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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

f‐ block MOFs: A Pathway to Heterometallic Transuranics

Abstract A novel series of heterometallic f ‐block‐frameworks including the first examples of transuranic heterometallic 238 U/ 239 Pu‐metal–organic frameworks (MOFs) and a novel monometallic 239 Pu‐analog are reported. In combination with theoretical calculations, we probed the kinetics and thermodynamics of heterometallic actinide(An)‐MOF formation and reported the first value of a U‐to‐Th transmetallation rate. We concluded that formation of uranyl species could be a driving force for solid‐state metathesis. Density of states near the Fermi edge, enthalpy of formation, band gap, proton affinity, and thermal/chemical stability were probed as a function of metal ratios. Furthermore, we achieved 97 % of the theoretical maximum capacity for An‐integration. These studies shed light on fundamental aspects of actinide chemistry and also foreshadow avenues for the development of emerging classes of An‐containing materials, including radioisotope thermoelectric generators or metalloradiopharmaceuticals.

Park, Kyoung Chul↗
DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

Chanakian, Sevan↗
DOE OSTI2022

f‐ block MOFs: A Pathway to Heterometallic Transuranics

Abstract A novel series of heterometallic f ‐block‐frameworks including the first examples of transuranic heterometallic 238 U/ 239 Pu‐metal–organic frameworks (MOFs) and a novel monometallic 239 Pu‐analog are reported. In combination with theoretical calculations, we probed the kinetics and thermodynamics of heterometallic actinide(An)‐MOF formation and reported the first value of a U‐to‐Th transmetallation rate. We concluded that formation of uranyl species could be a driving force for solid‐state metathesis. Density of states near the Fermi edge, enthalpy of formation, band gap, proton affinity, and thermal/chemical stability were probed as a function of metal ratios. Furthermore, we achieved 97 % of the theoretical maximum capacity for An‐integration. These studies shed light on fundamental aspects of actinide chemistry and also foreshadow avenues for the development of emerging classes of An‐containing materials, including radioisotope thermoelectric generators or metalloradiopharmaceuticals.

Park, Kyoung Chul↗
DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

36 MATERIALS SCIENCE↗
DOE OSTI2024

On‐Surface Synthesis and Determination of the Open‐Shell Singlet Ground State of Tridecacene**

Abstract The character of the electronic structure of acenes has been the subject of longstanding discussion. However, convincing experimental evidence of their open‐shell character has so far been missing. Here, we present the on‐surface synthesis of tridecacene molecules by thermal annealing of octahydrotridecacene on a Au(111) surface. We characterized the electronic structure of the tridecacene by scanning probe microscopy, which reveals the presence of an inelastic signal at 126 meV. We attribute the inelastic signal to spin excitation from the singlet diradical ground state to the triplet excited state. To rationalize the experimental findings, we carried out many‐body ab initio calculations as well as model Hamiltonians to take into account the effect of the metallic substrate. Moreover, we provide a detailed analysis of how the dynamic electron correlation and virtual charge fluctuation between the molecule and metallic surface reduces the singlet‐triplet band gap. Thus, this work provides the first experimental confirmation of the magnetic character of tridecacene.

Chemistry↗
DOE OSTI2025

High Pressure Synthesis of Pr 2 O 5 – A Unique Lanthanoid(IV) Oxide Peroxide

Abstract Reacting praseodymium(IV) oxide with oxygen at 27 GPa in a diamond anvil cell yielded the oxide peroxide Pr 2 IV (O 2 )O 3 , which was characterized by single crystal X‐ray diffraction on multi‐grain samples, Raman spectroscopy and quantum theoretical calculations at various pressure points. The presence of tetravalent praseodymium ions is supported by electronic structure calculations, showing a band gap of ca. 1.2 eV, which is consistent with the anticipated chemical model of an ionic solid. Pr 2 (O 2 )O 3 thus far represents the most oxygen rich phase of any binary compound of a lanthanoid and oxygen and is the first example of a peroxide anion next to Pr 4+ . Additionally, these results demonstrate that instead of oxidizing the praseodymium ions past their +IV oxidation state, oxygen undergoes a comproportionation to form peroxide anions. Direct oxidation of the oxide anions by Pr 4+ ‐ions was ruled out by a control experiment in argon instead of oxygen, where no oxidation of oxide ions was observed.

Chemistry↗
DOE OSTI2023

Transparent Conductive Encapsulants for Photoelectrochemical Applications

Utilizing sunlight to directly perform photoelectrochemical reactions is a promising route to renewable, net carbon-neutral fuels. However, a common problem with solar fuel production is semiconductor degradation in aqueous environments. An ideal protection layer should (1) prevent solution from reaching the semiconductor, (2) maintain charge transfer to and from solution, and (3) be transparent to light above the semiconductor band gap. While there have been substantial advances toward layers that meet these requirements, they are not easily adapted to new surfaces or new reactions, which can make protection difficult for newly developed photoabsorbers and (photo)electrochemical reaction pairings. In this work, we demonstrate the use of transparent conductive encapsulants (TCEs) to meet these requirements while also allowing for photoelectrode- and reaction-agnostic adaptability. TCEs are composed of an ethyl-vinyl acetate matrix with embedded conductive metal-coated microspheres that can be laminated to semiconductors. First, the electrochemical behavior of TCE-coated electrodes for the reduction of methyl viologen is characterized, demonstrating through-TCE electrical conduction. Then, photoelectrochemical measurements on TCE-protected semiconductors demonstrate the flexibility of this protection scheme. Finally, long-term photoelectrochemical measurements probe the efficacy of TCEs as protection layers. These findings demonstrate the potential of TCEs as adaptable protection layers in various photoelectrochemical applications.

14 SOLAR ENERGY↗
DOE OSTI2024

Impact of Dihedral Angle in Conjugated Organic Cation on the Structures and Properties of Organic‐Inorganic Lead Iodides

Abstract Conjugated organic cations are intriguing for organic‐inorganic halide perovskites due to their direct participation in the optoelectronic properties of hybrid materials. In conjugated cations, the dihedral angle, or torsion angle, between adjacent aromatic rings is a critical secondary structural element. This angle influences not only the shape of the cations but also the overlap between the π‐orbitals on adjacent rings, thereby affecting their electronic properties. Understanding how variations in the dihedral angle impact the structure and properties of hybrid organic‐inorganic metal halides (HOIMHs) is fundamentally important. In this study, we utilized 2,2′‐dimethyl bipyridinium as the organic cation, reacting it with PbI₂ to form hybrid lead iodides. Remarkably, variations in the dihedral angle between the two pyridinium rings resulted in the formation of two distinct crystal structures with different band gaps. Our findings demonstrate that manipulating the dihedral angle offers a novel approach to controlling the structures and properties of hybrid metal halides with conjugated cations.

Chandra Patra, Bidhan [Department of Chemistry and↗
DOE OSTI2024

Iodide Double Perovskites and the Limits of their Structural Stability

AbstractWhile halide double perovskites A2M(I)M(III)X6 have attracted significant attention, examples involving iodides are rare. We examine the limits of the structural stability of iodide double perovskites, presenting the synthesis and single‐crystal structures of Cs2NaScI6 and Cs2NaYI6. Bypassing the common expectation that iodides have small band gaps, these compounds display optical gaps of 3.10 eV [M(III) = Sc] and 3.65 eV [M(III) = Y]. Cs2NaScI6 is the only iodide double perovskite to exhibit a cubic crystal structure at room temperature. Density functional theory‐based electronic structure calculations help understand the role of competing Cs3M(III)2I9 (3 : 2 : 9) phases and provide possible reasons for why iodide double perovskites based around In(III), Sb(III) and Bi(III) cations have proved elusive. We confirm design rules for halide double perovskites based around concepts of the tolerance factor and the radius ratio of the smaller, trivalent ion, but also point to situations such as what is observed for Cs2NaScI6 where a double perovskite can be trapped in a metastable structure.

Mulligan, Anya S.↗
DOE OSTI2025

Photocatalytic Activity and Stability of Carbon Nitride‐Pyrite Composites

Abstract In photocatalysis, the photoabsorber plays a crucial role in the reaction. The most important parameters are stability, cost and optical band gap. In this work, a prominent class of absorbers, namely carbon nitrides (CN), has been investigated. In the literature, CN is most often described as stable, although photodegradation has been observed. In order to retain the beneficial properties of CN while improving stability, a crystalline phase poly(triazine imide) (PTI) of carbon nitride was investigated and compared to polymeric CN in photocatalytic hydrogen generation experiments. In order to improve the charge separation for the photoinduced hydrogen evolution reaction, pyrite (FeS 2 ) was used as a surface co‐catalyst with a loading of 1, 5 and 10 wt %. At the same time, any photodegradation products in solution were investigated by ion chromatography. Interestingly, PTI shows hardly any photocorrosion compared to defective carbon nitride, indicating its higher photostability in hydrogen evolution experiments. However, FeS 2 produces ammonium as a degradation product when synthesised from nitrogen‐containing precursors. When made from nitrogen‐free precursors, FeS 2 together with photostable PTI releases little ammonia, making it a photostable, earth‐abundant composite for photocatalytic hydrogen generation.

Kaulbersch, Julian [University of Bayreuth 95440 B↗
DOE OSTI2024

Graphitic Carbon Nitride Quantum Dots (g‐C 3 N 4 QDs): From Chemistry to Applications

Since their emergence in 2014, graphitic carbon nitride quantum dots (g-C 3 N 4 QDs) have attracted much interest from the scientific community due to their distinctive physicochemical features, including structural, morphological, electrochemical, and optoelectronic properties. Owing to their desirable characteristics, such as non-zero band gap, ability to be chemically functionalized or doped, possessing tunable properties, outstanding dispersibility in different media, and biocompatibility, g-C 3 N 4 QDs have shown promise for photocatalysis, energy devices, sensing, bioimaging, solar cells, optoelectronics, among other applications. As these fields are rapidly evolving, it is very strenuous to pinpoint the emerging challenges of the g-C 3 N 4 QDs development and application during the last decade, mainly due to the lack of critical reviews of the innovations in the g-C 3 N 4 QDs synthesis pathways and domains of application. Herein, an extensive survey is conducted on the g-C 3 N 4 QDs synthesis, characterization, and applications. Scenarios for the future development of g-C 3 N 4 QDs and their potential applications are highlighted and discussed in detail. In conclusion, the provided critical section suggests a myriad of opportunities for g-C 3 N 4 QDs, especially for their synthesis and functionalization, where a combination of eco-friendly/single step synthesis and chemical modification may be used to prepare g-C 3 N 4 QDs with, for example, enhanced photoluminescence and production yields.

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

Benzimidazolone-Dioxazine Pigments-Based Conjugated Polymers for Organic Field-Effect Transistor

Molecules based on benzimidazolone-dioxazine are known as blue/violet pigments and have been commercialized for decades. However, unfavorable solubility limits the application of these structures as building blocks of conjugated polymers despite their low band gaps. Herein, a series of donor–acceptor conjugated polymers containing soluble benzimidazolone-dioxazine structures as the acceptors and oligothiophene as donors are synthesized and investigated. With increasing numbers of thiophene rings, the steric hindrance diminishes and high molecular weight polymers can be achieved, leading to an improved performance in organic field effect transistor devices. The hole mobility of polymers with three to six thiophene units is in the order of 10 -1 cm2 V -1 s -1 . Among all the polymers, polymer P3 with three thiophene units between benzimidazolone-dioxazine structures shows the best hole mobility of 0.4 cm2 V -1 s -1 . Grazing-incidence wide-angle X-ray scattering results reveal that the high mobility of organic field-effect transistors (OFETs) can be accredited by matched donor–acceptor packing in the solid thin films.

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

The Crystal Structure of BaZn 2 Se 2 (OH) 2 Featuring Brownmillerite‐Type Layers

Abstract A bimetallic hydroxychalcogenide, BaZn 2 Se 2 (OH) 2 , was synthesized through hydrothermal pouch methods. The single crystal X‐ray diffraction and electron diffraction indicates that the phase crystallizes in the orthorhombic space group Pnma and is composed of anionic layers [ZnSe 3/3 (OH) 1/1 ] − that are separated and charged balanced by Ba 2+ cations. The [ZnSe 3/3 (OH) 1/1 ] – layer comprises two unique Zn sites, which form interpenetrating zigzag chains with an in‐plane dipole moment and adopts a brownmillerite‐type structural motif. The adjacent layers contain tetrahedrally coordinated Zn chains of opposite handedness related by an inversion center, which cancel the microscopic dipoles to minimize the macroscopic electric polarization. The adoption of a brownmillerite structural motif in BaZn 2 Se 2 (OH) 2 can be rationalized by the distinct charge difference between Se 2− and OH − anions, which creates a sufficient dipole moment in the ZnSe 3 (OH) tetrahedra to allow the occurrence of twisted chains. FTIR spectroscopy confirms the existence of OH − anions and DFT calculations indicate that BaZn 2 Se 2 (OH) 2 is a semiconductor with a direct band gap. This work expands the chemistry of the brownmillerite family from traditional homoanionic oxides to multianion hydroxychalcogenides, offering a new opportunity to explore tunable structural complexity for better design of functional materials.

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

Synthesis, crystal and electronic structure of the Zintl phase Ba 16 Sb 11 . A case study uncovering greater structural complexity via monoclinic distortion of the tetragonal Ca 16 Sb 11 structure type

The binary Zintl phase Ba 16 Sb 11 has been synthesized and structurally characterized. Detailed studies via single-crystal X-ray diffraction methods indicate that although Ba 16 Sb 11 appears to crystallize in the tetragonal Ca 16 Sb 11 structure type (space group $P\bar{4}2_1$m with a=13.5647(9) Å, c=12.4124(12)Å, Z=2, R 1 = 3.14%; wR 2 = 4.77%), there exists an extensive structural disorder. Some Ba 16 Sb 11 crystals were found to be monoclinic and the structure was solved and refined in space group P2 1 (a=18.3929(12) Å, b=13.5233(8) Å, c=18.3978(12) Å, β=94.6600(10)°; Z=4, R 1 =5.84 %; wR 2 =9.58 %). The latter corresponds to a 2-fold superstructure of the tetragonal one, which provides a disorder-free structural model. In both descriptions, the disordered tetragonal and the ordered monoclinic superstructure, the basic building units that make up the structure of this Ba-rich compound are pairs of face-shared square antiprisms of Ba atoms, which are centered by Sb atoms. The dimerized antiprisms are linked into parallel chains via square prisms of Ba atoms, which are also centered by Sb atoms. The Zintl concept can be applied in a straightforward manner and as result, the structure of Ba 32 Sb 22 (=2×Ba 16 Sb 11 ) can be rationalized as (Ba 2+ ) 32 (Sb 3– ) 20 [Sb 2 ] 4– . Notably, the partitioning of the valence electrons is done taking into an account the homoatomic Sb–Sb contacts (d=3.01 Å), which can be clearly distinguished in the lower symmetry space group. Electronic structure calculations of Ba 16 Sb 11 are in good accordance with the Zintl rationalization and predict a semiconductor with a band gap of 0.77 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Structural and optical properties of Sm{sup 3+}-doped borate glasses for luminescent applications

In this work, most interesting optical investigations are carried out with the glass composition (20 − x) CdCO{sub 3} + 15 Li{sub 2}CO{sub 3} + 15 Na{sub 2}CO{sub 3} + 50 H{sub 3}BO{sub 3} + xSm{sub 2}O{sub 3} (where x = 0.1, 0.5, 1.0, 3.0, 4.0 and 5.0 mol%) prepared with conservative melt-quenching technique. DSC verifies the annealing temperature and thermal stability of the prepared glass. X-ray diffraction technique is employed to inspect the amorphous structure of the glasses. FTIR technique helps to view the vibrational band positions of the host material. For optical studies, optical absorption and emission measurements are used. From the absorption spectra, Judd–Ofelt (JO) parameters (Ω{sub 2}, Ω{sub 4}, Ω{sub 6}), rms value of oscillator strength (δ{sub rms}), nephelauxetic parameter ( β-bar ), bonding parameter (δ) and optical band gaps are evaluated. Radiative properties such as radiative transition probabilities (A{sub T}), branching ratios (β{sub cal}, β{sub exp}) and stimulated emission cross section (σ{sub emi}) are calculated using JO theory. The higher stimulated emission cross section (14.39 × 10{sup −22} cm{sup 2}), optical gain (36.69 × 10{sup −25} cm{sup 2} s) indicate that 1.0 mol% of Sm{sup 3+}-doped borate glass would be a potential gain medium for solid state laser application at 603 nm. Color coordinates of the glasses located in the reddish-orange region could be useful for white light-emitting application.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Influence of structural defects on charge density waves in 1T-TaS2

Abstract The influence of intrinsic defects of 1T-TaS 2 on charge density waves (CDWs) is studied using scanning tunneling microscopy and spectroscopy (STM, STS), angle-resolved photoelectron spectroscopy (ARPES), and density functional theory (DFT). We identify several types of structural defects and find that most have a local character limited to a single CDW site, with a single exception which effectively behaves as a dopant, leading to band-bending and affecting multiple neighboring sites. While only one type of defect can be observed by STM topographic imaging, all defects are easily resolved in STS mapping. Our results indicate modulation of the Mott band gap commensurate with the CDW and breaking of the three-fold symmetry of electronic states. DFT calculations (with included Coulomb interactions) are used to investigate the electronic structure, focusing on both sulfur vacancy and oxygen-sulfur substitution. The sulfur vacancy system, characterized with a metallic behavior, is identified as the origin of one of the experimentally observed defects. Additionally, the effect of oxidation of 1T-TaS 2 depends on the substitution site, leading to the heterogeneity of electronic properties.

Chemistry↗