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Results for “doping mechanism”

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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Doping Bottleneck in Hematite: Multipole Clustering by Small Polarons

We report highly effective doping in transition metal oxides is critical to fundamentally overcome low carrier conductivity due to small polaron formation and reach their ideal efficiency for energy conversion applications. However, the optimal doping concentration in polaronic oxides such as hematite has been extremely low, for example, less than a percent, which hinders the benefits of doping for practical applications. In this work, we investigate the underlying mechanism of low optimal doping concentration with group IV (Ti, Zr, and Hf) and XIV (Si, Ge, Sn, and Pb) dopants from first-principles calculations. We find that novel dopant-polaron clustering occurs even at very low dopant concentrations and resembles electric multipoles. These multipoles can be very stable at room temperature and are difficult to fully ionize compared to separate dopants, and thus they are detrimental to carrier concentration improvement. This allows us to uncover mysteries of the doping bottleneck in hematite and provide guidance for optimizing doping and carrier conductivity in polaronic oxides toward highly efficient energy conversion applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Superconductivity in a doped valley coherent insulator in magic angle graphene: Goldstone-mediated pairing and Kohn-Luttinger mechanism

We consider magic angle graphene in the doping regime around charge neutrality and study the connection between a recently proposed inter-valley coherent insulator at zero doping and the neighboring superconducting domes. The breaking of the valley U(1) symmetry leads to massless Goldstone modes, which couple to the doped charge carriers. We derive the effective interaction between these Goldstone modes and the conduction electrons and study its role in mediating superconductivity. Combining it with the screened Coulomb potential, we find weak-coupling superconducting instabilities in the two-component p- and d-wave channels. The competition between the two channels is set by the distance between the bilayer graphene device and the metallic gates. We find that the p-wave instability originates from the attraction mediated by the Goldstone modes, while the d-wave pairing is caused purely by the screened Coulomb interaction, similarly to the Kohn-Luttinger mechanism.

Kozii, Vladyslav↗
DOE OSTI2024

Proton Conducting Neuromorphic Materials and Devices

Neuromorphic computing and artificial intelligence hardware generally aims to emulate features found in biological neural circuit components and to enable the development of energy-efficient machines. In the biological brain, ionic currents and temporal concentration gradients control information flow and storage. It is therefore of interest to examine materials and devices for neuromorphic computing wherein ionic and electronic currents can propagate. Protons being mobile under an external electric field offers a compelling avenue for facilitating biological functionalities in artificial synapses and neurons. In this review, we first highlight the interesting biological analog of protons as neurotransmitters in various animals. We then discuss the experimental approaches and mechanisms of proton doping in various classes of inorganic and organic proton-conducting materials for the advancement of neuromorphic architectures. Since hydrogen is among the lightest of elements, characterization in a solid matrix requires advanced techniques. We review powerful synchrotron-based spectroscopic techniques for characterizing hydrogen doping in various materials as well as complementary scattering techniques to detect hydrogen. First-principles calculations are then discussed as they help provide an understanding of proton migration and electronic structure modification. Outstanding scientific challenges to further our understanding of proton doping and its use in emerging neuromorphic electronics are pointed out.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Template-stabilized oxidic nickel oxygen evolution catalysts

Earth-abundant oxygen evolution catalysts (OECs) with extended stability in acid can be constructed by embedding active sites within an acid-stable metal-oxide framework. In this paper, we report stable NiPbO x films that are able to perform oxygen evolution reaction (OER) catalysis for extended periods of operation (>20 h) in acidic solutions of pH 2.5; conversely, native NiO x catalyst films dissolve immediately. In situ X-ray absorption spectroscopy and ex situ X-ray photoelectron spectroscopy reveal that PbO 2 is unperturbed after addition of Ni and/or Fe into the lattice, which serves as an acid-stable, conductive framework for embedded OER active centers. The ability to perform OER in acid allows the mechanism of Fe doping on Ni catalysts to be further probed. Catalyst activity with Fe doping of oxidic Ni OEC under acid conditions, as compared to neutral or basic conditions, supports the contention that role of Fe 3+ in enhancing catalytic activity in Ni oxide catalysts arises from its Lewis acid properties.

25 ENERGY STORAGE↗
DOE OSTI2026

Formation of Non-Doped Cubic Lithium Lanthanum Zirconium Oxide Nanofibers: Insights from In Situ Synchrotron X-Ray Scattering

This study investigates the formation mechanism of non-doped cubic lithium lanthanum zirconium oxide (c-LLZO) nanofibers using in situ synchrotron X-ray scattering techniques. Electrospun polymer precursor nanofibers were annealed at temperatures up to 800 °C, enabling real-time tracking of phase transitions via simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and evolved CO 2 gas analysis. The results reveal a three-step transformation pathway: polymer decomposition, formation of La 2 Zr 2 O 7 (LZO), and direct conversion of LZO to c-LLZO without intermediate tetragonal phases detected within the sensitivity of our in situ WAXS measurement. Cryo-electron energy loss spectroscopy (EELS) further elucidates the role of lithium diffusion, showing Li enrichment at fiber surfaces and Li deficiency in the interior, which stabilizes the cubic phase. This Li segregation effect in nanostructured LLZO materials extends beyond the previously reported size effect. This work advances the understanding of c-LLZO formation mechanisms and provides practical insights for optimizing synthesis routes to achieve phase-pure c-LLZO for solid-state battery applications.

LLZO phase stability↗
DOE OSTI2020

Isolating p- and n-Doped Fingers With Intrinsic Poly-Si in Passivated Interdigitated Back Contact Silicon Solar Cells

Polycrystalline silicon on silicon oxide (poly-Si/SiO x ) passivating contacts enable ultra high efficiency interdigitated back contact silicon solar cells. To prevent shunt between n- and p-type doped fingers, an insulating region is required between them. We evaluate the use of intrinsic poly Si for this isolation region. Interdigitated fingers were formed by plasma deposition of doped hydrogenated amorphous silicon through mechanically aligned shadow masks, on top of a full-area intrinsic amorphous silicon layer. High temperature annealing then crystallized the a-Si:H to poly Si and drove in the dopants. Two mechanisms were identified which cause contamination of the intrinsic poly Si gap during processing. During deposition of doped fingers, we show using secondary ion mass spectrometry and conductivity measurements that the intrinsic gap becomes contaminated by doped a-Si:H tails several nanometers thick to concentrations of ~10 20 cm -3 . Another source of contamination occurs during high-temperature annealing, where dopants desorb from doped regions and readsorb onto intrinsic a Si:H. Both pathways reduce the resistivity of the intrinsic gap from ~10 5 Ω·cm to ~10 -1 Ω·cm. We show that plasma etching of the a-Si:H surface before crystallizing with a capping layer can eliminate the contamination of the intrinsic poly-Si, maintaining a resistivity of ~10 5 Ω·cm. Lastly, this demonstrates masked plasma deposition as a dopant patterning method for Si solar cells.

14 SOLAR ENERGY↗
DOE OSTI2020

P-Type Electrochemical Doping Can Occur by Cation Expulsion in a High Performing Polymer for Organic Electrochemical Transistors

We investigate the mechanism of ion-dependent charge compensation during electrochemical oxidation (doping) of the model mixed ionic/electronic transporting polythiophene derivative poly(3-{[2-(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT). Using a combination of electrochemical quartz microbalance gravimetry and glow discharge optical emission spectroscopy, we show that charge compensation during polymer redox processes proceeds via a cation-dependent mechanism. For p-type polymer oxidation in certain electrolytes, charge compensation is achieved by both eventual injection of anions into the film, as well as initial expulsion of cations from the film. We compare doping mechanisms for a variety of electrolyte salts including potassium chloride, tetrabutylammonium chloride, potassium hexafluorophosphate (KPF6), and tetrabutylammonium hexafluorophosphate. For the electrolyte KPF6, both the cations and anions coexist in the water-swelled polymer even prior to application of electrical bias. Our data indicate that electrochemical doping (hole injection into the polymer and ionic charge compensation) proceeds via the following mechanism: (1) hydration of the neutral film by electrolyte (water, cations, anions), (2) cation (K+) expulsion from the film upon initial application of an oxidative bias, and (3) anion injection into the film at higher oxidation/doping levels (>~2 × 1020/cm3). Understanding the mechanism of charge compensation during the doping process should allow for the design of improved mixed ionic/electronic conductors for use in applications ranging from organic supercapacitors and redox flow batteries to bioelectronic sensors, thermoelectrics, and devices for neuromorphic computing.

Flagg, Lucas Q.↗
DOE OSTI2020

Mapping the structural, magnetic and electronic behavior of (Eu 1-x Ca x ) 2 Ir 2 O 7 across a metal-insulator transition

In this study, we employ bulk electronic properties characterization and x-ray scattering/spectroscopy techniques to map the structural, magnetic and electronic properties of (Eu 1-x Ca x ) 2 Ir 2 O 7 as a function of Ca-doping. As expected, the metal-insulator transition temperature, T-MIT, decreases with Ca-doping until a metallic state is realized down to 2 K. In contrast, T-AFM becomes decoupled from the MIT and (likely short-range) AFM order persists into the metallic regime. This decoupling is understood as a result of the onset of an electronically phase separated state, the occurrence of which seemingly depends on both synthesis method and rare earth site magnetism. PDF analysis suggests that electronic phase separation occurs without accompanying chemical phase segregation or changes in the short-range crystallographic symmetry while synchrotron x-ray diffraction confirms that there is no change in the long-range crystallographic symmetry. X-ray absorption measurements confirm the $J_{eff}$ = 1/2 character of (Eu 1-x Ca x ) 2 Ir 2 O 7 . Surprisingly these measurements also indicate a net electron doping, rather than the expected hole doping, indicating a compensatory mechanism. Lastly, XMCD measurements show a weak Ir magnetic polarization that is largely unaffected by Ca-doping. Keywords: term, term, term.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Orbital Hybridization and Magnetic Coupling at Cuprate–Manganite Interfaces Driven by Manganite Doping

Fundamental understanding of interface mechanisms that generate unexpected physical properties in the cuprate/manganite heterostructures is essential for possible applications as spintronic devices. In this study, CaCuO 2 /La 1–x Sr x MnO 3 (CCO/LSMO) superlattices are investigated, where the infinite-layer cuprate CCO does not have apical oxygen for Cu in the Ca-plane so that the Mn–O–Cu superexchange coupling can be only present at the MnO 2 –(La,Sr)O–CuO 2 interface. Two different doping states for the manganites are studied, namely x = 0.1 and x = 0.3, corresponding to the ferromagnetic insulating and ferromagnetic metallic states, respectively, in the manganite phase diagram. Linear and circular dichroism in X-ray absorption by synchrotron radiation at Cu and Mn L-edges clearly demonstrate that in the absence of apical oxygen in the cuprate block, the magnetic coupling between the LSMO and CCO is weakly ferromagnetic when LSMO is metallic, while it is antiferromagnetic when LSMO is insulating, along with an increased Cu 3d(3z 2 –r 2 ) and Mn 3d(3z 2 –r 2 ) orbital occupation. It is proposed that the Mn 3d valence band upward shift driven by the enhancement of Mn 3+ content in the underdoped sample enhances the orbital hybridization. The stronger hybridization at the interface MnO 2 –(La,Sr)O–CuO 2 gives rise to the antiferromagnetic coupling between cuprate and underdoped manganite.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Precipitation Hardening in Ferroelectric Ceramics

Domain wall motion in ferroics, similar to dislocation motion in metals, can be tuned by well-concepted microstructural elements. In demanding high-power applications of piezoelectric materials, the domain wall motion is considered as a lossy hysteretic mechanism that should be restricted. Current applications for so-called hard piezoelectrics are abundant and hinge on the use of an acceptor-doping scheme. However, this mechanism features severe limitations due to enhanced mobility of oxygen vacancies at moderate temperatures. By analogy with metal technology, the authors present here a new solution for electroceramics, where precipitates are utilized to pin domain walls and improve piezoelectric properties. Through a sequence of sintering, nucleation, and precipitate growth, intragranular precipitates leading to a fine domain structure are developed as shown by transmission electron microscopy, piezoresponse force microscopy, and phase-field simulation. This structure impedes the domain wall motion as elucidated by electromechanical characterization. As a result, the mechanical quality factor is increased by ≈50% and the hysteresis in electrostrain is suppressed considerably. This is even achieved with slightly increased piezoelectric coefficient and electromechanical coupling factor. This novel process can be smoothly implemented in industrial production processes and is accessible to simple laboratory experimentation for microstructure optimization and implementation in various ferroelectric systems.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Nitrogen: A promising doping strategy for high-performance ovonic threshold switching selectors

The Ovonic Threshold Switching (OTS) selector serves as an essential component in the development of three-dimensional high-density memory integration technology. Nevertheless, the state-of-the-art high-performance OTS materials usually contain toxic elements such as arsenic (As), posing significant risks to both environmental and human health. Nitrogen (N), which belongs to the same group as arsenic (As), has emerged as a highly promising alternative for As doping. However, the underlying mechanisms that govern N-based OTS materials have not yet been extensively investigated. In this study, we delve into the effects of N doping on the structural, bonding, and electronic properties of amorphous GeSe (a-GeNSe) by ab initio molecular dynamics simulations to bridge the knowledge gap. Our findings indicate that upon N doping in a-GeSe, the formation of robust Ge-N bonds, along with N-centered tetrahedral and triangular structures, resulting in the sluggish atomic movement that enhances the thermal stability and endurance of a-GeNSe. The OTS characteristics are significantly influenced by the material’s electronic band structure, and thus the relatively slow performance drift can be attributed to the stabilization of mid-gap states, a result of N doping which effectively slows down the aging process of chalcogenide glass. Moreover, the increased mobility gap in a-GeNSe raises the threshold voltage (V th ), making it more compatible with commercially available phase-change memory materials. Furthermore, our findings reveal the extensive impact of the N element on a typical OTS material and offer valuable perspectives for alternative doping strategies that could potentially supplant As practices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electron Transfer Going the Distance: Mn-Doped ZnSe as a Model Photocatalytic System

In this work, time-resolved photoluminescence and transient absorption spectroscopy are used to examine the influence that long excited state lifetimes of Mn-doped nanocrystals have on the mechanisms of photo-induced electron transfer (PET) in photocatalytic systems. Mn-doped ZnSe nanocrystals can undergo PET over their nearly millisecond excited state lifetime, which enables electron transfer to viologens in solution over large encounter distances in comparison to molecular length scales. The long excited state lifetimes also enable diffusion of the excited nanocrystals over hundreds of nanometers in solution, allowing them to react with molecular species at nanomolar concentrations. The ability to capture and sustain optical energy in spin-forbidden transitions among crystal field states of the Mn ions opens opportunities to explore the influence that long excited state lifetimes have on photocatalytic reaction mechanisms involving molecular species such as CO 2 that can be difficult to concentrate or attach to nanocrystal surfaces.

14 SOLAR ENERGY↗
DOE OSTI2025

Resistivity distribution and donor properties of antimony-doped n -type Czochralski silicon ingots

We investigate antimony (Sb)-doped Czochralski-grown silicon as an alternative n-type substrate for photovoltaic applications, and characterize their axial resistivity distribution, donor properties, and mechanical strength. We find that Sb-doped ingots can achieve a more uniform resistivity distribution along the axial direction compared to P-doped counterparts. Dopant concentration profiles in P-doped ingots can be accurately modelled using the standard Scheil's equation, accounting only for dopant segregation during solidification. In contrast, modelling Sb-doped ingots requires consideration of both dopant segregation and evaporation effects to fit the dopant distribution accurately. Using electron paramagnetic resonance spectroscopy at 9 K, we observe two hyperfine lines in P-doped samples, and six hyperfine lines for Sb 121 and eight for Sb 123 isotopes, with the number of hyperfine lines governed by the nuclear spins. We further identify two-atom Sb clustering in the Sb-doped wafers, confirmed through simulations of the additional weak electron paramagnetic resonance peaks. Finally, we find that 140 μm as-cut planar Sb-doped wafers exhibit slightly higher mechanical strength compared to P-doped wafers.

14 SOLAR ENERGY↗
DOE OSTI2022

Luminescence from Self‐Trapped Excitons and Energy Transfers in Vacancy‐Ordered Hexagonal Halide Perovskite Cs 2 HfF 6 Doped with Rare Earths for Radiation Detection

Abstract Compared to halides Cs 2 HfX 6 (X = Cl, Br, I) with a vacancy‐ordered cubic double perovskite structure, the halide Cs 2 HfF 6 (CHF), with a hexagonal Bravais lattice, possesses a higher mass density and chemical stability for radiation detection. Luminescence properties and energy transfer mechanisms of rare‐earths‐doped CHF materials are studied here. The structure of CHF is identified as a new type of vacancy‐ordered hexagonal perovskite, with the same type of building blocks of the double perovskite but stacked with single layers. Density‐functional theory calculations reveal a large bandgap of CHF. A broad emission is observed from the pristine CHF host, which is suggested to be associated with self‐trapped excitons (STEs). A series of rare‐earths‐doped materials are designed utilizing the STE emissions, and efficient energy transfers from STEs and Tb 3+ to Eu 3+ are achieved for tunable emissions. The codoped material shows stable emission under X‐ray irradiation, with 10.2% reduction from its initial emission intensity, associated with possible structural evolution by radiation‐induced deformation of the soft host. The radiation responses of singly and codoped materials are evaluated, and the codoped material is found to be more sensitive to the radiation energy than the singly doped or pristine CHF for radiation detection.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Working principles of doping-well structures for high-mobility two-dimensional electron systems

Suppressing electron scattering is essential to achieve high-mobility two-dimensional electron systems (2DESs) that are clean enough to probe exotic interaction-driven phenomena. In heterostructures it is common practice to utilize modulation doping, where the ionized dopants are physically separated from the 2DES channel. The doping-well structure augments modulation doping by providing additional screening for all types of charged impurities in the vicinity of the 2DES, which is necessary to achieve record-breaking samples. Despite its prevalence in the design of ultrahigh-mobility 2DESs, the working principles of the doping-well structure have not been reported. As such, here we elaborate on the mechanics of electron transfer from doping wells to the 2DES, focusing on GaAs/AlGaAs samples grown by molecular beam epitaxy. Based on this understanding we demonstrate how structural parameters in the doping well can be varied to tune the properties of the 2DES.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Evidence of Reversible Oxidation at CuInSe2 Grain Boundaries

Effects related to alkali metal doping and grain boundaries have puzzled the chalcopyrite photovoltaics community for a long time. This study is the first to report reversible oxidation of grain boundaries in CuInSe2 thin films. The phenomenon is observed in sodium-doped films, but not in undoped ones. Cathodoluminescence imaging, secondary ion mass spectrometry and Kelvin probe force microscopy analyses are performed on CuInSe2 thin films before and after exposure to vacuum. The findings suggest the existence of yet unidentified solid-gas equilibria. Resolving the nature of such reactions will provide new insights into the mechanism of alkali metal doping and passivation in chalcopyrite solar cells.

14 SOLAR ENERGY↗
DOE OSTI2021

Spin-orbital coupling and slow phonon effects enabled persistent photoluminescence in organic crystal under isomer doping

When periodically packing the intramolecular donor-acceptor structures to form ferroelectric-like lattice identified by second harmonic generation, our CD49 molecular crystal shows long-wavelength persistent photoluminescence peaked at 542 nm with the lifetime of 0.43 s, in addition to the short-wavelength prompt photoluminescence peaked at 363 nm with the lifetime of 0.45 ns. Interestingly, the long-wavelength persistent photoluminescence demonstrates magnetic field effects, showing as crystalline intermolecular charge-transfer excitons with singlet spin characteristics formed within ferroelectric-like lattice based on internal minority/majority carrier-balancing mechanism activated by isomer doping effects towards increasing electron-hole pairing probability. Our photoinduced Raman spectroscopy reveals the unusual slow relaxation of photoexcited lattice vibrations, indicating slow phonon effects occurring in ferroelectric-like lattice. Here, we show that crystalline intermolecular charge-transfer excitons are interacted with ferroelectric-like lattice, leading to exciton-lattice coupling within periodically packed intramolecular donor-acceptor structures to evolve ultralong-lived crystalline light-emitting states through slow phonon effects in ferroelectric light-emitting organic crystal.

36 MATERIALS SCIENCE↗