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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 973 records · Page 54
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Ultrahigh‐Flux X‐ray Detection by a Solution‐Grown Perovskite CsPbBr 3 Single‐Crystal Semiconductor Detector

Abstract Solution‐processed perovskites are promising for hard X‐ray and gamma‐ray detection, but there are limited reports on their performance under extremely intense X‐rays. Here, a solution‐grown all‐inorganic perovskite CsPbBr 3 single‐crystal semiconductor detector capable of operating at ultrahigh X‐ray flux of 10 10 photons s −1 mm −2 is reported. High‐quality solution‐grown CsPbBr 3 single crystals are fabricated into detectors with a Schottky diode structure of eutectic gallium indium/CsPbBr 3 /Au. A high reverse‐bias voltage of 1000 V (435 V mm − 1 ) can be applied with a small and stable dark current of ≈60–70 nA (≈9–10 nA mm − 2 ), which enables a high sensitivity larger than 10 000 µC Gy air −1 cm − 2 and a simultaneous low detection limit of 22 nGy air s − 1 . The CsPbBr 3 semiconductor detector shows an excellent photocurrent linearity and reproducibility under 58.61 keV synchrotron X‐rays with flux from 10 6 to 10 10 photons s − 1 mm − 2 . Defect characterization by thermally stimulated current spectroscopy shows a similar low defect density of a synchrotron X‐ray and a lab X‐ray irradiated device. Solid‐state nuclear magnetic resonance spectroscopy suggests that the excellent performance of the solution‐grown CsPbBr 3 single crystal may be associated with its good short‐range order, comparable to the spectrometer‐grade melt‐grown CsPbBr 3 .

Pan, Lei↗
DOE OSTI2025

Structure Prediction of Ionic Epitaxial Interfaces with Ogre Demonstrated for Colloidal Heterostructures of Lead Halide Perovskites

Colloidal epitaxial heterostructures are nanoparticles composed of two different materials connected at an interface, which can exhibit properties different from those of their individual components. Combining dissimilar materials offers exciting opportunities to create a wide variety of functional heterostructures. However, assessing structural compatibility–the main prerequisite for epitaxial growth–is challenging when pairing complex materials with different lattice parameters and crystal structures. This complicates both the selection of target heterostructures for synthesis and the assignment of interface models when new heterostructures are obtained. Here, we demonstrate Ogre as a powerful tool to accelerate the design and characterization of colloidal heterostructures. To this end, we implemented developments tailored for the high-efficiency prediction of epitaxial interfaces between ionic/polar materials, which encompass most colloidal semiconductors. These include the use of pre-screening candidate models based on charge balance at the interface and the use of a classical potential for fast energy evaluations, with parameters automatically calculated based on the input bulk structures. These developments are validated for perovskite-based CsPbBr 3 /Pb 4 S 3 Br 2 heterostructures, where Ogre produces interface models in excellent agreement with density functional theory and experiments. Furthermore, we use Ogre to rationalize the templating effect of CsPbCl 3 on the growth of lead sulfochlorides, where perovskite seeds induce the formation of Pb 4 S 3 Cl 2 rather than Pb 3 S 2 Cl 2 due to better epitaxial compatibility. Finally, combining Ogre simulations with experimental data enables us to unravel the structure and composition of the hitherto unsolved CsPbBr 3 /Bi x Pb y S z interface, and to assign a structure to several other reported metal halide- and oxide-based interfaces. The Ogre package is available on GitHub or via the OgreInterface desktop application, available for Windows, Linux, and Mac.

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

Phonon-Driven Femtosecond Dynamics of Excitons in Crystalline Pentacene from First Principles

Nonradiative exciton relaxation processes are critical for energy transduction and transport in optoelectronic materials, but how these processes are connected to the underlying crystal structure and the associated electron, exciton, and phonon band structures, as well as the interactions of all these particles, is challenging to understand. Here, in this work, we present a first-principles study of exciton-phonon relaxation pathways in pentacene, a paradigmatic molecular crystal and optoelectronic semiconductor. We compute the momentum- and band-resolved exciton-phonon interactions, and use them to analyze key scattering channels. We find that both exciton intraband scattering and interband scattering to parity-forbidden dark states occur on the same ~100 fs timescale as a direct consequence of the longitudinal-transverse splitting of the bright exciton band. Consequently, exciton-phonon scattering exists as a dominant nonradiative relaxation channel in pentacene. We further show how the propagation of an exciton wave packet is connected with crystal anisotropy, which gives rise to the longitudinal-transverse exciton splitting and concomitant anisotropic exciton and phonon dispersions. Our results provide a framework for understanding the role of exciton-phonon interactions in exciton nonradiative lifetimes in molecular crystals and beyond.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Alkaline Earth-Based Ternary Chalcogenide Nanocrystals: Cadmium- and Lead-Free Optical Materials

As some of the most abundant elements on the Earth’s crust, alkaline earths bear great potential in Cd- and Pb-free catalytic, energy conversion, and light-emitting devices. Further, because large electropositive alkaline earths (Ae) can adopt high coordination numbers, they offer structural complexity beyond the tetrahedral (c-Si, II–VI, III–V) or octahedral (IV–VI) atomic sites that─with the notable exception of halide perovskites─are ubiquitous among nanocrystalline semiconductors. Here, in this work, we present a general route to photoluminescent 6–125 nm nanocrystals of ternary AeIn 2 Ch 4 semiconductors (Ae = Sr, Ba; Ch = S, Se) featuring eight- (Ae) as well as four-coordinate (In and Ch) atomic sites. Powder X-ray diffraction, electron microscopy, optical absorption, photoluminescence, and solid-state (ss)NMR spectroscopies attest to the ternary composition and phase purity of the nanocrystals. Continuous shape measures pinpoint the degree of distortion of individual crystallographic sites from ideal coordination polyhedra, allowing us to identify two very distinct types of Se coordination environments. Based on their higher order, pseudo-3-fold rotational symmetry, we assign 77 Se ssNMR peaks with negligible chemical shift anisotropy (CSA) to pyramidalized Se sites with vacant-trigonal bipyramid-like geometries and those with larger CSA to lower symmetry Se sites with seesaw-distorted geometries. Calculations help to better understand the electronic band structure of these materials. The nanocrystals are stable at room temperature in open air for several weeks, and some compositions up to 1000 °C under Ar, making them strong candidates for practical applications.

alkaline earth↗
DOE OSTI2026

Bandgap Engineering of Ga 2 O 3 by MOCVD Through Alloying with Indium

Ga 2 O 3 and In 2 O 3 are vital semiconductors with current and future electronic device applications. Here, we study the alloying of In 2 O 3 and Ga 2 O 3 (IGO) and the associated changes in structure, morphology, band gap, and electrical transport properties. Undoped films of IGO were deposited on sapphire substrates with varying indium (In) percentage from zero to 100% by metal-organic chemical vapor deposition (MOCVD). Some films were annealed in H 2 to induce electrical conductivity. The measurements showed the optical band gap decreased by adding In; this was confirmed by density functional (DFT) calculations, which revealed that the nature of the valence band maximum and conduction band minimum strongly relate to the chemistry and that the band gap drops by adding In. The as-grown films were highly resistive except for pure In 2 O 3 , which possesses p-type conductivity, likely arising from In vacancy-related acceptor states. N-type conductivity was induced in all films after H-anneal. DFT calculations revealed that the presence of In decreases the electron effective mass, which is consistent with the electrical transport measurements that showed higher electron mobility for higher In percentage. The work revealed the successful band gap engineering of IGO and the modification of its band structure while maintaining high-quality films by MOCVD.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Polaritonic Chern Insulators in Monolayer Semiconductors

Systems with strong light-matter interaction open up new avenues for studying topological phases of matter. Examples include exciton polaritons, mixed light-matter quasiparticles, where the topology of the polaritonic band structure arises from the collective coupling between matter wave and optical fields strongly confined in periodic dielectric structures. Distinct from light-matter interaction in a uniform environment, the spatially varying nature of the optical fields leads to a fundamental modification of the well-known optical selection rules, which were derived under the plane wave approximation. Here, in this study, we identify polaritonic Chern insulators by coupling valley excitons in transition metal dichalcogenides to photonic Bloch modes in a dielectric photonic crystal slab. We show that polaritonic Dirac points, which are markers for topological phase transition points, can be constructed from the collective coupling between valley excitons and photonic Dirac cones in the presence of both time-reversal and inversion symmetries. Lifting exciton valley degeneracy by breaking time-reversal symmetry leads to gapped polaritonic bands with nonzero Chern numbers. Through numerical simulations, we predict polaritonic chiral edge states residing inside the topological gaps. Our Letter paves the way for the further study of strong exciton-photon interaction in nanophotonic structures and for exploring polaritonic topological phases and their practical applications in polaritonic devices.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

The underappreciated lone pair in halide perovskites underpins their unusual properties

The presence of 6s 2 (5s 2 ) lone-pair electrons on the B-site Pb (Sn) in all-inorganic and hybrid halide ABX 3 perovskites distinguishes these materials from the familiar tetrahedral semiconductors traditionally employed in optoelectronics and is key to many of their appealing properties. These electrons are stereochemically active, albeit often in a hidden fashion, resulting in unusual and highly anharmonic lattice dynamics that are linked to many of the special optoelectronic properties displayed by this material class. This article describes the connections between this atypical electronic configuration and the electronic structure and lattice dynamics of these compounds. We illustrate how the lone pair leads to favorable bandwidths and band alignments, mobile holes, large ionic dielectric response, large positive thermal expansion, and even possibly defect-tolerant electronic transport. Taken together, the evidence suggests that other high-performing semiconductors may be found among compounds with lone-pair-bearing cations in high symmetry environments and a high degree of connectivity between atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2022

MgZnO High‐Voltage Transparent Thin‐Film Transistors Built on Glass

Recently, there has been increasing interest in building‐integrated photovoltaic (BIPV), which enables harvesting solar energy effectively. Large‐area glass components such as windows are widely used in modern constructions. In addition to the PV on opaque components such as walls and roofs, the transparent photovoltaic (TPV) directly built on glass is complementary to fully utilize the PV energy in BIPV. A high‐voltage transparent thin‐film transistor (HVTTFT) built on glass is an ideal option for distributed microinverters for TPV modules. A wide‐bandgap oxide‐based HVTTFT on glass for this purpose is reported. The HVTTFT on glass uses ZnO‐based materials with different functions for two roles: a semiconductor Mg 0.01 Zn 0.99 O (MZO) as TFT channel and Al‐doped ZnO (AZO) as transparent conductive oxide (TCO) electrodes. The centrosymmetric circular structure of the MZO HVTTFT with a high‐k‐stacking gate dielectric enables a blocking voltage as high as ≈1 kV and an on/off ratio of 10 6 . The device exhibits an average optical transmittance of 81% over the visible spectrum.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Synthesis, Crystal and Electronic Structure of La 2 SiP 4

La 2 SiP 4 (mP-28, Z=4, Wyckoff sequence e 7 , space group P2 1 /c (No. 14), a=10.8230(6) Å, b=7.5208(4) Å, c=7.9189(4) Å, and β=105.389(2)°) which crystallizes in the La 2 CuS 4 structure type is reported here. Instead of isolated CuS 3 triangles bridged by disulfide anions, in the crystal structure of La 2 SiP 4 , one-dimensional zig-zag chains composed of SiP 4 tetrahedra connected by P–P bonds are present. Lanthanum cations fill the voids between the chains and are coordinated by either 9P or 8P+Si atoms. La2SiP4 is a narrow bandgap semiconductor with calculated and measured optical bandgaps of 0.35 eV and 0.85(1) eV, respectively.

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

Short-Range Order Tunes Optical Properties in Long-Range Disordered ZnSnN2-ZnO Alloy

Local site ordering offers a new paradigm for property control in functional materials. However, systems that exhibit a propensity for local order and global disorder are often challenging to characterize, and demonstrations of ordering-induced property tuning are few and far between. Here, we demonstrate that short-range ordering tunes the optical absorption edge in the long-range disordered alloy system (ZnSnN2)1-x(ZnO)2x at x = 0.25. We use combinatorial cosputtering to synthesize a set of thin-film samples spanning this alloy space. X-ray diffraction demonstrates lattice contraction as a function of alloy composition, confirming that a mixed-anion and -cation alloy has been synthesized. Using N and O K-edge X-ray absorption near-edge structure in conjunction with simulations of cation-disordered supercell structures, we find that samples exhibit octet-rule-breaking motifs around both anions. Upon annealing at an alloy composition of x = 0.25, X-ray absorption analysis suggests that local motif structure shifts toward octet-rule-conserving while long-range disorder is maintained. Spectroscopic ellipsometry reveals that local ordering increases the absorption edge energy at constant composition. Additionally, alloy-induced optical absorption edge tuning is demonstrated. This work paves the way toward property tuning with short-range ordering in (ZnSnN2)1-x(ZnO)2x and beyond.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Doping Effects on Multivalence States, Electronic Structure, and Optical Band Gap in LaCrO 3 under Varied Atmospheres: An Integrated Experimental and Density Functional Theory Study

Doping effects on the valence state, electronic structure, and optical band and the effects on electrical conductivity were studied on the doped lanthanum chromite (LaCrO 3 ) system. The specific compositions studied were La 1–x Ca x CrO 3 (LCCx), La 1–x Sr x CrO 3 (LSCx), and La 0.8 Sr 0.2 Cr 1–x Mn x O 3 (LSCMx) (0.1 ≤ x ≤ 0.4). The powders were synthesized using a modified Pechini sol–gel method, and the ceramic samples were densified using a reactive sintering method resulting in densities >96% theoretical. X-ray photoelectron spectroscopy (XPS) was completed to characterize the defect states and cationic valence compensation as a result of divalent (Ca 2+ or Sr 2+ ) and trivalent (Mn 3+ ) substitutions. XPS was completed for samples tested in oxidizing and reducing atmospheres (up to 1500 °C), which provided insights into the oxidation state transitions induced by the Ca 2+ and Sr 2+ dopants. The work notably demonstrated, for the first time, the oxidation/reduction transitions of Cr 4+ to Cr 3+ in Sr 2+ /Mn 3+ co-doped samples under reducing atmospheres. Reflectance UV–vis spectrophotometry optical band gap measurements were also completed for the same materials; a decrease in the optical band gap (2.81–3.12 eV) was shown with increased substitution, suggesting electronic structure modifications in the LaCrO 3 perovskite. Density functional theory calculations validated experimental trends, predicting a diminishing band gap with a rising dopant concentration. The transition in Cr oxidation states was attributed to the presence of divalent/trivalent cations. These findings contribute some insights into methods to tune the LaCrO 3 electrical properties for various low- and high-temperature applications.

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

Synthesis of Interface-Driven Tunable Bandgap Metal Oxides

Mixed bandgap and bandgap tunability in semiconductors is critical in expanding their use. Composition altera-tions through single-crystal epitaxial growth and formation of multi-layer tandem structures are often employed to achieve mixed bandgaps albeit with limited tunability. In this study, self-assembled one-dimensional coordination polymers provide a fac-ile synthon and template for graphitic C-doped mesoporous oxides, gC-β-Ga 2 O 3 or gC-In 2 O 3 via controlled oxidative ligand ablation. These materials have mixed bandgaps, and colors, depending on amount of gC present. The carbon-oxide interface leads to induced gap states, hence, a stoichiometrically tunable band structure. Structurally, a multi-scale porous network percolating throughout the material is realized. The nature of heat-treatment and top-down process allows for facile tunabil-ity and formation of mixed bandgap metal oxides through controlled carbon deposition. As a proof of concept, gC-β-Ga 2 O 3 was utilized as a photocatalyst for CO 2 reduction, which demonstrated excellent selectivity and conversion rates into CH 4 and CO.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Exploring the Structural Origins of Optically Efficient One-Dimensional Lead Halide Perovskite Nanostructures

Metal halide perovskites have excellent optoelectronic properties. This study aims to determine how the optoelectronic properties of a model perovskite, cesium lead bromide (CsPbBr3), change with length and thickness in one dimension (1D). By examining the photophysics of CsPbBr3 quantum dots (QDs), nanowires (NWs), and nanorods (NRs), we observe the influence of confinement, exciton diffusion, and trapping on their optical properties. Our findings reveal that exciton diffusion to trap states limits the photoluminescence quantum yield (PLQY) of 1D CsPbBr3 in the weakly confined regime (8-14 nm) and explains their long-lived exciton dynamics, while enhanced radiative rates contribute to achieving near-unity PLQY in the strongly confined regime (<7 nm). Consequently, blue-emitting, 2.4 nm-thick CsPbBr3 NRs were 3.6X more emissive than the conventional CsPbBr3 QDs. This study underscores how structural optimization can improve the optoelectronic performance of CsPbBr3 and provides insight into the complex interplay of radiative and nonradiative processes in 1D ionic semiconductors.

Oddo, Alexander M↗
DOE OSTI2020

Gapped metals as thermoelectric materials revealed by high-throughput screening

The typical strategy to design high performance thermoelectric materials is to dope a semiconducting material until optimal properties are obtained. However, some known thermoelectric materials such as La 3 Te 4 , Mo 3 Sb 7 , Yb 14 MnSb 11 , and NbCoSb are actually gapped metals,i.e., their band structure displays a gap slightly above or below the band crossed by the Fermi level. This key feature makes these metals comparable to degenerate semiconductors and thus suitable for thermoelectric applications. In this work, we perform a computational high-throughput search for such gapped metals exhibiting attractive thermoelectric properties. Several thousands of metals are found to present this key feature, and about one thousand of them show decent thermoelectric properties as evaluated by a computed zT. We present the different chemistry of gapped metals we discovered such as clathrates, Chevrel phases, or transition metal dichalcogenides and discuss their previous studies as thermoelectric and their potential as new thermoelectric materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Amorphous zinc–molybdenum–sulfide chalcogel as a long-cycle, high-capacity electrode for lithium-ion batteries

The inherent limitations of intercalation-based electrodes in lithium-ion batteries have prompted the search for alternative materials with higher specific capacities and robust electrochemical stability. Sulfur-based electrodes, despite their high theoretical capacities (1672 mAh g −1 ), typically suffer from poor cycling performance. In this work, zinc molybdenum polysulfide (Zn x Mo 3 S 13 , 0.5 ≤ x), an amorphous semiconductor chalcogel, exhibits high specific capacity and excellent cycling stability. Synchrotron X-ray pair distribution function and extended X-ray absorption fine structure analyses reveal a short-range atomic structure comprising Mo–Mo, M–S (M = Mo, Zn), and S–S bonding motifs. The coordination environment of Mo and S closely resembles that of Mo 3 S 13 clusters, interconnected via S–S bridges and Zn 2+ cations. The Li/Zn x Mo 3 S 13 cell delivers an initial discharge capacity of 844 mAh g −1 at C/3, and retains 386.2 mAh g −1 after 1000 cycles with an average coulombic efficiency of 99.99%. The distribution of relaxation times analysis confirms the formation of a stable solid electrolyte interphase, which underpins the cell's long-term stability. In conclusion, this outstanding performance is attributed to the synergistic effects of the chalcogel's unique amorphous framework, semiconductive character, Zn-mediated polysulfide anchoring, and structural resilience, positioning Zn x Mo 3 S 13 chalcogel among the most durable pure metal sulfide cathodes reported for next-generation LIBs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Baking and plasma pretreatment of sapphire surfaces as a way to facilitate the epitaxial plasma-enhanced atomic layer deposition of GaN thin films

The growth of high-quality epitaxial gallium nitride (GaN) thin films is achieved by using a baking and plasma pretreatment of the substrate prior to the GaN plasma-enhanced atomic layer deposition (PE-ALD). In this work, it is found that such pretreatment makes the GaN films grow coherently on sapphire substrates, following a layer-by-layer growth mechanism. The deposited GaN film shows high crystalline quality, a sharp GaN/sapphire interface, and a flat surface. The possibility of growing high-quality GaN epilayers in this way broadens the range of applications for PE-ALD in GaN-based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Prospects for n -type doping of (Al x Ga 1– x ) 2 O 3 alloys

Here we systematically explore the properties of group-IV (C, Si, Ge, and Sn) and transition metal (Hf, Zr, and Ta) dopants substituting on the cation site in (Al x Ga 1– x ) 2 O 3 (AlGO) alloys using first-principles calculations with a hybrid functional. In Ga 2 O 3 , each of these dopants acts as a shallow donor. In Al 2 O 3 , they are deep defects characterized by the formation of either DX centers or positive- U (+/0) levels. Combining our calculations of dopant charge-state transition levels with information of the AlGO alloy band structure, we estimate the critical Al composition at which each dopant transitions from being a shallow to a deep donor. We identify Si to be the most efficient dopant to achieve n -type conductivity in high Al-content AlGO alloys, acting as a shallow donor over the entire predicted stability range for AlGO solid solution alloys.

36 MATERIALS SCIENCE↗