AVS 72 Session AS1+CA+EL-ThM: Emerging Methods for Materials Characterization and Functional Properties

Single crystal Mg₂SiO₄, also called forsterite or peridot in its gem form, has been measured via spectroscopic ellipsometry over a spectral range of 0.78—8.5 eV to determine its anisotropic dielectric response over its a- and c-crystallographic axes. These axes are investigated because the available (010) crystal cut does not allow optical sensitivity to the b-axis. Forsterite has potential to serve as a substrate for epitaxial superconducting RuO₂ film growth and as a bioceramic in bone and dental applications. Single crystal forsterite has an orthorhombic crystal structure belonging to space group Pbnm.Collecting multiple measurements and rotating the sample between each measurement varies the Euler angles and increases sensitivity to anisotropy. Within the measured spectral range, oscillators describing optical absorption are used to model the imaginary part of the dielectric spectra, and Kramers-Kronig integration of these oscillators plus Sellmeier functions model the real part of the dielectric spectra. Absorption onset occurs at 7.5 eV along the a-axis and 6.6 eV along the c-axis.

Magnetic-dipole transitions of paramagnetic spins provide a largely unexplored route for controlling thermal radiation at cryogenic temperatures. We demonstrate this concept using Cr³⁺ centers in β-Ga₂O₃ by combining polarization-resolved millimeter-wave spectroscopy with a spin-Hamiltonian/Bloch electrodynamic model [1–3]. Measurements at T = 15 K, magnetic fields of approximately 6.5–8 T, and 190–230 GHz validate the three field-dependent Cr³⁺ spin transitions through direct comparison of experiment and calculation.

The measured complex permeability is incorporated into a full-wave 4×4 Berreman propagation treatment [4,5] to obtain reflected and transmitted powers and hence spin-resolved absorptivity. Because magnetic bias makes the response nonreciprocal, emissivity is evaluated using the field-reversed adjoint relation rather than the conventional same-state Kirchhoff law [6,7]. With μ(−B)=μᵀ(B), the circular-polarization channels satisfy εR(+B)=AL(−B) and εL(+B)=AR(−B), while for unpolarized radiation εunp(+B)=Aunp(−B).

Within this framework, magnetic field tunes the spin-resonance frequencies through Zeeman coupling, while temperature redistributes the spin populations and modifies their radiative strength. Planck-weighted calculations show narrow, magnetically tunable thermal-emission channels whose spectral positions are set by the field-dependent Cr³⁺ transitions and whose strengths depend on population differences and overlap with the blackbody spectrum. Maps versus magnetic field and temperature identify operating conditions where one or more resonances contribute most strongly to the emitted thermal power.

The calculations further show that same-state absorption and emission are generally unequal, while the field-reversed adjoint relations are recovered. These results establish paramagnetic spin resonance as an intrinsic, contactless mechanism for spectrally selective thermal-emission control without patterned resonators or cavities, linking experimentally validated magnetic-resonance spectroscopy with nonreciprocal thermal photonics.

References
[1] V. Rindert et al., Phys. Rev. B 110, 054413 (2024).
[2] V. Rindert et al., Phys. Rev. Lett. 134, 086703 (2025).
[3] V. Rindert et al., Appl. Phys. Lett. 126, 082105 (2025).
[4] D. W. Berreman, Opt. Soc. Am. 62, 502 (1972).
[5] M. Schubert, Phys. Rev. B 53, 4265 (1996).
[6] C. Guo et al., Phys. Rev. X 12, 021023 (2022).
[7] C. Khandekar et al., New J. Phys. 22, 123005 (2020).

Demand in high-power switching electronics has driven the need for wider and wider bandgap semiconductors. Recent developments have moved from Gallium Oxide towards solid solutions of Aluminum-Gallium Oxide. Alpha-phase Aluminum-Gallium Oxide is a colossal bandgap semiconductor with a rhombohedral crystal structure. Multilayer heterostructures and a MESFET with layers of varying Aluminum concentration were grown by Suboxide Molecular-Beam Epitaxy on commercial sapphire substrates. General Spectroscopic Ellipsometry was performed on the samples and the template in the spectral range of 0.7 to 9.2 eV. The anisotropic dielectric functions were modeled using the direct band-to-band transition critical-point model developed by Hilfiker et al.. The channel layer of the MESFET was found to have an optical bandgap of 6.71 eV. Thick, relaxed epitaxial layers were found to have bandgaps up to 7 eV.

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a workhorse of local chemical characterization across a broad range of materials and systems, with more than 5,000 publications reporting its use since 2016. Yet the technique requires constant supervision by an expert to watch data acquisition, adjust sputter and analysis parameters, and decide when something has gone wrong. Here, we present PACE-SIMS (Pause–Assess–Correct–Execute for SIMS), an autonomous workflow on an unmodified commercial TOF.SIMS 5 instrument in which those judgments are delegated to an artificial-intelligence (AI) agent under human-approved limits. The researcher specifies the study in plain language; the agent builds the measurement plan and, after a go/no-go review, executes it, pausing at checkpoints after every acquisition to assess data quality the way an operator would (layer thickness, signal levels, crater depth consistency, detector saturation) and to accept the measurement, correct parameters and remeasure, retry, or escalate to the human. We validated the approach in a randomized, blind, two-polarity study of ¹⁸O-enriched WOₓ films with a buried tracer layer: 35 measurements in 8.1 hours from about 40 minutes of total human attention. The agent recalibrated per-sample sputter settings that differed by 44% across the films, held isotope ratios steady through a 32% swing in absolute yields, and correctly attributed a sustained 30% intensity drop to a primary-beam emission fault rather than the sample, escalating once for source realignment before resuming without loss of the sample budget. Beyond demonstrating autonomous operation, the experiment also provided insight into the dynamic properties of the WOₓ films, whose oxygen stoichiometry controls the resistive-switching and electrochromic behavior that motivates these materials. The proposed architecture is instrument-portable and can be applied to any analytical technique, especially destructive ones.

Water-lean solvents are a promising technology for capturing acid gases like carbon dioxide (CO₂).In situ liquid time-of-flight secondary ionization mass spectroscopy (ToF-SIMS) is used to study a representative solvent N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2-EEMPA) with different CO₂ loadings to reveal the complex solvent structure upon CO₂ capture.Characteristic peaks of 2-EEMPA, such as m/z^- 215 C₁₁H₂₃N₂O₂^- (deprotonated 2-EEMPA) and m/z⁺ 217 C₁₁H₂₅N₂O₂⁺ (protonated 2-EEMPA), are detected due to acid gas uptake.Also, solvent molecules and carboxylate ion pairs, such as m/z^- 259 C₁₂H₂₃N₂O₄^- [(deprotonated 2-EEMPA∙∙∙CO₂)] and m/z⁺ 261 C₁₂H₂₅N₂O₄⁺ (protonated 2-EEMPA∙∙∙CO₂), are observed.Interestingly, more than one CO₂ molecule can be captured per each solvent molecule as evidenced in SIMS mass spectra, for example, m/z^- 321 C₁₃H₂₅N₂O₇ [(deprotonated 2-EEMPA)∙∙∙2CO₂∙∙∙H₂O], m/z⁺ 305 C₁₃H₂₅N₂O₄⁺ [(protonated 2-EEMPA)∙∙∙2CO₂], m/z^- 389 C₁₇H₂₉N₂O₈ [(deprotonated 2-EEMPA)∙∙∙3CO₂∙∙∙3CH₂], and m/z⁺ 373 C₁₆H₂₅N₂O₈⁺ [(protonated 2-EEMPA)∙∙∙3CO₂∙∙∙2C].However, the monomer of 2-EEMPA and CO₂ seems to be most prevalent.Furthermore, solvent clusters are detected in loaded solvents, for instance m/z⁺ 433 C₂₂H₄₉N₄O₄⁺ [(2-EEMPA)₂∙∙∙H] and m/z⁺ 646 [(2-EEMPA)₃-2H], while capturing CO₂ at different amounts.Relative abundance of cluster ions provides a semi-qualitative venue to assess the free energies gas capture energetics, indicating the relative stability trend within the same solvent system, previously impossible.These observed ion clusters are verified with molecular modeling, where dimer, trimer, and cluster ions are validated for their presence either due to weak molecular interactions or hydrogen bonds.In situ molecular imaging of ionic liquids and molecular modeling reveals that the acid gas capture mechanism by ionic liquids includes both physical adsorption and chemical bonding with multiple reaction pathways, engaging cluster formation and alteration of solvent structures.

The adsorption geometry of carboxylic acids on metal oxides controls key steps in ketonization. While molecular and deprotonated binding modes of acetic (and formic) acid on anatase TiO₂(101) are known, how co-adsorbed water influences adsorption, proton transfer, and surface mobility on this facet remains unclear. Here, we combine angle- and energy-resolved molecular beam scattering with scanning tunneling microscopy to probe acetic-acid binding motifs in the presence of water on anatase TiO₂(101). To first build a molecularly resolved understanding of the structure of water on a-TiO₂(101), precise surface coverages of water are prepared revealing complex coverage dependent adsorption motifs. Water molecules in the second layer are coordinated to the surface through hydrogen bonding and are shown through timelapse STM to participate in coordinated surface diffusion. The site-specificity and stability of acetic acid co-adsorbed with water is then visualized through STM. Overall, this work provides a molecularly resolved description of the dynamics and structure of water, acetic acid, and acetate intermediates on a-TiO₂(101) with results relevant for ketonization on metal oxides.

The ceria-supported ruthenium materials are promising catalysts for a variety of chemical reactions. Here, we report results from spin-polarized density functional theory (DFT) calculations on geometry, electronic structures, and reactivity of CeO₂(110)-supported 3-atoms Ru cluster (Ru₃/CeO₂) and single Ru atom (Ru₁/CeO₂). On the reduced CeO₂ support, a single Ru atom forms a squared planar geometry and 3-atoms Ru cluster forms a non-planar geometry with under-coordinated Ru atoms. The 3-atoms cluster does not induce the metallicity but introduces multiple Ru electronic states within the energy gap of reduced CeO₂. Oxidation state of a Ru atom of Ru₃/CeO₂ is lower than that of Ru₁/CeO₂, and subsequently influence the adsorption and activation of CO₂ and H₂ at active Ru sites. The CO₂-Ru/CeO₂ interaction is dominated by C-Ru 2p-4d hybridization over O-Ce 2p-4f hybridization and thus facilitated by stronger bonding of Ru with C than of Ce with CO₂ oxygen atoms. In addition, we find that more occupied spin-up and spin-down states are available closer to the Fermi level of Ru₃/CeO₂ than those of Ru₁/CeO₂ in the presence of adsorbed CO₂ and thus influencing their reactivity towards H₂ adsorption and dissociation. The higher activity of Ru₃/CeO₂ as compared to Ru₁/CeO₂ towards H₂ dissociation can be attributed to the lower oxidation states of cluster Ru atoms and multiple Ru 4d states near the Fermi level. In addition, based on DFT-calculated energetics of reaction pathways for reverse water gas shift reaction, we find that the hydrogen-assisted C-O cleavage pathway is more thermodynamically favorable than the redox pathway on both model catalyst systems, in consistent with experimental observation of formate species shown by spectroscopic measurements. These findings provide a basic understanding of structure-property relationships and thereby can help optimize ceria-based Ru catalysts toward the rational design of active sites for reactions involving carbon dioxide and hydrogen.

This work is supported by NSF grant CHE-2400068.