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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

The Zintl phases A In 2 As 2 (A = Ca, Sr, Ba): new topological insulators and thermoelectric material candidates

Recently, there has been a lot of interest in topological insulators (TIs), being electronic materials, which are insulating in their bulk but with the gapless exotic metallic state on their surface. The surface states observed in such materials behave as a perfect conductor thereby making them more suited for several cutting-edge technological applications such as spintronic devices. Here, we report the synthesis and structural characterization of the Zintl phases AIn 2 As 2 (A = Ca, Sr, Ba), which could become a new class of TIs. Crystal structure elucidation by single-crystal X-ray diffraction reveals that CaIn 2 As 2 and SrIn 2 As 2 are isostructural and crystallize in the EuIn 2 P 2 structure type (space group P6 3 /mmc, no. 194, Z = 2) with unit cell parameters a = 4.1482(6) Å, c = 17.726(4) Å; and a = 4.2222(6) Å, c = 18.110(3) Å, respectively. Their hexagonal structure is made up of alternating [In 2 As 2 ] 2– layers separated by slabs of A 2+ cations. BaIn 2 As 2 on the other hand crystallizes in the monoclinic EuGa 2 P 2 structure type (space group P2/m, no. 10, Z = 4) with unit cell parameters a = 10.2746(11) Å, b = 4.3005(5) Å, c = 13.3317(14) Å and β = 95.569(2)°. This structure is also layered, and it is made up of different type of polyanionic [In 2 As 2 ] 2– units and Ba 2+ cations. The valence electron count for all three compounds adheres to the Zintl-Klemm formalism, and all elements achieve closed-shell electronic configurations. Bulk electronic structure calculations indicate the opening of a bandgap E g ~ 0.03 eV (CaIn 2 As 2 and Sr 2 In 2 As 2 ), and E g ~0.21 eV (BaIn 2 As 2 ) in the absence of strain and spin–orbit coupling (SOC). Furthermore, these results argue in favor of the realization of a nontrivial topological insulator state under the influence of tensile strain and SOC. Preliminary transport properties on BaIn 2 As 2 are suggestive of a degenerate p-type semiconductor—a behavior which is sought after in thermoelectric (TE) materials. Since both TIs and excellent TE materials are known to favor the same material properties such as narrow bandgap, heavy elements, and strong SOC, these three Zintl phases are also projected as candidates TE materials.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Electrochemical quantification of phosphonic acid passivated surface sites of NiO x for perovskite solar cells

Nickel oxide (NiO x ) is among the few p-type metal oxide semiconductors considered a strong candidate for hole transport layers in halide perovskite solar cells (PSCs). However, its reactivity with perovskite ions poses significant challenges to achieving high efficiency and long-term stability. Here, we investigate passivation of detrimental reactive surface sites on NiO x by carbazole phosphonic acids. We leverage electrochemical cyclic voltammetry (CV) of NiO x electrodes as a proxy measure for the redox activity that afflicts PSCs. From the CVs, we derive a metric, N (units cm −2 ), that relates to the number of redox active sites on NiO x surfaces. We observe a statistically significant negative correlation between PSC efficiency and N-value that indicates PSCs are more efficient on NiO x with lower electrochemical reactivity. The new mechanistic insight into NiO x passivation demonstrates it requires a reducing agent and Brønsted acid combination, providing a broadly applicable approach for evaluating and enhancing the stability and performance of NiO x -based interfaces in photovoltaics.

14 SOLAR ENERGY↗
DOE OSTI2014

Electric field effect in ultrathin black phosphorus

Black phosphorus exhibits a layered structure similar to graphene, allowing mechanical exfoliation of ultrathin single crystals. Here, we demonstrate few-layer black phosphorus field effect devices on Si/SiO{sub 2} and measure charge carrier mobility in a four-probe configuration as well as drain current modulation in a two-point configuration. We find room-temperature mobilities of up to 300 cm{sup 2}/Vs and drain current modulation of over 10{sup 3}. At low temperatures, the on-off ratio exceeds 10{sup 5}, and the device exhibits both electron and hole conduction. Using atomic force microscopy, we observe significant surface roughening of thin black phosphorus crystals over the course of 1 h after exfoliation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Synthesis of Bi{sub 2}S{sub 3} with different sulfur content by conventional high temperature solid state solvothermal route

Bismuth sulfide (Bi{sub 2}S{sub 3}) is a binary chalcogenide compound material belonging to V-VI group of semiconductors. Because of its direct band gap of 1.3 eV and high figure of merit (ZT) value, it is widely used as a thermo electronic-cooling material based on the Peltier effect. The electrical and optical property of Bi{sub 2}S{sub 3} material is strongly dependent on stoichiometric composition, defect chemistry and structure. In this study, we have synthesized Bi{sub 2}S{sub x} (x = 3.15, 3.30, 3.45) compound material with different sulfur content by conventional high temperature solid state solvothermal reaction of bismuth and sulfur. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDXS) analysis of synthesized compound materials were carried out to observe crystallinity, surface morphology and composition of elements in the compound. The optical analysis revealed that energy band gap decreases with increase of sulfur content.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Holographic and time-resolving ability of pulse-pair two-dimensional velocity interferometry

Previous velocity interferometers used at research laboratories for shock physics experiments measured target motion at a point or many points on a line on the target. Recently, a two-dimensional (2d) version (2d-velocity interferometer system for any reflector) has been demonstrated using a pair of ultrashort (3 ps) pulses for illumination, separated by 268 ps. We have discovered new abilities for this instrument, by treating the complex output image as a hologram. For data taken in an out of focus configuration, we can Fourier process to bring narrow features such as cracks into sharp focus, which are otherwise completely blurred. This solves a practical problem when using high numerical aperture optics having narrow depth of field to observe moving surface features such as cracks. Furthermore, theory predicts that the target appearance (position and reflectivity) at two separate moments in time are recorded by the main and conjugate images of the same hologram, and are partially separable during analysis for narrow features. Hence, for the cracks we bring into refocus, we can make a two-frame movie with a subnanosecond frame period. Longer and shorter frame periods are possible with different interferometer delays. Since the megapixel optical detectors we use have superior spatial resolution over electronic beam based framing cameras, this technology could be of great use in studying microscopic three-dimensional-behavior of targets at ultrafast times scales. Demonstrations on shocked silicon are shown.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Predicted roles of defects on band offsets and energetics at CIGS (Cu(In,Ga)Se 2 /CdS) solar cell interfaces and implications for improving performance

The laboratory performance of CIGS (Cu(In,Ga)Se 2 ) based solar cells (20.8% efficiency) makes them promising candidate photovoltaic devices. However, there remains little understanding of how defects at the CIGS/CdS interface affect the band offsets and interfacial energies, and hence the performance of manufactured devices. To determine these relationships, we use density functional theory with the B3PW91 hybrid functional that we validate to provide very accurate descriptions of the band gaps and band offsets. This confirms the weak dependence of band offsets on surface orientation observed experimentally. We predict that the conduction band offset (CBO) of perfect CuInSe2/CdS interface is large, 0.79 eV, which would dramatically degrade performance. Moreover we show that band gap widening induced by Ga adjusts only the valence band offset, and we find that Cd impurities do not significantly affect the CBO. Thus we show that Cu vacancies at the interface play the key role in enabling the tunability of CBO. We predict that Na further improves the CBO through electrostatically elevating the valence levels to decrease the CBO, explaining the observed essential role of Na for high performance. Moreover we find that K leads to a dramatic decrease in the CBO to 0.05 eV, much better than Na. We suggest that the efficiency of CIGS devices might be improved substantially by tuning the ratio of Na to K, with the improved phase stability of Na balancing phase instability from K. All these defects reduce interfacial stability slightly, but not significantly.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Mobility of indium on the ZnO(0001) surface

The mobility of indium on the Zn-polar (0001) surface of single crystal ZnO wafers was investigated using real-time x-ray photoelectron spectroscopy. A sudden transition in the wettability of the ZnO(0001) surface was observed at ∼520 °C, with indium migrating from the (0001{sup ¯}) underside of the wafer, around the non-polar (11{sup ¯}00) and (112{sup ¯}0) sidewalls, to form a uniform self-organized (∼20 Å) adlayer. The In adlayer was oxidized, in agreement with the first principles calculations of Northrup and Neugebauer that In{sub 2}O{sub 3} precipitation can only be avoided under a combination of In-rich and Zn-rich conditions. These findings suggest that unintentional In adlayers may form during the epitaxial growth of ZnO on indium-bonded substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Yield strength of Cu and a CuPb alloy (1% Pb)

With PBX9501 we explosively loaded fully annealed OFHC-Cu and an OFHC-CuPb (extruded with 1% Pb that ag- gregates at the Cu grain boundaries) to study the effects of the 1% Pb on the elastic-plastic yield Y of Cu. The yield-stress Y was studied through observation of surface velocimetry and total ejected mass ρ A from periodic surface perturbations machined onto the sample surfaces. The perturbation’s wavelengths were λ $\approx$ 65 µm, and their amplitudes h were varied to determine the wavenumber (2π/λ) amplitude product kh at which ejecta production for the Cu and CuPb begins, which relates to Y. The Y of the two materials is apparently different.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Reduced electronic correlation effects in half substituted Ba(Fe 1-x Co x ) 2 As 2

In this work, we report a comprehensive study of the tridimensional nature and orbital character of the low-energy electronic structure in 50% Cobalt doped Ba(Fe 1-x Co x ) 2 As 2 ( d 6.5 ), by using polarization- and photon energy-dependent angle-resolved photoemission spectroscopy. An extra electron-like Fermi surface is observed around the Brillouin zone boundary compared with isoelectronic K y Fe 2-x Se 2 ( d 6.5 ). The bands near the Fermi level (E F ) are mainly derived from Fe/Co 3 d t 2g orbitals, revealing visible dispersions along the k z direction. In combination with the local density approximation and the dynamical mean-field theory calculations, we find that the As 4 p bands are non-renormalized and the whole 3d band needs to be renormalized by a “single” factor of ~1.6, indicating moderate electronic correlation effects. The “single” factor description of the correlation strength among the different 3 d orbitals is also in sharp contrast to orbital-dependent correlation effects in BaFe 2 As 2 . Our findings indicate a remarkable reduction of correlation effects with little difference among 3 d orbitals in BaFeCoAs 2 , due to the increased filling of the electronic 3 d shell in the presence of significant Hund's coupling. Finally, the results support that the electronic correlation effects and multiple orbital physics play an important role in the superconductivity of the 122 system and in other ferropnictides.

36 MATERIALS SCIENCE↗
DOE OSTI2019

In situ X-ray imaging of heterogeneity in dynamic compaction of granular media

Dynamic compaction of granular materials is a complex process wherein the material response at the continuum scale arises from fracture, comminution, and fragment ow at the grain scale. Granular materials have heterogeneity due to variation in grain properties, as well as variation in local packing density and structure. These heterogeneities may affect the dynamic compaction response, producing a structured and time-varying compaction front. Methodology used to characterize the shock behavior in solid materials may be inappropriate for granular materials, because of this unsteady behavior and interactions between the granular material and measurement surfaces. To observe the compaction front heterogeneity, in-situ x-ray imaging of granular compaction is conducted at a scale between the grain- and continuum scales. To allow for sufficient x-ray transmission, a thin sample geometry is used, for which boundary affects may result in a significantly different load path than would occur in a 1-D compaction configuration. Numerical simulations of the experimental geometry support analysis of the results so that the feature of the compaction front can be distinguished from artifacts of the experimental configuration. The results suggest compaction front heterogeneity may have structure with a length scale of tens of particle diameters, and demonstrate that finite grain size can inhibit the formation shear-induced features that would arise in a homogenized representation of the same material.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The nature and source of solar magnetic phenomena

The heliosphere appears to be powered by coaxial helicity injection from a negative helicity injector in the northern hemisphere and a positive one in the southern hemisphere. The injector magnetic flux for both is the measured solar polar magnetic flux, and the injector voltage is generated by a simple dynamo effect due to the differential rotation of the solar surface with the polar magnetic flux. The dynamo current is estimated from the solar motion that it causes. This current also appears to sustain a thin, shallow global magnetic structure over most of the solar surface that has the form of a 1D minimum energy state. The current channel appears to be destroyed and reformed every 11 years. The currents and magnetic fields reverse in this solar cycle. A brief discussion of surface phenomena observed during this cycle is given. Plasma self-organization is briefly discussed and used in this analysis of solar data. The magnetic phenomena discussed include torsional oscillations, the heat source for the chromosphere and the corona, filaments, meridional flow, the solar cycle, sunspots, CMEs, and flares.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Understanding attenuated solvent reorganization energies near electrode interfaces

In this work, we examine the role of image charge effects on the electrostatic potential fluctuations experienced by ionic species in the vicinity of an electrode surface. We combine simulation and theory to quantify these fluctuations and how they vary with distance from the electrode surface. We observe that the potential distribution narrows significantly for species within a few electrolyte screening lengths of the electrode. We attribute this narrowing to the effects of image charge fluctuations originating from the polarization response of the electrode. We show that the physical consequences of these image charge effects can be captured in the context of a simple analytical field theory with anti-symmetric boundary conditions. We contextualize these results by discussing their implications for rates of Marcus-like outer-sphere interfacial electron transfer.

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

Understanding aerosol properties in convective outflows during TRACER

Convective outflow boundaries commonly form across the southeastern U.S., often originating from storms initiated along sea-breeze fronts. As outflows spread, merge, and collide, they frequently trigger secondary convection in air masses already modified by primary storm outflow. This resulting air mass carries a distinctive aerosol population, which can influence the development of secondary convective cells. During the U.S. DOE TRACER field campaign in summer 2022, we investigated how outflows affect aerosol size, composition, and hygroscopicity across Greater Houston, Texas. Using a novel detection algorithm, we identified and verified 72 outflow boundaries from surface meteorological observations. In the relatively simple continental environment northwest of Houston, a common relationship emerged between CCN-inferred hygroscopicity (κ) and size-resolved aerosol concentrations. The κ of aerosols typically decreased in size ranges where aerosol concentrations increased following outflow passage and increased where concentrations decreased. In contrast, at the Atmospheric Radiation Measurement’s (ARM) fixed site in La Porte, Texas, the aerosol response depended on outflow direction, shaped by concentrated industrial and shipping activity to the north and south/southeast along the Houston Ship Channel. Concurrent size distribution and composition changes suggested case-specific variability in κ, with either reinforcing or offsetting effects. Outflows passing over industrial/shipping sectors often promoted less hygroscopic aerosol populations while cleaner residential outflows often favored more hygroscopic aerosol populations. These results highlight key aerosol changes following outflow passage, providing a foundation for improved understanding of aerosol-cloud interactions related to secondary convection initiation along or in the wake of outflow boundaries.

Thompson, Seth A. [Texas A & M Univ., College Stat↗
DOE OSTI2014

Steady-state magnetohydrodynamic flow around an unmagnetized conducting sphere

The noncollisional interaction between conducting obstacles and magnetized plasma winds can be found in different scenarios, from the interaction occurring between regions inside galaxy clusters to the interaction between the solar wind and Mars, Venus, and active comets, or even the interaction between Titan and the Saturnian magnetospheric flow. These objects generate, through several current systems, perturbations in the streaming magnetic field leading to its draping around the obstacle's effective conducting surface. Recent observational results suggest that several properties associated with magnetic field draping, such as the location of the polarity reversal layer of the induced magnetotail, are affected by variations in the conditions of the streaming magnetic field. To improve our understanding of these phenomena, we perform a characterization of several magnetic field draping signatures by analytically solving an ideal problem in which a perfectly conducting magnetized plasma (with frozen-in magnetic field conditions) flows around a spherical body for various orientations of the streaming magnetic field. In particular, we compute the shift of the inverse polarity reversal layer as the orientation of the background magnetic field is changed.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Hierarchical off-diagonal low-rank approximation of Hessians in inverse problems, with application to ice sheet model initialization

Obtaining lightweight and accurate approximations of discretized objective functional Hessians in inverse problems governed by partial differential equations (PDEs) is essential to make both deterministic and Bayesian statistical large-scale inverse problems computationally tractable. The cubic computational complexity of dense linear algebraic tasks, such as Cholesky factorization, that provide a means to sample Gaussian distributions and determine solutions of Newton linear systems is a computational bottleneck at large-scale. These tasks can be reduced to log-linear complexity by utilizing hierarchical off-diagonal low-rank (HODLR) matrix approximations. In this work, we show that a class of Hessians that arise from inverse problems governed by PDEs are well approximated by the HODLR matrix format. In particular, we study inverse problems governed by PDEs that model the instantaneous viscous flow of ice sheets. In these problems, we seek a spatially distributed basal sliding parameter field such that the flow predicted by the ice sheet model is consistent with ice sheet surface velocity observations. Here, we demonstrate the use of HODLR Hessian approximation to efficiently sample the Laplace approximation of the posterior distribution with covariance further approximated by HODLR matrix compression. Computational studies are performed which illustrate ice sheet problem regimes for which the Gauss–Newton data-misfit Hessian is more efficiently approximated by the HODLR matrix format than the low-rank (LR) format. We then demonstrate that HODLR approximations can be favorable, when compared to global LR approximations, for large-scale problems by studying the data-misfit Hessian associated with inverse problems governed by the first-order Stokes flow model on the Humboldt glacier and Greenland ice sheet.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2019

Structure and reactivity of iridium oxide layers grown on Ir(1 0 0) by oxidation at sub-ambient O 2 pressures

We used low energy electron diffraction (LEED) and temperature programmed desorption (TPD) to investigate the structure and reactivity of iridium oxide layers prepared by oxidizing Ir(100) at 765 K and O 2 pressures ranging from 0.05 to 5 Torr. Our LEED results provide evidence that Ir(100) oxidation at O 2 pressures up to 0.20 Torr produces a mixture of Ir oxide structures present as small domains, including a commensurate IrO 2 (101) structure that coexists with other structures. Oxidizing from 0.50 to 1 Torr causes formation of a commensurate IrO 2 (110)R27° structure and a sharp rise in the oxygen uptake from ~8 to 20 ML (monolayer) as the films exhibit signs of roughening. Further increasing the O 2 pressure from 1 to 5 Torr causes the IrO 2 (110)R27° structure to be replaced with a so-called IrO 2 (110)-aligned structure, for which the IrO 2 (110) lattice vectors align with those of the Ir(100) substrate. We find that the oxidized Ir(100) surfaces become increasingly reactive toward the dissociation and oxidation of CH 4 as IrO 2 (110) develops on the surface, and observe that the IrO 2 (110)-aligned structure is more reactive than the IrO 2 (110)R27° phase. Furthermore, our findings demonstrate that the oxide phase evolution on Ir(100) is sensitive to the O 2 pressure in the range from 0.05 to 5 Torr, and that the development of reactive IrO 2 (110) structures requires elevated O 2 pressures (> 0.5 Torr) and temperature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Neutron diffraction study of magnetism in van der Waals layered MnBi 2n Te 3n+1

Two-dimensional van der Waals MnBi 2 n Te 3 n +1 ( n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi 2 n Te 3 n +1 ( n = 1, 2, 3, 4) while there is no detectable Mn at the non-magnetic atomic sites within the resolution of neutron diffraction experiments. The occupancy of Mn monotonically decreases with the increase of n . The polarized neutron diffraction on the representative MnBi 4 Te 7 reveals that its magnetization density is exclusively accumulated at the Mn site, in good agreement with the results from the unpolarized neutron diffraction. The defects of Bi at the Mn site naturally explain the continuously reduced saturated magnetic moments from n = 1 to n = 4. The experimentally estimated critical exponents of all the compounds generally suggest a three-dimensional character of magnetism. Our work provides material-specified structural parameters that may be useful for band structure calculations to understand the observed topological surface states and for designing quantum magnetic materials through chemical doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Broad background in electron diffraction of 2D materials as a signature of their superior quality

Abstract An unusually broad bell-shaped component (BSC) has been previously observed in surface electron diffraction on different types of 2D systems. It was suggested to be an indicator of uniformity of epitaxial graphene (Gr) and hexagonal boron nitride (hBN). In the current study we use low-energy electron microscopy and micro-diffraction to directly relate the BSC to the crystal quality of the diffracting 2D material. Specially designed lateral heterostructures were used to map the spatial evolution of the diffraction profile across different 2D materials, namely pure hBN, BCN alloy and pure Gr, where the alloy region exhibits deteriorated structural coherency. The presented results show that the BSC intensity has a minimum in the alloyed region, consequently showing that BSC is sensitive to the lateral domain size and homogeneity of the material under examination. This is further confirmed by the presence of a larger number of sharp moiré spots when the BSC is most pronounced in the pure hBN and Gr regions. Consequently, it is proposed that the BSC can be used as a diagnostic tool for determining the quality of the 2D materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗