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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Persistent Room-Temperature Photodarkening in Cu-Doped β - Ga 2 O 3

β–Ga 2 O 3 is an ultrawide band gap semiconductor with emerging applications in power electronics. Here, the introduction of acceptor dopants yields semi-insulating substrates necessary for thin-film devices. In the present work, exposure of Cu-doped β–Ga 2 O 3 to UV light > 4 eV is shown to cause large, persistent photo-induced darkening at room temperature. Electron paramagnetic resonance spectroscopy indicates that light exposure converts Cu 2+ to Cu 3+ , a rare oxidation state that is responsible for the optical absorption. The photodarkening is accompanied by the appearance of O–H vibrational modes in the infrared spectrum. Hybrid function calculations show that Cu acceptors can favorably complex with hydrogen donors incorporated as interstitial (Hi) or substitutional (HO) defects. When Cu Ga –HO complexes absorb light, hydrogen is released, contributing to the observed Cu 3+ species and O–H modes.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local Structural Disorder in Metavanadates MV 2 O 6 (M = Zn and Cu) Synthesized by the Deep Eutectic Solvent Route: Photoactive Oxides with Oxygen Vacancies

Metavanadates MV 2 O 6–δ (M = Zn and Cu) are synthesized by using a deep eutectic solvent (DES), a mixture of hydrogen bond donor and acceptor, as a reaction medium. Dissolution of stable binary metal oxides in a DES followed by a heat treatment yields phase-pure vanadates. According to in situ powder X-ray diffraction, ternary phases (α-Zn 2 V 2 O 7 and β-Cu 2–x V 2 O 7 , x ~ 0.27) are intermediates in the reaction pathway taken. Identifying a polymorphic phase transformation temperature for CuV 2 O 6 as well as the narrow temperature range between formation and decomposition for both metavanadates allows for tackling the challenge of the synthesis of these materials. The oxygen vacancy introduced by the DES route is accompanied by the formation of reduced V 4+ and Cu + in the oxide matrix, based on X-ray photoelectron spectroscopy. These oxygen vacancies modify the vibrational modes in the corresponding Raman spectra and are also responsible for broad optical absorptions in the 1.8–1.1 eV range. The optical band gaps of the materials are found at 1.8 eV (CuV 2 O 6 ) and 2.2 eV (ZnV 2 O 6 ), approximately 0.1–0.3 eV below the values reported in the literature. The reduced band gaps and sub-band gap photon absorption are key features of the oxygen-deficient metavanadates. Surface photovoltage spectroscopy reveals that all the synthesized vanadates are n-type materials with electrons as the majority charge carriers, and photoelectrochemical measurements confirm photoanodic currents for methanol oxidation. Furthermore, these results provide insight into the synthesis and structure–property relationship of the metavanadates for their potential use as photoanodes.

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

Restored Passivation after Complete Removal of Front Poly-Si Between the Grid in Poly-Si/SiO2 Front/Back Cells

This work explores the use of wet chemical etching to completely remove the front poly-Si layer between grid lines of a front/back poly-Si/SiO2 passivated contact device. The purpose of the work is to provide excellent passivation under the metal grid, but to also maximize Jsc by eliminating optical absorption in the front poly-Si. We show that after a high temperature anneal to distribute carriers in the poly-Si/SiO2/c-Si interface all of the poly-Si can be removed selectively with TMAH, using the SiO2 as an etch-stop layer. The etched surface can be repassivated back to its original level with the addition of a SiNx/Al2O3 stack grown on the preserved tunnelling oxide layer followed by a short forming gas anneal. Using symmetric 100 - 200 nm intrinsically doped poly-Si/SiO2 on textured n-type wafer templates we show absolute device efficiency enhancement from 15% to 21% by increasing the Jsc by 12 mA/cm2 after removing the front poly-Si and repassivating while effectively preserving the Voc, and FF of the device. IQE values are similar to PERC devices in the short wavelength range.

etching↗
DOE OSTI2025

Many-body perturbation theory with hybrid density functional theory starting points accelerated by adaptively compressed exchange

We report on the use of the adaptively compressed exchange (ACE) operator to accelerate many-body perturbation theory (MBPT) calculations, including G 0 W 0 and the Bethe–Salpeter equation (BSE), for hybrid density functional theory starting points. We show that by approximating the exact exchange operator with the low-rank ACE operator, substantial computational savings can be achieved with systematically controllable errors in the quasiparticle energies computed with full-frequency G 0 W 0 and the optical absorption spectra and vertical excitation energies computed by solving the BSE within density matrix perturbation theory. Our implementation makes use of the ACE-accelerated electronic Hamiltonian to carry out both G 0 W 0 and BSE without explicitly computing empty states. We show the robustness of the approach and present the computational gains obtained on both the central processing unit and graphics processing unit nodes. In conclusion, our work will facilitate the exploration and evaluation of fine-tuned hybrid starting points aimed at enhancing the accuracy of MBPT calculations without involving computationally demanding self-consistency in Hedin’s equations.

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

Efficient Perovskite Solar Cells Achieved via Rapid Photonic Annealing of all Stacking Layers: Unveiling the Crystallization Energy Window

In this study, we report high-performance perovksite solar cells (PSCs) with rapid photonic annealing (RPA) of all stacking layers, enabled by ultraviolet (UV) light-emitting diode (LED) sources, to replace lengthy and energy-intensive thermal annealing (TA). The UV-LED annealing technique allows for layer-specific annealing, where the selected light source provides a precise UV wavelength for maximizing the amount of light absorption by the target layer. The disparity in optical absorption between the target layer and the underlying films allows the stack of the underlying films to remain relatively unaffected, making this process ideal for heating sensitive substrates. Along with a systematic investigation into the layer-specific annealing mechanism of RPA, the results demonstrated that this UV-LED-based photonic annealing of all stacking layers (7 s for perovskite absorber) can produce PSCs with the power conversion efficiency (PCE) of over 23%, the highest reported among optically annealed PSCs. Moreover, the RPA device retains over 80% of the initial PCE over 1000 h under continuous 1 sun illumination at 55 °C and 30%–60% relative humidity (RH), while TA control device drops to 50% of its initial efficiency. Furthermore, these findings represent significant strides toward achieving rapid, cost-effective, and scalable manufacturing of commercial perovskite photovoltaics (PV).

14 SOLAR ENERGY↗
DOE OSTI2024

Probing time-resolved plasma-driven solution electrochemistry in a falling liquid film plasma reactor: Identification of HO$^−_2$ as a plasma-derived reducing agent

Many applications involving plasma–liquid interactions depend on the reactive processes occurring at the plasma–liquid interface. In this article, we report on a falling liquid film plasma reactor allowing for in situ optical absorption measurements of the time-dependence of the ferricyanide/ferrocyanide redox reactivity, complemented with ex situ measurement of the decomposition of formate. We found excellent agreement between the measured decomposition percentages and the diffusion-limited decomposition of formate by interfacial plasma-enabled reactions, except at high pH in thin liquid films, indicating the involvement of previously unexplored plasma-induced liquid phase chemistry enabled by long-lived reactive species. We also determined that high pH facilitates a reduction-favoring environment in ferricyanide/ferrocyanide redox solutions. In situ conversion measurements of a 1:1 ferricyanide/ferrocyanide redox mixture exceed the measured ex situ conversion and show that conversion of a 1:1 ferricyanide/ferrocyanide mixture is strongly dependent on film thickness. We identified three dominant processes: reduction faster than ms time scales for film thicknesses >100 µm, •OH-driven oxidation on time scales of <10 ms, and reduction on 15 ms time scales for film thickness <100 µm. We attribute the slow reduction and larger formate decomposition at high pH to HO$^−_2$ formed from plasma-produced H 2 O 2 enabled by the high pH at the plasma–liquid interface as confirmed experimentally and by computed reaction rates of HO$^−_2$ with ferricyanide. Overall, this work demonstrates the utility of liquid film reactors in enabling the discovery of new plasma-interfacial chemistry and the utility of atmospheric plasmas for electrodeless electrochemistry.

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

Rate Coefficients for OH + NO (+N 2 ) in the Fall-off Regime and the Impact of Water Vapor

The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N 2 and in N 2 –H 2 O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO 2 or HONO impurities. Our rate coefficients (k 1 ) in N 2 bath gas are parametrized in terms of low-pressure (k o ) and high-pressure (k ∞ ) rate coefficients and a fall-off parameter (F C ) with k 1,0 N 2 = 7.24 × 10 –31 (T/300 K) –2.17 cm 6 molecule –2 s –1 , k 1,∞ = 3.3 × 10 –12 (T/300 K) –3 cm 3 molecule –1 s –1 , and F C = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H 2 O bath gas is a factor 5–6 larger than in N 2 , at room temperature (k 1,o H 2 O = 4.55 × 10 –30 (T/300 K) –4.85 cm 6 molecule –2 s –1 ) indicating that H 2 O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N 2 –H 2 O mixtures, a parametrization of k 1 including both N 2 and H 2 O as third-body quenchers was derived. Neglecting the effect of H 2 O results, e.g., in an underestimation of k 1 by >10% in the tropical boundary layer.

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

Stoichiometric Control and Optical Properties of BaTiO 3 Thin Films Grown by Hybrid MBE

BaTiO 3 is a technologically relevant material in the perovskite oxide class with above-room-temperature ferroelectricity and a very large electro-optical coefficient, making it highly suitable for emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO 3 thin films with sufficient control over the film's stoichiometry to achieve reproducible thin film properties. Here the growth of BaTiO 3 thin films by hybrid molecular beam epitaxy is reported. A self-regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO 3 films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO 3 crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO 3 thin films, degradation of optical properties is observed, accompanied by the appearance of a wide optical absorption peak in the IR spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO 3 can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO 3 films.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Optically induced quantum transitions in direct probed mesoscopic NbSe 2 for prototypical bolometers

Superconducting transition-edge sensors (TES) have emerged as fascinating devices to detect broadband electromagnetic radiation with low thermal noise. The advent of metallic transition metal dichalcogenides, such as NbSe 2 , has also created an impetus to understand their low-temperature properties, including superconductivity. Interestingly, NbSe 2 -based sensor within the TES framework remains unexplored. In this work, direct-probed superconducting NbSe 2 absorbers led to a proof-of-concept demonstration for the transduction of incoming light to heat, where a thermodynamic superconducting phase transition in NbSe 2 was evident to switch it to the normal-state, when biased below its superconducting transition temperature. A wavelength-dependent response of its optical absorption properties was observed, based on the incident optical excitation source used. Furthermore, extensive optical characterization studies were conducted using Raman spectroscopy, where the in-plane and out-of-plane thermal conductivity was empirically determined. Our results open new possibilities for the use of NbSe 2 in superconducting radiation detectors, including in a TES framework

47 OTHER INSTRUMENTATION↗
DOE OSTI2020

Tunable Perovskite-Derived Bismuth Halides: Cs 3 Bi 2 (Cl 1– x I x ) 9

Bismuth-based perovskites are of interest as safer alternatives to lead-based optoelectronic materials. Prior studies have reported on the compounds Cs 3 Bi 2 Cl 9 , Cs 3 Bi 2 I 9 , and Cs 3 Bi 2 Cl 3 I 6 . Here we examine a range of compounds of the formula Cs 3 Bi 2 (Cl 1–x I x ) 9 , where x takes values from 0.09 to 0.52. Powder and single-crystal X-ray diffraction were used to determine that all of these compounds adopt the layered vacancy-ordered perovskite structure observed for Cs 3 Bi 2 Cl 3 I 6 , which is also the high-temperature phase of Cs 3 Bi 2 Cl 9 . We find that, even with very small iodine incorporation, the structure is switched to that of Cs 3 Bi 2 Cl 3 I 6 , with I atoms displaying a distinct preference for the capping sites on the BiX 6 octahedra. Optical absorption spectroscopy was employed to study the evolution of optical properties of these materials, and this is complemented by density functional theory electronic structure calculations. Finally, three main absorption features were observed for these compounds, and with increasing x, the lowest-energy features are red-shifted.

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

Optimization and Prediction of Spectral Response of Metasurfaces Using Artificial Intelligence

Hot-electron generation has been a topic of intense research for decades for numerous applications ranging from photodetection and photochemistry to biosensing. Recently, the technique of hot-electron generation using non-radiative decay of surface plasmons excited by metallic nanoantennas, or meta-atoms, in a metasurface has attracted attention. These metasurfaces can be designed with thicknesses on the order of the hot-electron diffusion length. The plasmonic resonances of these ultrathin metasurfaces can be tailored by changing the shape and size of the meta-atoms. One of the fundamental mechanisms leading to generation of hot-electrons in such systems is optical absorption, therefore, optimization of absorption is a key step in enhancing the performance of any metasurface based hot-electron device. Here we utilized an artificial intelligence-based approach, the genetic algorithm, to optimize absorption spectra of plasmonic metasurfaces. Using genetic algorithm optimization strategies, we designed a polarization insensitive plasmonic metasurface with 90% absorption at 1550 nm that does not require an optically thick ground plane. We fabricated and optically characterized the metasurface and our experimental results agree with simulations. Finally, we present a convolutional neural network that can predict the absorption spectra of metasurfaces never seen by the network, thereby eliminating the need for computationally expensive simulations. Our results suggest a new direction for optimizing hot-electron based photodetectors and sensors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Investigations of radiation damaged arranged scintillating particle composites

Composites consisting of periodically arranged scintillating particles within an acrylic (poly methyl methacrylate) matrix are excellent templates for the gamma-insensitive detection of fast neutrons. In order to study the impact of radiation damage on acrylic-based scintillating particle composites, we exposed acrylic samples to different doses of fast neutrons up to 119.7 kGy and investigated the change in optical characteristics using optical absorption spectroscopy in the 200–800 nm wavelength range. The experimental results are compared to coupled MCNP6 and optical ray-tracing (FRED) simulations, and qualitative agreement is found. Neutron-induced radiation damage in the acrylic matrix manifests as a red-shift in the ultra-violet absorption edge of the acrylic and a corresponding decrease in the light propagation efficiency, which leads to detector performance degradation. We show for the first time that the effects of radiation damage can be mitigated by the use of wavelength-shifting coatings. We predict that composites with wavelength-shifting coatings can enable neutron detectors with high tolerance against neutron-induced radiation damage, a property that is particularly desired for neutron detection in high radiation environments.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Circularly Polarized Optical Stark Effect in CdSe Colloidal Quantum Wells

Colloidal quantum wells, or nanoplatelets, exhibit large, circularly polarized optical Stark effects under sub-band-gap femtosecond illumination. The optical Stark effect is measured for CdSe colloidal quantum wells of several thicknesses and separately as a measure of pump photon energy, pump fluence, and temperature. These measurements show that optical Stark effects in colloidal quantum wells shift the absorption features up to 5 meV, at the intensities up to 2.9 GW.cm -2 and large detuning (>400 meV) of the pump photon energy from the band edge absorption. Optical Stark shifts are underpinned by large transition dipoles of the colloidal quantum wells (mu = 15-23 D), which are larger than those of any reported colloidal quantum dots or epitaxial quantum wells. The rapid (<500 fs), narrow band blue shift of the excitonic features under circular excitation indicates the viability of these materials beyond light emission such as spintronics or all-optical switching.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Flat Bands and Chiral Optical Response of Moiré Insulators

Here, we present a low-energy model describing the reconstruction of the electronic spectrum in twisted bilayers of honeycomb crystals with broken sublattice symmetry. The resulting moiré patterns are classified into two families with different symmetry. In both cases, flat bands appear at relatively large angles without any magic-angle condition. Transitions between them give rise to sharp resonances in the optical absorption spectrum at frequencies well below the gap of the monolayer. Owing to their chiral symmetry, twisted bilayers display circular dichroism, that is, different absorption of left and right circularly polarized light. This optical activity is a nonlocal property determined by the stacking. In hexagonal boron nitride, sensitivity to the stacking leads to strikingly different circular dichroism in the two types of moirés. Our calculations exemplify how subtle properties of the electronic wave functions, encoded in current correlations between the layers, control physical observables of moiré materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Cleavable quaternary oxychlorides with high magnetic ordering temperatures

Quaternary oxychlorides derived from Ruddlesden–Popper 3d transition metal oxides offer a route to cleavable crystals with bulk antiferromagnetic ordering temperatures reaching at least 550 K. Here, we study the magnetic, optical, and mechanical behavior of Sr 2 FeO 3 Cl, Ca 2 FeO 3 Cl, Ca 3 Fe 2 O 5 Cl 2 , and Sr 3 Fe 2 O 5 Cl 2 . Through optical absorption measurements, we show that these antiferromagnetic semiconductors have optical band gaps of ≈2.1(1) eV. The magnetic ordering symmetries and temperatures were probed by neutron powder diffraction and Mössbauer spectroscopy on polycrystalline samples, demonstrating Néel temperatures (T N ) near room temperature in the single layer Sr 2 FeO 3 Cl (T N ≈ 311 K) and Ca 2 FeO 3 Cl (T N ≈ 360 K), and the double-layer compound Sr 3 Fe 2 O 5 Cl 2 has T N ≈ 545 K. The high-spin moments of Fe 3+ lie within the basal plane and the magnetic structures are compensated within each magnetic layer and characterized by magnetic propagation vectors k = ($\frac{1}{2}$ $\frac{1}{2}$ 0). Magnetization results demonstrate the quasi-2D nature of the magnetism, with a broad maximum in the susceptibility near 2T N for Sr 2 FeO 3 Cl. Scotch tape tests and mechanical exfoliation onto SiO 2 confirm the micaceous nature of these crystals with cleavage down to a single unit cell (two magnetic layers) achieved for Sr 3 Fe 2 O 5 Cl 2 . In conclusion, this paper highlights strong antiferromagnetic interactions, semiconducting band gaps, and cleavability of quaternary Fe-based oxychlorides and motivates future work on crystals and exfoliated flakes of these and related oxyhalide systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Opto-thermal laser detonator

An opto-thermal laser detonator uses resonantly absorptive tuned nano-material associated with secondary explosives for optical absorption and initiation by an integral laser diode. The opto-thermal laser detonator includes main explosive material; resonantly absorptive tuned nano-material; secondary explosive material, wherein the resonantly absorptive tuned nano-material and the secondary explosive material are associated to form associated material made of the resonantly absorptive tuned nano-material and the secondary explosive material; and a laser diode operatively connected to the associated material, wherein the laser diode initiates the associated material which in turn initiates the main explosive material.

Wilkins, Paul R.↗
DOE OSTI2023

Opto-thermal laser detonator

An opto-thermal laser detonator uses resonantly absorptive tuned nano-material associated with secondary explosives for optical absorption and initiation by an integral laser diode. The opto-thermal laser detonator includes main explosive material; resonantly absorptive tuned nano-material; secondary explosive material, wherein the resonantly absorptive tuned nano-material and the secondary explosive material are associated to form associated material made of the resonantly absorptive tuned nano-material and the secondary explosive material; and a laser diode operatively connected to the associated material, wherein the laser diode initiates the associated material which in turn initiates the main explosive material.

Wilkins, Paul R.↗