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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Turn-On Conductivity with Proton-Coupled Electron Transport in Metal–Organic Frameworks

Proton-coupled electron transfer (PCET) has been studied for decades in the context of molecular reactivity, but its impact on long-range electron transport is barely understood. When defined broadly as ion-coupled charge transport (ICCT), relevant systems include lithium-ion battery electrodes, electrochromic coatings, and myriad electrocatalysts. Despite ample evidence that ion-electron coupling enhances or diminishes the performance of these devices, little is known about the experimental signatures of ICCT and the microscopic factors that govern its mechanism. Here, we expect that ion-electron coupling becomes especially relevant in high surface area materials, such as the layered electrodes of intercalation batteries, due to the close proximity of itinerant electrons and electrolyte. Here, we report an electrochemical investigation into a family of metal-organic frameworks (MOFs) that serves as a well-defined platform for understanding the effect of ICCT on both elec- tronic and ionic conductivity. Through photochemical doping of e - –H + pairs and introduction of solvent guest molecules, the Ti-containing MOFs convert from electronic-only insulators conductors (σ e ≈ 10 -12 S cm -1 ) to mixed ion-electron semiconduc- tors (σ e ≈ 10 -7 S cm -1 , σ ion ≈ 10 -5 S cm -1 ). Direct current and alternating current techniques support the existence of proton- electron coupling and, critically, that improved ionic conductivity enhances electronic conductivity. Taken together, these results provide direct evidence that PCET enables long-range charge transport and generalized electrochemical tools and synthetic methods for studying ion-electron coupling in materials broadly.

Charge transport↗
DOE OSTI2024

Structural and physical properties of two distinct 2D lead halides with intercalated Cu( II )

Transition metal cation intercalation between the layers of two-dimensional (2D) metal halides is an underexplored research area. In this work we focus on the synthesis and physical property characterizations of two layered hybrid lead halides: a new compound [Cu(O 2 C–CH 2 –NH 2 ) 2 ]Pb 2 Br 4 and the previously reported [Cu(O 2 C–(CH 2 ) 3 –NH 3 ) 2 ]PbBr 4 . These compounds exhibit 2D layered crystal structures with incorporated Cu 2+ between the metal halide layers, which is achieved by combining Cu(II) and lead bromide with suitable amino acid precursors. The resultant [Cu(O 2 C–(CH 2 ) 3 –NH 3 ) 2 ]PbBr 4 adopts a 2D layered perovskite structure, whereas the new compound [Cu(O 2 C–CH 2 –NH 2 ) 2 ]Pb 2 Br 4 crystallizes with a new structure type based on edge-sharing dodecahedral PbBr5O3 building blocks. [Cu(O 2 C–CH 2 –NH 2 ) 2 ]Pb 2 Br 4 is a semiconductor with a bandgap of 3.25 eV. It shows anisotropic charge transport properties with a semiconductor resistivity of 1.44 × 10 10 Ω cm (measured along the a-axis) and 2.17 × 10 10 Ω cm (along the bc-plane), respectively. The fabricated prototype detector based on this material showed response to soft low-energy X-rays at 8 keV with a detector sensitivity of 1462.7 μCGy –1 cm –2 , indicating its potential application for ionizing radiation detection. Finally, these encouraging results are discussed together with the results from density functional theory calculations, optical, magnetic, and thermal property characterization experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Reconfiguring the band-edge states of photovoltaic perovskites by conjugated organic cations

The band edges of metal-halide perovskites with a general chemical structure of ABX 3 (A, usually a monovalent organic cation; B, a divalent cation; and X, a halide anion) are constructed mainly of the orbitals from B and X sites. Hence, the structural and compositional varieties of the inorganic B–X framework are primarily responsible for regulating their electronic properties, whereas A-site cations are thought to only help stabilize the lattice and not to directly contribute to near-edge states. We report a π-conjugation–induced extension of electronic states of A-site cations that affects perovskite frontier orbitals. The π-conjugated pyrene-containing A-site cations electronically contribute to the surface band edges and influence the carrier dynamics, with a properly tailored intercalation distance between layers of the inorganic framework. Lastly, the ethylammonium pyrene increased hole mobilities, improved power conversion efficiencies relative to that of a reference perovskite, and enhanced device stability.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Single-Crystal-to-Single-Crystal Post-Synthetic Modifications

Chalcogenides are the cornerstone of the semiconductor and thermoelectric industries and are up-and-coming materials for superconductors, catalysis, and battery applications. Challenges in synthesizing those materials emerge from the chalcogen's volatility and the tendencies of chalcogenides to react with even trace quantities of oxygen. Many techniques have been applied to the growth of chalcogenide single crystals, which are convenient for structure determinations and intrinsic property measurements. One of the recent advances in chalcogenide chemistry is the intriguing single-crystal-to-single-crystal (SCSC) transformation, leading to new metastable compositions. Post-synthetic transformations are well-known and studied for chalcogenide powders; however, examples of post-synthetic conversions that retain single crystallinity are rare. To date, the scope of SCSC reactions includes (de)intercalation in the layered compositions and ion exchange in open-framework materials, salt-inclusion chalcogenides, and layered structures. This poster will discuss the successful examples of SCSC modifications monitored by single-crystal X-ray diffraction (SC-XRD), emphasizing how post-synthetic transformations affect materials' properties.

chalcogenide↗
DOE OSTI2022

Highly Asymmetric Graphene Layer Doping and Band Structure Manipulation in Rare Earth–Graphene Heterostructure by Targeted Bonding of the Intercalated Gadolinium

Heterostructures consisting of vertically stacked two-dimensional (2D) materials have recently gained large attention due to their highly controllable electronic properties and resulting quantum phases. In contrast to the mechanically stacked multilayered systems, which offer exceptional control over a stacking sequence or interlayer twist angles, the epitaxially grown 2D materials express unprecedented quality and stability over wafer-scale lengths. However, controlling the growth conditions remains a major obstacle toward the formation of complex, epitaxial heterostructures with well-defined electronic properties. Here, we synthesized a trilayer graphene heterostructure on the SiC(0001) substrate with two specific interlayer locations occupied by gadolinium. We applied multitechnique methodology based on low-temperature scanning tunneling microscopy/spectroscopy (STM/S) and angle-resolved photoelectron spectroscopy (ARPES) to determine the intercalant’s locations in the complex, epitaxial graphene heterostructure. Our approach relies on very high quality and large, micrometer-scale homogeneity of the synthesized system. The experimentally determined electronic structure is dominated by the two topmost graphene layers. Overall, our spectroscopic results show quantitative agreement between global ARPES, local STM/S, and density functional theory predictions. The characterized electronic properties primarily reflect highly anisotropic doping levels between the two corresponding graphene layers, which significantly affect the band structure topology. Two pairs of hybridized massive Dirac bands from our initial synthesis–the bilayer graphene on the SiC(0001) substrate–are transformed upon Gd intercalation into two pairs of massless Dirac bands with a new hybridization region in between. Our results open perspectives in the realization of exotic 2D quantum materials via atomically precise synthesis of epitaxial, multilayered graphene–rare earth heterostructures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Preventing Electrolyte Decomposition on a Ca Metal Electrode Interface Using an Artificial Solid‐Electrolyte Interphase

Abstract Calcium ion batteries are gaining attention as alternatives to lithium‐ion technology because they offer comparable properties at reduced cost and improved safety. However, progress has been limited because of the inability to efficiently and reversibly plate and strip Ca metal anodes in organic electrolytes. Moreover, the inorganic components of the solid‐electrolyte interphase (SEI) that form via decomposition of the electrolyte often do not allow for the diffusion of Ca ions. In this work, an approach combining density functional theory and ab initio molecular dynamics (AIMD) simulations is utilized to show that the use of a preformed artificial SEI layer of amorphous can potentially prevent electrolyte decomposition. First, Ca is shown to be able to intercalate into an amorphous layer (up to Ca 1.5 Al 2 O 3 ) and diffuse through on a reasonable time scale. Through calculation of the density of states, the system is found to remain insulating up to the equilibrium stoichiometry. Finally, AIMD simulations with a realistic organic electrolyte environment are used to show that this calcinated layer completely prevents the decomposition of solvent molecules. This approach can provide a route to efficient rechargeable Ca ion batteries, paving the way for cheap large‐scale energy storage.

Young, Joshua↗
DOE OSTI2024

Interlayer engineering of Fe 3 GeTe 2 : From 3D superlattice to 2D monolayer

The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546 , 265–269 (2017) and B. Huang et al., Nature 546 , 270–273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14 , 408–419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe 3 GeTe 2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature ( T C ), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Dual‐Ion Intercalation and High Volumetric Capacitance in a Two‐Dimensional Non‐Porous Coordination Polymer

Abstract Intercalation is a promising ion‐sorption mechanism for enhancing the energy density of electrochemical capacitors (ECs) because it offers enhanced access to the electrochemical surface area. It requires a rapid transport of ions in and out of a host material, and it must occur without phase transformations. Materials that fulfil these requirements are rare; those that do intercalate almost exclusively cations. Herein, we show that Ni 3 (benzenehexathiol) (Ni 3 BHT), a non‐porous two‐dimensional (2D) layered coordination polymer (CP), intercalates both cations and anions with a variety of charges. Whereas cation intercalation is pseudocapacitive, anions intercalate in a purely capacitive fashion. The excellent EC performance of Ni 3 BHT provides a general basis for investigating similar dual‐ion intercalation mechanisms in the large family of non‐porous 2D CPs.

Banda, Harish↗
DOE OSTI2021

Dual-Ion Intercalation and High Volumetric Capacitance in a Two-Dimensional Non-Porous Coordination Polymer

Intercalation is a promising ion-sorption mechanism for enhancing the energy density of electrochemical capacitors (ECs) because it offers enhanced access to the electrochemical surface area. It requires a rapid transport of ions in and out of a host material, and it must occur without phase transformations. Materials that fulfil these requirements are rare; those that do intercalate almost exclusively cations. Herein, we show that Ni3(benzenehexathiol) (Ni 3 BHT), a non-porous two-dimensional (2D) layered coordination polymer (CP), intercalates both cations and anions with a variety of charges. Whereas cation intercalation is pseudocapacitive, anions intercalate in a purely capacitive fashion. The excellent EC performance of Ni3BHT provides a general basis for investigating similar dual-ion intercalation mechanisms in the large family of non-porous 2D CPs.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Thermodynamics and kinetics of Pb intercalation under graphene on SiC(0001)

SiC-supported graphene intercalated by a two-dimensional Pb monolayer can provide an appealing platform for spintronic applications. Such a monolayer structure is thermodynamically ultrastable, as observed in recent experiments. However, important fundamentals such as the structure of intercalated phases, locations of intercalated atoms, thermodynamic preference for intercalation, and intercalation pathways for this system have not yet been understood conclusively. Here, in this work, extensive density functional theory calculations are performed to assess Pb intercalation thermodynamics and kinetics under graphene on SiC(0001). We find that intercalation of isolated Pb atoms is strongly disfavored over adsorption on top of graphene. However, intercalation of complete Pb layers in the gallery between SiC and graphene buffer layer is strongly favored over supported Pb monolayers and moderately favored over formation of supported large three-dimensional Pb islands. We also find that initiation of intercalation either by individual Pb atoms directly penetrating graphene or by hopping under a static graphene step edge is energetically prohibitive at experimental temperatures. Consequently, more complex intercalation pathways are operative and further analyzed. We demonstrated that once an intercalated Pb monolayer forms around a graphene step edge, a facile Pb mass transport by Pb vacancy-mediated diffusion can be triggered for continued growth of the intercalated monolayer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Highly Efficient Uniaxial In-Plane Stretching of a 2D Material via Ion Insertion

On-chip dynamic strain engineering requires efficient micro-actuators that can generate large in-plane strains. Inorganic electrochemical actuators are unique in that they are driven by low voltages (≈1 V) and produce considerable strains (≈1%). However, actuation speed and efficiency are limited by mass transport of ions. Minimizing the number of ions required to actuate is thus key to enabling useful “straintronic” devices. Here, it is shown that the electrochemical intercalation of exceptionally few lithium ions into WTe 2 causes large anisotropic in-plane strain: 5% in one in-plane direction and 0.1% in the other. This efficient stretching of the 2D WTe 2 layers contrasts to intercalation-induced strains in related materials which are predominantly in the out-of-plane direction. The unusual actuation of Li x WTe 2 is linked to the formation of a newly discovered crystallographic phase, referred to as Td', with an exotic atomic arrangement. On-chip low-voltage (<0.2 V) control is demonstrated over the transition to the novel phase and its composition. Within the Td'-Li 0.5-δ WTe 2 phase, a uniaxial in-plane strain of 1.4% is achieved with a change of δ of only 0.075. This makes the in-plane chemical expansion coefficient of Td'-Li 0.5-δ WTe 2 far greater than of any other single-phase material, enabling fast and efficient planar electrochemical actuation.

36 MATERIALS SCIENCE↗
DOE OSTI2022

2D Oxides Realized via Confinement Heteroepitaxy

Novel confinement techniques facilitate the formation of non-layered 2D materials. Here it is demonstrated that the formation and properties of 2D oxides (GaO x , InO x , SnO x ) at the epitaxial graphene (EG)/silicon carbide (SiC) interface is dependent on the EG buffer layer properties prior to element intercalation. Using 2D Ga, it is demonstrated that defects in the EG buffer layer lead to Ga transforming to GaO x with non-periodic oxygen in a crystalline Ga matrix via air oxidation at room temperature. However, crystalline monolayer GaO 2 and bilayer Ga 2 O 3 with ferroelectric wurtzite structure(FE-WZ') can then be formed via subsequent high-temperature O 2 annealing. Furthermore, the graphene/X/SiC (X = 2D Ga or Ga 2 O 3 ) junction is tunable from Ohmic to a Schottky or tunnel barrier depending on the interface species. Finally, using vertical transport measurements and electron energy loss spectroscopy analysis, the bandgap of 2D gallium oxide is identified as 6.6 ± 0.6 eV, significantly larger than that of bulk β-Ga 2 O 3 (≈4.8 eV), suggesting strong quantum confinement effects at the 2D limit. Furthermore, the study presented here is foundational for development of atomic-scale, vertical 2D/3D heterostructure for applications requiring short transit times, such as GHz and THz devices.

2D gallium oxides↗
DOE OSTI2023

Targeted Dy intercalation under graphene/SiC for tuning its electronic band structure

Metal intercalation of graphene is a promising method to tune its electronic band structure and generate novel electronic and topological phases. The tuning depends critically on the ability to bond the intercalated atoms at predesigned, subsurface interlayer locations because the emerging band structure depends on metal location. In this work, we have studied Dy intercalation under single-layer graphene (SLG) on SiC using spot profile analysis–low-energy electron diffraction and scanning tunneling microscopy (STM). The experimental work is complemented with density-functional theory (DFT) analysis. Because different diffraction spots originate from different subsurface interlayer regions, it is possible to identify changes in the intercalation location by monitoring the spot intensity as a function of growth conditions. DFT calculations of the chemical potential as a function of intercalated Dy coverage support the variation of the stability of the intercalated phase at different intercalated locations. The preferred location is confirmed from STM studies showing the removal of the 6 × 6 moiré corrugation at the preferred location, observed at higher Dy coverage.

2-dimensional systems↗
DOE OSTI2020

Emerging Magnetic Interactions in van der Waals Heterostructures

Vertical van der Waals (vdWs) heterostructures based on layered materials are attracting interest as a new class of quantum materials, where interfacial charge-transfer coupling can give rise to fascinating strongly correlated phenomena. Transition metal chalcogenides are a particularly exciting material family, including ferromagnetic semiconductors, multiferroics, and superconductors. Here, we report the growth of an organic–inorganic heterostructure by intercalating molecular electron donating bis(ethylenedithio)tetrathiafulvalene into (Li,Fe)OHFeSe, a layered material in which the superconducting ground state results from the intercalation of hydroxide layer. Molecular intercalation in this heterostructure induces a transformation from a paramagnetic to spin-glass-like state that is sensitive to the stoichiometry of molecular donor and an applied magnetic field. Besides, electron-donating molecules reduce the electrical resistivity in the heterostructure and modify its response to laser illumination. In conclusion, this hybrid heterostructure provides a promising platform to study emerging magnetic and electronic behaviors in strongly correlated layered materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct Observation of Alternating Octahedral and Prismatic Sodium Layers in O3-Type Transition Metal Oxides

The oxygen stacking of O3-type layered sodium transition metal oxides (O3-NaTMO 2 ) changes dynamically upon topotactic Na extraction and reinsertion. While the phase transition from octahedral to prismatic Na coordination that occurs at intermediate desodiation by transition metal slab gliding is well understood, the structural evolution at high desodiation, crucial to achieve high reversible capacity, remains mostly uncharted. In this work, the phase transitions of O3-type layered NaTMO 2 at high voltage are investigated by combining experimental and computational approaches. An OP2-type phase that consists of alternating octahedral and prismatic Na layers is directly observed by in situ X-ray diffraction and high-resolution scanning transmission electron microscopy. The origin of this peculiar phase is explained by atomic interactions involving Jahn–Teller active Fe 4+ and distortion tolerant Ti 4+ that stabilize the local Na environment. Finally, the path-dependent desodiation and resodiation pathways are also rationalized in this material through the different kinetics of the prismatic and octahedral layers, presenting a comprehensive picture about the structural stability of the layered materials upon Na intercalation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1973

High field superconductivity in alkali metal intercalates of MoS2

In the search for better high temperature, high critical field superconductors, a class of materials was found which have layered structures and can be intercalated with various elements and compounds. Since a large number of compounds can be formed, intercalation provides a method of control of superconducting properties. They also provide the possible medium for excitonic superconductivity. Results of magnetic field studies are presented on alkali metal (Na, K, Rb, and Cs) intercalated MoS2 (2H polymorph).

Woollam, J. A.↗
DOE OSTI2022

Thermodynamically Driven Formation of Intercalated Cu Carpets from Supported Cu Pyramids on MoS 2

Thermodynamic and kinetic analyses based on our first-principles density functional theory calculations are used to interpret the experimentally observed formation of Cu carpets intercalated under the top layer of a 2H-MoS 2 substrate. Spontaneous Cu transport from Cu pyramids on top of the MoS 2 substrate through surface point defects to the growing Cu carpet is shown to be driven by a slightly lower chemical potential for the Cu carpet. We demonstrate that the competition between a preference for a thicker Cu carpet and the cost of elastic stretching of the top MoS 2 layer results in a selected Cu carpet thickness. Furthermore, we also propose that Cu transport occurs primarily via vacancy-mediated diffusion through constricting point defect portals.

36 MATERIALS SCIENCE↗