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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.

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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Direct Evidence of Graphene-Induced Molecular Reorientation in Polymer Films

For the first time, direct evidence of graphene-induced molecular reorientation in polymer films using polarization modulated infrared reflection absorption spectroscopy (PMIRRAS) are presented. By creating favorable electrostatic interactions, graphene-polymer interfaces can be controlled by varying polymer and solvent composition. After trans of unmodified graphene from copper onto a polymer substrate, polymer chain rearrangement relative to the orientation at the polymer-copper interface is observed using PM-IRRAS. Transfer success is characterized using both optical transmission measurements and Raman spectroscopy to quantify the transfer fidelity, i.e. graphene coverage fraction. Taken together, oxygencontaining poly(ethylene-co-vinyl acetate) shows more polymer chain rearrangement and better graphene coverage compared to oxygen-free polyethylene. Polymer composition seems to dominate graphene-polymer interactions while solvent choice has a smaller effect on transfer quality. These results are the first direct measurement of this effect and point towards the possibility of engineering graphene-polymer interactions for specific applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2017

Field Exploration and Life Detection Sampling Through Planetary Analogue Sampling (FELDSPAR).

Exploration missions to Mars rely on rovers to perform analyses over small sampling areas; however, landing sites for these missions are selected based on large-scale, low-resolution remote data. The use of Earth analogue environments to estimate the multi-scale spatial distributions of key signatures of habitability can help ensure mission science goals are met. A main goal of FELDSPAR is to conduct field operations analogous to Mars sample return in its science, operations, and technology from landing site selection, to in-field sampling location selection, remote or stand-off analysis, in situ analysis, and home laboratory analysis. Lava fields and volcanic regions are relevant analogues to Martian landscapes due to desiccation, low nutrient availability, and temperature extremes. Operationally, many Icelandic lava fields are remote enough to require that field expeditions address several sampling constraints that are experienced in robotic exploration, including in situ and sample return missions. The Fimmvruhls lava field was formed by a basaltic effusive eruption associated with the 2010 Eyjafjallajkull eruption. Mlifellssandur is a recently deglaciated plain to the north of the Myrdalsjkull glacier. Holuhraun was formed by a 2014 fissure eruptions just north of the large Vatnajkull glacier. Dyngjusandur is an alluvial plain apparently kept barren by repeated mechanical weathering. Informed by our 2013 expedition, we collected samples in nested triangular grids every decade from the 10 cm scale to the 1 km scale (as permitted by the size of the site). Satellite imagery is available for older sites, and for Mlifellssandur, Holuhraun, and Dyngjusandur we obtained overhead imagery at 1 m to 200 m elevation. PanCam-style photographs were taken in the field by sampling personnel. In-field reflectance spectroscopy was also obtained with an ASD spectrometer in Dyngjusandur. All sites chosen were 'homogeneous' in apparent color, morphology, moisture, grain size, and reflectance spectra at all scales greater than 10 cm. Field lab assays were conducted to monitor microbial habitation, including ATP quantification, qPCR for fungal, bacterial, and archaeal DNA, and direct cell imaging using fluorescence microscopy. Home laboratory analyses include Raman spectroscopy and community sequencing. ATP appeared to be significantly more sensitive to small changes in sampling location than qPCR or fluorescence microscopy. Bacterial and archaeal DNA content were more consistent at the smaller scales, but similarly variable across more distant sites. Conversely, cell counts and fungal DNA content have significant local variation but appear relatively homogeneous over scales of 1 km. ATP, bacterial DNA, and archaeal DNA content were relatively well correlated at many spatial scales. While we have observed spatial variation at various scales and are beginning to observe how that variation fluctuates over time as biodiversity recovers after an eruption, we do not yet fully understand what parameters lead to the observed spatial variation. Home laboratory analyses will help us further understand the elemental and structural composition of the basaltic matrices, but further field analyses are vital for the understanding how temperature, moisture, incident radiation, and so forth influence the habitability of a microclimate.

Field↗
DOE OSTI2025

Thermal decomposition of neptunyl ammonium nitrate: mechanistic insights and structural characterization of the Np 2 O 5 intermediate phase

Neptunium (Np) possesses a rich and unique chemistry that often diverges from other actinide elements yet remains relatively underexplored compared with the other light actinides. A resurgence of interest in Np has been spurred by the application of 237 Np for plutonium-238 ( 238 Pu) production for use in radioisotope thermoelectric generators (RTGs), necessitating evaluation of Np chemical reactions and materials. The work presented here studied the thermal decomposition of neptunyl ammonium nitrate (NH 4 Np VI O 2 (NO 3 ) 3 ) for synthesis of neptunium dioxide (NpO 2 ), which is the target material used for production of 238 Pu. Additionally, structural characterization of the intermediate solid Np pentoxide (Np 2 O 5 ) was performed. Advanced solid-state characterization techniques, including simultaneous thermal analysis (STA), powder X-ray diffraction (pXRD), Raman spectroscopy, and density functional theory (DFT) modeling have been combined to study the reaction pathways. Analysis revealed that NH 4 Np VI O 2 (NO 3 ) 3 thermally decomposes to a proposed neptunyl nitrate intermediate, followed by Np 2 O 5 and finally NpO 2 , all within the temperature range of 150 °C–600 °C. Further characterization of the pentoxide intermediate provided the first Raman spectra of pure-phase Np 2 O 5 and associated DFT modeling confirmed Raman peak assignments for this phase. These findings provide mechanistic information to advance production of the critical radioisotope 238 Pu and advance the state of knowledge on Np materials chemistry using modern characterization techniques.

Lawson, Kathryn M. [Oak Ridge National Laboratory ↗
NASA NTRS2015

Survival of Organic Materials in Ancient Cryovolcanically-Produced Halite Crystals

Spectroscopic evidence supports the presence of Mg-Na-K salts derived from cryovolcanism on the surface of Europa. Halite (NaCl) is effective at very long-term preservation of organic phases and structures. Collection of salt crystals from Europan plumes would provide solid inclusions of organics, potentially also biomaterials, all suitable for analysis. Two thermally-metamorphosed ordinary chondrite regolith breccias (Monahans 1998 (H5) and Zag (H3-6)) contain fluid and solid inclusion-bearing halite crystals, dated to approximately 4.5 billion years, and thus the trapped aqueous fluids and solids are at least as old. Heating/freezing studies of the aqueous fluid inclusions in these halites demonstrated that they were trapped near 25 degrees Centigrade, and their continued presence in the halite grains requires that their incorporation into the H chondrite asteroid occurred after that body's metamorphism ended, since heating would have dessicated the halite. O and H isotopes of the trapped fluids are consistent with mixing of asteroidal and cometary water. Cryovolcanic Origin of the Halite: We hypothesize that these meteoritic halites derive from ancient cryovolcanism based on the following points. (1) Salts crystals are observed as products of current cryovolcanism on Enceladus. (2) In-situ spacecraft analysis of some of the icy grains associated with the Enceladus salt found minor organic or siliceous components, including methane, also found in the Monahans halite. (3) Cryovolcanic fluids are observed to be in chemical disequilibrium, reflecting incomplete reactions between interior volatiles and rocky materials. The coexistence of N2 and HCN in Enceladus' cryovolcanic fluids requires that the plume consists of a mixture of materials whose sources experienced different degrees of aqueous processing, including primordial material trapped in ice that has not been in contact with liquid water. The observed mineral assemblage within the Monahans and Zag halites is also far from equilibrium. Cryovolcanoes on Ceres are a potential source of our halite, however the processes that form halite should also be operating within Europa. Dissolution of Monahans halite grains has revealed a remarkable variety of organics, which dominate the population of solid inclusions. Thermal alteration of this macromolecular carbon (measured by Raman spectroscopy) shows remarkable diversity. We have identified highly-condensed aromatics, diamond, carbonates and chloromethane. Light organic compounds like methane tend to be water soluble and require cold formation temperatures at high hydrogen fugacity - i.e. require water ice. Another indication that these halites have not been heated is that light organics readily volatilize or aromaticize into PAHs. We are currently analyzing the organics by Raman and C-XANES, and measuring the content and exploring the potential chirality of amino acids in the halite. Implications for Europa Plumes: Organic materials and structures erupted by a Europa cryovolcano should be similarly preserved within halite, and other salts, which will be a convenient form for capture and analysis, since halite will serve to encapsulate and protect the organics from spacecraft contamination. Also, being transparent at many wavelengths halite will permit analysis by spacecraft-mounted spectroscopic techniques. In addition, halite is readily dissolved, permitting further analysis of entrained organics.

Zolensky, M.↗
DOE OSTI2025

Characterizing sample degradation from synchrotron based X-ray measurements of ultra-thin exfoliated flakes

It is undeniable that novel 2D devices and heterostructures will have a lasting impact on the advancement of future technologies. However, the inherent instability of many exfoliated van der Waals (vdW) materials is a well-known hurdle yet to be overcome. Thus, the sustained interest in exfoliated vdW materials underscores the importance of understanding the mechanisms of sample degradation to establish proactive protective measures. Here, the impact of prolonged synchrotron-based X-ray beam exposure on exfoliated flakes of two contemporary vdW materials, NiPS 3 and α-RuCl 3 , is explored using resonant inelastic X-ray scattering (RIXS) and total fluorescence yield X-ray absorption spectroscopy (XAS). In NiPS 3 , the resulting RIXS and XAS spectra show a suppression, then vanishing, of NiS 6 multiplet excitations coupled with an upward shift of the peak energy of the XAS as a function of X-ray dose. In α-RuCl 3 , the signs of beam damage from the RIXS spectra are less evident. However, the post-experiment characterization of both materials using Raman spectroscopy exhibits signals of an amorphous and disordered system compared to pristine flakes; in addition, energy-dispersive X-ray spectroscopy of NiPS 3 shows evidence of ligand vacancies. As synchrotron radiation is fast becoming a required probe to study 2D vdW materials, these findings lay the groundwork for the development of future protective measures for synchrotron-based prolonged X-ray beam exposure, as well as for X-ray free electron laser.

2D magnetism↗
DOE OSTI2021

Importance of multimodal characterization and influence of residual Li 2 S impurity in amorphous Li 3 PS 4 inorganic electrolytes

Amorphous Li 3 PS 4 (LPS) solid-state electrolytes are promising for energy-dense lithium metal batteries. LPS glass, synthesized from a 3:1 mol ratio of Li 2 S and P 2 S 5 , has high ionic conductivity and can be synthesized by ball milling or solution processing. Ball milling has been attractive because it provides the easiest route to access amorphous LPS with a conductivity of 3.5 × 10 –4 S cm –1 (20 °C). However, achieving the complete reaction of precursors via ball milling can be difficult, and most literature reports use X-ray diffraction (XRD) or Raman spectroscopy to confirm sample purity, both of which have limitations. Furthermore, the effect of residual precursors on ionic conductivity and lithium metal cycling is unknown. In this work, we illustrate the importance of multimodal characterization to determine LPS phase and chemical purity. To determine the residual Li 2 S content in LPS, we show that (1) XRD and 31 P solid state nuclear magnetic resonance (ssNMR) are insufficient and (2) Raman loses sensitivity at concentrations below 12 mol% Li 2 S. Most importantly, we show that 7 Li ssNMR is highly sensitive. Using 7 Li ssNMR, we investigate the effect of ball milling parameters and develop a robust and highly reproducible procedure for pure LPS synthesis. We find that as the residual Li 2 S precursor content increases, LPS conductivity decreases and lithium metal batteries exhibit higher overpotentials and poor cycle life. Furthermore, our work reveals the importance of multimodal characterization techniques for amorphous solid-state electrolyte characterization and will enable better synthetic strategies for highly conductive electrolytes for efficient energy-dense solid-state lithium metal batteries.

25 ENERGY STORAGE↗
DOE OSTI2020

Calibration of Raman Spectrometer for Gas Phase Measurements

Raman spectroscopy is currently used at SRNL to measure the concentration of gaseous species evolved from chemical reactions. This report documents the methods used to calibrate the spectroscopic systems in both frequency and intensity spaces.

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

Pressure-driven symmetry transitions in dense H 2 O ice

X-ray diffraction and Raman spectroscopy of H 2 O (ice) structures are measured under static compression in combination with grain normalizing heat treatment via direct laser heating. Here, we report the transition from cubic ice-VII to a structure of tetragonal symmetry, ice-VII t at 5.1 ± 0.5 GPa. This is succeeded by the H-bond symmetrization transition occurring at a pressure of 30.9 ± 3 GPa. Both experimental observations are supported by simulated Raman spectra from density-functional theory quantum calculations. The transition to H-bond symmetrization is evidenced by the reversible emergence of its characteristic Raman mode and a 2.5-fold increase in bulk modulus, implying a significant increase in bonding strength.

58 GEOSCIENCES↗
NASA NTRS1979

Investigation of the feasibility of CARS measurements in scramjet combustion

Results are presented of analytical and experimental investigations to determine the feasibility of using coherent anti-Stokes Raman spectroscopy (CARS) to measure temperature and species concentrations in supersonic combustion experiments. Experimental CARS spectra of O2 and H2 in a laboratory flat H2 - air diffusion flame are presented. Temperatures deduced from both species generally agree with radiation corrected thermocouple measurements. Oxygen concentrations were determined from the shape of the experimental spectra. Measured H2 CARS signal magnitudes are in good agreement with analytical predictions based on H2 concentration profiles determined from spontaneous Raman scattering. Based on these results, CARS diagnostics appear to be feasible if problems associated with the supersonic flow are not too severe.

Shirley, J. A.↗
DOE OSTI2025

Cavity-Assisted Coherent Phonon Generation and Control in a WSe 2 /Au Structure

Coherent phonons in the Terahertz (THz) regime have gained attention as potential candidates for next-generation high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. Nevertheless, achieving efficient control over THz coherent phonons continues to pose a considerable challenge. In this work, we explore THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of WSe 2 on Au (WSe 2 /Au) and Si (WSe 2 /Si) using time-resolved pump-probe spectroscopy. The generation of THz coherent phonons was studied as a function of WSe 2 layer thickness and laser wavelength. Notably, a significant enhancement in THz coherent phonon generation was observed in the WSe 2 /Au structure, but only within specific ranges of WSe 2 thickness and laser wavelength. Further, the results from numerical simulations, which consider a self-hybridized optical cavity depending on WSe 2 thickness, along with optical reflectance and Raman spectroscopy measurements, aligned well with the time-domain observations of THz coherent phonon generation. We propose that the observed enhancement in THz coherent phonon generation is strongly influenced by light-matter interaction in the WSe 2 cavity, a mechanism that may be applicable to a broader range of vdW materials. These findings offer promising insights for the development of THz phononic or phonon-integrated devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Characterization of thermally heat-treated polyacrylonitrile carbon fibers

This study investigates the graphitization process of polyacrylonitrile (PAN) carbon fibers by subjecting commercial fibers to thermal heat treatment at temperatures ranging from 1400 to 2100 °C in 100 °C increments, using either argon or nitrogen gas atmospheres. Changes in crystallinity, surface morphology, and lattice parameters were analyzed for two commercial carbon fibers using X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Results indicated minimal changes in surface morphology with increasing heat-treatment temperature; however, crystallinity significantly increased. Crystallinity changes were more strongly dependent on temperature rather than gas atmosphere or fiber type. At intermediate heat-treatment temperatures (1600–1800 °C), fibers treated in argon showed a slight preference for graphitization. The highest level of graphitization was measured at 2100 °C. Crystallite size increased as the intensity ratio of the D1 to G Raman peaks increased, reaching a peak around ~1800 °C, after which the ratio started to decrease. This behavior aligns with Ferrari's three-stage model of carbon crystallization and is consistent with both the Marie-Mering degree of graphitization and Brubaker's Integrated Absolute Differential models, all of which describe the transformation from an amorphous to a more graphitic structure. At the higher heat-treatment temperatures, the changes between atmospheres and fiber types were measured to converge to similar levels of graphitization. In conclusion, this study evaluates the progressive change in commercial grade carbon fibers when heat-treated.

Characterization↗
NASA NTRS1996

Evaluation of Boundary-Enhancement Additives for Perfluoropolyethers

Six additives were synthesized and evaluated as boundary lubrication enhancers for perfluoropolyethers. These additives included a phosphonate, a thiophosphonate, a beta-diketone, a benzothiazole, an amide and a sulfite. These additives were evaluated in a vacuum four-ball apparatus, at a one weight percent concentration in a perfluoropolyether based on hexafluoropropene oxide. Tests were performed in vacuum (less than 5.0 x 10(exp 6) Torr), at room temperature (approx. 23 C), at an initial Hertzian stress of 3.5 GPa (200N load), and a sliding velocity of 28.8 mm/sec (100 rpm). Infrared (IR) and Raman spectroscopies were used to analyze the 440 C specimens after testing. Wear rates for each formulation were determined from the slope of wear volume as a function of sliding distance. All additives yielded reductions in mean wear rates of at least 55 percent, with the exception of the benzothiazole which had no effect. Two of the additives, an amide and a sulfite, reduced the mean wear rate by at least 80 percent. IR and Raman analysis indicated the severity of wear can be correlated to the amount of surface fluorinated polymeric acid species (R(sub f)COOH) and amorphous carbon, in and around the wear scar.

Shogrin, Bradley A.↗
DOE OSTI2021

A comparison of strategies for state-selective coherent Raman excitation

Here we report on the theoretical and experimental evaluation of state-selective bond activation for coherent Raman spectroscopy and control of chemical reactivity. We compare six different strategies for state-selective activation using shaped broadband femtosecond laser pulses. Results for a pair of long pulses with different wavelengths are also considered. For each of these approaches, we report on their theoretical and experimental ability to excite a desired vibrational mode while minimizing excitation of other modes including harmonics. We find that the double parabola spectral phase function gives the most intense Raman excitation with 10 cm -1 selectivity with the lowest background. The sinusoidal phase function produces intense Raman excitation with high selectivity but gives a series of harmonic excitations which could hinder selectivity. Implications of this study on several fields ranging from sensing, to controlling chemical reactions, and selective excited state excitation by X-ray pulses are discussed.

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

The S solubility of Cr and Al containing simulated low-activity waste glass

Here, the presence of sulfur-containing molten salt phases during vitrification is a particular problem which can be detrimental to the melter. Borosilicate compositions were designed to explore both the effects of a fixed amount of Cr, as Cr 2 O 3 or Na 2 CrO 4 , and the impact of Al 2 O 3 , on sulfur solubility as determined by progressively adding elemental S. The NABS series (with Al 2 O 3 ) has a higher SO 3 solubility of ~3.5 mol% while the NBS series (without Al 2 O 3 ) has a SO 3 solubility of ~2 mol%. Crystalline Cr 2 O 3 formed in the NABS series upon the addition of S but is not present in the NBS series. At the highest S addition, a separate salt phase always forms, and was identified as Na 2 (S,Cr)O 4 via electron dispersive spectroscopy, ultraviolet-visible spectroscopy, x-ray diffraction, and thermal analysis. Magic angle spinning nuclear magnetic resonance identified no change in the B coordination in either series as S is added, suggesting that the Na is scavenged from other locations, such as the Si sites, to charge compensate sulfate, whether in the glass or in the salt. The Cr precursor identity did not affect the observed overall behavior regarding sulfur, but did impact Cr redox as identified with Raman spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Regioisomer-Specific Crystallization of Diglycidyl Ether of Bisphenol F Epoxy Resin

Diglycidyl ether of bisphenol F (DGEBF) is a common industrial epoxy resin, desired for its possible replacement of the bisphenol A (BPA) analogue DGEBA due to potential health hazards of BPA. During curing, crystallization of the DGEBF resin can cause inaccurate resin-to-hardener ratios, resulting in incomplete crosslinking; therefore, understanding the crystallization of DGEBF is important for proper industrial control of the material. Here, in this work, we utilized a combination of single-crystal x-ray diffraction, Raman spectroscopy, and 1 H-NMR to investigate the crystallization of DGEBF. We observed crystallization of two DGEBF regioisomers: ortho-ortho’-DGEBF and ortho-para’-DGEBF, and report their crystal structures. Notably, para-para’-DGEBF crystallization is not observed, indicating crystallization of this regioisomer does not occur under ambient conditions. Density functional theory calculations were performed to describe the phonon modes of the ortho-ortho’- and ortho-para’-DGEBF structures, corroborating Raman signatures for the respective isomers. Using a phonon mode analysis, we identify the presence of three specific optical vibrational modes at 631.3, 344.1, and 345.6 cm -1 that are unique to the para monomer. This work offers insights into the crystallization behavior of DGEBF resins under ambient conditions and may inform potential industrial application of this material.

Neu, Jennifer [Oak Ridge National Laboratory (ORNL↗
DOE OSTI2023

Observation of flat and weakly dispersing bands in the van der Waals semiconductor Nb 3 Br 8 with breathing kagome lattice

Niobium halides, Nb 3 ⁢X 8 (X=Cl,Br,I), which are predicted two-dimensional magnets, have recently gotten attention due to their breathing kagome geometry. Here we have studied the electronic structure of Nb 3⁢ Br 8 by using angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. ARPES results depict the presence of multiple flat and weakly dispersing bands. These bands are well explained by the theoretical calculations, which show they have Nbd character indicating their origination from the Nb atoms forming the breathing kagome plane. This van der Waals material can be easily thinned down via mechanical exfoliation to the ultrathin limit and such ultrathin samples are stable as depicted from the time-dependent Raman spectroscopy measurements at room temperature. Finally, these results demonstrate that Nb 3 ⁢Br 8 is an excellent material not only for studying breathing kagome induced flat band physics and its connection with magnetism but also for heterostructure fabrication and for practical application.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Magnetic-Core/Gold-Shell Nanoparticles for the Detection of Hydrophobic Chemical Contaminants

Magnetic-core/gold-shell nanoparticles (MAuNPs) are of interest for enabling rapid and portable detection of trace adulterants in complex media. Gold coating provides biocompatibility and facile functionalization, and a magnetic core affords analyte concentration and controlled deposition onto substrates for surface-enhanced Raman spectroscopy. Iron oxide cores were synthesized and coated with gold by reduction of HAuCl4 by NH2OH. MAuNPs were grafted with polyethylene glycol (PEG) and/or functionalized with 4-mercaptobenzoic acid (4-MBA) and examined using a variety of microscopic, spectroscopic, magnetometric, and scattering techniques. For MAuNPs grafted with both PEG and 4-MBA, the order in which they were grafted impacted not only the graft density of the individual ligands, but also the overall graft density. Significant Raman signal enhancement of the model analyte, 4-MBA, was observed. This enhancement demonstrates the functionality of MAuNPs in direct detection of trace contaminants. The magnetic deposition rate of MAuNPs in chloroform and water was explored. The presence of 4-MBA slowed the mass deposition rate, and it was postulated that the rate disparity originated from differing NP-substrate surface interactions. These findings emphasize the importance of ligand choice in reference to the medium, target analyte, and substrate material, as well as functionalization procedure in the design of similar sensing platforms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Direct observation of two-dimensional magnons in atomically thin CrI 3

Magnons are collective spin excitations in crystals with long-range magnetic order. The emergent van der Waals magnets provide a highly tunable platform to explore magnetic excitations in the two-dimensional limit with intriguing properties, manifesting from their honeycomb lattice structure and switchable magnetic configurations. Here, we report the direct observation of two-dimensional magnons through magneto-Raman spectroscopy with optical selection rules determined by the interplay between crystal symmetry, layer number and magnetic states in atomically thin CrI 3 . In monolayers, we observe an acoustic magnon mode at ~0.3 meV. It has strict cross-circularly polarized selection rules locked to the magnetization direction that originates from the conservation of angular momentum of photons and magnons dictated by three-fold rotational symmetry. Additionally, we reveal optical magnon modes at ~17 meV. This mode is Raman silent in monolayers, but optically active in bilayers and bulk due to a relaxation of the parity criterion resulting from the layer index. In the layered antiferromagnetic states, we directly resolve two degenerate optical magnon modes with opposite angular momentum and conjugate optical selection rules. From these measurements, we quantitatively extract the spin-wave gap, magnetic anisotropy and intralayer and interlayer exchange constants, and establish two-dimensional magnets as a new platform for exploring magnon physics.

36 MATERIALS SCIENCE↗