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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Widely Tunable Optical and Thermal Properties of Dirac Semimetal Cd 3 As 2

In this paper, a detailed analysis of the temperature-dependent optical properties of epitaxially grown cadmium arsenide (Cd 3 As 2 ), a newly discovered 3D Dirac semimetal is reported. Fermi level tuning—instigated from Pauli-blocking in the linear Dirac cone—and varying Drude response, generate large variations in the mid- and far-infrared optical properties. Thermo-optic shifts larger than those of traditional III–V semiconductors are demonstrated. Electron scattering rate, plasma frequency edge, Fermi level shift, optical conductivity, and electron effective mass analysis of Cd 3 As 2 thin-films are quantified and discussed in detail. The ab initio density functional study and experimental analysis of epitaxially grown Cd 3 As 2 promise applications for nanophotonic and nanoelectronic devices, such as reconfigurable metamaterials and metasurfaces, nanoscale thermal emitters, and on-chip directional antennas.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Dark Exciton in 2D Hybrid Halide Perovskite Films Revealed by Magneto-Photoluminescence at High Magnetic Field

Here, a comprehensive study of the exciton fine structure (EFS) is presented in 2D-phenethylammonium lead iodide films using magnetic field-induced polarization of photoluminescence (PL) in both Faraday and Voigt configurations at fields up to 25 Tesla. Three exciton bands are identified in the PL spectrum associated with bound, dark, and bright excitons, respectively. Under a high magnetic field in Faraday/Voigt configuration, large field-induced circular/linear polarization is observed in the PL band related to the dark exciton, which is magnetically activated. Furthermore, it is found that the dark exciton has an anomalous field-induced circular polarization, which cannot be explained by the classical Boltzmann distribution of spin-polarized species. These findings are well explained by an effective mass model that includes exchange terms unique to the monoclinic symmetry as a perturbation of the EFS in the approximate tetragonal symmetry. It is also confirmed that the field-induced linear polarization is sensitive to the monoclinic exchange term, whereas the field-induced circular polarization is immune to such term.

14 SOLAR ENERGY↗
DOE OSTI2021

High Thermoelectric Performance through Crystal Symmetry Enhancement in Triply Doped Diamondoid Compound Cu 2 SnSe 3

The presence of high crystallographic symmetry and nanoscale defects are favorable for thermoelectrics. With proper electronic structures, a highly symmetric crystal tends to possess multiple carrier channels and promote electrical conductivity without sacrificing Seebeck coefficient. In addition, nanoscale defects can effectively scatter acoustic phonons to suppress thermal conductivity. Here, we report that the triple doping of Cu 2 SnSe 3 leads to a high ZT value of 1.6 at 823 K for Cu 1.85 Ag 0.15 (Sn 0.88 Ga 0.1 Na 0.02 )Se 3 , and a decent average ZT (ZT ave ) value of 0.7 is also achieved for Cu 1.85 Ag 0.15 (Sn 0.93 Mg 0.06 Na 0.01 )Se 3 from 475 to 823 K. Our study reveals: (1) Ag doping on Cu sites generates numerous point defects and greatly decreases lattice thermal conductivity. (2) Doping Mg or Ga converts the monoclinic Cu 2 SnSe 3 into a cubic structure. This symmetry enhancing leads to increase in the effective mass from 0.8 m e to 2.6 m e (m e , free electron mass) and the power factor from 4.3 μW/cm –1 K –2 for Cu 2 SnSe 3 to 11.6 μW/cm –1 K –2 . (3) Na doping creates dense dislocation arrays and nanoprecipitates, which strengthens the phonon scattering. (4) Pair distribution function analysis shows localized symmetry breakdwon in the cubic Cu 1.85 Ag 0.15 (Sn 0.88 Ga 0.1 Na 0.02 )Se 3 . Furthermore, the present work provides a standpoint to design promising thermoelectric materials by synergistically manipulating crystal symmetry and nanoscale defects.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Strong Valence Band Convergence to Enhance Thermoelectric Performance in PbSe with Two Chemically Independent Controls

Abstract We present an effective approach to favorably modify the electronic structure of PbSe using Ag doping coupled with SrSe or BaSe alloying. The Ag 4d states make a contribution to in the top of the heavy hole valence band and raise its energy. The Sr and Ba atoms diminish the contribution of Pb 6s 2 states and decrease the energy of the light hole valence band. This electronic structure modification increases the density‐of‐states effective mass, and strongly enhances the thermoelectric performance. Moreover, the Ag‐rich nanoscale precipitates, discordant Ag atoms, and Pb/Sr, Pb/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZT avg of around 0.86 over 400–923 K.

Luo, Zhong‐Zhen↗
DOE OSTI2023

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

Chanakian, Sevan↗
DOE OSTI2020

Strong Valence Band Convergence to Enhance Thermoelectric Performance in PbSe with Two Chemically Independent Controls

We present an effective approach to favorably modify the electronic structure of PbSe using Ag doping coupled with SrSe or BaSe alloying. The Ag 4d states make a contribution to in the top of the heavy hole valence band and raise its energy. The Sr and Ba atoms diminish the contribution of Pb 6s2 states and decrease the energy of the light hole valence band. This electronic structure modification increases the density-of-states effective mass, and strongly enhances the thermoelectric performance. Furthermore, the Ag-rich nanoscale precipitates, discordant Ag atoms, and Pb/Sr, Pb/Ba point defects in the PbSe matrix work together to reduce the lattice thermal conductivity, resulting a record high average ZT avg of around 0.86 over 400–923 K.

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

Investigating the Role of Vacancies on the Thermoelectric Properties of EuCuSb‐Eu 2 ZnSb 2 Alloys

Abstract AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M ‐site in the planar MX ‐layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu 1− x Zn 0.5 x Sb solid solution. The transition from a fully‐occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn 0.5 Sb) leads to non‐linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb‐sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK −1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn 0.5 Sb.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Band gap dependence of exciton Lande factor in lead halide perovskites

The optical properties of lead halide perovskite semiconductors in vicinity of the bandgap are controlled by excitons, so that investigation of their fundamental properties is of critical importance. The exciton Landé or g-factor g X is the key parameter, determining the exciton Zeeman spin splitting in magnetic fields. The exciton, electron, and hole carrier g-factors provide information on the band structure, including its anisotropy, and the parameters contributing to the electron and hole effective masses. Here, g X is measured by reflectivity in magnetic fields up to 60 T for lead halide perovskite crystals. The materials band gap energies at a liquid helium temperature vary widely across the visible spectral range from 1.520 up to 3.213 eV in hybrid organic–inorganic and fully inorganic perovskites with different cations and halogens: FA 0.9 Cs 0.1 PbI 2.8 Br 0.2 , MAPbI 3 , FAPbBr 3 , CsPbBr 3 , and MAPb(Br 0.05 Cl 0.95 ) 3 . The exciton g-factors are found to be nearly constant, ranging from +2.3 to +2.7. Thus, the strong dependences of the electron and hole g-factors on the bandgap roughly compensate each other when combining to the exciton g-factor. The same is true for the anisotropies of the carrier g-factors, resulting in a nearly isotropic exciton g-factor. The experimental data are compared favorably with model calculation results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

New avenues for |∆ B | = 2 processes beyond neutron-antineutron oscillations

We explore baryon-number-violating (|∆ B | = 2) processes beyond the well-known neutron-antineutron ($n - \bar{n}$) oscillations, focusing on the $Λ - \bar{Λ}$ system. The presence of a strange quark in the Λ baryon introduces a new set of six-quark operators roughly of the form (uds) 2 , which are different from the (udd) 2 operators responsible for oscillations. Using the Standard Model Effective Field Theory (SMEFT), we classify all dimension-9 operators that cause |∆ B | = 2 transitions and study their UV completions mediated by exotic scalar fields with trilinear interactions. We demonstrate that in these models, oscillations can occur at tree level, with $n - \bar{n}$ mixing potentially appearing at higher loop levels. We employ a chiral effective theory to constrain the effective mass mixing δm Λ , deriving bounds from current experimental limits on $n - \bar{n}$ oscillations and dinucleon decays such as pp → K + K + . These bounds indicate that $Λ - \bar{Λ}$ oscillations probe a complementary parameter space, sensitive to baryon-number violation at scales up to 10 2 − 10 3 TeV. We show that the existing indirect bounds make it challenging to provide a competitive bound on δm Λ at BESIII.

Baryon/Lepton Number Violation↗
DOE OSTI2025

Quantum oscillation studies of the nodal line semimetal Ni 3 In 2 S 2-x Se x

Ternary shandite compounds with the general formula T 3 M 2 X 2 (T = Ni, Co, Rh or Pd; M = Sn, In, or Pb; and X = S or Se) have emerged as a large pool of topological semimetals. This family of compounds hosts different topological phases for various combinations of T, M and X. This paper reports the observation of quantum oscillations under the high magnetic fields in Ni 3 In 2 S 2-x Se x single crystals. Angular dependence of oscillation frequency suggests an evolution of the Fermi surface from three-dimensional to two-dimensional on Se substitution for S in Ni 3 In 2 S 2 . Here, the effective mass obtained for each composition by fitting the oscillation amplitude with the Lifshitz-Kosevich formula, shows no significant change, suggesting that the topological phase might be relatively robust against enhanced spin-orbit coupling upon Se doping in Ni 3 In 2 S 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Investigations on the electronic properties and effect of chitosan capping on the structural and optical properties of zinc aluminate quantum dots

Quantum confined uncapped and chitosan capped nanoparticles of ZnAl 2 O 4 synthesized by microwave-assisted sol–gel method were investigated by structural analysis and optical techniques. X-ray diffraction studies confirmed the formation of cubic spinels with crystallite size 4.5 nm and 3.4 nm, respectively for uncapped and chitosan capped ZnAl 2 O 4 nanoparticles. Chitosan capping produced a blue shift in bandgap energy (3.84 to 3.94 eV) agreeing with size effects according to the Brus equation. A blue shift in emission peak and enhancement in photoluminescence intensity was also observed upon chitosan capping. The electronic band structure and the density of states of the bulk spinel were also calculated using density functional theory. The effective masses of electrons and holes estimated based on the band structure were used to extract the excitonic Bohr radius.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Electronic structure of p -type transparent conducting oxide CuAlO 2

We report copper-based delafossite oxides are excellent candidates for the p-type transparent conducting oxide (TCO), which is essential in realizing transparent semiconductor applications. Using angle-resolved photoemission spectroscopy (ARPES), we report the low-energy electronic structure of CuAlO 2 . We found that the band structure near the valence band top is characterized by hole bands with their maxima along the Brillouin zone boundary. Furthermore, the effective masses along the Γ–M and Γ–K directions were found to be (0.6 ± 0.1) m0 and (0.9 ± 0.1) m 0 , respectively, which impose an important benchmark against the existing band calculations.

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

Guiding centre motion for particles in a ponderomotive magnetostatic end plug

The Hamiltonian dynamics of a single particle in a rotating plasma column, interacting with an magnetic multipole is perturbatively solved for up to second order, using the method of Lie transformations. First, the exact Hamiltonian is expressed in terms of canonical action-angle variables, and then an approximate integrable Hamiltonian is introduced, using another set of actions and angles, which describe the centre of oscillation for the particle. The perturbation introduces an effective ponderomotive potential, which to leading order is positive. At the second order, the pseudopotential consists of a sum of terms of the Miller form, and can have either sign. Additionally, at second order, the ponderomotive interaction introduces a modification to the particle effective mass, when considering the motion along the column axis. It is found that particles can be axially confined by the ponderomotive potentials, but acquire radial excursions which scale as the confining potential. The radial excursions of the particle along its trajectory are investigated, and a condition for the minimal rotation frequency for which the particle remains radially confined is derived. Last, we comment on the changes to the aforementioned solution to the pseudopotentials and particle trajectory in the case of resonant motion, that is, a motion which has the same periodicity as the perturbation.

Rubin, T. (ORCID:0000000164850096)↗
DOE OSTI2020

Ultralow Thermal Conductivity and Thermoelectric Properties of Rb 2 Bi 8 Se 13

Complex bismuth chalcogenides are narrow band gap semiconductors with intrinsic low thermal conductivity, which exhibit high potential as highly efficient thermoelectric materials. Here, we assess the basic thermoelectric properties of polycrystalline Rb 2 Bi 8 Se 13 in the temperature range 300-823 K as well as performance-optimizing strategies. We find that the as-made bulk samples are single phase with the monoclinic crystal structure (P2 1 /m) and crystallize in layer morphology. The pristine sample exhibits an exceedingly low lattice thermal conductivity of 0.6- 0.46 W·m -1 ·K -1 at 300-823 K, which derives from strong lattice anharmonicity, large Grüneisen parameters, and low phonon velocities induced by the complex crystal structure with heavy atom Bi and a large unit cell. Here, the Cl doping successfully enhances the carrier concentration in Rb 2 Bi 8 Se 13 with a negligible impact on the electronic band structure, displaying common doping behaviors. Se vacancy, on the contrary, leads to n-type doping and enhances the effective mass and power factors more significantly. Consequently, a maximum ZT of ~0.75 at 823 K for the 0.3% Se-vacancy-doped sample is obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Ligand-Dependent Tuning of Interband and Intersubband Transitions of Colloidal CdSe Nanoplatelets

Although surface ligands of colloidal nanocrystals are known to adjust the absolute energy levels of valence and conduction bands of semiconductor nanocrystals, they typically have only minor influence on the band gap or effective masses. This changes in nanoplatelets. Ligand exchange of CdSe colloidal nanoplatelets induces large (up to 300 meV) bathochromic shifts of both interband and intersubband transitions. Here, three families of ligands-halides, thiolates, and phosphonates-are used to tune interband transitions, reflecting electron and hole confinement, across visible wavelengths and intersubband transitions, reflecting electron confinement, across the near-infrared spectral window. Furthermore, careful examination shows that delocalization from expansion of the nanoplatelet short axis, which was reported previously, cannot alone explain observed red shifts. Instead, comparison of intersubband, interband, and hole energy levels shows that ligand head group chemistry confers specific, idiosyncratic adjustments of the contribution of conduction and valence bands to the observed bathochromic shifts. Phosphonate ligands show the largest band gap reductions but the smallest red shift of intersubband transition energies; halide-exchanged samples displayed smaller reductions in band gap but large red shifts of intersubband transitions; thiolates fall in between. A related specificity is observed in hole states, which implicates ligand-responsive valence band curvature as an additional contribution driving optical changes. For nanoplatelets, surface ligand chemistry offers not only a tool to adjust the absolute energy level of conduction and valence bands but also an alternative route to preferential electron or hole band engineering that is normally achieved with inorganic shells.

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

Crystal Structure and Atomic Vacancy Optimized Thermoelectric Properties in Gadolinium Selenides

Thermoelectric materials enable the energy conversion of waste heat into electricity, helpful to relieve global energy crisis. Here, we report a systematic investigation on high-temperature thermoelectric gadolinium selenides, cubic Gd 3-x Se 4 (x = 0.16, 0.21 and 0.25) and orthorhombic Gd 2 Se 3-y (y = 0.02, 0.06 and 0.08). High energy synchrotron x-ray diffraction and total scattering have been used to investigate the crystallographic and local structures. Atomic-scale clusters of Gd vacancy in the cubic phase are observed by employing the reverse Monte Carlo simulation. For cubic Gd 3-x Se 4 , adjusting Gd vacancy triggers the effect of multiple conduction bands, confirmed by the increase in effective masses and theoretical calculations. A reasonable peak zT value of 0.27 is achieved at 850 K for Gd 3-x Se 4 (x = 0.16). On the other hand, tuning Se vacancy enables the optimization of electron concentration for the orthorhombic Gd 2 Se 3-y . More significantly, its low deformation potential (Ξ = 12eV) gives rise to enhanced electron mobility and higher peak zT value of 0.54 at 850 K for Gd 2 Se 3-y (y = 0.02). Intriguingly, a higher zT of 1.2 at 1200 K is reasonably predicted by quality factor analysis. Finally, this work extends the scope of high-temperature thermoelectric materials and facilitates the exploration of novel high-temperature thermoelectric materials

36 MATERIALS SCIENCE↗
DOE OSTI2023

Impact of Chiral Symmetry Breaking on Spin-Texture and Lone Pair Expression in Chiral Crystals of Hybrid Antimony and Bismuth Halides

Chiral organic building blocks can be incorporated into hybrid organic-inorganic metal-halide crystalline semiconductors so as to control and impact the interconversion between light, charge, and spin. Here, we report a series of hybrid antimony and bismuth halide materials of the general formula MBA 4 B 2 X 10 (B = Sb 3+ , Bi 3+ ; X = Br - , I - ) and study how chiral symmetry breaking imposed by the templating organic chiral methylbenzylammonium (MBA) cations induces symmetry breaking within the inorganic sublattice and leads to a unique spin-texture. The chiral MBA cations introduce two structural modifications to the metal-halide sublattice that consists of isolated edge-sharing dimers of B 2 X 10 octahedra: (1) the dimers have a chiral spatial arrangement with respect to one another and (2) there is an asymmetric distortion of the two subunits within the individual dimers with a higher distortion caused by strong stereochemical activity of the Sb 5s 2 lone pair electrons. The structural distortions and chiral arrangement result in circular dichroism (CD) at the band edge of the inorganic framework with dissymmetry factors in the range of 10 -4 . Chiral spin-splitting of the inorganic states, caused by breaking of the inversion symmetry and large spin-orbit coupling, is studied via density functional theory (DFT), and a multiband effective mass theory was developed that links the DFT-derived spin-splitting of helical character (i.e., spin expectation value not perpendicular to the crystal momentum) to the observed CD. Furthermore, we also find broad red photoluminescence from the MBA 4 Sb 2 Br 10 compounds, which we attribute to self-trapped excitonic emission driven by the large distortion due to the lone pair expression.

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

Organizing Chaos: Boosting Thermoelectric Properties by Ordering the Clathrate Framework of Ba 8 Cu 16 As 30

The type I clathrate, Ba 8 Cu 16 As 30 , is reinvestigated and found to have a low-temperature polymorph mP 108-Ba 8 Cu 16 As 30 with ordered Cu and As sites. In situ temperature-dependent powder X-ray diffraction experiments guided synthetic efforts toward the synthesis of the ordered monoclinic ( mP 108) and disordered cubic ( cP 54) polymorphs with high phase purity. While a transition from mP 108-Ba 8 Cu 16 As 30 to cP 54-Ba 8 Cu 16 As 30 is not directly observed, cP 54-Ba 8 Cu 16 As 30 is stabilized through quenching from high temperatures and is confirmed through high-resolution synchrotron powder X-ray diffraction. Further, combined theoretical predictions and experimental observations of the thermoelectric properties of both polymorphs reveal that the ordering of Cu and As atoms in the clathrate framework simultaneously enhances the Seebeck coefficient and electronic conductivity by increasing the hole effective mass and reducing the electronic scattering events. Consequently, the zT of mP 108-Ba 8 Cu 16 As 30 reaches a maximum of 0.2 at 575 K, an order of magnitude higher than that of cP 54-Ba 8 Cu 16 As 30 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗