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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Combinatorial investigation of structural and optical properties of cation-disordered ZnGeN 2

Cation-disordered ZnGeN 2 shows promise for application as a blue-green emitter in light-emitting devices, but more foundational work is necessary to understand structure–property relationships. In this work, we present a combinatorial exploration of the experimental phase space of wurtzite (cation-disordered) ZnGeN 2 using high-throughput co-sputtering. Structure, morphology and optical properties are explored as a function of cation composition and synthesis temperature. ZnGeN 2 is found to crystallize in the wurtzite structure ranging from Zn-rich to Ge-rich compositions. X-ray diffraction refinements reveal a continuous shift in cell volume with off-stoichiometry, indicating alloy-like structural behavior. Furthermore, the optical absorption of all films examined is lower in energy than the value predicted for cation-ordered ZnGeN 2 , suggesting that cation disorder is decreasing the bandgap. Additionally, the absorption threshold shifts continuously to higher energy for Ge-rich samples, consistent with bandgap shifts due to alloy-like structural behavior. Defect formation energy diagrams are calculated to help guide understanding of off-stoichiometry from a defect complex perspective. This work paves the way toward use of ZnGeN 2 as a bandgap-tunable optoelectronic semiconductor.

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

Triplet photodynamic and up-conversion luminescence in donor–acceptor dyads with slip-stacked vs. co-facial arrangement

We report the design/synthesis and characterization of organic donor–acceptor (D–A) dyads can provide precious data allowing to improve the efficiency of classical photo-induced bimolecular interactions/processes. In this report, two novel triplet D–A dyads (4 and 5) were synthesized and fully characterized. While the optical absorption and emission profiles of these new systems exhibit similar spectral structures as that of the triplet donor/sensitizer quinoidal thioamide (QDN), the transient absorption (TA) spectra of these two dyads produced new features that can be associated with triplet transients and charge transfer species. However, the kinetics of the excited-state processes/dynamics is significantly influenced by the geometrical arrangement(s) of donor/acceptor chromophores. Further analysis of the TA data suggests that the dyad with slip-stack geometry (4) is less effective in undergoing both intra- and inter-dyad triplet energy transfer than the dyad with co-facial geometry (5). Subsequently, triplet sensitization of 9,10-diphenylanthracene (DPA) using both dyads led to upconverted photoluminescence via triplet–triplet annihilation of DPA triplet transients. But, it was found that a maximum upconversion quantum yield could be achieved at a low power density using the co-facial type dyad 5. Altogether, these results provide valuable guidance in the design of triplet donor–acceptor dyads, which could be used for light-harvesting/modulation applications.

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

Sequentially regular polymer acceptors featuring flexible spacers for high-performance and mechanically robust all-polymer solar cells

Developing high-performance and mechanically robust polymer solar cells (PSCs) is crucial for realizing wearable power sources. While efficient all-polymer solar cells (all-PSCs) can be fabricated from polymerized small-molecule acceptors (PSMAs) with high optical absorption and electron mobilities, they still show limited mechanical robustness. Here, we achieve highly efficient and mechanically robust all-PSCs by designing a PSMA (PYFS-Reg) containing sequence-regular flexible spacers (FSs). The regular incorporation of the FS units into PSMAs is essential in simultaneously improving the electrical and mechanical properties of blend films. As a result, all-PSCs featuring PYFS-Reg achieve a high power conversion efficiency (PCE = 16.1%) and stretchability (crack onset strain (COS) = 22.4%), outperforming PSMAs without FSs (i.e., PYBDT, PCE = 12.6% and COS = 11.7%) or with randomly distributed FSs (i.e., PYFS-Ran, PCE = 12.2% and COS = 18.1%). Importantly, these all-PSCs are fabricated by an environmentally benign, non-halogenated solvent process. To further demonstrate their feasible applications in wearable devices, we construct intrinsically stretchable (IS) all-PSCs by using PYFS-Reg-based active layers, which exhibit a high PCE (10.6%) and excellent device stretchability (strain at PCE 80% = 36.7%).

14 SOLAR ENERGY↗
DOE OSTI2023

Probing single-chain conformation and its impact on the optoelectronic properties of donor–accepter conjugated polymers

The chain conformation of donor–acceptor conjugated polymers (D–A CPs) is critical to their optical and electronic properties. However, probing the conformation of D–A CPs (e.g., persistence length and contour length) at a single-chain level is challenging due to the formation of aggregates in dilute solution, even in a good solvent. In this work, we studied the chain conformation and corresponding optical spectra for high-performance D–A CPs in the single-chain state by multimodal variable-temperature scattering and spectroscopy techniques, as well as by molecular dynamics simulations. Here we found a critical role of the side-chain length and branch point in the persistence length and optical absorption due to steric effects. Hence, it is important to consider both the chain rigidity and coplanarity of the polymer backbone to achieve desirable optoelectronic properties. Our findings bridge the fundamental knowledge gaps to design new CPs with desired optoelectronic properties via molecular engineering for next-generation electronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Size-controlled synthesis of ultrasmall Cu 3 VS 4 nanocrystals via direct ternary nucleation

Here, we report the size-controlled synthesis of ultrasmall (3–8 nm), phase-pure, and stoichiometric Cu 3 VS 4 nanocrystals via direct nucleation of the ternary sulvanite phase. An unusual redshift in optical absorption is observed with decreasing size and attributed to the plasmonic-like nature of valence band–intermediate band transitions.

Sun, Zhaohong [University of Southern California, ↗
DOE OSTI2026

350 ps Ultrafast room-temperature scintillation realized on CsPbBr 3 -based single crystals via Br 2 over-doping

Ultrafast scintillators are essential for next-generation radiation detection, positron emission tomography, and high-speed medical imaging. All-inorganic CsPbBr 3 perovskites are attractive candidates because of their high stopping power, and excellent optical quality, yet their long carrier lifetimes result in slow scintillation responses on the order of hundreds of nanoseconds. Here, we demonstrate that controlled over-doping with Br 2 produces CsPbBr 3.03 single crystals with sub-nanosecond scintillation at room temperature while preserving crystal quality. Single crystals grown by the Bridgman method exhibit high transparency and maintain the orthorhombic perovskite structure. Br 2 over-doping induces a slight lattice expansion (about 0.42% increase in unit-cell volume) while maintaining the orthorhombic perovskite phase and high optical transparency. Optical absorption reveals a slight redshift of the absorption edge after Br 2 introduction, indicating a modified defect landscape. Time-resolved photoluminescence and radioluminescence measurements show that Br 2 doping creates dense and efficient recombination centers that reduce the scintillation decay time from more than 100 ns in undoped crystals to 350 ps under 5.486 MeV α-particle excitation, and the scintillation decay time decreases by two orders of magnitude. The doped crystals also achieve a spatial resolution of 12 lp mm −1 in X-ray imaging. These results reveal a defect-engineering route for achieving ultrafast scintillation in halide perovskites and highlight the potential of Br 2 -modified CsPbBr 3 for fast timing applications.

Li, Zongxiao [Chinese Academy of Sciences (CAS), N↗
DOE OSTI2020

Initial-state preparation effects in time-resolved electron paramagnetic resonance experiments

In this paper, we explain a recent experimental observation that the time-resolved electron paramagnetic resonance spectra of an organic molecule for optical excitation within a highly absorbing region of the molecule has similar intensities to the spectra for optical excitation in a nonabsorbing region [D. L. Meyer et al., J. Phys. Chem. Lett. 8, 1677 (2017)]. We demonstrate that this phenomenon is due to an initial-state preparation effect of photoexcitation that leads to similar initial populations of triplet states for both optical excitation regions. Due to the low intersystem crossing rates, the initial triplet populations are not perturbed on the time scale of the experiment, so they determine the relative intensities of the paramagnetic resonance spectra. The effect is surprising given the weak spin-orbit interactions of organic molecules. Such initial-state preparation effects are likely to occur in systems where the intersystem crossing time scales are long compared to the time scale of the experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Spectral solar irradiance on inclined surfaces: A fast Monte Carlo approach

Estimating spectral plane-of-array (POA) solar irradiance on inclined surfaces is an important step in the design and performance evaluation of both photovoltaic and concentrated solar plants. This work introduces a fast, line-by-line spectral, Monte Carlo (MC) radiative transfer model approach to simulate anisotropic distributions of shortwave radiation through the atmosphere as photon bundles impinge on inclined surfaces. This fast Monte Carlo approach reproduces the angular distribution of solar irradiance without the undesirable effects of spatial discretization and thus computes detailed POA irradiance values on surfaces at any orientation and also when surfaces are subjected to the anisotropic ground and atmospheric scattering. Polarization effects are also easily incorporated into this approach that can be considered as direct numerical simulation of the physics involved. Here, we compare our Monte Carlo radiative transfer model with the most widely used empirical transposition model, Perez4, under various conditions. The results show that the Perez4 model reproduces the more detailed Monte Carlo simulations with less than 10% deviation under clear skies for all relevant surface tilt and azimuth angles. When optically thin clouds are present, observed deviations are larger, especially when the receiving surface is strongly tilted. Deviations are also observed for large azimuth angle differences between the receiving surface and the solar position. When optically thick clouds are present, the two models agree within 15% deviation for nearly all surface orientation and tilt angles. The overall deviations are smaller when compared with cases for optically thin clouds. The Perez4 model performs very well (6.0% deviation) in comparison with the detailed MC simulations for all cases, thus validating its widespread use for practical solar applications. When detailed atmospheric profiles and cloud optical properties are available, the proposed fast Monte Carlo radiative model reproduces accurate spectral and angular POA irradiance levels for various atmospheric and cloud cover conditions, surface orientations, and different surface and ground properties.

42 ENGINEERING↗
DOE OSTI2020

Anisotropic band structure of TiS 3 nanoribbon revealed by polarized photocurrent spectroscopy

A re-discovered member of the layered material family, Titanium Trisulfide (TiS 3 ), has attracted intense research interest recently for the possibility of realizing an exciton insulator in the monolayer limit. However, due to their quasi-one-dimensional nature, thin TiS 3 flakes are typically in the form of nanoribbons that are challenging to characterize by optical absorption spectra, due to their small cross section. Here, we employ a sensitive photocurrent spectroscopy technique to probe the absorption of a thin TiS 3 nanoribbon in a field-effect transistor configuration. We have found a significant modification of the band structure of the thin TiS 3 nanoribbon, compared to its bulk counterpart. In addition, the polarization-dependent photocurrent spectra of thin TiS 3 nanoribbons exhibit greatly enhanced anisotropy compared to that of a thick TiS 3 device. The atomically thin TiS 3 flake, thus, not only provides an exciting platform for investigating many-body physics but also enables anisotropic quantum optoelectronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Robust method for broadband efficiency enhancement of electron photocathodes using optical interferences

In this work, we demonstrate the key features of an interference cathode using both simulations and experiments. We deposit Cs 3 Sb photocathodes on Ag to produce an interference enhanced photocathode with 2–5× quantum efficiency (QE) enhancement using a robust procedure that requires only a smooth metal substrate and QE monitoring during growth. We grow both an interference cathode (Ag substrate) and a typical photocathode (Si reference substrate) simultaneously to confirm that the effects are due to optical interactions with the substrate rather than photocathode composition or surface electron affinity differences. Growing the cathodes until the QE converges shows both the characteristic interference peaks during growth and the identical limiting case where the cathode is “infinitely thick,” in agreement with simulations. We also grow a cathode until the QE on Ag peaks and then stop the growth, demonstrating broadband QE enhancement.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Optical constants of polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As determined by variable-angle spectroscopic ellipsometry

The optical constants of single-crystal Al x Ga 1-x P and Al x Ga 1-x As have been widely studied at various Al compositions but have not yet been thoroughly investigated as polycrystalline materials. Using variable-angle spectroscopic ellipsometry, we estimate and analyze the optical constants of polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As that were non-epitaxially deposited at deposition temperatures of either 250 or 420 °C, and with various dopant species, dopant concentrations, and V/III flux ratios. Here, for wavelengths of 350–400 nm, decreasing the V/III ratio increases the refractive index of Be-doped polycrystalline Al 0.25 Ga 0.75 P. For wavelengths >500 nm, as the targeted Be dopant concentration increases for polycrystalline Al 0.25 Ga 0.75 P, the amount the extinction coefficient increases and the indirect bandgap decreases depends on the V/III ratio. Furthermore, Si-doped polycrystalline Al 0.25 Ga 0.75 P has significantly smaller extinction coefficient values than when it is Be-doped, for the same V/III ratio and targeted doping concentration, at wavelengths >500 nm. Based on our findings, we hypothesize that the dopant species, dopant concentration, and the V/III ratio dictate various types of defect concentrations, which then affect the optical constants and indirect bandgap of polycrystalline Al 0.25 Ga 0.75 P. Additionally, for wavelengths >500 nm, we find that polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As have the lowest extinction coefficient values when deposited at 420 °C, as opposed to 250 °C. Thus, we hypothesize that the deposition temperature is the most significant factor in dictating the optical constants of these polycrystalline III-Vs. Ultimately, this work demonstrates alternative methods to tune the optical constants of polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As, as opposed to tuning the Al composition.

42 ENGINEERING↗
DOE OSTI2024

Four-wave mixing with anti-parity-time symmetry in hot 85 Rb vapor

Here, we report an experimental demonstration of anti-parity-time symmetric optical four-wave mixing in thermal rubidium vapor, where the propagation of probe and stokes fields in a double-Λ scheme is governed by a non-Hermitian Hamiltonian. We are particularly interested in studying quantum intensity correlations between the two fields near the exceptional point, taking into account loss and accompanied Langevin noise. Our experimental measurements of classical four-wave mixing gain and the associated two-mode relative-intensity squeezing are in reasonable agreement with the theoretical predictions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Charge state transition levels of Ni in $β$-Ga 2 O 3 crystals from experiment and theory: An attractive candidate for compensation doping

Nickel-doped β-Ga 2 O 3 crystals were investigated by optical absorption and photoconductivity, revealing Ni-related deep levels. Here, the photoconductivity spectra were fitted using the phenomenological Kopylov and Pikhtin model to identify the energy of the zero-phonon transition (thermal ionization), Franck–Condon shift, and effective phonon energy. The resulting values are compared with the predicted ones by first-principle calculations based on the density functional theory (DFT). An acceptor level (0/-) of 1.9 eV and a donor level (+/0) of 1.1 eV above the valence band minimum are consistently determined for Ni Ga , which preferentially incorporates on the octahedrally coordinated Ga site. Temperature-dependent resistivity measurements yield a thermal activation energy of ~2.0 eV that agrees well with the determined Ni acceptor level. Conclusively, Ni is an eminently suitable candidate for compensation doping for producing semi-insulating β-Ga 2 O 3 substrates due to the position of the acceptor level (below and close to the mid-bandgap).

36 MATERIALS SCIENCE↗
DOE OSTI2024

PigmentHunter: A point-and-click application for automated chlorophyll-protein simulations

Chlorophyll proteins (CPs) are the workhorses of biological photosynthesis, working together to absorb solar energy, transfer it to chemically active reaction centers, and control the charge-separation process that drives its storage as chemical energy. Yet predicting CP optical and electronic properties remains a serious challenge, driven by the computational difficulty of treating large, electronically coupled molecular pigments embedded in a dynamically structured protein environment. To address this challenge, we introduce here an analysis tool called PigmentHunter, which automates the process of preparing CP structures for molecular dynamics (MD), running short MD simulations on the nanoHUB.org science gateway, and then using electrostatic and steric analysis routines to predict optical absorption, fluorescence, and circular dichroism spectra within a Frenkel exciton model. Inter-pigment couplings are evaluated using point-dipole or transition-charge coupling models, while site energies can be estimated using both electrostatic and ring-deformation approaches. The package is built in a Jupyter Notebook environment, with a point-and-click interface that can be used either to manually prepare individual structures or to batch-process many structures at once. Here, we illustrate PigmentHunter’s capabilities with example simulations on spectral line shapes in the light harvesting 2 complex, site energies in the Fenna–Matthews–Olson protein, and ring deformation in photosystems I and II.

14 SOLAR ENERGY↗
DOE OSTI2024

Selective incorporation of antimony into gallium nitride

Dilute concentrations of antimony (Sb) incorporation into GaN induce strong bandgap bowing and tunable room-temperature photoluminescence from the UV to the green spectral regions. However, the atomistic details of the incorporation of Sb into the GaN host remain unclear. In this work, we use first-principles calculations to understand the thermodynamics of Sb substitution into GaN and its effect on the optical and Raman spectra. Although it is empirically considered that Sb is preferentially incorporated as an anion (Sb3−) into the N sublattice, we demonstrate that Sb can also be incorporated as a cation (Sb3+, Sb5+) into the metal sublattice. Our thermodynamic analysis demonstrates that SbN0, SbGa2+, and SbGa0 can co-exist under Ga-rich conditions in n-type samples. We further confirm the dual incorporation of Sb by calculating the vibrational frequencies of different anionic and cationic substitutions to explain the origins of experimentally observed additional Raman peaks of Sb-doped GaN. Moreover, the calculated band structures of different Sb substitutions into GaN explain the experimental photoluminescence and optical absorption spectra. Overall, our analysis suggests that the coexistence of Sb3−, Sb3+, and Sb5+ substitutions into GaN explains the totality of experimental measurements. Our results demonstrate that the selective incorporation of Sb into GaN (and potentially other group-V elements such as As, P, or Bi) by tuning the growth conditions can drastically modify the electronic properties, for applications in visible light emitters and photocatalysis.

Physics↗
DOE OSTI2024

Ligand field exciton annihilation in bulk CrCl 3

The layered van der Waals material CrCl 3 exhibits very strongly bound ligand field excitons that control optoelectronic applications and are connected with magnetic ordering by virtue of their d-orbital origin. Time-resolved photoluminescence of these exciton populations at room temperature shows that their relaxation is dominated by exciton–exciton annihilation and that the spontaneous decay lifetime is very long. Furthermore, these observations allow the rough quantification of the exciton annihilation rate constant and contextualization in light of a recent theory of universal scaling behavior of the annihilation process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Shock equation of state experiments in MgO up to 1.5 TPa and the effects of optical depth on temperature determination

Laser-driven shock compression enables an experimental study of phase transitions at unprecedented pressures and temperatures. One example is the shock Hugoniot of magnesium oxide (MgO), which crosses the B1–B2-liquid triple point at 400–600 GPa, 10 000–13 000 K (0.86–1.12 eV). MgO is a major component within the mantles of terrestrial planets and has long been a focus of high-pressure research. Here, we combine time-resolved velocimetry and pyrometry measurements with a decaying shock platform to obtain pressure–temperature data on MgO from 300 to 1500 GPa and 9000 to 50 000 K. Pressure–temperature–density Hugoniot data are reported at 1500 GPa. These data represent the near-instantaneous response of an MgO [100] single crystal to shock compression. We report on a prominent temperature anomaly between 400 and 460 GPa, in general agreement with previous shock studies, and draw comparison with equation-of-state models. We provide a detailed analysis of the decaying shock compression platform, including a treatment of a pressure-dependent optical depth near the shock front. We show that if the optical depth of the shocked material is larger than 1 μm, treating the shock front as an optically thick gray body will lead to a noticeable overestimation of the shock temperature.

36 MATERIALS SCIENCE↗