90 Search Results

Hexagonal boron nitride (h-BN) is a promising material for a range of emerging applications in electronics, quantum information technology, and energy storage. Soft X-ray absorption spectroscopy (XAS) is powerful to reveal atomic details of BN, especially in the presence of defects. However, correlating XAS spectral features with specific defect types remains elusive. In this Letter, we report B K-edge XAS measurements of sputter-deposited turbostratic h-BN films and use a combination of first-principles spectroscopic simulations and analysis of detailed electronic structure and local charge transfer characteristics to elucidate their unique spectroscopic features. Our results show that the two main defect-related peaks,more » between the main π* resonances of h-BN and B2O3, as typically observed in BN films deposited by energetic condensation or bombarded with energetic ions, are associated with electronic states of H-passivated B atoms bonded to one and two oxygen impurity atoms, respectively. These conclusions hold significant implications for applications relying on defect-mediated properties of h-BN.« less

Ethanol is an important source of clean hydrogen, acetaldehyde, acetic acid, acetate esters, and light hydrocarbons. Controlling the divergent reaction pathways to these products requires understanding how different active sites influence the elementary steps involved. Herein, we present a combined surface science, theory, and nanoparticle catalysis study demonstrating how two single-atom dopants (Rh and Ni) in a Cu host can distinctively alter the selectivity of alcohol conversion. Specifically, our model studies reveal that ethanol reacts on Ni₁Cu single-atom alloys to selectively produce acetaldehyde, whereas methane and CO are also formed on Rh₁Cu single-atom alloys. Interestingly, these different reactivities are inmore » contrast to the behavior of the pure metals as Ni(111) and Rh(111) surfaces favor methane/CO and surface carbon/CO, respectively. DFT calculations of reaction pathways and simulated product desorption based on microkinetic analyses explain these reactivity differences, demonstrating that C–C cleavage leading to methane formation has a lower barrier on Rh single-atom sites. To test the catalytic relevance of these fundamental results we synthesized and characterized supported Ni₁Cu and Rh₁Cu single-atom alloy nanoparticles with dopant:Cu ratios of 1:200. Flow reactor results revealed that both Ni and Rh increased ethanol conversion over Cu and that Ni₁Cu catalysts were >99.9% selective to acetaldehyde, while Rh₁Cu also produced 0.6%–2.6% of equimolar methane and CO between 433 and 493 K, demonstrating that C–C bond cleavage is enabled by isolated Rh sites. Furthermore, these catalytic results bridge the pressure and materials gaps, and together, this study provides insights into how different isolated dopant sites promote different catalytic pathways.« less

Doping effects on the valence state, electronic structure, and optical band and the effects on electrical conductivity were studied on the doped lanthanum chromite (LaCrO₃) system. The specific compositions studied were La_1–xCa_xCrO₃ (LCCx), La_1–xSr_xCrO₃ (LSCx), and La_0.8Sr_0.2Cr_1–xMn_xO₃ (LSCMx) (0.1 ≤ x ≤ 0.4). The powders were synthesized using a modified Pechini sol–gel method, and the ceramic samples were densified using a reactive sintering method resulting in densities >96% theoretical. X-ray photoelectron spectroscopy (XPS) was completed to characterize the defect states and cationic valence compensation as a result of divalent (Ca²⁺ or Sr²⁺) and trivalent (Mn³⁺) substitutions. XPS was completed formore » samples tested in oxidizing and reducing atmospheres (up to 1500 °C), which provided insights into the oxidation state transitions induced by the Ca²⁺ and Sr²⁺ dopants. The work notably demonstrated, for the first time, the oxidation/reduction transitions of Cr⁴⁺ to Cr³⁺ in Sr²⁺/Mn³⁺ co-doped samples under reducing atmospheres. Reflectance UV–vis spectrophotometry optical band gap measurements were also completed for the same materials; a decrease in the optical band gap (2.81–3.12 eV) was shown with increased substitution, suggesting electronic structure modifications in the LaCrO₃ perovskite. Density functional theory calculations validated experimental trends, predicting a diminishing band gap with a rising dopant concentration. The transition in Cr oxidation states was attributed to the presence of divalent/trivalent cations. These findings contribute some insights into methods to tune the LaCrO₃ electrical properties for various low- and high-temperature applications.« less

Humans experience widespread exposure to anthropogenic per- and polyfluoroalkyl substances (PFAS) through various media, which can lead to a wide range of negative health impacts. Tap water is an important source of exposure in communities with any degree of contamination but routine or large-scale PFAS monitoring often depends on targeted analytical methods limited to measuring specific PFAS. We analyzed 680 tap water samples from the American Healthy Homes Survey II for PFAS using non-targeted analysis (NTA) to expand the range of detectable PFAS. Based on detection frequency and relative abundance, about half of the identified PFAS were found only bymore » NTA. We identified (with varying degrees of confidence) 75 distinct PFAS, including 57 exclusively detected by NTA. The identified PFAS are members of seven structural subclasses differentiated by their head groups and degree of fluorination. Clustering analysis categorized the PFAS into four coabundance groups dominated by specific PFAS subclasses. One group uniquely identified by NTA contains zwitterionic PFAS and other PFAS transformation products which are likely associated with aqueous firefighting foam contaminants in a small number of spatially correlated samples. These results help further characterize the scope of exposure to emerging PFAS experienced by the U.S. population via tap water and augment nationwide targeted-PFAS monitoring programs.« less

Wurtzite LaN (wz-LaN) is a semiconducting nitride that has piezoelectric and ferroelectric properties, making it promising for applications in electronics, either as a binary compound or in alloys such as LaAlN. The prospects for wz-LaN in devices are influenced by the properties of point defects and impurities; here, we use first-principles density functional theory with a hybrid functional to calculate their formation energies, as well as their atomic and electronic structures. Among native point defects, we find that nitrogen-related defects, both vacancies ($$V^+_N$$) and interstitials ($$N^-_i$$), are energetically most favorable under most relevant chemical potentials and positions of the Fermimore » level; $$V^0_N$$ may additionally be observed under N-poor conditions, and $$N^0_i$$ may be prominent under N-rich conditions. We also investigate the incorporation of oxygen and hydrogen, which will likely be present as unintentional impurities. We find that the $$O^+_N$$ substitutional species readily forms, but oxygen will not lead to n-type conductivity due to formation of DX centers and compensation by interstitial defects. Similarly, substitutional HN and interstitial H_i can compensate both p- and n-type dopants. Our results provide detailed, microscopic guidance for the development of electronic devices based on wz-LaN.« less

High-quality CsPbBr₃ crystals hold significant potential for gamma-ray detection due to their remarkable optoelectronic properties. This study details an optimized production process using the Bridgman method to achieve highly pure CsPbBr₃ crystals. By implementing rigorous synthesis and purification techniques, we successfully reduced the total impurity levels to 9 ppm, as confirmed by glow discharge mass spectroscopy (GDMS). The resulting CsPbBr₃ crystals demonstrate exceptional performance, including high transparency, intense photoemission, and prolonged photoluminescence decay times. These properties facilitate superior gamma-ray detection with an energy resolution of 1.4% for the ¹³⁷Cs 662 keV gamma-rays, comparable to commercial Cd_1-x Zn_xTe (CZT) detectors. Ourmore » findings underscore the critical relationship between material purity and detector performance, highlighting the potential of CsPbBr₃ as a cost-effective alternative in radiation detection applications. Further studies on defect origins and electronic states are necessary to fully leverage the capabilities of CsPbBr₃ crystals in practical high-energy radiation detection systems.« less

MnBi₂Te₄ (MBT) is an intrinsically magnetic topological material that possesses unique properties due to the quantum Hall effect. However, the low Mn–Bi mixed antisite formation energy is detrimental to these properties as it alters the long-range magnetic ordering in the Mn layer. Therefore, it is crucial to destabilize the antisite defects in MBT while preserving the favorable electronic properties. Here, to this end, we utilized a screening approach to understand the role of dopants in the Mn and Bi sites. We find that Sc, Y, and La dopings prefer substitutions on the Bi site and significantly increase the Mn–Bi antisitemore » defect formation energy. We find that the contribution from the empty s and d states of Sc, Y, and La around the Fermi level is insignificant. However, at the high concentration limit, the Bi–Te octahedra are significantly modified with doping, leading to important changes in the band structure.« less

A critical challenge to the commercialization of clean and high-efficiency solid oxide fuel cell (SOFC) technology is the insufficient stack lifespan caused by a variety of degradation mechanisms, which are associated with cell components and chemical feedstocks. Cell components related degradation refers to thermal/chemical/electrochemical deterioration of cell materials under operating conditions, whereas the latter regards impurities in feedstocks of oxidant (air) and reductant (fuel). This article provides a thermodynamic perspective on the understanding of the impurities-induced degradation mechanisms in SOFCs. The discussion focuses on using thermodynamic analysis to elucidate poisoning mechanisms in cathodes by impurity species such as Cr, CO₂,more » H₂O, and SO₂ and in the anode by species such as S (or H₂S), SiO₂, and P₂ (or PH₃). The author hopes the presented fundamental insights can provide a theoretical foundation for searching for better technical solutions to address the critical degradation challenges.« less

The carburizing supercritical CO₂ (sCO₂) environment limits the use of lower cost steels in the lower temperature (450–650°C) portions of the sCO₂ Brayton cycle because of concerns about internal carburization and embrittlement. Results on a ferritic–martensitic steel and conventional and advanced austenitic steels at 450–650°C in 30 MPa sCO₂ with and without 1% O₂ and 0.1% H₂O additions have indicated that sCO₂ environments will have lower maximum operating temperatures compared to steam plants. Pack Al and Cr coatings were evaluated at 650°C on T91 and 316H substrates and showed some benefit for up to 2000 h at 650°C, especially withoutmore » impurities. However, characterization indicated Al₂O₃ was not formed and Cr-rich carbides formed in the Cr coatings. With the addition of impurities in the sCO₂, the coatings were less protective at 650°C. Subsequent exposures at 600°C in sCO₂ showed similar behavior. Postexposure evaluations included measuring the bulk C content and room temperature tensile properties. Finally, improvements were indicated but the tensile results were complicated by the high temperature pack coating process affecting the substrate properties.« less