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At least 235 records · Page 13

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DOE OSTI2026

Space-Charge-Limited van der Waals Spin Transistor

Integrating semiconducting and magnetic materials could combine transistorlike operation with nonvolatility and enable architectures such as logic in memory. Here, in this work, we employ correlated electrical transport and scanning nitrogen-vacancy center magnetic imaging to elucidate a spin transistor concept that amalgamates vertical and lateral hopping transport inside a 2D antiferromagnetic semiconductor, mechanistically distinct from vertical tunneling devices. Our device, based on a monolayer-bilayer junction in CrSBr, displays giant, gate-tunable magnetoresistance driven by the dual action of electrostatic doping on space-charge-limited lateral conduction and interlayer exchange coupling. Moreover, we visualize a field-trainable, layer-sharing effect that selects between coherent or domain-wall reversal at the spin-flip transition, enabling multilevel, memristive conductance states. Our layer-dependent space charge mechanism for convergent electrical and magnetic control opens opportunities to address limitations in contemporary computing.

2-dimensional systems↗

DOE OSTI2023

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two‐Dimensional Metal–Organic Frameworks

Abstract Metal–organic frameworks (MOFs) with mobile charges have attracted significant attention due to their potential applications in photoelectric devices, chemical resistance sensors, and catalysis. However, fundamental understanding of the charge transport pathway within the framework and the key properties that determine the performance of conductive MOFs in photoelectric devices remain underexplored. Herein, we report the mechanisms of photoinduced charge transport and electron dynamics in the conductive 2D M−HHTP (M=Cu, Zn or Cu/Zn mixed; HHTP=2,3,6,7,10,11‐hexahydroxytriphenylene) MOFs and their correlation with photoconductivity using the combination of time‐resolved terahertz spectroscopy, optical transient absorption spectroscopy, X‐ray transient absorption spectroscopy, and density functional theory (DFT) calculations. We identify the through‐space hole transport mechanism through the interlayer sheet π–π interaction, where photoinduced hole state resides in HHTP ligand and electronic state is localized at the metal center. Moreover, the photoconductivity of the Cu−HHTP MOF is found to be 65.5 S m −1 , which represents the record high photoconductivity for porous MOF materials based on catecholate ligands.

Wang, Denan↗

DOE OSTI2023

Dominant Role of Hole Transport Pathway in Achieving Record High Photoconductivity in Two‐Dimensional Metal–Organic Frameworks

14 SOLAR ENERGY↗

DOE OSTI2016

Molecular Simulation Models of Carbon Dioxide Intercalation in Hydrated Sodium Montmorillonite

Carbon capture and storage (CCS) technologies provide effective options for reducing carbon dioxide (CO 2 ) emissions and mitigating global climate change. Geologic CO 2 storage involves the injection of supercritical CO 2 into deep geologic formations overlain by sealing formations and geologic traps that prevent the CO 2 from escaping. The ability of seals to retain injected CO 2 depends on their permeability and geomechanical property evolution. Caprocks are often comprised of shale or mudstone enriched with swelling clay minerals that can expand or contract upon interaction with CO 2 . Intercalation of CO 2 into interlayer of swelling clay causes evolution of the geomechanical stress and can promote fracture formation. The interlayer of expanded clay provides ample space for additional permanent CO 2 storage and its subsequent transformation into carbonates.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Synergistic Coupling of CO2 and H2O during Expansion of Clays in Supercritical CO2-CH4 Fluid Mixtures

A combination of in operando IR and XRD methods were used to investigate the interaction of variably hydrated supercritical CO2-CH4 fluids with Na/NH4/Cs substituted montmorillonites. Comparing the behavior of Na-clay exposed to CH4 versus CO2 dominant fluid phases demonstrated that CO2 disrupts the H-bond network of intercalated H2O. At relatively low potentials of H2O, CO2 facilitates H2O intercalation and expansion of the interlayer region of the Na-clay. In contrast, CO2 inhibited H2O intercalation but promoted expansion of the Cs-clay. The NH4-clay displayed intermediated behavior. Methane likely intercalated the clays but opportunistically, filling unoccupied space after CO2 and H2O actively expanded the interlayer region. By comparing and contrasting the behavior of the Cs and NH4 clays to the Na Clay we conclude that, whereas H2O is required for initial Na-clay expansion, CO2 synergistically facilitated expansion and hydration of Na-clay by mixing with H2O (the entropic contribution) and participating in the outer solvation sphere of Na (the enthalpic contribution). In contrast, CO2 can directly solvate interlayer Cs and NH4 and cause expansion in the absence of H2O; nonetheless, H2O and CO2 also act synergistically to facilitate expansion of the Cs and NH4 clays at low H2O concentrations.

Loring, John S.↗

DOE OSTI2024

Pairing and Pair Breaking by Gauge Fluctuations in Bilayer Composite Fermion Metals

We study interlayer pairing of composite fermions in the total $\nu=1/2+1/2$ quantum Hall bilayer as a possible framework for understanding the experimentally observed transition from a compressible state at large layer spacing to a bilayer quantum Hall state at small layer spacing. We consider a model in which the effective interlayer composite fermion pairing interaction mediated by the Chern-Simons gauge fields in the two layers is singular with both attractive (out-of-phase) and repulsive (in-phase) components diverging at low frequency. If only the more singular attractive interaction is included the pairing gap obtained by solving the gap equation is proportional to the inverse of the layer spacing squared. In the so-called local approximation, we find that when the less singular repulsive interactions are also included the pairing gap still falls off as inverse layer spacing squared, consistent with recent analyses, but is strongly suppressed to a degree that may account for the fact that this predicted inverse square dependence is not observed experimentally. The analytically obtained local approximation solutions are then used as a starting point to numerically iterate the full gap equation to assess the validity of the approximation in this limit.

Deng, Haoyun↗

DOE OSTI2020

Effects of lithium intercalation in twisted bilayer graphene

In this work, we investigate the effects of lithium intercalation in twisted bilayers of graphene, using first-principles electronic structure calculations. To model this system we employ commensurate supercells that correspond to twist angles of 7.34° and 2.45°. From the energetics of lithium absorption we demonstrate that for low Li concentration the intercalants cluster in the AA regions with double the density of a uniform distribution. The charge donated by the Li atoms to the graphene layers results in modifications to the band structure that can be qualitatively captured using a continuum model with modified interlayer couplings in a region of parameter space that has yet to be explored either experimentally or theoretically. Thus, the combination of intercalation and twisted layers simultaneously provides the means for spatial control over material properties and an additional knob with which to tune moiré physics in twisted bilayers of graphene, with potential applications ranging from energy storage and conversion to quantum information.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers

In this paper, we present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moire pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moire Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moire potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. Finally, we discuss the possibility of using the moire pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Twist Angle-Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe 2 /WSe 2 twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1° to 3.5°. Additionally, using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of “twistronics”.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Characterization of nonlinear ultrasonic waves behavior while interacting with poor interlayer bonds in large-scale additive manufactured materials

Over the past decades, researchers have developed several nonlinear ultrasonic techniques for quality control of materials commonly used in different applications. Owing to the superior sensitivity of nonlinear ultrasound waves to small defects such as micro-cracks, their applicability in different nondestructive testing (NDT) problems has been investigated in numerous studies. These studies utilize frequency domain analysis to detect the generation of higher harmonics because of the formation of defects in the inspected medium. Frequency domain analysis based on the Fourier transform is a significant approach used in linear systems; however, it may not perform adequately on nonlinear systems. Hence, studies on nonlinear dynamics and physics consider analyzing systems' behavior in the phase-space domain. In contrast to the frequency domain analysis, which can result in information loss, analysis in the phase-space domain retains all the information regarding a system's states. Here, we investigate the nonlinearities induced by poor interlayer bonding in polymer-based additive manufactured parts in the phase-space domain. It is convenient to characterize the nonlinearity in the phase-space domain because it provides a geometrical representation of a system's states. Two types of low quality interlayer bond are considered. The first type is simulated artificially while the second type is manufactured by reducing the bond quality during the printing process. The analysis verified that the received ultrasonic signals exhibit classical nonlinear behavior in the phase-space domain while interacting with simulated poor interlayer bonds. In addition, the results showed that the behavior of ultrasonic waves is amplitude-dependent and evolves into models that have not been previously reported. Furthermore, Largest Lyapunov Exponent (LLE) is used to quantify the behavior of nonlinear ultrasonic waves while interacting with poor interlayer bonds. Using LLE, it was observed that the divergence rate of the phase-space trajectories depends on the amplitude of the excitation. This observation quantitatively proves that nonlinear behavior of ultrasound while interacting with poor interlayer bonds can be amplitude-dependent. The results of both simulated and inherent poor interlayer bond cases showed that LLE can be used as a reliable quantitative damage-sensitive feature to detect and potentially characterize weak bonds, which are difficult to detect using conventional approaches. Additionally, the reported results in the phase-space domain provide a basis for proposing a new mathematical model for ultrasonic waves interacting with poor interlayer bonds.

36 MATERIALS SCIENCE↗

DOE OSTI2015

CdS-pillared CoAl-layered double hydroxide nanosheets with superior photocatalytic activity

Graphical abstract: - Highlights: • CdS nanocrystals were intercalated into CoAl-LDH interlayer. • The nanohybrid display superior visible-light photocatalytic activity. • A photoexcitation model for the pillared heterostructured system was proposed. - Abstract: A new nanohybrid was synthesized by mixing the positively charged 2D nanosheets of CoAl-layered double hydroxide (CoAl-LDH) and the negatively charged CdS nanosol suspensions. It was revealed that the CdS nanoparticles were intercalated into the interlayer region of CoAl-LDH with a spacing of 2.62 nm. The obtained nanohybrid exhibited a mesoporous texture with an expanded specific surface area of 62 m{sup 2} g{sup −1} and a superior photocatalytic activity in the degradation of acid red with a reaction constant of 1.26 × 10{sup −2} min{sup −1} under visible-light radiation, which is more than 2 times those of his parents CoAl-LDH and CdS.

36 MATERIALS SCIENCE↗

DOE OSTI2018

CsMn 4 As 3 : A Layered Tetragonal Transition-Metal Pnictide Compound with an Antiferromagnetic Ground State

We report the synthesis and properties of a new layered tetragonal ternary compound CsMn4As3 (structure: KCu 4 S 3 -type, space group: P4/mmm, No. 123 and Z = 2). The material is a small band-gap semiconductor and exhibits an antiferromagnetic ground state associated with Mn spins. The compound exhibits a signature of a distinct magnetic moment canting event at 150(5) K with a canting angle of ≈ 0:3°. Although, some features of the magnetic characteristics of this new compound are qualitatively similar to those of the related BaMn 2 As 2 , the underlying Mn sublattices of the two materials are quite different. While the Mn square-lattice layers in BaMn 2 As 2 are equally spaced along the c-direction with the interlayer distance d L Ba = 6.7341(4) Å, the Mn sublattice forms bilayers in CsMn 4 As 3 with the interlayer distance within a bilayer d L Cs = 3.1661(6) Å and the distance between the two adjacent bilayers d B = 7:290(6) Å. This difference in the Mn sublattice is bound to significantly alter the energy balance between the J 1 , J 2 and J c exchange interactions within the J 1 -J 2 -J c model compared to that in BaMn 2 As 2 and the other related 122 compounds including the well-known iron-arsenide superconductor parent compound BaFe 2 As 2 . Owing to the novelty of its transition metal sublattice, this new addition to the family of tetragonal materials related to the iron-based superconductors brings prospects for doping and pressure studies in the search of new superconducting phases as well as other exciting correlated-electron properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Layer-by-layer growth of graphene oxide multilayers using robust interlayer linking chemistry. 2 Zr–bissulfates

We report on the facile layer-by-layer growth of graphene oxide sulfate (GO-S) multilayer structures. The layers are bonded to a modified planar silica support using Zr 4+ ions. Optical ellipsometry shows step-by-step layer growth with layer thickness consistent with that expected from molecular mechanics calculations. The reaction to form Zr–bissulfate linkages is facile, with multistep layer deposition occurring within six to ten minutes per layer. The GO-S layers can be deposited directly on GO-S underlayer(s) or interlayer spacers can be incorporated to control the spacing with GO-S layers. X-ray photoelectron spectroscopy (XPS) data provides information on the density of the sulfonated groups present at the graphene oxide surface and on the Zr : S ratio. This is the first report we are aware of that demonstrates robust layer-by-layer growth of graphene oxide structures.

Graphene oxide↗

DOE OSTI2019

Tuning the Hydrophobicity of Layer-Structure Silicates To Promote Adsorption of Nonaqueous Fluids: Effects of F – for OH – Substitution on CO 2 Partitioning into Smectite Interlayers

The intercalation of nonaqueous fluids in the nanopores of organic and inorganic materials is of significant interest, particularly in the energy science community. Recently, X-ray powder diffraction and computational modeling results have shown that structural F – for OH – substitution in layered silicates makes them more hydrophobic. Here, we use grand canonical molecular dynamics calculations to investigate how increasing the F – /(F – + OH – ) ratio of a prototypical layered silicate (the smectite Na-hectorite) impacts the intercalation behavior of CO 2 and H 2 O at elevated temperature and pressure. At the conditions of this study ( T = 323 K, P = 90 bar, water-saturated CO 2 ), increasing F – for OH – substitution causes decreasing total CO 2 + H 2 O intercalation, increasing CO 2 /(CO 2 + H 2 O) ratios in the interlayer galleries, and an increasing energy barrier to CO 2 and H 2 O intercalation. CO 2 intercalation is the greatest at monolayer basal spacings, and the results support the idea that with Na + as the exchangeable cation, the interlayers must be propped open by some H 2 O molecules to allow CO 2 to enter the interlayer galleries. The computed immersion energies suggest that the bilayer or a more expanded structure is the stable state under these conditions, in agreement with experimental results, and that the basal spacings of the minimum energy 2L structures increase with increasing F – for OH – substitution. Finally, these results are consistent with a wide range of experimental data for smectites at ambient conditions and elevated pressures and temperatures and suggest that F – for OH – substitution in conjunction with reduced structural charge and exchange with large, low charge cations may increase the ability of smectite minerals to incorporate hydrophobic species such as CH 4 , CO 2 , H 2 , and other organic compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Non-Faradaic Li + Migration and Chemical Coordination across Solid-State Battery Interfaces

Efficient and reversible charge transfer is essential to realizing high-performance solid-state batteries. Efforts to enhance charge transfer at critical electrode–electrolyte interfaces have proven successful, yet interfacial chemistry and its impact on cell function remains poorly understood. Using X-ray photoelectron spectroscopy combined with electrochemical techniques, we elucidate chemical coordination near the LiCoO 2 –LIPON interface, providing experimental validation of space-charge separation. Space-charge layers, defined by local enrichment and depletion of charges, have previously been theorized and modeled, but the unique chemistry of solid-state battery interfaces is now revealed. Here we highlight the non-Faradaic migration of Li+ ions from the electrode to the electrolyte, which reduces reversible cathodic capacity by ~15%. Inserting a thin, ion-conducting LiNbO 3 interlayer between the electrode and electrolyte, however, can reduce space-charge separation, mitigate the loss of Li+ from LiCoO 2 , and return cathodic capacity to its theoretical value. This work illustrates the importance of interfacial chemistry in understanding and improving solid-state batteries.

Gittleson, Forrest S.↗

DOE OSTI2021

Light-activated interlayer contraction in two-dimensional perovskites for high-efficiency solar cells

Understanding and tailoring the physical behaviour of halide perovskites under practical environments is critical for designing efficient and durable optoelectronic devices. Here, we report that continuous light illumination leads to >1% contraction in the out-of-plane direction in two-dimensional hybrid perovskites, which is reversible and strongly dependent on the specific superlattice packing. X-ray photoelectron spectroscopy measurements show that constant light illumination results in the accumulation of positive charges in the terminal iodine atoms, thereby enhancing the bonding character of inter-slab I–I interactions across the organic barrier and activating out-of-plane contraction. Correlated charge transport, structural and photovoltaic measurements confirm that the onset of the light-induced contraction is synchronized to a threefold increase in carrier mobility and conductivity, which is consistent with an increase in the electronic band dispersion predicted by first-principles calculations. Flux-dependent space-charge-limited current measurement reveals that light-induced interlayer contraction activates interlayer charge transport. Lastly, the enhanced charge transport boosts the photovoltaic efficiency of two-dimensional perovskite solar cells up to 18.3% by increasing the device’s fill factor and open-circuit voltage.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Electrochemical reactivity of atomic and molecular species under solid-state confinement

The nanoconfinement of electrochemically-active guest species in host solid state electrode materials provides opportunities to tune mass transport between the bulk electrolyte and inner surface of the electrode, enhance electron-transfer rates, and/or improve the stability and dispersion of active material. Here, this review summarizes recent experimental and theoretical electrochemical studies of three types of nanoconfined guest species: (1) ion adsorption of electrolyte ions, (2) confined redox-active molecules, and (3) electrocatalytic reactions of confined ions/solvents and catalytic particles. The examples discussed in this review illustrate how the confinement of guest species within enclosed spaces with nanoscale dimensions – such as pores, pockets, channels, and interlayers – can lead to improved electrochemical performance.

25 ENERGY STORAGE↗

DOE OSTI2024

Phase-Selective Synthesis of Rhombohedral WS 2 Multilayers by Confined-Space Hybrid Metal–Organic Chemical Vapor Deposition

Rhombohedral polytype transition metal dichalcogenide (TMDC) multilayers exhibit non-centrosymmetric interlayer stacking, which yields intriguing properties such as ferroelectricity, a large second-order susceptibility coefficient χ (2) , giant valley coherence, and a bulk photovoltaic effect. These properties have spurred significant interest in developing phase-selective growth methods for multilayer rhombohedral TMDC films. Here, in this study, we report a confined-space, hybrid metal–organic chemical vapor deposition method that preferentially grows 3R-WS 2 multilayer films with thickness up to 130 nm. We confirm the 3R stacking structure via polarization-resolved second-harmonic generation characterization and the 3-fold symmetry revealed by anisotropic H 2 O 2 etching. The multilayer 3R WS 2 shows a dendritic morphology, which is indicative of diffusion-limited growth. Multilayer regions with large, stepped terraces enable layer-resolved evaluation of the optical properties of 3R-WS 2 via Raman, photoluminescence, and differential reflectance spectroscopy. These measurements confirm the interfacial quality and suggest ferroelectric modification of the exciton energies.

3R-WS2↗