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At least 1,099 records · Page 61
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Light-Driven Iodine Loss and Photoluminescence Homogenization in Mixed-Halide Perovskite Semiconductors

Carrier-induced instabilities in lead halide perovskites are often investigated as either transient phenomena, e.g., photoinduced halide segregation or permanent performance changes, e.g., photodegradation, while the mechanistic links between them remain unclear. Here, we aim to connect these observations by studying a model mixed-halide system, MAPb(Br x I 1–x ) 3 . By combining grazing-incidence X-ray diffraction, photothermal deflection spectroscopy, and photoluminescence measurements with hyperspectral microscopy, we investigate the role of mobile halide defects and local chemistry on reversible and long-term instabilities in these materials. Our results show that mixed-halide perovskites are uniquely susceptible to photoinduced changes with illumination driving initial iodide redistribution (i.e., halide segregation), eventual selective iodine expulsion, and subsequent changes in photoinduced halide segregation behavior. By quantifying structural and compositional changes, we estimate an approximately 3–5% iodine loss in our mixed-halide samples after only 24 h of illumination. Further, using microscale measurements, we identify pre-existing iodide-rich domains as key contributors to both the observed transient photostability and permanent iodine loss in MAPbBrI 2 , and see evidence that extended light soaking results in iodide redistribution that improves optoelectronic homogeneity. Overall, our results emphasize the importance of carrier-induced halide oxidation in creating a dynamic defect landscape in mixed-halide perovskites and provide a framework for interpreting apparent light-driven changes in optoelectronic behavior through the lens of permanent compositional changes.

electrical conductivity↗
DOE OSTI2014

Fabrication of polycrystalline Cu{sub 2}ZnSnSe{sub 4} layers with strongly preferential grain orientation via selenization of Sn/Cu/ZnSe(001)/GaAs(001) structures

We report on a simple approach for the fabrication of polycrystalline Cu{sub 2}ZnSnSe{sub 4} films on GaAs with strongly preferential grain orientation. Such layers are important for both fundamental spectroscopic investigations as well as studies of the impact of grain boundaries on the performance of solar cells. Cu{sub 2}ZnSnSe{sub 4} (CZTSe) layers were fabricated by growing a Sn/Cu/ZnSe(001) stack on a GaAs(001) substrate in a molecular-beam epitaxy system followed by selenization in a tube furnace. Raman spectroscopy as well as X-ray diffraction measurements combined with a microstructural investigation indicate the presence of the kesterite phase. Further, X-ray analysis such as rocking curves and ϕ-scans proves a preferential grain orientation of the obtained CZTSe films in all three dimensions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Observation of atomic ordering of triple-period-A and -B type in GaAsBi

We report the observation of atomic ordering of triple-period (TP)-A and -B type in low temperature (LT) grown GaAsBi alloy using transmission electron microscopy (TEM). In addition to previous reports, where only TP-A ordering was identified in III-V alloys, here, we confirm by electron diffraction, high-resolution (HR) TEM, and HR Z-contrast scanning TEM that two ordering variants coexists for LT-GaAsBi. We find that the TP-A ordering variant dominates over the TP-B variant. TP-A domains extend over 50–100 nm (projected lateral width) and are of higher perfection compared to TP-B domains. HR Z-contrast scanning TEM on different domains reveals a variation in the Bi occupancy in the (111) planes with triple period sequence. Since the formation of ordered phases has been directly linked to the occurrence of specific surface reconstructions, our results suggest a correlation between the TP-A and B type domains and the multiple stability of n × 3 and 3 × n reconstructions on the (001) surface of GaAsBi under low temperature growth.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

X-ray diffraction at the National Ignition Facility

We report details of an experimental platform implemented at the National Ignition Facility to obtain in situ powder diffraction data from solids dynamically compressed to extreme pressures. Thin samples are sandwiched between tamper layers and ramp compressed using a gradual increase in the drive-laser irradiance. Pressure history in the sample is determined using high-precision velocimetry measurements. Up to two independently timed pulses of x rays are produced at or near the time of peak pressure by laser illumination of thin metal foils. The quasi-monochromatic x-ray pulses have a mean wavelength selectable between 0.6 Å and 1.9 Å depending on the foil material. The diffracted signal is recorded on image plates with a typical 2 θ x-ray scattering angle uncertainty of about 0.2° and resolution of about 1°. Analytic expressions are reported for systematic corrections to 2 θ due to finite pinhole size and sample offset. A new variant of a nonlinear background subtraction algorithm is described, which has been used to observe diffraction lines at signal-to-background ratios as low as a few percent. Variations in system response over the detector area are compensated in order to obtain accurate line intensities; this system response calculation includes a new analytic approximation for image-plate sensitivity as a function of photon energy and incident angle. This experimental platform has been used up to 2 TPa (20 Mbar) to determine the crystal structure, measure the density, and evaluate the strain-induced texturing of a variety of compressed samples spanning periods 2–7 on the periodic table.

47 OTHER INSTRUMENTATION↗
DOE OSTI2025

Filling data analysis gaps in time-resolved crystallography by machine learning

There is a growing understanding of the structural dynamics of biological molecules fueled by x-ray crystallography experiments. Time-resolved serial femtosecond crystallography (TR-SFX) with x-ray Free Electron Lasers allows the measurement of ultrafast structural changes in proteins. Nevertheless, this technique comes with some limitations. One major challenge is the quality of data from TR-SFX measurements, which often faces issues like data sparsity, partial recording of Bragg reflections, timing errors, and pixel noise. To overcome these difficulties, conventionally, large volumes of data are collected and grouped into a few temporal bins. The data in each bin are then averaged and paired with the mean of their corresponding jittered timestamps. This procedure provides one structure per bin, resulting in a limited number of averaged structures for the entire time interval spanned by the experiment. Therefore, the information on ultrafast structural dynamics at high temporal resolution is lost. This has initiated research for advanced methods of analyzing experimental TR-SFX data beyond the standard binning and averaging method. To address this problem, we use a machine learning algorithm called Nonlinear Laplacian Spectral Analysis (NLSA), which has emerged as a promising technique for studying the dynamics of complex systems. In this work, we demonstrate the power of this algorithm using synthetic x-ray diffraction snapshots from a protein with significant data incompleteness, timing uncertainties, and noise. Our study confirms that NLSA is a suitable approach that effectively mitigates the effects of these artifacts in TR-SFX data and recovers accurate structural dynamics information hidden in such data.

Trujillo, Justin (ORCID:0000000285505360)↗
DOE OSTI2014

Astrometric performance of the Gemini multiconjugate adaptive optics system in crowded fields

The Gemini multiconjugate adaptive optics system (GeMS) is a facility instrument for the Gemini South telescope. It delivers uniform, near-diffraction-limited image quality at near-infrared wavelengths over a 2 arcmin field of view. Together with the Gemini South Adaptive Optics Imager (GSAOI), a near-infrared wide-field camera, GeMS/GSAOI's combination of high spatial resolution and a large field of view will make it a premier facility for precision astrometry. Potential astrometric science cases cover a broad range of topics including exoplanets, star formation, stellar evolution, star clusters, nearby galaxies, black holes and neutron stars, and the Galactic Centre. In this paper, we assess the astrometric performance and limitations of GeMS/GSAOI. In particular, we analyse deep, mono-epoch images, multi-epoch data and distortion calibration. In this work, we find that for single-epoch, undithered data, an astrometric error below 0.2 mas can be achieved for exposure times exceeding 1 min, provided enough stars are available to remove high-order distortions. We show however that such performance is not reproducible for multi-epoch observations, and an additional systematic error of ~0.4 mas is evidenced. This systematic multi-epoch error is the dominant error term in the GeMS/GSAOI astrometric error budget, and it is thought to be due to time-variable distortion induced by gravity flexure.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Efficient First-Principles Methodology for the Calculation of the All-Phonon Inelastic Scattering in Solids

Inelastic scattering experiments are key methods for mapping the full dispersion of fundamental excitations of solids in the ground as well as nonequilibrium states. A quantitative analysis of inelastic scattering in terms of phonon excitations requires identifying the role of multiphonon processes. Here, we develop an efficient first-principles methodology for calculating the all-phonon quantum mechanical structure factor of solids. We demonstrate our method by obtaining excellent agreement between measurements and calculations of the diffuse scattering patterns of black phosphorus, showing that multiphonon processes play a substantial role. The present approach constitutes a step towards the interpretation of static and time-resolved electron, x-ray, and neutron inelastic scattering data.

36 MATERIALS SCIENCE↗
DOE OSTI2020

X-ray atomic mapping of quantum dots

We report a model-independent atomic-mapping technique for quantum dots (QDs) by combining Bragg reflection x-ray standing wave (XSW) and grazing incidence diffraction (GID) measurements. In this study, we choose GaAs capped InGaAs QDs/GaAs(001) as a model system to show the locations and arrangements of indium atoms within the QDs along various [hkl] directions. This technique directly reveals the actual amount of positional anisotropy and ordering fraction of indium atoms within the QDs by probing the (1¯11), (111), (311), (1¯31), (113), and (1¯13) crystallographic planes. We find that indium atoms are outwardly shifted along the [001] direction by small fractions of the lattice constant, 0.04a GaAs and 0.06a GaAs from Ga sites for 50- and 150-Å GaAs capped InGaAs QDs, respectively. We observe that an improved coherency factor of the indium atoms within the QDs by 45–60% along the [001] and [011] directions reduces the photoluminescence linewidth by 22%, thus making the QDs efficient for QD-laser and optoelectronic device applications. Here, we also find that the position and ordering of In atoms along the (113) and (1¯13) planes are most sensitive to the thickness of the GaAs cap layer. Our XSW-based results are supported by numerical calculations using a QD-macroscopic structural model based on our GID study. We thus show that this atomic-mapping technique will be useful for studying various quantum structures and tuning their properties.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Novel Polyoxovanadate K{sub 2}ZnV{sub 5}O{sub 14}: Crystal Structure and Peculiarities of Crystal Chemistry

A novel structure type has been established as a result of studying a non-merohedral microtwin of polyoxovanadate (K{sub 2}ZnV{sub 5}O{sub 14}) by X-ray diffractometry (R = 0.0595). The new compound, synthesized under hydrothermal conditions in the ZnCl{sub 2}–K{sub 2}CO{sub 3}–V{sub 2}O{sub 5}–H{sub 2}O system, is characterized as follows: a = 8.066(5) Å, b = 8.117(5) Å, c = 9.236(5) Å, β = 105.287(5)°, sp. P2{sub 1}/m, Z = 2, ρ{sub calcd} = 3.54 g/cm{sup 3}. Edge-shared five-core “clusters” consisting of vanadium octahedra, between which ZnO{sub 4} tetrahedra (sharing vertices with octahedra) are located, form two-dimensional two-layer anion packets of the (ZnV{sub 5}O{sub 14}){sup 2–} composition, alternating along the c axis with layers of potassium atoms. Structural peculiarities determine the morphology and color of new-phase crystals.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Two-dimensional antiferromagnetic correlations in an La{sub 1.4}Sr{sub 1.6}(Mn{sub 0.9}Co{sub 0.1}){sub 2}O{sub 7} single crystal

The temperature and field dependences of the magnetization, the electrical resistivity, and the magnetostriction of bilayer lanthanum manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} single crystals and cobalt-doped La{sub 1.4}Sr{sub 1.6}(Mn{sub 0.9}Cu{sub 0.1}){sub 2}O{sub 7} are measured. The magnetostriction of the cobalt-doped compound increases as compared to the initial La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} compound, and the magnetization and the magnetoresistance of the former compound change substantially. Powder and single-crystal neutron diffraction patterns are used to detect ferromagnetic ordering in La{sub 1.4}Sr{sub 1.6}(Mn{sub 0.9}Co{sub 0.1}){sub 2}O{sub 7} at a temperature below T{sub C} ~ 45(2) K, and this ordering coexists with antiferromagnetic correlations, which develop at temperatures below T{sub C} ~ 80(5) K.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Resonant Optical Reflection from AsSb–AlGaAs Metamaterials and Structures

The optical reflection in periodic structures based on a semiconductor AlGaAs matrix containing two-dimensional arrays of plasmonic AsSb nanoinclusions was studied. The number of periods was 12 or 24. The spatial period was near 110 nm in both cases. In the experimental optical reflection spectra at normal incidence we observed resonant Bragg diffraction with the main peaks at wavelengths of 757 or 775 nm, depending on the spatial period of the nanostructure. The magnitudes of the resonance peaks reached 19 and 31% for the systems of 12 and 24 AsSb–AlGaAs layers, while the volume fraction of the nanoinclusions was much less than 1%. In the case of light incident at inclined angles, the Bragg-diffraction pattern shifted according to Wulff-Bragg’s law. Numerical calculations of the optical reflection spectra were performed using the transfer-matrix method by taking into account the spatial geometry of the structures and the resonance characteristics of the plasmonic AsSb layers.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Thermal Expansion and Luminescent Properties of Triorthogermanates CaLa{sub 2-x}Eu{sub x}Ge{sub 3}O{sub 10} (x = 0.0–0.6)

Solid solutions CaLa{sub 2-x}Eu{sub x}Ge{sub 3}O{sub 10} (x = 0.0–0.6, Δx = 0.1) have been synthesized for the first time. The compounds are isostructural to CaLa{sub 2}Ge{sub 3}O{sub 10}, they crystallize in the monoclinic system, space group P2{sub 1}/c, Z = 4. The low-temperature X-ray diffraction studies have revealed the strain anisotropy of germanate CaLa{sub 2}Ge{sub 3}O{sub 10} crystal lattice in the temperature range 80–298 K, and the linear thermal expansion coefficients have been calculated. The optical properties of the activated phases have been studied, and the influence of the dopant concentration and the excitation wavelength on the luminescence characteristics of the synthesized compounds has been established.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Optical Manipulation with Plasmonic Beam Shaping Antenna Structures

Near-field optical trapping of objects using plasmonic antenna structures has recently attracted great attention. However, metal nanostructures also provide a compact platform for general wavefront engineering of intermediate and far-field beams. Here, we analyze optical forces generated by plasmonic beam shaping antenna structures and show that they can be used for general optical manipulation such as guiding of a dielectric particle along a linear or curved trajectory. This removes the need for bulky diffractive optical components and facilitates the integration of optical force manipulation into a highly functional, compact system.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

To what extent can space be compressed? Bandwidth limits of spaceplates

Spaceplates are novel flat-optic devices that implement the optical response of a free-space volume over a smaller length, effectively “compressing space” for light propagation. Together with flat lenses such as metalenses or diffractive lenses, spaceplates have the potential to enable the miniaturization of any free-space optical system. While the fundamental and practical bounds on the performance metrics of flat lenses have been well studied in recent years, a similar understanding of the ultimate limits of spaceplates is lacking, especially regarding the issue of bandwidth, which remains as a crucial roadblock for the adoption of this platform. In this work, we derive fundamental bounds on the bandwidth of spaceplates as a function of their numerical aperture and compression ratio (ratio by which the free-space pathway is compressed). The general form of these bounds is universal and can be applied and specialized for different broad classes of space-compression devices, regardless of their particular implementation. Our findings also offer relevant insights into the physical mechanism at the origin of generic space-compression effects and may guide the design of higher performance spaceplates, opening new opportunities for ultra-compact, monolithic, planar optical systems for a variety of applications.

Shastri, Kunal (ORCID:0000000169282753)↗
DOE OSTI2019

Density-driven ionization potential depression and its effects on material properties

Changes in ionization potential at high density drive changes in material transport properties and often in crystal structure and equation of state. Current models used to describe and predict these effects have not been tested in regimes much beyond ~200 GPa, because of limitations in experimental techniques. Now, however, high-energy laser facilities with flexible pulse-shaping capabilities have made it possible to reach pressures in the 1000 GPa range, while maintaining temperatures below 1 eV. We have used these new methods to ramp-compress cobalt and magnesium and have measured shifts in the k-shell emission lines to infer changes in ionization stage, and x-ray diffraction to measure changes in crystal structure. In the case of cobalt, commonly-used detailed models for effective ionization based on isolated-atom electronic structure are shown to give the wrong answer, while self-consistent density-functional-theory-based models reproduce the data well. In the case of magnesium, zero-kelvin density functional theory predictions of density-driven electron localization in lattice interstitials (so-called electride phases) at high pressure are found to be too simple to describe material evolution along a laser ramp-compression pathway. Instead, by a combination of experiment, first-principles theory and evolutionary structure-searching algorithms, we have discovered several unexpected lower-symmetry metastable structures. First principles results find that the density of localized interstitial charge in the high-pressure magnesium phases is too low to be measurable with x-ray diffraction, and the body of theory work which has predicted high pressure electride phases for many systems, based on analysis of electron localization function, needs to be re-examined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

High Ms Fe16N2 thin film with Ag under layer on GaAs substrate

(001) textured Fe16N2 thin film with Ag under layer is successfully grown on GaAs substrate using a facing target sputtering (FTS) system. After post annealing, chemically ordered Fe16N2 phase is formed and detected by X-ray diffraction (XRD). High saturation magnetization (Ms) is measured by a vibrating sample magnetometer (VSM). In comparison with Fe16N2 with Ag under layer on MgO substrate and Fe16N2 with Fe under layer on GaAs substrate, the current layer structure shows a higher Ms value, with a magnetically softer feature in contrast to the above cases. In addition, X-ray photoelectron spectroscopy (XPS) is performed to characterize the binding energy of N atoms. To verify the role of strain that the FeN layer experiences in the above three structures, Grazing Incidence X-ray Diffraction (GIXRD) is conducted to reveal a large in-plane lattice constant due to the in-plane biaxial tensile strain. INTRODUCTION

Allard, Jr, Lawrence Frederick↗
DOE OSTI2021

Visualizing Intrinsic 3D-Strain Distribution in Gold Coated ZnO Microstructures by Bragg Coherent X-Ray Diffraction Imaging and Transmission Electron Microscopy with Respect to Piezotronic Applications

Novel devices ranging from bio magnetic field sensors to energy harvesting nano machines utilize the piezotronic effect. For optimal function, understanding the interaction of electrical and strain phenomena within the semiconductor crystal is necessary. Here, studies of a model piezotronic system are presented, consisting of a ZnO microrod coated by a thin layer of gold, which forms a Schottky contact with the piezoelectric ZnO material. Coherent X-ray diffraction imaging (CXDI) and transmission electron microscopy (TEM) are used to visualize the structure and strain distribution, showing that the ZnO microrod exhibits strains of multiple origins in the bulk and at the interface. Strain values of -6 × 10 -4 have been measured by CXDI at the ZnO/Au interface. The origin is shown to be a combination of an interface strain, possibly caused by the Schottky contact formation, and distinct, localized electrical fields inside the crystal which are assigned to electron depletion and screening in a bent ZnO/Au piezotronic rod. These findings will contribute to sensor development and to a better understanding of piezotronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Phase Diagram and Dielectric Properties of (1–x)Ba(Ti{sub 1–y}Zr{sub y})O{sub 3} · xPbTiO{sub 3} Ceramics

Ceramic (1–x)Ba(Ti1–yZry)O{sub 3} · xPbTiO{sub 3} (0 ≤ x, у ≤ 1) samples have been characterized by X-ray diffraction and dielectric measurements. The results have been used to map out the phase diagram of the system, which demonstrates the variation in the phase composition of the samples. It has been shown that, in the composition regions adjacent to the BaTiO{sub 3}–PbTiO{sub 3} side and BaZrO{sub 3} corner of the composition triangle, the samples consist of perovskite solid solutions that have tetragonal and cubic structures, respectively, at room temperature. In the intermediate composition region, the samples consist of different perovskite solid solutions similar in composition and structure. We have obtained composition dependences of the unit-cell symmetry and parameters for the solid solutions, their ferroelectric Curie temperature T{sub C}, characteristic dielectric relaxation temperatures, dielectric permittivity ε, and dielectric loss tangent tanδ (at temperatures from 100 to 800 K and frequencies from 25 to 106 Hz) and analyzed the evolution of their dielectric properties with increasing BaZrO{sub 3} content: from ferroelectric to properties of ferroelectric relaxors, reentrant relaxors, and dielectric relaxors of the dipole glass type.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗