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DOE OSTI2024

Estimation of mean electron energy in helium surface ionization waves on dielectric substrates

Abstract The determination of basic plasma parameters in atmospheric pressure discharges is critical to advancing their use in applications. Atmospheric pressure plasma jets have found use in the fields of medicine, agriculture, material modification and others. Atmospheric pressure plasma jets often generate plasma surface ionization waves (SIW) which interact with and propagate over surfaces. Electrical diagnostics are challenging in SIW due to high collision frequencies and small scale of the plasma discharge. This work employs a passive optical emission line ratio technique to estimate the mean electron energy in SIW over planar dielectric substrates. The method uses an intensity ratio of two helium triplet lines: He( 3 3 S ) at 706.5 nm and He( 3 3 D ) at 587.56 nm. A collisional-radiative model is used to correlate line ratio to mean electron energy and determine dependencies on electron density and He/air gas mixture. Mean electron energies ranging from 3–8 eV are determined in He/air mixtures and are found to remain constant as the surface wave propagates radially. This work provides a 2D, time-resolved, mean electron energy diagnostic for surface ionization wave propagation and validation of numerical modeling in atmospheric pressure systems with spatially varying He/air gas mixtures. The model in question is designed for use with any He line ratio in the n = 3 excitation level.

Morsell, Joshua (ORCID:0000000331073002)↗

DOE OSTI2025

Broadband mm-Wave Substrate Integrated Waveguides [Poster]

At mm-wave frequencies, signals need to be supported by specialized, low-loss transmission lines such as waveguides. Substrate-integrated waveguides (SIWs) combine the good electrical properties of rectangular waveguides with the compactness of planar transmission lines. Additionally, SIWs can be fabricated using standard PCB manufacturing techniques and can be easily integrated with other mm-wave components. This study explores SIWs in the 50-75 GHz range, with designs optimized for simple fabrication.

42 ENGINEERING↗

DOE OSTI2026

Tuning Uranium Redox Chemistry in Asymmetric Polyoxometalate Complexes: Access to U(IV), U(V), and Transient U(VI)

We report the synthesis and characterization of asymmetric sandwich-type complexes [TpU IV EW 11 O 39 ] x– (E = P; x = 4, E = Si; x = 5, Tp = trispyrazolylborate). These complexes represent rare examples of the selective formation of asymmetric actinide-polyoxometalate (An-POM) complexes. Oxidation studies show that the U V analogues are stable and accessible, with both complexes isolated. Attempts to access U VI with this framework yield mixed results. Attempts to produce [TpU VI PW 11 O 39 ] 2– only led to decomposition, while electrochemical oxidation experiments gave evidence of transient formation of [TpU VI SiW 11 O 39 ] 3– in solution. Rapid decomposition of [TpU VI SiW 11 O 39 ] 3– inhibits full characterization. These studies indicate that the redox properties and stability of An-POM complexes containing actinides in higher valencies are dependent on the anionic charge of the assembly.

Actinides↗

DOE OSTI2024

From +I to +IV, Alkalis to Actinides: Capturing Cations across the Periodic Table with Keggin Polyoxometalate Ligands

Coordination chemistry trends across the periodic table are often difficult to probe experimentally due to limitations in finding a versatile but consistent chelating platform that can accommodate various elements without changing its coordination mode. Herein, we present new metal/ligand systems covering a wide range of ionic radii, charges, and elements. Five different ligands derived from the Keggin structure (HBW 11 O 39 8– , PW 11 O 39 7– , SiW 11 O 39 8– , GeW 11 O 39 8– , and GaW 11 O 39 9– ) were successfully crystallized with six different cations (Na + , Sr 2+ , Ba 2+ , La 3+ , Ce 4+ , and Th 4+ ) and characterized by single-crystal X-ray diffraction. Twenty-five new compounds were obtained by using Cs + as the counterion, yielding a consistent base formula of Cs x [M(XW 11 O 39 ) 2 ]·nH 2 O. Despite having a similar first-coordination sphere geometry (i.e., 8-coordinated), the nature of the central cation was found to impact the long-range geometry of the complexes. This unique crystallographic data set shows that, despite the traditional consensus, the local geometry of the cation (i.e., metal–oxygen bond distance) is not enough to depict the full impact of the complexed metal ion. The bending and twisting of the complexes, as well as ligand–ligand distances, were all impacted by the nature of the central cation. We also observed that counterions play a critical role by stabilizing the geometry of the M(XW 11 ) 2 complex and directing complex–complex interactions in the lattice. We also define certain structural limits for this type of complex, with the large Ba 2+ ion seemingly approaching those limits. Finally, this study thus lays the foundation for capturing the coordination chemistry of other rarer elements across the periodic table such as Ra 2+ , Ac 3+ , Bk 4+ , Cf 3+ , etc.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Morphology Effects on Free Energies of Proton-Coupled Electron Transfer in Polyoxotungstates

Polyoxotungstates have previously been established to facilitate the hydrogenation of small molecule substrates via hydrogen atom transfer from reactive hydroxyl groups formed at the assembly surface. Understanding structure−function relationships that dictate the thermochemistry and kinetics of protoncoupled electron transfer is key to controlling this chemistry. In this work, we combine comprehensive electrochemical experiments and density functional theory calculations to address how different polyoxotungstate morphologies, specifically W 6 O 19 −2 , W 10 O 32 −4 , SiW 12 O 40 −4 , and P 2 W 18 O 62 −6 , affect the bond dissociation free energies of surface hydroxides (BDFE(O−H)) formed upon reduction of the assembly in acidic media. Our results reveal increasing hydroxide bond strengths with increasing cluster size, and that anisotropic cluster geometries result in substantial thermodynamic differentiation of H-binding sites. We demonstrate an excellent agreement between theory and experiments on the reported BDFE(O−H) values and, importantly, we elucidate how cluster size and shape affect electronic properties (local charges and frontier molecular orbitals), giving rise to sites with increased preference for hydrogen binding, demonstrated in higher BDFE(O−H). Overall, this work aids the understanding and design of polyoxometalates exhibiting surface sites with tailored interaction strengths.

anions↗

DOE OSTI2026

Polyoxometalate-Assisted Crystallization: A General Strategy Enabling Structural Characterization of Molecular Radium Complexes

A fundamental understanding of radium (Ra) coordination chemistry has been hindered by the scarcity and radiological hazards of 226 Ra, leaving the structural characterization of molecular Ra complexes almost entirely unexplored. Here, we introduce a polyoxometalate (POM)-assisted crystallization strategy that enables single-crystal X-ray diffraction analysis of Ra–chelator complexes from microgram-scale samples. Employing the plenary Keggin anion [SiW 12 O 40 ] 4– , we isolated and structurally characterized Ra 2+ complexes of 18-crown-6 and the bis-picolinate macrocycle macropa, together with their Sr 2+ and Ba 2+ analogues. The resulting structures reveal systematic, size-dependent trends in coordination number and metal-donor distances across the alkaline earth series and provide the first direct experimental measurements of Ra–N and Ra–OCOO bond distances. Together, these results establish POM-assisted crystallization as a robust approach for obtaining solid-state structural information on Ra 2+ complexes of organic chelators, opening new opportunities to advance the coordination chemistry needed to fully realize radium’s potential in isotope production and targeted radiotherapy. More broadly, this approach expands the experimental toolkit available for studying scarce, highly radioactive elements accessible only in microgram quantities.

Anions↗

DOE OSTI2025

Two-Dimensional Silk Crystal Films as Matrix Layer for High-Performance Microelectronics

This study explores a bio-inspired approach for memristive devices by combining Keggin-type polyoxometalates (POMs)-[SiW 12 O 40 ] 4 (POM-T) and [PW 12 O 40 ] 3 (POM-P), with silk fibroin (SF) to create 2D SF–POM layers on highly ordered pyrolytic graphite (HOPG) as resistive switching layers for memristors. We propose that the ordered SF layer template 0D POMs facilitate the formation of conductive filaments, thereby enhancing the variability of the manufactured memristors. AFM analysis revealed that both SF and SF–POM layers shared similar morphologies, while SF–POM–T formed larger aggregates, likely due to the stronger acidity of POM-T, which probably caused SF to aggregate and alter its secondary structure. Scanning Kelvin probe microscopy (SKPM) revealed that POMs reduced the contact potential difference of HOPG, resulting in lower work functions. Compared to an SF device, the SF–POM–P device showed improved memristive behavior, with a larger current gap and good repeatability over multiple sweeps; whereas the SF–POM–T device did not exhibit memristor activity, likely due to acidity-induced disruption of the SF template’s order and CF formation. More importantly, SF–POM–P devices also demonstrated programmable memristive states. Finally, combining simulation-driven memristor modeling, we showcase a co-design workflow for advancing bioinspired memristors through new materials design, synthesis, and device modeling and development.

36 MATERIALS SCIENCE↗