Search NASA⌕ Search

SEARCH · Search NASA

Results for “selective 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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Magnetocaloric Effect in Lightly‐Doped Fe 5 Si 3 Single Crystals

Abstract Development of promising new materials for above room temperature magnetic cooling applications relies on careful balancing of structure and composition to maximize accessible metastable phases that can drive a strong magnetocaloric effect (MCE). However, the working temperatures of these materials may fall outside of desired application windows. In this work, it is shown that it is possible to control metastable phase stability temperatures of Fe 5 Si 3 through selection of appropriate spin and charge doping. Here, the parent material's desired structure appears only within a narrow temperature range from 1098 to 1303 K. Doping with Mn and P is shown to allow stabilization of the parent's high temperature phase and resulting MCE to room temperature. The structural and magnetic properties, and the magnetocaloric effect of single crystal Fe 4.83 Mn 0.16 Si 2.91 P 0.09 (FMSP) are investigated experimentally and theoretically. A first‐order magneto‐elastic transition is observed at 348 K, where magnetic onset is accompanied by a change in lattice volume without an apparent change in crystal symmetry. Although the trace Mn and P doping are found to decrease the T C , the maximum magnetic entropy change Δ S Max ( T ) and the relative cooling power (RCP) of FMSP are enhanced compared to polycrystalline Fe 5 Si 3 . As a result, an intrinsically broader entropy change over a larger temperature span is generated in the lightly doped single crystal of Fe 5 Si 3 . The magnetic moment of the system is also enhanced. Density functional theory (DFT) calculations are performed to gain microscopic insights into the experimental findings. The results suggest that the hexagonal Fe 5 Si 3 is a new giant room temperature MCE material that is on par with La–Fe–Si and Fe‐Mn‐P‐Si systems.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Synthesis and characterization of Sn-doped hematite as visible light photocatalyst

Highlights: • Sn-doped hematite nanoparticles are prepared by SCS in one step. • The Sn doping have the ability to inhibit particle growth of hematite. • Sn can enhance visible light harvesting and e{sup −}/h{sup +} separation. • Sn-doped hematite degrades MB under visible light effectively. • The products with 5 mol% Sn have the highest photocatalytic activity. - Abstract: Sn-doped hematite nanoparticles are prepared by solution combustion synthesis. The products are characterized with various analytical and spectroscopic techniques to determine their structural, morphological, light absorption and photocatalytic properties. The results reveal that all the samples consist of nanocrystalline hematite with mesoporous structures, and Sn has the ability to inhibit the growth of hematite particle. Compared to pure hematite, the doped hematite samples with appropriate amount of Sn show better activities for degradation of methylene blue under visible light irradiation. The highest activity is observed for 5% Sn doped hematite and this product has long-term stability and no selectivity for dye degradation. The enhanced performance of 5% Sn doped hematite is ascribed to the smaller particle size, increased ability to absorb in visible light, efficient charge separation as well as improved e{sup −} transfer associated with the effects of appropriate amount of Sn doped sample.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Cobalt Oxide on N-Doped Carbon for 1-Butene Oligomerization to Produce Linear Octenes

Cobalt oxide supported on N-doped carbon catalysts were investigated for 1-butene oligomerization. The materials were synthesized by treating activated carbon with nitric acid and subsequently with NH3 at 200, 400, 600, and 800 °C, followed by impregnation with cobalt. The 1-butene oligomerization selectivity increased with ammonia treatment temperature of the carbon support. The oligomerization selectivity of cobalt oxide on N-doped carbon synthesized at 800 °C (800A-CoOx/N-C) is 2.6 times higher than previously reported cobalt oxide on N-doped carbon synthesized with NH 4 OH (2A-CoOx/N-C). Over 70% of the butene dimers were linear C8 olefins for all catalysts. The oligomerization selectivity increased with 1-butene conversion. The catalysts were characterized by elemental analysis, N2 adsorption, X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). The nitrogen content of the catalysts increases with ammonia treatment temperature as confirmed by elemental analysis. The surface content of pyridinic nitrogen with a binding energy of 398.4 ± 0.1 eV increased with ammonia treatment temperature as evidenced by deconvolution of N 1s XPS spectra.

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

Tuning hydrogenation chemistry of Pd-based heterogeneous catalysts by introducing homogeneous-like ligands

Abstract Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H 2 activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (<50%) in hydrotreating of diphenyl ether (DPE, modelling the strongest C-O linkage in lignin) and enhanced ethene selectivity (>90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.

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

Orbital-selective metal skin induced by alkali-metal-dosing Mott-insulating Ca2RuO4

Abstract Doped Mott insulators are the starting point for interesting physics such as high temperature superconductivity and quantum spin liquids. For multi-band Mott insulators, orbital selective ground states have been envisioned. However, orbital selective metals and Mott insulators have been difficult to realize experimentally. Here we demonstrate by photoemission spectroscopy how Ca 2 RuO 4 , upon alkali-metal surface doping, develops a single-band metal skin. Our dynamical mean field theory calculations reveal that homogeneous electron doping of Ca 2 RuO 4 results in a multi-band metal. All together, our results provide evidence for an orbital-selective Mott insulator breakdown, which is unachievable via simple electron doping. Supported by a cluster model and cluster perturbation theory calculations, we demonstrate a type of skin metal-insulator transition induced by surface dopants that orbital-selectively hybridize with the bulk Mott state and in turn produce coherent in-gap states.

Physics↗
DOE OSTI2016

Realizing stable fully spin polarized transport in SiC nanoribbons with dopant

Intrinsic half-metallicity recently reported in zigzag edged SiC nanoribbons is basically undetectable due to negligible energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations. In this Letter, by density functional theory calculations, we demonstrate a scheme of N doping at the carbon edge to selectively close the edge state channel at this edge and achieve 100% spin filtering, no matter whether it is in an AFM state or FM state. This turns SiC nanoribbon into a promising material for obtaining stable and completely spin polarized transport and may find application in spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2003

High-Temperature Sprayable Phosphor Coating Developed for Measuring Surface Temperatures

The use of phosphor thermography for noncontact temperature measurements in harsh environments has been proven over the last decade, but it has suffered from difficult application procedures such as vapor deposition or sputtering techniques. We have developed a high-temperature-sensitive paint that is easily applied with commercially available paint-spraying equipment and have successfully demonstrated it to temperatures up to 1500 C. Selected phosphors have also shown measurable signals to 1700 C, thus allowing a combination of phosphors to be used in high-temperature binders to make surface temperature measurements from ambient to over 1500 C. Phosphor thermography is an optical technique that measures the time response of fluorescence light, which is a function of the phosphor temperature. The phosphors are excited with short wavelength light (ultraviolet or blue), and they emit light at a longer wavelength. This technique has a benefit over other temperature measurements, such as thermocouples and infrared thermography, in difficult environments such as high blackbody backgrounds, vibration, flames, high electromagnetic noise, or where special windows may be needed. In addition, the sprayable phosphor paints easily cover large or complicated structures, providing full-surface information with a single measurement. Oak Ridge National Laboratories developed and tested the high-temperature binders and phosphors under the direction of the NASA Glenn Research Center. Refractory materials doped with rare earth metals were selected for their performance at high temperature. Survivability, adhesion, and material compatibility tests were conducted at high temperatures in a small furnace while the fluorescent response from the phosphors was being measured. A painted sample in a furnace with a clearly visible fluorescing dot excited by a pulsed laser is shown. Measuring the decay time of this fluorescence yields the surface temperature. One new paint was recently tested in a rocket test stand at Glenn. The floor of a square duct nozzle was painted, and full-field lifetime decay measurements were acquired for multiple firings of the rocket. Good agreement with predicted results was obtained, matching temperature gradients along the length of the nozzle and clearly showing shock structures. These good results were very satisfactory given that the measurements were made looking through the combustion plume. Infrared pyrometry was incapable of making the surface measurements because of the interference from the rocket exhaust, which contaminated the infrared signature.

Bencic, Timothy J.↗
DOE OSTI2019

A novel reversible fluorescent probe for the highly sensitive detection of nitro and peroxide organic explosives using electrospun BaWO 4 nanofibers

Fabrication of highly stable, reversible, and efficient portable sensors for the detection of explosives for safety and security is challenging due to the robustness of the currently available detection tools, limiting their mass deployment to the explosion prone areas. This paper reports a new direction towards the sensing of nitro- and peroxide-based explosives using highly stable rare-earth-doped BaWO 4 nanofibers with remarkable sensitivity and reversibility. Here, BaWO 4 nanofibers doped with Tb 3+ and Eu 3+ ions are fabricated through a sol–gel electrospinning process, and their emission characteristics and application as a fluorescent probe for the sensing of 2-nitrotoluene and H 2 O 2 , explosive taggants representing a broad class of explosives, are studied in detail. Scheelite structured BaWO 4 nanofibers exhibit excellent luminescence characteristics, and the rare-earth ion doping in the polycrystalline BaWO 4 nanofibers is tailored to achieve blue, green, red, and white light emissions. These nanofibers are extremely sensitive to 2-nitrotoluene and H 2 O 2 with rapid response time, and sensitivity is observed within the range of 1–400 ppb and 1–10 ppm, towards 2-nitrotoluene and H 2 O 2 , respectively. The fluorescence quenching of BaWO 4 nanofibers in the presence of 2-nitrotoluene and H 2 O 2 is exponential with the quenching constants up to 1.73 × 10 6 and 2.73 × 10 4 L mol –1 , respectively, which are significantly higher than those of most of the fluorescent probes based on metal–organic frameworks and conjugated organic materials. After exposing to 2-nitrotoluene, the luminescence of the nanofibers is retained completely upon heating at 120 °C for 10 min and the sensory response is retained as fresh nanofibers, and currently available fluorescent explosive sensors could not achieve such a recovery. The high sensitivity and selectivity of scalable rare-earth-doped BaWO 4 nanofibers provide a new platform for the simultaneous detection of two classes of explosives.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Superior lithium adsorption and required magnetic separation behavior of iron-doped lithium ion-sieves

The recent research on adsorption-based lithium recovery from lithium-containing solutions has been centred on adsorption capacity and separation of lithium ion-sieves powder from solutions. Herein, an effective iron-doped lithium titanium oxide (Fe-doped Li 2 TiO 3 ) was synthesized by Fe-doping via solid state reactions followed by acid treatment to form iron-doped lithium ion-sieves (Fe/Ti-x(H)). The resulting solid powder displays both superior adsorption capacity of lithium and high separation efficiency of the adsorbent from the solutions. SEM imaging and BET surface area measurement results showed that at Fe doping levels x ≤ 0.15, Fe-doping led to grain shrinkage as compared to Li 2 TiO 3 and at the same time the BET surface area increased. The Fe/Ti-0.15(H) exhibited saturated magnetization values of 13.76 emu g -1 , allowing effective separation of the material from solid suspensions through the use of a magnet. Consecutive magnetic separation results suggested that the Fe/Ti-0.15(H) powders could be applied at large-scale and continuously removed from LiOH solutions with separation efficiency of 96% or better. Lithium adsorption studies indicated that the equilibrium adsorption capacity of Fe/Ti-0.15(H) in LiOH solutions (1.8 g L -1 Li, pH 12) reached 53.3 mg g -1 within 24 h, which was higher than that of pristine Li 2 TiO 3 (50.5 mg g-1) without Fe doping. Competitive adsorption and regeneration results indicated that the Fe/Ti-0.15(H) possessed a high selectivity for Li with facile regeneration. Therefore, it could be expected that the iron-doped lithium ion-sieves have practical applicability potential for large scale lithium extraction and recovery from lithium-bearing solutions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Energy Shifts and Broadening of Excitonic Resonances in Electrostatically Doped Semiconductors

Tuning the density of resident electrons or holes in semiconductors through electrostatic doping provides crucial insight into the composition of the many excitonic complexes that are routinely observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers (e.g., spin and valley) of the resident carriers with applied magnetic fields and doping levels, as well as by selecting the quantum numbers of the photoexcited or recombining electron-hole (𝑒−ℎ) pair through optical polarization. Here, we discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable 𝑒−ℎ pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. Based on the carrier density, magnetic field, and light polarization, all optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the 𝑒−ℎ pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the broadening and decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moiré heterobilayer semiconductor systems, we relate the energy shift and amplitude of the neutral-exciton resonance to the compressibility of the resident carrier gas.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Establishing Self-Dopant Design Principles from Structure–Function Relationships in Self-n-Doped Perylene Diimide Organic Semiconductors

Self-doping is a particular doping method that has been applied to a wide range of organic semiconductors. However, there is a lack of understanding regarding the relationship between dopant structure and function. A structurally diverse series of self-n-doped perylene diimides (PDIs) is investigated to study the impact of steric encumbrance, counterion selection, and dopant/PDI tether distance on functional parameters such as doping, stability, morphology, and charge-carrier mobility. The studies show that self-n-doping is best enabled by the use of sterically encumbered ammoniums with short tethers and Lewis basic counterions. Additionally, water is found to inhibit doping, which concludes that thermal degradation is merely a phenomenological feature of certain dopants, and that residual solvent evaporation is the primary driver of thermally activated doping. In situ grazing-incidence wide-angle X-ray scattering studies show that sample annealing increases the π–π stacking distance and shrinks grain boundaries for improved long-range ordering. These features are then correlated to contactless carrier-mobility measurements with time-resolved microwave conductivity before and after thermal annealing. In conclusion, the collective relationships between structural features and functionality are finally used to establish explicit self-n-dopant design principles for the future design of materials with improved functionality.

36 MATERIALS SCIENCE↗
NASA NTRS2018

Method of Making Dye-Doped Polystyre Microparticles

Various embodiments provide dye-doped polystyrene microspheres generated using dispersion polymerization. Polystyrene microspheres may be doped with fluorescent dyes, such as xanthene derivatives including kiton red 620 (KR620), using dispersion polymerization. Certain functionalities, such as sodium styrene sulfonate, may be used to shift the equilibrium distribution of dye molecules to favor incorporation of the dye into the particles. Polyelectrolyte materials, such as poly(diallyldimethyl ammonium chloride), PolyDADMAC, may be used to electrostatically trap and bind dye molecules within the particles. A buffer may be used to stabilize the pH change of the solution during dye-doped polystyrene microsphere generation and the buffer may be selected depending on the pKa of the dye being incorporated. The various embodiments may provide dye-doped polystyrene microspheres, such as KR620-doped polystyrene microspheres that are non-toxic and non-carcinogenic. These non-toxic and non-carcinogenic dye-doped polystyrene microspheres may be suitable for use in wind tunnel testing.

Wohl, Jr., Christopher J.↗
DOE OSTI2022

n-Type doping of a solution processed p-type semiconductor using isoelectronic surface dopants for homojunction fabrication

The p-n junction is one of the fundamental requirements for a practical semiconductor-based electronic device. Designing a heterojunction comprising of dissimilar p-type and n-type semiconductors calls for careful energy level considerations, both when selecting the semiconductor materials as well as the metal contacts. A homojunction based on a single semiconductor simplifies this task, as energy levels of the p-type and n-type materials are already fairly similar, allowing for easier selection of contacts. Traditionally, homojunctions rely on doping of a bulk semiconductor to achieve p- and n-type transport through controlled addition of aliovalent dopants via energy-intensive processes such as ion implantation or thermal annealing. Exact control of doping in nanocrystalline semiconductors is significantly more challenging, due to self-purification effects. However, owing to their large surface areas, surface moieties can be utilized to both dope the nanostructures as well as tune their energy levels. Here, we present a facile technique based on an isoelectronic surface dopant in order to achieve p- and n-type materials based on the same semiconductor. We show that thin p-type colloidal Bi 2 Te 3 nanowires can be switched to n-type through surface functionalization, thus increasing the availability of new nanocrystalline solution-processable p-n homojunctions.

42 ENGINEERING↗
DOE OSTI2024

Molecular Origins of Near-Infrared Luminescence in Molybdenum and Tungsten Oxyhalide Perovskites

Materials with near-infrared (near-IR) luminescence are desirable for applications in communications and sensing, as well as biomedical diagnostics and imaging. The most used inorganic near-IR emitters rely on precise doping of host crystal structures with select rare-earth or transition metal ions. Recently, another class of materials with intrinsic near-IR emission has been reported. The compositions of these materials were initially described as vacancy-ordered halide double perovskites Cs 2 MoCl 6 and Cs 2 WCl 6 , but further investigation by some of us on the compound reported as Cs2WCl6 revealed an oxyhalide instead, with a composition Cs 2 WO x Cl 6–x , where 1 < x < 2. Here we demonstrate that the Mo compounds similarly possess the composition Cs 2 MoO x Cl 6–x or Cs 2 MoO x Br 6–x where 1 < x < 2. Preparing the pure halide appears harder for Mo than for W, and we have not succeeded in doing so. The distinctly different composition requires the coordination environment and oxidation state for the Mo and W centers to be reconsidered from what was assumed for the pure halides. In this work, we examine the mechanism for near-IR emission in these materials given their true structures and compositions. We demonstrate that the luminescence is due to the specific d-orbital splitting caused by the presence of oxygen in the distorted [MOX 5 ] 2– octahedra (X is Cl or Br). The fine structure in the emission spectra at low temperatures has been resolved and is attributed to vibronic coupling to the Mo–O and W–O bond stretches. Understanding the true structure and composition of these interesting materials, besides explaining the near-IR luminescence, suggests how this desirable emission can be realized and manipulated.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nucleation Effects in the Atomic Layer Deposition of Nickel–Aluminum Oxide Thin Films

Atomic layer deposition (ALD) has become an important technique to synthesize a wide variety of materials on the subnanometer length scale. Expanding the library of ALD for ternary materials is vital for applications in which ternary materials allow for tuning of physical, optical, and electronic properties. In this work reported herein, we demonstrate the first report of nickel–aluminum oxide (Ni–Al–O) films deposited by ALD using nickeloceneozone and trimethylaluminum-water as reactants. While deposition of a wide range of compositions is achieved, the observed growth per cycle (GPC) did not follow the simple combination of GPCs measured for the binary ALD processes. Nucleation studies performed to better understand this behavior reveal that the deposition of aluminum oxide is greatly enhanced on a NiO surface prepared by nickelocene and ozone. A model is developed that considers nucleation effects to predict composition and thickness as a function of supercycle recipe. Characterization of the deposited films shows that Al-doping of NiO results in contraction of the NiO lattice, decreased crystallinity, and reduced density, and that films become completely amorphous at compositions with less than 50% Ni. In addition, Al-doped NiO films deposited by ALD are investigated as a hole-selective transport layer in lead-based perovskite solar cells. An Al doping of ~4% improves power conversion efficiency of the perovskite-based devices over that of NiO primarily through increases in fill factor.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Solution Chemistry to Control Boron-Containing Monolayers on Silicon: Reactions of Boric Acid and 4-Fluorophenylboronic Acid with H- and Cl-terminated Si(100)

In this work, the reactions of boric acid and 4-fluorophenylboronic acid with H- and Cl-terminated Si(100) surfaces in solution were investigated. X-ray photoelectron spectroscopy (XPS) studies reveal that both molecules react preferentially with Cl–Si(100) and not with H–Si(100) at identical conditions. On Cl–Si(100), the reactions introduce boron onto the surface, forming a Si–O–B structure. The quantification of boron surface coverage demonstrates that the 4-fluorophenylboronic acid leads to ~2.8 times higher boron coverage compared to that of boric acid on Cl–Si(100). Consistent with these observations, density functional theory studies show that the reaction of boric acid and 4-fluorophenylboronic acid is more favorable with the Cl- versus H-terminated surface and that on Cl–Si(100) the reaction with 4-fluorophenylboronic acid is ~55.3 kJ/mol more thermodynamically favorable than the reaction with boric acid. The computational studies were also used to demonstrate the propensity of the overall approach to form high-coverage monolayers on these surfaces, with implications for selective-area boron-based monolayer doping.

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

Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi 8 Te 13 and Sb-doped MnBi 2n Te 3n+1 (n = 2, 3)

MnBi$_{2n}$Te$_{3n+1}$ (MBT) is the first intrinsic magnetic topological insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having n ≥ 4 or with Sb doping. In this work, we studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compounds, including MnBi 8 Te 13 and Sb doped MnBi$_{2n}$Te$_{3n+1}$ ($n = 2, 3)$ using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all three compounds, suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the saturation fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare 'double-peak' behavior observed in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗