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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

The Role of Cation Coordination in the Electrical and Optical Properties of Amorphous Transparent Conducting Oxides

Amorphous oxide semiconductor materials have demonstrated numerous advantages without compromise of electrical properties as compared to their crystalline counterparts, yet understanding of the fundamental principles allowing this has remained elusive. To study the origins of enhanced optoelectronic properties, we apply high-throughput, combinatorial sputtering, structural and spectral mapping, and computationally intensive ab initio molecular dynamics simulations with density functional theory to a ternary, post-transition metal oxide system, namely, zinc tin oxide. The deposited thin films exhibit a high figure of merit, achieving carrier densities in the range of 1019 to 1020 cm–3 and carrier mobilities up to 35 cm2/Vs. These results highlight the role of local distortions and cation coordination in determining the microscopic origins of carrier generation and transport. In particular, we identify the strong likelihood of Sn undercoordination in both Zn-poor and Zn-rich phases leading to the high carrier concentrations observed. This not only diverges from the still widespread historical indictment of oxygen vacancies controlling carrier population in crystalline oxides but also provides a comprehensive framework to describe the unique structure–property relationships using specific structural and electronic descriptors in disordered phase materials.

CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS,M↗
DOE OSTI2020

Tunable semiconductor laser with two acousto-optic tunable filters in its external cavity

We report a laser with a linear external cavity containing a near-IR injection semiconductor optical amplifier as a gain element and two acousto-optic tunable filters (AOTFs) with quasi-collinear interaction of acoustic and light waves. At identical frequencies of RF signals that control the filters and ensure compensation for the Doppler frequency shift of light passing through the AOTFs, the steady-state laser emission linewidth can be reduced to about 25 MHz, which is almost three orders of magnitude smaller than that in the case of one filter (20 GHz). The position of the emission line, which remains narrow, slowly fluctuates within a spectral range about 3 GHz wide, which seems to be due to the large length of the external cavity and insufficient thermal stabilization of its components. This does not prevent us from obtaining light with a large coherence length. In emission wavelength sweep mode, the instantaneous emission linewidth increases with tuning rate, reaching 0.022 nm (8.8 GHz) at the highest tuning rate: 10{sup 4} nm s{sup −1}. In the case of automatic control over the output optical power at a level of 3 mW, the tuning range is 815 – 875 nm. Such instruments are of practical interest for optical coherence tomography, spectroscopy, optical metrology, and other application areas. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2025

Efficiency droop contributors in InGaN green light emitting diodes

Here, efficiency droop contributors (i.e., inherent Auger–Meitner recombination, polarization-induced effects, thermal effects, and light extraction) in InGaN green light emitting diodes (LEDs) are decoupled and quantified. First, a modified ABC model is developed, and external quantum efficiency measurements are taken under constant and pulsed currents (⁠ EQE Constant and EQE Pulsed ⁠, respectively). The LED internal quantum efficiency with and without thermal effects (⁠ IQE$^{ABC}_{Constant}$ and IQE$^{ABC}_{Pulsed}$⁠, respectively) is extracted using the modified model. Then, using Raman spectroscopy, the LED junction temperature is extracted. Finally, using the optical-electrical model (OEM), the polarization- and temperature-independent LED internal quantum efficiency (⁠ IQE OEM ) is calculated from the modified ABC model and the extracted junction temperature. By comparing external (⁠ EQE Constant ⁠) and the three internal quantum efficiencies (⁠ IQE$^{ABC}_{Constant}$, IQE$^{ABC}_{Pulsed}$⁠, and IQE OEM ), the impacts of inherent Auger–Meitner recombination, polarization-induced effects, thermal effects, and light extraction on the efficiency droop are decoupled and quantified. It is found that inherent Auger–Meitner recombination-induced droop is approximately 49% of the total efficiency droop in commercial green LEDs, while polarization-induced effects contribute about 35%, and thermal droop accounts for nearly 16%. Lastly, these findings suggest, to quash the green gap, it is critical to search for materials and device designs with low inherent Auger–Meitner coefficients and polarization fields, respectively.

42 ENGINEERING↗
DOE OSTI2022

Room Temperature Lasing from Semiconducting Single-Walled Carbon Nanotubes

Miniaturized near-infrared semiconductor lasers that are able to generate coherent light with low energy consumption have widespread applications in fields such as optical interconnects, neuromorphic computing, and deep-tissue optogenetics. With optical transitions at near-infrared wavelengths, diameter-tunable electronic structures, and superlative optoelectronic properties, semiconducting single-walled carbon nanotubes (SWCNTs) are promising candidates for nanolaser applications. However, despite significant efforts in this direction and recent progress toward enhancing spontaneous emission from SWCNTs through Purcell effects, SWCNT-based excitonic lasers have not yet been demonstrated. Leveraging an optimized cavity-emitter integration scheme enabled by a self-assembly process, here we couple SWCNT emission to the whispering gallery modes supported by polymer microspheres, resulting in room temperature excitonic lasing with an average lasing threshold of 4.5 kW/cm 2 . The high photostability of SWCNTs allows stable lasing for prolonged duration with minimal degradation. Finally, this experimental realization of excitonic lasing from SWCNTs, combined with their versatile electronic and optical properties that can be further controlled by chemical modification, offers far-reaching opportunities for tunable near-infrared nanolasers that are applicable for optical signal processing, in vivo biosensing, and optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Spin–orbit coupling proximity effect in MoS 2 /Fe 3 GeTe 2 heterostructures

Layered two-dimensional (2D) magnet/semiconductor heterostructures combine spintronic and optoelectronic properties of constituent materials, leading to new magneto-optical and magnetoelectric phenomena such as spontaneous emission of helical light and enhanced Zeeman splitting in single photon emission. While prior focus was mostly on the magnetic proximity effect, where properties of 2D magnets are transferred to nonmagnetic 2D materials, the inverse effect of 2D semiconductors altering 2D magnets is much less understood. Here, we fabricated and studied van der Waals (vdW) heterostructures of 2D magnet Fe 3 GeTe 2 (FGT) and 2D semiconductor MoS 2 . With reflectance magnetic circular dichroism, we found that the coercive field of MoS 2 -covered FGT reduces compared with uncovered FGT, agreeing well with our first-principles calculations. With its strong spin–orbit coupling (SOC), MoS 2 effectively alters the crystal field of the adjacent FGT and its magnetic anisotropy. Furthermore, an unconventional two-step hysteresis loop emerges in MoS 2 /FGT as a result of the superposition of two regions of FGT: at the interface and away from the interface. Our experimental elucidation of the SOC proximity effect that MoS 2 exerts on FGT provides fundamental understanding for the rational development of 2D magnet/semiconductor heterostructures.

2D materials↗
DOE OSTI2021

Controlled n –Doping of Naphthalene Diimide–Based Two–Dimensional Polymers

Two-dimensional polymers (2DPs) are promising as structurally well-defined, permanently porous, organic semiconductors. However, 2DPs are nearly always isolated as closed shell organic species with limited charge carriers, which leads to low bulk conductivities. Here, we enhance the bulk conductivity of two naphthalene diimide (NDI)-containing 2DP semiconductors by controllably n-doping the NDI units using cobaltocene (CoCp 2 ). Optical and transient microwave spectroscopy reveals that both as-prepared NDI-containing 2DPs are semiconducting with sub-2 eV optical bandgaps and photoexcited charge-carrier lifetimes of tens of nanoseconds. Following reduction with CoCp 2 , both 2DPs largely retain their periodic structures and exhibit optical and electron-spin resonance spectroscopic features consistent with the presence of NDI-radical anions. Here, while the native NDI-based 2DPs are electronically insulating, maximum bulk conductivities of >10 –4 S cm –1 are achieved by substoichiometric levels of n-doping. Density functional theory calculations show that the strongest electronic couplings in these 2DPs exist in the out-of-plane (π-stacking) crystallographic directions, which indicates that cross-plane electronic transport through NDI stacks is primarily responsible for the observed electronic conductivity. Taken together, this study underlines that controlled molecular doping is a useful approach to access structurally well-defined, paramagnetic, 2DP n-type semiconductors with measurable bulk electronic conductivities of interest for electronic or spintronic devices.

2D polymers↗
DOE OSTI2022

Optimizing molecular light absorption in the strong coupling regime for solar energy harvesting

The strong coupling of optical absorbers (e.g., molecules or semiconductors) to confined photonic modes fundamentally alters the physical properties of the coupled system via the formation of hybrid light-matter states. One potential application of strong light-matter coupling relies on exploiting it to localize light-induced charge excitation processes to small volumes of material. Applications that would benefit from this localization include thin-film photovoltaics, photodetection, photocatalysis, and others, where the overall performance depends on the ability of a material to amplify light absorption (i.e., the formation of electron-hole pairs) within specific locations in space. This contribution investigates how strong light-matter coupling affects light absorption rates in molecular absorbers coupled to photonic nanostructures. Here, our results show that the molecular light absorption efficiencies are highest in configurations where the strongly coupled molecules interact directly with the incoming photon flux. We also identify a nonlinear dependence in the molecular absorption as a function of concentration, unique to the strongly coupled systems. Based on these results, we propose design principles for engineering nanostructured systems that allow for high efficiencies of charge carrier localization into strongly coupled absorbers.

14 SOLAR ENERGY↗
DOE OSTI2022

Fast Exciton Diffusion in Monolayer PtSe2

Recently, 2D noble metal dichalcogenides have drawn considerable attention due to their thickness-tunable electronic and optical properties. However, the dynamical properties of photocarriers in these materials are less studied. In this report photocarrier dynamics in monolayer and bilayer PtSe 2 samples prepared by chemical vapor deposition are studied by transient absorption microscopy. Spatially and temporally resolved differential reflectance measurements yield room-temperature exciton lifetimes of 25 and 50 ps for monolayer and bilayer samples, respectively. The exciton diffusion coefficient in monolayer PtSe 2 is found to be as large as 48 cm 2 s -1 . This value is higher than exciton diffusion coefficients of most known monolayer semiconductors. The deduced exciton mobility is close to the theoretical limit of charge carrier mobility of monolayer PtSe 2 . The superior exciton transport property is unique to monolayers, as the exciton diffusion coefficient drops to 6.7 cm 2 s -1 in bilayers PtSe 2 . The novel exciton transport properties, along with its high air stability, make monolayer PtSe 2 an attractive material for ultrathin excitonic devices. These results provide insights on the exciton dynamic properties of 2D PtSe 2 and help develop fundamental understanding on the performance of various optoelectronic devices based on 2D PtSe 2 . .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Origins of the high-energy electroluminescence peaks in long-wavelength (~495–685 nm) InGaN light-emitting diodes

We investigate the unexpected high-energy electroluminescence (EL) peaks observed in long-wavelength InGaN light-emitting diodes (LEDs) with ground state emission peaks between ~495 and 685 nm by studying the EL spectra of LEDs with varying quantum well (QW) thicknesses and indium compositions. In addition to the ground state emission, two high-energy emission peaks were observed in the LEDs with thick QWs and high indium compositions. The less energetic high-energy emission peak (2.4–2.6 eV) is attributed to the optical transitions involving excited states. Factors influencing the excited state transitions, such as the QW thickness and indium compositions, were also examined by simulations to better understand the occurrence of these transitions. The more energetic high-energy emission peak (2.8–3.1 eV) originates from V-defect sidewalls and was verified through micro-photoluminescence measurements. Identification of the high-energy emission peaks is essential as it enables targeted epitaxial or growth optimizations to minimize or eliminate these undesirable emission peaks. In conclusion, this work demonstrates the importance of using thin QWs to suppress the unwanted high-energy emissions due to excited state transitions and V-defect sidewalls for long-wavelength InGaN LEDs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Scalable ultrafast epitaxy of large-grain and single-crystal II-VI semiconductors

A general problem for semiconductor applications is that very slow deposition on expensive single-crystal substrates yields high crystalline quality with excellent electro-optical properties, but at prohibitive costs and throughput for many applications. In contrast, rapid deposition on inexpensive substrates or nanocrystalline films yields low costs, but comparatively inferior crystallinity, carrier transport, and recombination. Here, we present methods to deposit single-crystal material at rates 2-3 orders of magnitude faster than state-of-the-art epitaxy with low-cost methods without compromising crystalline or electro-optical quality. For example, single-crystal CdTe and CdZnTe films that would take several days to grow by molecular-beam epitaxy are deposited in 8 minutes by close-spaced sublimation, yet retain the same crystalline quality measured by X-ray diffraction rocking curves. The fast deposition is coupled with effective n- and p-type in-situ doping by In, P, and As. The epitaxy can be extended to nanocrystalline substrates. For example, we recrystallize thin CdTe films on glass to deposit large grains with low defect density. The results provide new research paths for photovoltaics, detectors, infrared imaging, flexible electronics, and other applications.

14 SOLAR ENERGY↗
DOE OSTI2024

Free-electron crystals for enhanced X-ray radiation

Abstract Bremsstrahlung—the spontaneous emission of broadband radiation from free electrons that are deflected by atomic nuclei—contributes to the majority of X-rays emitted from X-ray tubes and used in applications ranging from medical imaging to semiconductor chip inspection. Here, we show that the bremsstrahlung intensity can be enhanced significantly—by more than three orders of magnitude—through shaping the electron wavefunction to periodically overlap with atoms in crystalline materials. Furthermore, we show how to shape the bremsstrahlung X-ray emission pattern into arbitrary angular emission profiles for purposes such as unidirectionality and multi-directionality. Importantly, we find that these enhancements and shaped emission profiles cannot be attributed solely to the spatial overlap between the electron probability distribution and the atomic centers, as predicted by the paraxial and non-recoil theory for free electron light emission. Our work highlights an unprecedented regime of free electron light emission where electron waveshaping provides multi-dimensional control over practical radiation processes like bremsstrahlung. Our results pave the way towards greater versatility in table-top X-ray sources and improved fundamental understanding of quantum electron-light interactions.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Atomistically resolved hot exciton relaxation dynamics in CdSe quantum dots: Experiment and theory

Semiconductor quantum dots (QDs) are well known to give rise to a quantum confined structure of excitons. Because of this quantum confinement, new physics of hot exciton relaxation dynamics arises. Decades of work using transient absorption (TA) spectroscopy have yielded initial simple observations, such as estimates of the cooling rate from single pump photon energy experiments. More detailed TA experiments employed variable pump photon energies to measure excitonic state-resolved transition rates. These TA measurements, usually the simplest form, have been employed to characterize QDs and their relaxation dynamics to this day. Yet, these TA measurements are fundamentally lacking in their ability to measure energy-resolved hot exciton cooling, which requires observation of the full cooling history through the real excitonic manifold. Here, we employ coherent multi-dimensional spectroscopy (CMDS) to perform an atomistically directed study of hot exciton cooling in CdSe QDs, revealing energy resolved relaxation dynamics. CMDS experiments are compared with simulations and prior TA measurements and simpler theories. Our findings reveal a hot exciton relaxation dynamics landscape. This relaxation dynamics landscape is a linear or sub-linear function of excess energy for different structures of QDs, with a strong size dependence. Our model simulations parameterized by the empirical pseudopotential model reproduces the experimental functional form and the dependence upon QD diameter and shell.

Atomistic simulations↗
DOE OSTI2024

Control of Quantized Spontaneous Emission from Single GaAs Quantum Dots Embedded in Huygens’ Metasurfaces

Advancements in photonic quantum information systems (QIS) have driven the development of high-brightness, on-demand, and indistinguishable semiconductor epitaxial quantum dots (QDs) as single photon sources. Strain-free, monodisperse, and spatially sparse local-droplet-etched (LDE) QDs have recently been demonstrated as a superior alternative to traditional Stranski–Krastanov QDs. However, integration of LDE QDs into nanophotonic architectures with the ability to scale to many interacting QDs is yet to be demonstrated. Here, we present a potential solution by embedding isolated LDE GaAs QDs within an Al 0.4 Ga 0.6 As Huygens’ metasurface with spectrally overlapping fundamental electric and magnetic dipolar resonances. We demonstrate for the first time a position- and size-independent, 1 order of magnitude increase in the collection efficiency and emission lifetime control for single-photon emission from LDE QDs embedded within the Huygens’ metasurfaces. Our results represent a significant step toward leveraging the advantages of LDE QDs within nanophotonic architectures to meet the scalability demands of photonic QIS.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Excitonic Spin-Coherence Lifetimes in CdSe Nanoplatelets Increase Significantly with Core/Shell Morphology

Here, we report spin-polarized transient absorption for colloidal CdSe nanoplatelets as functions of thickness (2–6 monolayer thickness) and core/shell motif. Using electro-optical modulation of co- and cross-polarization pump–probe combinations, we sensitively observe spin-polarized transitions. Core-only nanoplatelets exhibit few-picosecond spin lifetimes that weakly increase with layer thickness. The spectral content of differenced spin-polarized signals indicate biexciton binding energies that decrease with increasing thickness and smaller values than previously reported. Shell growth of CdS with controlled thicknesses, which partially delocalize the electron from the hole, significantly increases the spin lifetime to ~49 ps at room temperature. Implementation of ZnS shells, which do not alter delocalization but do alter surface termination, increased spin lifetimes up to ~100 ps, bolstering the interpretation that surface termination heavily influences spin coherence, likely due to passivation of dangling bonds. Spin precession in magnetic fields both confirms long coherence lifetime at room temperature and yields the excitonic g factor.

2D semiconductor↗
DOE OSTI2021

Exciton-driven antiferromagnetic metal in a correlated van der Waals insulator

Abstract Collective excitations of bound electron-hole pairs—known as excitons—are ubiquitous in condensed matter, emerging in systems as diverse as band semiconductors, molecular crystals, and proteins. Recently, their existence in strongly correlated electron materials has attracted increasing interest due to the excitons’ unique coupling to spin and orbital degrees of freedom. The non-equilibrium driving of such dressed quasiparticles offers a promising platform for realizing unconventional many-body phenomena and phases beyond thermodynamic equilibrium. Here, we achieve this in the van der Waals correlated insulator NiPS 3 by photoexciting its newly discovered spin–orbit-entangled excitons that arise from Zhang-Rice states. By monitoring the time evolution of the terahertz conductivity, we observe the coexistence of itinerant carriers produced by exciton dissociation and a long-wavelength antiferromagnetic magnon that coherently precesses in time. These results demonstrate the emergence of a transient metallic state that preserves long-range antiferromagnetism, a phase that cannot be reached by simply tuning the temperature. More broadly, our findings open an avenue toward the exciton-mediated optical manipulation of magnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Impact of Alloy-Disorder-Induced Localization on Hole Diffusion in Highly Excited c -Plane and m -Plane (In, Ga) N Quantum Wells

The diffusion coefficient of holes can provide knowledge about carrier localization in (In, Ga)N, where the carrier dynamics are altered by randomly fluctuating potential landscape. In group-III nitrides, the diffusivity of holes is difficult to measure by electrical methods but it can be studied using optical techniques. Here, we investigate the dependence of the hole diffusion coefficient on direction and carrier density in c-plane and m-plane (In, Ga)N structures by employing the light-induced transient-grating technique. We show that the hole diffusion coefficient is anisotropic in the m-plane structure, where it is several times larger along the a crystallographic direction than along the c direction. Such anisotropy is observed within the broad range of carrier densities from 10 18 to 10 20 cm –3 . The diffusivity changes nonmonotonously with increasing photoexcitation, this dependence being different in thick and thin layers. We argue that an unexpectedly high diffusion coefficient at low carrier densities in thick quantum wells can be a signature of efficient hole transport via percolative paths occurring due to compositional disorder. In turn, a decrease of diffusivity with the excitation can reflect the effect of Coulomb blockade of these paths. Lastly, we demonstrate that disorder impacts carrier diffusivity even at carrier densities above 10 19 cm –3 , where the overflow of localized states must be included to explain the observed increase of the diffusion coefficient with the carrier density.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Nanocrystal‐Enabled Perovskite Heterojunctions in Photovoltaic Applications and Beyond

Abstract Heterojunctions are used to tailor the properties of semiconductors in optoelectronic devices, yet for emerging devices composed of metal halide perovskites, fabricating perovskite/perovskite heterojunctions has proved challenging due to solvent incompatibilities and rapid homogenization due to ion migration. Recent studies have demonstrated various strategies for using perovskite nanocrystals as a component to fabricate perovskite/perovskite heterojunctions, either with a perovskite thin film or a second nanocrystal layer. Heterojunctions such as these can impart many advantages of both bulk and nanocrystalline perovskite morphologies. This perspective focuses on recent developments of solution‐processed perovskite heterojunctions for solar cells and novel optoelectronic devices, in particular, highlighting the demonstrated and potential advantages of nanocrystal‐enabled fabrication strategies. A central tenet of this perspective is that the synthesis and dispersion of perovskite nanocrystals in non‐polar organic solvents offers a key processing advantage over traditional perovskite precursor solutions in polar solvents since the former allows for layer‐by‐layer deposition without dissolving an underlying perovskite film or crystal. This processing advantage, coupled with nanocrystal size control and ligand chemistry, enables perovskite heterojunctions with highly tunable optical and electrical properties. Such heterojunctions may enable disruptive technological advances in broad classes of devices such as solar cells, photodetectors, sensors, and (in)coherent photon sources with tunable polarization.

14 SOLAR ENERGY↗
DOE OSTI2021

Optical and Magneto-Optical Properties of Donor-Bound Excitons in Vacancy-Engineered Colloidal Nanocrystals

Controlled insertion of electronic states within the band gap of semiconductor nanocrystals (NCs) is a powerful tool for tuning their physical properties. One compelling example is II–VI NCs incorporating heterovalent coinage metals in which hole capture produces acceptor-bound excitons. To date, the opposite donor-bound exciton scheme has not been realized because of the unavailability of suitable donor dopants. Here, we produce a model system for donor-bound excitons in CdSeS NCs engineered with sulfur vacancies (V S ) that introduce a donor state below the conduction band (CB), resulting in long-lived intragap luminescence. V S -localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the V S are not magnetically coupled to the NC bands and that the polarization properties are determined by the spin of the valence-band photohole, whose spin flip is massively slowed down due to suppressed exchange interaction with the donor-localized electron.

77 NANOSCIENCE AND NANOTECHNOLOGY↗