Search NASA⌕ Search

SEARCH · Search NASA

Results for “Electron doping”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

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

Fermi Level Equilibration and Charge Transfer at the Exsolved Metal-Oxide Interface

Exsolution is a promising approach for fabricating oxide-supported metal nanocatalysts through redox-driven metal precipitation. A defining feature of exsolved nanocatalysts is their anchored metal-oxide interface, which exhibits exceptional structural stability in (electro)catalysis. However, the electronic interactions at this unique interface remain unclear, despite their known impact on catalytic performance. In this study, we confirm charge transfer between the host oxide and the exsolved metal by demonstrating a two-stage Fermi level ( E F ) evolution on SrTi 0.65 Fe 0.35 O 3−δ (STF) during metallic iron (Fe 0 ) exsolution. Combining ambient pressure X-ray photoelectron spectroscopy with theoretical analysis, we show that E F initially rises due to electron doping from oxygen vacancy formation in STF. Subsequently, upon Fe 0 precipitation, E F stabilizes and becomes insensitive to further oxygen release in STF, driven by E F equilibration and charge transfer between STF and the exsolved Fe 0 . In conclusion, these findings highlight the importance of considering electronic metal–support interactions when optimizing exsolved nanocatalysts.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electronic structure, self-doping, and superconducting instability in the alternating single-layer trilayer stacking nickelates La 3 ⁢Ni 2 ⁢O 7

Motivated by the recently proposed alternating single-layer trilayer stacking structure for the nickelate La 3 ⁢Ni 2 ⁢O 7 , we comprehensively study this system using ab initio and random-phase approximation techniques. Here, our analysis unveils similarities between this novel La 3⁢ Ni 2 ⁢O 7 structure and other Ruddlesden-Popper nickelate superconductors, such as a similar charge-transfer gap value and orbital-selective behavior of the eg orbitals. Pressure primarily increases the bandwidths of the Ni e g bands, suggesting an enhancement of the itinerant properties of those e g states. By changing the cell volume ratio V/V 0 from 0.9 to 1.10, we found that the bilayer structure in La 3 ⁢Ni 2 ⁢O 7 always has lower energy than the single-layer trilayer stacking La 3 ⁢Ni 2 ⁢O 7 . In addition, we observe a “self-doping” effect (compared to the average 1.5 electrons per eg orbital per site of the entire structure) from the trilayer to the single-layer sublattices and this effect will be enhanced by overall electron doping. Moreover, we find a leading d x 2 -y 2 -wave pairing state that is restricted to the single layer. Because the effective coupling between the single layers is very weak, due to the nonsuperconducting trilayer in-between, this suggests that the superconducting transition temperature T c in this structure should be much lower than in the bilayer structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetic semiconductors from functionalized Cr 2 ⁢C MXenes

Here, we report an ab initio investigation of functionalized and 3⁢𝑑-electron-doped Cr 2 ⁢C MXenes. Upon functionalization, the Cr 2⁢ C becomes chemically, dynamically, and mechanically stable, and it exhibits magnetic semiconducting behavior. Cr 2 ⁢CF 2 stands out as a wide band gap semiconductor, possessing superexchange interaction mediated by F atoms within the layer; however, the applied strain transforms it from an indirect to a direct band gap semiconductor. Strong spin-phonon coupling found in Cr 2 ⁢CH 2 is supported by the distorted Cr spin density due to the hydrogen environment. Two magnon branches, associated with two sublattice spins, are found in the ferromagnetic Cr 2 ⁢CO 2 and antiferromagnetic Cr 2 ⁢CF 2 . Depending on the types of 3⁢𝑑-electron dopants and functionalization, Cr 2 ⁢C MXenes (except for Cr 2 ⁢CO 2 ) change from an indirect band gap magnetic semiconductor to different states of electronic and magnetic matter, including an exotic direct band gap magnetic bipolar semiconductor. In addition, we reveal a band inversion between the two highest valence bands in the Fe-doped Cr 2 ⁢CCl 2 .

2D materials↗
DOE OSTI2020

Expanded Analogs of Three‐Dimensional Lead‐Halide Hybrid Perovskites

Abstract Replacing the Pb−X octahedral building unit of A I PbX 3 perovskites (X=halide) with a pair of edge‐sharing Pb−X octahedra affords the expanded perovskite analogs: A II Pb 2 X 6 . We report seven members of this new family of materials. In 3D hybrid perovskites, orbitals from the organic molecules do not participate in the band edges. In contrast, the more spacious inorganic sublattice of the expanded analogs accommodates larger pyrazinium‐based cations with low‐lying π* orbitals that form the conduction band, substantially decreasing the band gap of the expanded lattice. The molecular nature of the conduction band allows us to electronically dope the materials by reducing the organic molecules. By synthesizing derivatives with A II =pyridinium and ammonium, we can isolate the contributions of the pyrazinium‐based orbitals in the band gap transition of A II Pb 2 X 6 . The organic‐molecule‐based conduction band and the inorganic‐ion‐based valence band provide an unusual electronic platform with localized states for electrons and more disperse bands for holes upon optical or thermal excitation.

Umeyama, Daiki↗
DOE OSTI2022

Intrinsic and doping-enhanced superconductivity in monolayer 1 H − TaS 2 : Critical role of charge ordering and spin-orbit coupling

The interplay of superconductivity with charge density wave (CDW) in metallic transition-metal dichalcogenides has been widely debated, and viable strategies manipulating these quantum states in the two-dimensional (2D) limit remain unclear. Using the ab initio anisotropic Migdal-Eliashberg theory, we successfully explain the superconductivity observed in monolayer 1H-TaS 2 by simultaneously determining its precise CDW structure and treating the marked modification of electron-phonon interaction and critical temperature T c by spin-orbit coupling effects. With this paradigm, we further show that electron doping weakens the CDW order leading to increased T c up to 11 K, along with a single-gap to two-gap superconductivity transition due to the suppression of the CDW gap. By contrast, a low hole doping barely affects the CDW but still yields a significantly enhanced superconducting order, implying their good coexistence. Combined with the synergistic behavior of CDW and superconductivity, which cooperate upon TaS 2 thickness reduction causing an unusual rise of T c , our results unravel diversified interactions between the two collective orders in ultrathin TaS 2 , being competition, coexistence or cooperation depending on external stimuli, which provide key clues for controlling correlated states in devices based on 2D CDW superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Flat-band-induced itinerant ferromagnetism in RbCo 2 Se 2

ACo 2 Se 2 (A=K,Rb,Cs) is a homologue of the iron-based superconductor, AFe 2 Se 2 . From a comprehensive study of RbCo 2 Se 2 via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principle calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin-splitting of the electronic dispersions. However, we identify the ordered moment to be dominated by a d x 2 –y 2 flatband near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this d x 2 –y 2 flatband to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Magnetic freeze-out and anomalous Hall effect in ZrTe 5

The ultra-quantum limit is achieved when a magnetic field confines an electron gas in its lowest spin-polarised Landau level. Here we show that in this limit, electron doped ZrTe 5 shows a metal-insulator transition followed by a sign change of the Hall and Seebeck effects at low temperature. We attribute this transition to a magnetic freeze-out of charge carriers on the ionized impurities. The reduction of the charge carrier density gives way to an anomalous Hall response of the spin-polarised electrons. This behavior, at odds with the usual magnetic freeze-out scenario, occurs in this Dirac metal because of its tiny Fermi energy, extremely narrow band gap and a large g-factor. We discuss the different possible sources (intrinsic or extrinsic) for this anomalous Hall contribution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Quantum magnetism of iron-based ladders: Blocks, spirals, and spin flux

Motivated by increasing experimental evidence of exotic magnetism in low-dimensional iron-based materials, we present a comprehensive theoretical analysis of magnetic states of the multiorbital Hubbard ladder in the orbital-selective Mott phase (OSMP). Here, the model we used is relevant for iron-based compounds of the AFe 2 X 3 family (where A=Cs, Rb, Ba, K are alkali metals and X=S, Se are chalcogenides). To reduce computational effort, and obtain almost exact numerical results in the ladder geometry, we utilize a low-energy description of the Hubbard model in the OSMP—the generalized Kondo-Heisenberg Hamiltonian. Our main result is the doping vs interaction magnetic phase diagram. We reproduce the experimental findings on the AFe 2 X 3 materials, especially the exotic block magnetism of BaFe 2 Se 3 (antiferromagnetically coupled 2×2 ferromagnetic islands of the ↑↑↓↓ form). As in recent studies of the chain geometry, we also unveil block magnetism beyond the 2×2 pattern (with block sizes varying as a function of the electron doping) and also an interaction-induced frustrated block-spiral state (a spiral order of rigidly rotating ferromagnetic islands). Moreover, we predict new phases beyond the one-dimensional system: a robust regime of phase separation close to half filling, incommensurate antiferromagnetism for weak interaction, and a quantum spin-flux phase of staggered plaquette spin currents at intermediate doping. Finally, exploiting the bonding/antibonding band occupations, we provide an intuitive physical picture giving insight into the structure of the phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Magnetic properties of intercalated quasi-2D Fe 3-x GeTe 2 van der Waals magnet

Among several well-known transition metal-based compounds, cleavable van der Waals (vdW) Fe 3-x GeTe 2 (FGT) magnet is a strong candidate for use in two-dimensional (2D) magnetic devices due to its strong perpendicular magnetic anisotropy, sizeable Curie temperature (T C ~154 K), and versatile magnetic character that is retained in the low-dimensional limit. While the T C remains far too low for practical applications, there has been a successful push toward improving it via external driving forces such as pressure, irradiation, and doping. Here we present experimental evidence of a room temperature (RT) ferromagnetic phase induced by the electrochemical intercalation of common tetrabutylammonium cations (TBA+) into quasi-2D FGT. We obtained Curie temperatures as high as 350 K with chemical and physical stability of the intercalated compound. The temperature-dependent Raman measurements, in combination with vdW-corrected ab initio calculations, suggest that charge transfer (electron doping) upon intercalation could lead to the observation of RT ferromagnetism. This work demonstrates that molecular intercalation is a viable route in realizing high-temperature vdW magnets in an inexpensive and reliable manner and has the potential to be extended to bilayer and few-layer vdW magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

H Energetics on Charged Graphene

These data include energy barriers for hydrogen adsorption and desorption on charged graphene, as well as the density of states for hydrogen-decorated graphene layers. Together, they highlight the role of applied charge and electronic doping on hydrogen chemisorption on graphene.

08 HYDROGEN↗
DOE OSTI2019

Interfacial-Redox-Induced Tuning of Superconductivity in YBa 2 Cu 3 O 7-δ

Solid-state ionic approaches for modifying ion distributions in getter/oxide heterostructures offer exciting potentials to control material properties. Here, we report a simple, scalable approach allowing for manipulation of the superconducting transition in optimally doped YBa 2 Cu 3 O 7-δ (YBCO) films via a chemically driven ionic migration mechanism. Using a thin Gd capping layer of up to 20 nm deposited onto 100 nm thick epitaxial YBCO films, oxygen is found to leach from deep within the YBCO. Progressive reduction of the superconducting transition is observed, with complete suppression possible for a sufficiently thick Gd layer. These effects arise from the combined impact of redox-driven electron doping and modification of the YBCO microstructure due to oxygen migration and depletion. Finaly, this work demonstrates an effective step toward total ionic tuning of superconductivity in oxides, an interface-induced effect that goes well into the quasi-bulk regime, opening-up possibilities for electric field manipulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Enabling hydrogen chemisorption on charged graphene

Two-dimensional (2D) materials, including graphitic derivatives, have long been of interest for hydrogen storage applications, due to their high theoretical storage capacity, low weight, and other useful properties. However, poor kinetics for hydrogen adsorption and surface diffusion as part of the proposed spillover process for hydrogenation have limited their technological potential. Here, we use first-principles calculations to study electronic doping as a means to improve hydrogen chemisorption on graphene, which we use here as a proxy for graphitic derivatives more broadly. We find that positively charged graphene sheets have vastly improved kinetics for hydrogen diffusion and adsorption, while they limit unwanted hydrogen desorption. This combination of effects should favor hydrogen chemisorption via spillover. We connect these trends to the C–H bond, which introduces states near the Fermi level. These states are depopulated as electrons are removed, thereby lowering the bond energy and permitting more facile movement of hydrogen. Our results suggest that spillover mechanisms for hydrogen chemisorption should be revisited if strategies to apply a large charge to graphitic systems can be realized. Moreover, switchable application of the charge may lead to the reversible chemisorption of hydrogen. While the large magnitude of charging required suggests that graphene itself may not be suitable for reversible hydrogen chemisorption, the factors we identify and discuss could significantly boost the prospects of graphitic derivatives and other 2D or layered materials for hydrogen storage applications.

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

Performance and reliability of β-Ga 2 O 3 Schottky barrier diodes at high temperature

Beta-gallium oxide (ß-Ga 2 O 3 ) is an ultrawide bandgap semiconductor that has potential for power electronic applications and devices operating at high temperatures. Particularly important for these applications are its 4.9 eV bandgap, facile electron doping, and the ability to grow ß-Ga 2 O 3 crystals from the melt. In this work, vertical ß-Ga 2 O 3 Schottky barrier diodes were fabricated using Pt Schottky and Ti-based Ohmic contacts and Au contact pads on unintentionally doped n-type, (2¯01)-oriented single crystal substrates. The diode’s temperature-dependent electrical properties up to 400°C were investigated, and the Pt/Ga 2 O 3 Schottky barrier height was determined to be close to 1.2 eV. The degradation of the contacts over multiple cycles up to 400°C was observed, resulting in a significant increase in series resistance of the diodes by 1000× at ambient temperature after they were cycled. According to electron microscopy measurements, this degradation is likely due in part to the migration and oxidation of Ti at the top surface of the Au contact pads. This degradation highlights the need for further research and development to ensure stable Ohmic and Schottky contacts to Ga 2 O 3 at temperatures above 400°C.

36 MATERIALS SCIENCE↗
DOE OSTI2020

n–Doping of a Low–Electron–Affinity Polymer Used as an Electron–Transport Layer in Organic Light–Emitting Diodes

n-Doping electron-transport layers (ETLs) increases their conductivity and improves electron injection into organic light-emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n-doping has been notoriously more difficult for these materials. In this work, n-doping of the polymer poly[(9,9-dioctylfluorene-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,7-diyl)] (F8BT) is demonstrated via solution processing, using the air-stable n-dopant (pentamethylcyclopentadienyl)(1,3,5-trimethylbenzene)ruthenium dimer [RuCp*Mes] 2 . Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes] 2 , the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n-doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. Here, the [RuCp*Mes] 2 -doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.

36 MATERIALS SCIENCE↗
DOE OSTI2020

From Trivial Kondo Insulator Ce 3 Pt 3 Bi 4 to Topological Nodal-Line Semimetal Ce 3 Pd 3 Bi 4

In this work, using the density functional theory combined with dynamical mean-field theory, we have performed systematic study of the electronic structure and its band topology properties of Ce 3 Pt 3 Bi 4 and Ce 3 Pd 3 Bi 4 . At high temperatures ( ~ 290 K ), the electronic structures of both compounds resemble the open-core 4 f density functional calculation results. For Ce 3 Pt 3 Bi 4 , clear hybridization gap can be observed below 72 K, and its coherent momentum-resolved spectral function below 18 K exhibits an topologically trivial indirect gap of ~ 6 meV and resembles density functional band structure with itinerant 4 f state. For Ce 3 Pd 3 Bi 4 , no clear hybridization gap can be observed down to 4 K, and its momentum-resolved spectral function resembles electron-doped open-core 4 f density functional calculations. The band nodal points of Ce 3 Pd 3 Bi 4 at 4 K are protected by the gliding-mirror symmetry and form ringlike structure. Therefore, the Ce 3 Pt 3 Bi 4 compound is topologically trivial Kondo insulator while the Ce 3 Pd 3 Bi 4 compound is topological nodal-line semimetal.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Imaging antiferromagnetic domain fluctuations and the effect of atomic scale disorder in a doped spin-orbit Mott insulator

Correlated oxides can exhibit complex magnetic patterns. Understanding how magnetic domains form in the presence of disorder and their robustness to temperature variations has been of particular interest, but atomic scale insight has been limited. We use spin-polarized scanning tunneling microscopy to image the evolution of spin-resolved modulations originating from antiferromagnetic (AF) ordering in a spin-orbit Mott insulator perovskite iridate Sr 3 Ir 2 O 7 as a function of chemical composition and temperature. We find that replacing only several percent of lanthanum for strontium leaves behind nanometer-scale AF puddles clustering away from lanthanum substitutions preferentially located in the middle strontium oxide layer. Thermal erasure and reentry into the low-temperature ground state leads to a spatial reorganization of the AF puddles, which nevertheless maintain scale-invariant fractal geometry in each configuration. Our experiments reveal multiple stable AF configurations at low temperature and shed light onto spatial fluctuations of the AF order around atomic scale disorder in electron-doped Sr 3 Ir 2 O 7 .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Weyl semimetal in the rare-earth hexaboride family supporting a pseudonodal surface and a giant anomalous Hall effect

Rare-earth hexaborides offer a rich tapestry for exploring the interplay of topological orders, magnetism, and electron-correlation effects associated with protected quantum phenomena. Using first-principles modeling, we identify in cerium hexaboride (CeB 6 ) a topological state that harbors a pseudonodal surface (PNS) along with double-Weyl fermions. An analysis of the electronic states in the vicinity of the Fermi level reveals the presence of M z -mirror-symmetry-protected band crossings that involve itinerant (d) and localized (f) states of Ce. These band crossings are found to remain nearly gapless as one goes away from the mirror plane, driving the formation of PNS states. We also demonstrate the presence of coexisting double-Weyl fermions with chiral charges of ±2 on the C 4z rotational axis. By analyzing the Berry curvature field associated with the topological states, we predict that CeB 6 supports a giant anomalous Hall conductivity of -1840Ω -1 cm -1 in the slightly electron-doped regime, which would be larger than the values reported to date in magnetic Weyl semimetals. Our study thus indicates that CeB 6 , which combines ferromagnetic order with topological physics in a heavy-fermion matrix, would provide a promising playground for investigating topological magnetic states with giant anomalous Hall responses.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Spin Correlations in the Parent Phase of Li 1–x Fe x ⁢ ODFeSe

Elucidating spin correlations in the parent compounds of high-temperature superconductors is crucial for understanding superconductivity. We used neutron scattering to study spin correlations in Li 1–x Fe x ⁢ ODFeSe, an insulating material with reduced electron carriers compared to its superconducting counterpart (T c = 41 K), serving as the undoped parent compound. Our findings show a reduced total fluctuating moment in this insulator relative to FeSe and 122 iron pnictides, likely due to increased interlayer distances from intercalation, which enhance fluctuations and reduce the intensity of spin excitations. Moreover, we observed a V-shaped spin wavelike excitation dispersion, contrasting with the twisted hourglass pattern in the superconducting counterpart. Electron doping shifts spin excitation from (π,0) point to an incommensurate position towards (π,π) direction below 65 meV. In conclusion, this transition from V-shaped to hourglasslike dispersion, akin to behaviors in hole-doped cuprates, suggests a potential shared mechanism in magnetism and superconductivity across these diverse systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗