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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Generation of Tunable Stochastic Sequences Using the Insulator–Metal Transition

Probabilistic computing is a paradigm in which data are not represented by stable bits, but rather by the probability of a metastable bit to be in a particular state. The development of this technology has been hindered by the availability of hardware capable of generating stochastic and tunable sequences of “1s” and “0s”. The options are currently limited to complex CMOS circuitry and, recently, magnetic tunnel junctions. Here, we demonstrate that metal–insulator transitions can also be used for this purpose. We use an electrical pump/probe protocol and take advantage of the stochastic relaxation dynamics in VO 2 to induce random metallization events. A simple latch circuit converts the metallization sequence into a random stream of 1s and 0s. The resetting pulse in between probes decorrelates successive events, providing a true stochastic digital sequence.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2022

Gate-Tunable Proximity Effects in Graphene on Layered Magnetic Insulators

The extreme versatility of van der Waals materials derives from their ability to exhibit new electronic properties when assembled in proximity with dissimilar crystals. 1 For example, although graphene is inherently non-magnetic, recent work has reported a magnetic proximity effect in graphene interfaced with magnetic substrates, potentially enabling a pathway towards achieving a high-temperature quantum anomalous Hall effect. Here, we investigate heterostructures of graphene and chromium trihalide magnetic insulators (CrI 3 , CrBr 3 , and CrCl 3 ). Surprisingly, we are unable to detect a magnetic exchange field in the graphene, but instead discover proximity effects featuring unprecedented gate-tunability. The graphene becomes highly hole-doped due to charge transfer from the neighboring magnetic insulator, and further exhibits a variety of atypical gate-dependent transport features. The charge transfer can additionally be altered upon switching the magnetic states of the nearest CrI 3 layers. In conclusion, our results provide a roadmap for exploiting proximity effects arising in graphene coupled to magnetic insulators.

2D magnet↗
DOE OSTI2023

Gate-Tunable Multiband Transport in ZrTe 5 Thin Devices

Interest in ZrTe 5 has been reinvigorated in recent years owing to its potential for hosting versatile topological electronic states and intriguing experimental discoveries. However, the mechanism of many of its unusual transport behaviors remains controversial: for example, the characteristic peak in the temperature-dependent resistivity and the anomalous Hall effect. Here, through employing a clean dry-transfer fabrication method in an inert environment, we successfully obtain high-quality ZrTe 5 thin devices that exhibit clear dual-gate tunability and ambipolar field effects. Such devices allow us to systematically study the resistance peak as well as the Hall effect at various doping densities and temperatures, revealing the contribution from electron–hole asymmetry and multiple-carrier transport. By comparing with theoretical calculations, we suggest a simplified semiclassical two-band model to explain the experimental observations. Finally, our work helps to resolve the longstanding puzzles on ZrTe 5 and could potentially pave the way for realizing novel topological states in the two-dimensional limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetically Tunable Polariton Cavities in van der Waals Heterostructures

Nanophotonic cavities are the foundation for a broad spectrum of applications, including quantum sensing, on-chip communication, and cavity quantum electrodynamics. In van der Waals (vdW) materials, these cavities can harness polaritons, which are quasiparticles emerging from photon interactions with excitons, plasmons, or phonons that are confined in microscopic sample flakes. Hybrid phonon–plasmon cavities leverage the long lifetimes of phonons and good tunability of plasmons, but their reconfigurability remains fundamentally limited. Here, in this work, we introduce a magnetic-field-tuning mechanism for polaritonic cavities in a vdW heterostructure. Specifically, we demonstrate that the primary Landau transition in magnetized charge-neutral graphene can be harvested for controlling polaritonic cavity modes in a graphene-based phononic heterostructure. Additionally, we predict a magnetic-field-induced topological transition in the polariton isofrequency contour, causing a nontrivial cavity mode profile redistribution. Our study underscores the versatility of Landau-based nanophotonic cavities, offering new paradigms for the design and manipulation of light–matter interactions at the nanoscale.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Polytypic Zn–(In,Ga)–Se Nanocrystals with Tunable Emission

While the polymorphism of chalcopyrite semiconductors has been widely studied, the wurtzite analogues of defect-chalcopyrite II–III 2 –VI 4 compositions, such as Zn(In,Ga) 2 (S,Se) 4 , remain underexplored. Here, we report the synthesis of polytypic Zn–(In,Ga)–Se multipods via cation exchange using ZnSe as a template. With cubic cores and wurtzite arms elongated along the hexagonal c-axis, the multipods retain the structure and morphology of the ZnSe template. Optical characterization reveals composition-dependent absorption and photoluminescence, tunable from the visible to the near-infrared region, with spectral features distinct from those of previously reported defect-chalcopyrite structures. Temperature-dependent measurements demonstrate strong emission at cryogenic temperatures, which is quenched near room temperature due to thermally activated nonradiative processes. Here, we illustrate the use of ZnSe as a platform for cation exchange toward wurtzite multinary chalcogenides, unlocking access to novel structures with colorful optical properties.

cation exchange↗
DOE OSTI2021

Orientation-Controlled Large-Area Epitaxial PbI 2 Thin Films with Tunable Optical Properties

Lead iodide (PbI 2 ) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI 2 with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI 2 remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI 2 thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI 2 thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI 2 on other substrates and heterostructures of PbI 2 on single-crystalline graphene. Here, the ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI 2 and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.

2D materials↗
DOE OSTI2022

Covert MOF-Based Photoluminescent Tags via Tunable Linker Energetics

We report that the optical anticounterfeiting tags utilize the photoluminescent properties of materials to encode unique patterns, enabling identification and validation of important items and assets. These tags must combine optical complexity with ease of production and authentication to both prevent counterfeiting and to remain practical for widespread use. Metal–organic frameworks (MOFs) based on polynuclear, rare earth clusters are ideal materials platforms for this purpose, combining fine control over structure and composition, with tunable, complex energy transfer mechanisms via both linker and metal components. Here we report the design and synthesis of a set of heterometallic MOFs based on combinations of Eu, Nd, and Yb with the tetratopic linker 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene. The energetics of this linker facilitate the intentional concealment of the visible emissions from Eu while retaining the infrared emissions of Nd and Yb, creating an optical tag with multiple covert elements. Unique to the materials system reported herein, we document the occurrence of a previously not observed 11-metal cluster correlated with the presence of Yb in the MOFs, coexisting with a commonly encountered 9-metal cluster. We demonstrate the utility of these materials as intricate optical tags with both rapid and in-depth screening techniques, utilizing orthogonal identifiers across composition, emission spectra, and emission decay dynamics. This work highlights the important effect of linker selection in controlling the resulting photoluminescent properties in MOFs and opens an avenue for the targeted design of highly complex, multifunctional optical tags.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Gate-Tunable Short-Wave Infrared Polycrystalline GeSn Phototransistors on Noncrystalline Substrates

GeSn is a group-IV alloy with immense potential to advance microelectronics technology due to its intrinsic compatibility with existing Si CMOS processes. With a sufficiently high Sn composition, GeSn is classified as a direct bandgap semiconductor. Polycrystalline GeSn holds several additional advantages, including its significantly lower synthesis cost compared to its epitaxial counterpart, as well as the versatility to grow these films on a variety of substrates. Here, in this work, we present a polycrystalline thin-film GeSn phototransistor on a fused silica substrate with a Sn composition of ~10%, showing a photoresponse in the short-wave infrared wavelength range, critical for emerging sensing applications. This device shows a gate-tunable response, with responsivities approaching up to 1.7 mA/W with only a 30 nm-thick GeSn layer. Furthermore, phototransistors offer additional adaptability through gating, which allows for the reduction of dark current. This not only enhances the signal-to-noise ratio but also offers more flexible integration with various image sensor readout implementations using different substrates. The specific detectivity of this phototransistor is within an order of magnitude of those of previously reported GeSn photodetectors grown by molecular beam epitaxy and chemical vapor deposition, even though the absorber is 3 to 20× thinner while the electrode spacing for photocarrier transport is approximately 15× longer than the carrier diffusion length in this work, showing great potential benefits of extending similar device structures to epitaxial GeSn layers. As these GeSn phototransistors utilize a noncrystalline substrate, our work establishes a fundamentally more versatile path toward monolithically integrated GeSn-based photodetectors for next-generation multimodal sensors.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Heterobinuclear Molecular Precursors Direct the Formation of Supported Subnanometer Cu–M Clusters with Tunable Catalytic Behavior

Subnanometer bimetallic clusters hold great promise for catalytic applications due to their unique electronic properties and high surface-to-volume ratios. However, precise control over their composition and size remains a major challenge, particularly for immiscible metal pairs. Here, we report a surface-anchored molecular approach for synthesizing ∼0.7–0.8 nm Cu–M (M = Ru, Mo, W, Fe) bimetallic clusters on mesoporous silica supports, using heterobinuclear N-heterocyclic carbene (NHC)-based complexes as precursors. The NHC ligand functionalized with an alkoxysilane anchor enables robust grafting to the silica interface. Controlled calcination and reduction lead to subnanometer clusters with tunable composition, dictated by the metal–metal bond stability in the precursor. In situ transmission electron microscopy reveals cluster growth proceeds via sintering of adjacent surface-bound units, while elevated temperatures above 300 °C triggering diffusion and phase separation. Catalytic testing in ethylene hydrogenation demonstrates composition-dependent activity and kinetics, with CuRu and CuW clusters exhibiting lower apparent activation energy barriers compared with monometallic Cu nanoparticles. This study establishes a generalizable strategy for the interfacial synthesis of alloyed clusters from molecular precursors and provides mechanistic insight into how precursor design governs nanostructure formation and catalytic behavior.

N-heterocyclic carbene↗
DOE OSTI2024

Millimeter-Scale Exfoliation of hBN with Tunable Flake Thickness for Scalable Encapsulation

As a two-dimensional (2D) dielectric material, hexagonal boron nitride (hBN) is in high demand for applications in photonics, nonlinear optics, and nanoelectronics. Unfortunately, the high-throughput preparation of macroscopic-scale, high-quality hBN flakes with controlled thickness is an ongoing challenge, limiting device fabrication and technological integration. In this report we present a metal thin-film exfoliation method to prepare hBN flakes with millimeter-scale dimension, near-unity yields, and tunable flake thickness distribution from 1-7 layers, a substantial improvement over scotch tape exfoliation. The single crystallinity and high quality of the exfoliated hBN are demonstrated with optical microscopy, atomic force microscopy, Raman spectroscopy, and second harmonic generation. We further explore a possible mechanism for the effectiveness and selectivity based on thin-film residual stress measurements, density functional theory calculations, and transmission electron microscopy imaging of the deposited metal films. We find that the magnitude of the residual tensile stress induced by thin film deposition plays a key role in determining exfoliated flake thickness in a manner which closely resembles 3D semiconductor spalling. Lastly, we demonstrate that our exfoliated, large-area hBN flakes can be readily incorporated as encapsulating layers for other 2D monolayers. Altogether, this method brings us one step closer to the high throughput, mass production of hBN-based 2D photonic, optoelectronic, and quantum devices.

2D materials↗
DOE OSTI2026

Control of Excitonic Energy Transfer in RGB Quantum Dot:Polymer Composites for Tunable White Emission

Tint-controlled white light is crucial for both illumination systems and display applications. Here, in this study, we demonstrate solution-processed quantum-dot light-emitting diodes (QD-LEDs) featuring a red–green–blue (RGB) QD–poly(methyl methacrylate) (PMMA) composite emissive layer (EML) for tunable white electroluminescence (EL). In this composite EML, PMMA functions as a dispersion matrix that modulates the interdot spacing (d), thereby controlling Förster resonance energy transfer (FRET) between QDs. By adjustment of the PMMA content, the balance of R, G, and B emissions is controlled, enabling systematic and continuous tuning of the EL color from greenish to reddish white at a fixed RGB ratio and constant driving bias. Time-resolved photoluminescence measurements confirm that the variation in the exciton lifetime with the PMMA fraction is the primary factor for tuning the EL color. Notably, nearly pure white EL with CIE coordinates close to (0.33, 0.33) is achieved using a diluted PMMA matrix without significant degradation of the electrical properties. Our results demonstrate d as an independent design parameter for decoupling color tuning from RGB composition and electrical operation, providing a versatile design framework for high-quality white- or tint-controlled QD-LEDs toward advanced solid-state lighting and display technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Carbonylative Co- and Terpolymerizations of 10-Undecen-1-ol: A Route to Polyketoesters with Tunable Compositions

A strategy to synthesize branched polyketoesters from the carbonylative polymerization of bifunctional α,ω-alkenols such as 10-undecen-1-ol is presented. This strategy hinges on the competitive application of two related catalytic manifolds, alternating alkene/CO copolymerization, and alkene hydroesterification, which share a common metal acyl intermediate. Small molecule model studies of cationic Pd-catalyzed alkene carbonylation in the presence of alcohols demonstrate that the relative rates of ketone formation (through alternating alkene/CO insertion) and ester formation (through metal acyl alcoholysis) can be tuned across a wide range through judicious bis(phosphine) ligand design. Carbonylative polymerization of 10-undecen-1-ol with a (dppp(3,5-CF 3 ) 4 )Pd(OTs) 2 catalyst (dppp(3,5-CF 3 ) 4 = 1,3-bis[bis[3,5-bis(trifluoromethyl)phenyl]-phosphino]propane) led to the formation of high molecular weight polyketoesters with intermediate dispersity (M n > 20,000 g/mol, D- = 2.6) and a ketone/ester microstructure ratio of approximately 1:2. In these polymerization reactions, deploying electron-deficient bis(phosphines) to suppress deleterious alkene isomerization was the key to accessing the high molecular weight polymer. Further, terpolymerization reactions of 1-hexene/10-undecen-1-ol/CO or 1-fluoro-10-undecene/10-undecen-1-ol/CO by (dppp(3,5-CF 3 ) 4 )Pd(OTs) 2 were also successful. Furthermore, this proof of concept polymerization unlocks access to tunable polymer microstructures without extensive postpolymerization treatment.

10-undecen-1-ol↗
DOE OSTI2021

Tunable Heteroassembly of a Plant Pseudoenzyme–Enzyme Complex

Pseudoenzymes have emerged as key regulatory elements in all kingdoms of life despite being catalytically nonactive. Yet many factors defining why one protein is active while its homologue is inactive remain uncertain. For pseudoenzyme–enzyme pairs, the similarity of both subunits can often hinder conventional characterization approaches. In plants, a pseudoenzyme, PDX1.2, positively regulates vitamin B 6 production by association with its active catalytic homologues such as PDX1.3 through an unknown assembly mechanism. Here we used an integrative experimental approach to learn that such pseudoenzyme–enzyme pair associations result in heterocomplexes of variable stoichiometry, which are unexpectedly tunable. We also present the atomic structure of the PDX1.2 pseudoenzyme as well as the population averaged PDX1.2–PDX1.3 pseudoenzyme–enzyme pair. Finally, we dissected hetero-dodecamers of each stoichiometry to understand the arrangement of monomers in the heterocomplexes and identified symmetry-imposed preferences in PDX1.2–PDX1.3 interactions. In this work, our results provide a new model of pseudoenzyme–enzyme interactions and their native heterogeneity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Tunable n- and p-Type Behavior of Ruddlesden–Popper and Dion–Jacobson 2D Perovskites

Interfacial band alignment in two-dimensional (2D)/three-dimensional (3D) perovskite heterostructures is important for maximizing separation, extraction, and collection of charge carriers, which, in turn, stabilize the perovskite layer during solar cell operation. Despite the wide use of 2D spacer cations for stabilizing perovskite solar cells, spacer cation exchange across the 2D/3D interface induces structural transformation and degradation in performance and stability. We have now examined the electrochemical and photoelectrochemical behavior of 2D Ruddlesden–Popper (RP) and Dion–Jacobson (DJ) perovskites to assess the impact of binding configuration on the interfacial charge separation and their operational stability. In-situ open circuit potential (OCP) responses of 2D perovskites under light illumination, paired with redox couples (ferrocene/ferrocenium), reveal different semiconducting polarizabilities of BA-RP (n-type) and BDA-DJ (p-type). The tunability of the Fermi energy level in 2D perovskites discussed in this study offers insight into the design of 2D/3D interfaces for optimizing charge transfer and enhancing charge neutrality and stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Hierarchical Epoxy Structures via Tunable Polymerization-Induced Phase Separation Combined with Additive Manufacturing

Polymerization-induced phase separation (PIPS) allows for the control of thermoset morphologies and properties, enabling the tuning of domain sizes and thermomechanical response. However, its use in generating substructural features in additively manufactured materials has been limited. In this work, we combine epoxy PIPS with UV curable acrylate and rheological modifiers to print nano- to macro-phase separating materials via a two-step, dual-cure approach. This method enables direct ink write printing of hierarchical structures with both controlled morphologies through phase separation and macroscale architecture through print design. We find that formulations for phase-separating materials require judicious incorporation of additives to enable printability and to provide sufficient green strength. Atomic force microscopy-nano infrared mapping reveals tunable, reticulated nano- to micron-scale domains of the resultant multiphase materials and their morphology changes due to additives, resulting in alterations to thermomechanical and tensile properties. Shape memory behavior is also demonstrated through multimaterial additive manufacturing of epoxies with functionally graded internal morphology using active mixing techniques, highlighting this method’s ability to fabricate complex architectures with controlled morphologies and thermomechanical response.

Van Meter, Kylie E [Organic Materials Science, San↗
DOE OSTI2020

Understanding Charge Storage in Hydrated Layered Solids MOPO 4 (M = V, Nb) with Tunable Interlayer Chemistry

Hydrated layered solids are interesting charge storage hosts with potentially high electrochemical activity and interlayer tunability. Although it is often possible to tune their interlayer distance by a pillaring strategy, the poor electrochemical stability of such artificial structures remains a major issue in device operation. Here we investigate the charge storage properties of MOPO 4 (M = V, Nb) hydrates with a nanosheet morphology to understand the influence of the interlayer environment on cycling stability, as well as ion selectivity. While in hydrated VOPO 4 interlayer H 2 O molecules act as compressible springs to enable fast Li-/Na-ion transport kinetics and reasonable structural reversibility, NbOPO 4 layers with bridging PO 4 groups serving as permanent linkers exhibit highly stable cyclability for Li-ions, owing to a zero-volume-change ion transport process. However, the latter suffers from a much larger migration energy barrier for Na-ions. Lastly, our findings not only highlight a structurally intriguing material system, but also provide insights into reviving materials with an originally unstable interlayer chemical environment, and shed light on the design principles for creating electrochemically stable charge storage hosts.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Tunable Mid-Infrared Interband Emission from Tensile-Strained InGaAs Quantum Dots

We demonstrate the ability to tailor self-assembled growth of In 0.5 Ga 0.5 As quantum dots (QDs) on GaSb(111)A surfaces by molecular beam epitaxy. Spontaneous formation via the Volmer–Weber growth mode produces QDs with excellent structural and optical quality. By harnessing tensile strain to reduce their band gap energy, these QDs are characterized by light emission that extends into the midwave infrared wavelength range of 3.2–3.9 μm (0.318–0.388 eV). As we increase QD size, we can tune the band alignment from type-III to type-II, where light emission occurs due to interband recombination between quantum confined electrons in the InGaAs QDs and holes in the GaSb barriers. Of particular interest is an unusual blue-shift in emission wavelength with increasing QD size, which we attribute to the incorporation of Sb into the InGaAs QDs from the GaSb barriers. By expanding this approach to produce tensile-strained QDs from other narrow band gap semiconductors, we anticipate the development of a range of highly tunable mid-infrared light sources.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Tunable Localized Charge Transfer Excitons in Nanoplatelet–2D Chalcogenide van der Waals Heterostructures

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D–2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate–2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe 2 and WSe 2 monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHs were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Finally, our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗