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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

2D Electrets of Ultrathin MoO 2 with Apparent Piezoelectricity

Since graphene, a variety of 2D materials have been fabricated in a quest for a tantalizing combination of properties and desired physiochemical behavior. 2D materials that are piezoelectric, i.e., that allow for a facile conversion of electrical energy into mechanical and vice versa, offer applications for sensors, actuators, energy harvesting, stretchable and flexible electronics, and energy storage, among others. Unfortunately, materials must satisfy stringent symmetry requirements to be classified as piezoelectric. Here, 2D ultrathin single-crystal molybdenum oxide (MoO 2 ) flakes that exhibit unexpected piezoelectric-like response are fabricated, as MoO 2 is centrosymmetric and should not exhibit intrinsic piezoelectricity. However, it is demonstrated that the apparent piezoelectricity in 2D MoO 2 emerges from an electret-like behavior induced by the trapping and stabilization of charges around defects in the material. Arguably, the material represents the first 2D electret material and suggests a route to artificially engineer piezoelectricity in 2D crystals. Specifically, it is found that the maximum out-of-plane piezoresponse is 0.56 pm V -1 , which is as strong as that observed in conventional 2D piezoelectric materials. The charges are found to be highly stable at room temperature with a trapping energy barrier of ≈2 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Mott localization in the van der Waals crystal CrI 3 : A GGA+DMFT study

Using the generalized gradient approximation plus dynamical mean-field theory (GGA+DMFT) we confirm the importance of multi-orbital dynamical correlations in determining the paramagnetic insulating state of CrI 3 . Furthermore, while the ferromagnetic phase reveals weak electronic correlation effects due to strong spin-orbital polarization, the Mott insulating state of paramagnetic CrI 3 crystal is shown to be driven by the interplay between orbital-dependent one-electron lineshape and multi-orbital electronic interactions. To probe the paramagnetic Mott insulating state we performed x-ray absorption spectroscopy (XAS) measurements for the two structural phases of CrI 3 . Our study is relevant to understanding the orbital-selective electronic structure reconstruction of Mott insulators and should be applicable to other van der Waals bonded materials from bulk to the ultrathin limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Ultrathin epitaxial MgB 2 on SiC: Substrate surface-polarity-dependent properties

High-quality, ultrathin superconducting films are required for advanced devices such as hot-electron bolometers, superconducting nanowire single-photon detectors, and quantum applications. Using hybrid physical-chemical vapor deposition, we show that MgB 2 films as thin as 4 nm can be fabricated on the carbon-terminated 6⁢H-SiC (0001) surface with a superconducting transition temperature above 33 K and a rms roughness of 0.7 nm. Remarkably, the film quality is a function of the SiC surface termination, with the C-terminated surface preferred to the Si-terminated surface. To understand the MgB 2 thin film/SiC substrate interactions giving rise to this difference, we characterized the interfacial structures using Rutherford backscattering spectroscopy/channeling, electron-energy-loss spectroscopy, and x-ray photoemission spectroscopy. The MgB 2 /SiC interface structure is complex and different for the two terminations. Both terminations incorporate substantial unintentional oxide layers influencing MgB 2 growth and morphology, but with a different extent, intermixing, and interface chemistry. In this paper, we report measurements of transport, resistivity, and the critical superconducting temperature of MgB 2 /SiC that are different for the two terminations, and they link interfacial structure variations to observed differences. The result shows that the C face of SiC is a preferred substrate for the deposition of ultrathin superconducting MgB 2 films.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Fabrication of ultrathin suspended membranes from atomic layer deposition films

In this report, ultrathin films suspended as freestanding membranes are critical to many microelectronic and materials science applications. However, fabrication methods are currently limited in either their flexibility, due to material selectivity issues during the final membrane release, or their scalability. Here, we demonstrate a novel fabrication process for suspending ultrathin films with thicknesses as low as 4 nm and lateral dimensions up to 20 x 1000 μm from a variety of materials grown by atomic layer deposition. A silicon nitride membrane serves as the support for a sacrificial polymer layer and an ultrathin atomic layer deposition film which, after plasma etching, will form the membrane. The high chemical selectivity between atomic layer deposition-grown transition metal nitrides and oxides and the sacrificial polymer means that ultrathin films of a variety of materials can be released without damage using a single process. Electrically conductive titanium nitride membranes can be produced by this method and are of significant interest for electron microscopy applications. Electron transparency of titanium nitride membranes was found to be ~14% higher than silicon nitride of the same thickness, and of similar conductivity to graphite, meaning that ultrathin, conductive, and electron transparent membranes can be fabricated at scale. These membranes are ideal supports for electron and photon characterization techniques, as well as microelectromechanical system applications that require a conductive membrane.

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 OSTI2021

Imaging Electrons in Two Dimensional Materials (Final report)

Two-dimensional (2D) materials have extraordinary characteristics that offer promising new approaches for science and technology. Electrons in graphene can move ballistically through a sheet, even though it is only a single atom thick. And transition metal dichalcogenide materials can be cleaved into 2D flakes of all types – metals, semiconductors, insulators, magnetic materials, and superconductors. To benefit from these discoveries, the science needs to be understood to transform 2D materials into useful new devices and systems. The evolution in time of atomic scale graphene structures has been studied in time with a transmission electron microscope (TEM), and ballistic transport of electrons in graphene was been imaged using a cooled scanning probe microscope (SPM), as well as electron motion in MoS 2 . Graphene is only one atom thick but is extremely strong, creating the opportunity to fabricate atomic scale structures in the lateral direction. Using an atomic resolution TEM, a suspended sheet of graphene can be shaped by a Si impurity atom on its surface that acts like a chisel to open up apertures, one atom at a time. The electrons in the TEM provide both the chiseling energy and the ability to image the results. Electrons and holes in graphene form a new type of electronic system with conical conduction and valence bands that meet at the Dirac point, with no energy gap. For moderate densities, the carriers form a Fermi liquid with ballistic transport over micron-scale distances. Using a cooled SPM the cyclotron orbit of electrons in graphene has been imaged. In the magnetic focusing regime, electrons leaving a point contact circle around and leave from a second point contact when the cyclotron diameter is equal to the contact spacing. The paths of electrons are focused - they enter at different angles but converge again at the exiting contact. When the SPM tip knocks an electron out of its orbit, an imaging signal is created. The ballistic motion of carriers in graphene opens the way for ballistic devices that manipulate beams of electrons and holes. Our collaborator Gil-Ho Lee, in Philip Kim's group, created a collimating contact by placing zig-zag absorbers on either side of entering electrons. Our cooled SPM was used to image the electron beam and determine its 9-degree halfwidth. By changing the gate voltage, a collimated beam of holes was also created, opening the way for colliding beam experiments. Coherent beams of carriers are desirable for quantum information processing. Via Andreev reflection, superconducting contacts can covert Cooper pairs in a superconductor to an ingoing and outgoing electrons and holes. Using our cooled SPM, Andreev reflection was imaged from a superconducting contact on a graphene device. Magnetic focusing was used to create an incoming beam of electrons and an outgoing beam of holes, detected by a third point contact. The images show a clear transition from normal reflection above the superconducting transition temperature to Andreev reflection as the device is cooled. Transition metal dichalcogenides offer a wide array of materials that can be exfoliated into ultrathin 2D sheets. A cooled SPM can be used to detect quantum dots as well as to image electron flow. The tip charge capacitively couples to electrons on the dot, acting as a gate to tune the dot conductance. Coulomb blockade peaks appear as a bullseye pattern in an SPM image as the tip is raster scanned above the dot. This approach was used to detect quantum dots in a MoS 2 channel as the carrier density was reduced and electrons pooled in low energy points. Through our DOE supported research, cooled SPM imaging has proven to be a very useful tool to uncover the motion of electrons and holes in the new quantum materials graphene and MoS 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Tailorable multifunctionalities in ultrathin 2D Bi-based layered supercell structures

Two-dimensional (2D) materials with robust ferromagnetic behavior have attracted great interest because of their potential applications in next-generation nanoelectronic devices. Aside from graphene and transition metal dichalcogenides, Bi-based layered oxide materials are a group of prospective candidates due to their superior room-temperature multiferroic response. In this work, an ultrathin Bi 3 Fe 2 Mn 2 O 10+δ layered supercell (BFMO322 LS) structure was deposited on an LaAlO 3 (LAO) (001) substrate using pulsed laser deposition. Microstructural analysis suggests that a layered supercell (LS) structure consisting of two-layer-thick Bi–O slabs and two-layer-thick Mn/Fe–O octahedra slabs was formed on top of the pseudo-perovskite interlayer (IL). A robust saturation magnetization value of 129 and 96 emu cm -3 is achieved in a 12.3 nm thick film in the in-plane (IP) and out-of-plane (OP) directions, respectively. The ferromagnetism, dielectric permittivity, and optical bandgap of the ultrathin BFMO films can be effectively tuned by thickness and morphology variation. In addition, the anisotropy of all ultrathin BFMO films switches from OP dominating to IP dominating as the thickness increases. This study demonstrates the ultrathin BFMO film with tunable multifunctionalities as a promising candidate for novel integrated spintronic devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2026

Magnon confinement in epitaxial antiferromagnetic oxide heterostructures.

Magnons, the quanta of spin waves, have been extensively studied in a range of materials for spintronics, particularly for non-volatile logic-in-memory devices. Controlling magnons in conventional antiferromagnets and harnessing them in practical applications, however, remains a challenge. Here, we demonstrate highly efficient magnon transport in a LaFeO3/ BiFeO3/ LaFeO3 all-antiferromagnetic system, which can be controlled electrically, making it highly desirable for energy-efficient computation. Leveraging spin-orbit-driven spin-charge transduction, we demonstrate that this material architecture permits magnon confinement in ultrathin antiferromagnets, enhancing the output voltage generated by magnon transport by several orders of magnitude, which provides a pathway to enable magnetoelectric memory and logic functionalities. Additionally, the non-volatility of output voltage enables ultralowpower logic-in-memory processing, where magnonic devices can be efficiently reconfigured via electrically controlled magnon spin currents within magnetoelectric channels.

Husain, Sajid↗
DOE OSTI2020

Spin-orbit-proximitized ferromagnetic metal by monolayer transition metal dichalcogenide: Atlas of spectral functions, spin textures, and spin-orbit torques in Co/MoSe 2 , Co/WSe 2 and Co/TaSe 2 heterostructures

In this work, the heterostructures composed of an ultrathin ferromagnetic metal (FM) and a material hosting strong spin-orbit (SO) coupling are the principal resource for SO torque and spin-to-charge conversion nonequilibrium effects in spintronics. The key quantity in theoretical description of these effects is nonequilibrium spin density, which can appear on any monolayer of the heterostructure through which the current is flowing as long as the monolayer bands are affected by the native or proximity induced SO coupling. Here we demonstrate how hybridization of wave functions of Co layer and a monolayer of transition metal dichalcogenides (TMDs)—such as semiconducting MoSe 2 and WSe 2 or metallic TaSe 2 —can lead to dramatic transmutation of electronic and spin structure of Co within some distance away from its interface with TMD, when compared to the bulk of Co or its surface in contact with a vacuum. This is due to proximity induced SO splitting of Co bands encoded in the spectral functions and spin textures on its monolayers, which we obtain using noncollinear density functional theory (ncDFT) combined with equilibrium Green function (GF) calculations. In fact, SO splitting is present due to structural inversion asymmetry of the bilayer—i.e., just the presence of the interface—even if SO coupling within TMD monolayer is artificially switched off in ncDFT calculations. However, switching it on makes the effects associated with proximity SO coupling within Co layer about five times larger. Injecting spin-unpolarized charge current through SO-proximitized monolayers of Co generates nonequilibrium spin density over them, so that its cross product with the magnetization of Co determines SO torque. The SO torque computed via first-principles quantum transport methodology, which combines ncDFT with nonequilibrium GF calculations, can be used as the screening parameter to identify optimal combination of materials and their interfaces for applications in spintronics. In particular, we identify heterostructure two-monolayer-Co/monolayer-WSe 2 as the most optimal one, at least in the clean limit.

2-dimensional systems↗
DOE OSTI2023

Doubling the superconducting transition temperature of ultraclean wafer-scale aluminum nanofilms

Here, we studied the role of reduced dimensionality and disorder in the superconducting properties of wafer-scale aluminum (Al) nanofilms. This new generation of ultrathin films were grown using molecular beam epitaxy and depict normal-state sheet resistance at least 20 times lower than the quantum resistance ℎ⁡/(4⁢𝑒 2 ). Defying general expectations, the superconducting transition temperature of our films increases with decreasing Al film thickness, reaching 2.4 K for a 3.5-nm-thick Al film grown on GaAs: twice that of bulk Al (1.2 K). Surface phonon softening is shown to impact superconductivity in pure ultrathin films, offering a route for materials engineering in two dimensions.

2-dimensional systems↗
DOE OSTI2022

Emergent ferroelectricity in subnanometer binary oxide films on silicon

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. Here, we report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO 2 ) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO 2 , conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

42 ENGINEERING↗
DOE OSTI2020

Broadening the Gas Separation Utility of Monolayer Nanoporous Graphene Membranes by an Ionic Liquid Gating

Ultrathin two-dimensional (2D) monolayer atomic crystal materials offer great potential for extending the field of novel separation technology due to their infinitesimal thickness and mechanical strength. One difficult and ongoing challenge is to perforate the 2D monolayer material with subnanometer pores with atomic precision for sieving similarly sized molecules. Here, we demonstrate the exceptional separation performance of ionic liquid (IL)/graphene hybrid membranes for challenging separation of CO 2 and N 2 . Notably, the ultrathin ILs afford dynamic tuning of the size and chemical affinity of nanopores while preserving the high permeance of the monolayer nanoporous graphene membranes. The hybrid membrane yields a high CO 2 permeance of 4000 GPU and an outstanding CO 2 /N 2 selectivity up to 32. Finally, this rational hybrid design provides a universal direction for broadening gas separation capability of atomically thin nanoporous membranes.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Suppressing Atmospheric Degradation of Sulfide-Based Solid Electrolytes via Ultrathin Metal Oxide Layers

Sulfide-based solid-state electrolytes (SSEs) are promising materials with superior Li-ion conductivity; however, their poor atmospheric stability limits commercial manufacturing at scale. Here, we investigate the impact of ultrathin metal oxide layers deposited via atomic layer deposition (ALD) on the stability of Li 6 PS 5 Cl (LPSCl). Al 2 O 3 layers grown directly on LPSCl particles significantly stabilize the surface chemistry and Li-ion transport properties relative to uncoated material upon exposure to both an ambient atmosphere (22% relative humidity, RH) and humidified O 2 (100% RH). Detailed investigations indicate that coatings impede the surface and bulk degradation kinetics of exposed materials, even for coatings as thin as ∼1 Å. Furthermore, this suggests that stabilization is due to more than just a physical barrier. Shifts in valence band edge positions of coated LPSCl indicate that ALD coatings alter the surface electronic structure and resulting oxidation tendency of underlying LPSCl, suggesting new avenues to improving the environmental stability of sulfide SSEs.

Atmospheric chemistry↗
DOE OSTI2021

Preparation of large ultrathin free-standing polymer films

A method of making large ultrathin free-standing polymer films without use of a sacrificial layer includes the steps of providing a substrate, applying a polyelectrolyte material to said substrate, applying a polymer material onto said substrate and onto said polyelectrolyte material, and directly delaminating said polymer material from said substrate and said polyelectrolyte to produce the ultrathin free-standing polymer film.

Stadermann, Michael↗
DOE OSTI2020

Robust natural nanocomposites realizing unprecedented ultrafast precise molecular separations

Synthetic polymer membranes can potentially reduce the large energy and carbon footprints that are typically associated with traditional chemical separation technologies. Unfortunately, current production protocols negate the green benefits of membrane separation. To address this bottleneck, here we report the use of natural materials monosaccharide – glucose and polydopamine and Zr-based metal organic frameworks (MOFs) to fabricate ultrathin nanocomposite membranes via interfacial polymerization reaction. The synergistic effect of these three materials on angstrom-scale molecular transport both in organic solvent and aqueous environment was elucidated using a series of complementary techniques. We demonstrate such nature-inspired nanocomposite membranes enable structural stability even in polar aprotic solvents, and unparalleled ultra-fast, low-pressure, precise separations in both nanofiltration modes, which easily surpass state-of-the-art membranes relying on unsustainable materials. Finally, the multi-functionality of saccharide nanocomposites was elegantly harnessed to impact separation applications that contribute towards a better living environment.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Metal fluoride passivation coatings prepared by atomic layer deposition for Li-ion batteries

The fabrication of robust interfaces between transition metal oxides and non-aqueous electrolytes is one of the great challenges of lithium ion batteries. Atomic layer deposition (ALD) of aluminum tungsten fluoride (AlWxFy) improves the electrochemical stability of LiCoO2. AlWxFy thin films were deposited by combining trimethylaluminum and tungsten hexafluoride. in-situ quartz crystal microbalance and transmission electron microscopy studies show that the films grow in a layer-by-layer fashion and are amorphous nature. Ultrathin AlWxFy coatings (<10 Å) on LiCoO2 significantly enhance stability relative to bare LiCoO2 when cycled to 4.4 V. The coated LiCoO2 exhibited superior rate capability (up to 400 mA/g) and discharge capacities at a current of 400 mA/g were 51% and 92% of the first cycle capacities for the bare and AlWxFy coated materials. These results open new possibilities for designing ultrathin and electrochemically robust coatings of metal fluorides via ALD to enhance the stability of Li-ion electrodes.

Mane, Anil U.↗