broadly to magnetism: i) Advancing understanding of broken spin-valley and sublattice symmetries in Magic Angle Twisted Bilayer Graphene and ii) Developing the theory of itinerant electron magnetism in Gamma-valley and K-valley transition metal dichalcogenide moire materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Enhanced Catalysis under 2D Silica: A CO Oxidation Study

Interfacially confined microenvironments have recently gained attention in catalysis, as they can be used to modulate reaction chemistry. The emergence of a 2D nanospace at the interface between a 2D material and its support can promote varying kinetic and energetic schemes based on molecular level confinement effects imposed in this reduced volume. We report on the use of a 2D oxide cover, bilayer silica, on catalytically active Pd(111) undergoing the CO oxidation reaction. We “uncover” mechanistic insights about the structure-activity relationship with and without a 2D silica overlayer using in situ IR and X-ray spectroscopy and mass spectrometry methods. We find that the CO oxidation reaction on Pd(111) benefits from confinement effects imposed on surface adsorbates under 2D silica. Finally, this interaction results in a lower and more dispersed coverage of CO adsorbates with restricted CO adsorption geometries, which promote oxygen adsorption and lay the foundation for the formation of a reactive surface oxide that produces higher CO 2 formation rates than Pd alone.

36 MATERIALS SCIENCE↗