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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Six Quaternary Chalcogenides of the Pavonite Homologous Series with Ultralow Lattice Thermal Conductivity

Six new quaternary chalcogenides belonging to the pavonite sulfosalt mineral family with the general formula M n+1 (Bi/ Sb) 2 Q n+5 (n = 2-7) were synthesized by direct reactions of the elements at high temperatures. The compounds include InPbBi 3 S 7 (n = 2), In 0.5 Mn 2 Bi 3.5 Se 8 (n = 3), CdPb 2 Bi 4 S 9 (n = 4), Ag 1.5 CdBi 5.5 Se 10 (n = 5), Ag 2 CdBi 6 Se 11 (n = 6), and Ag 2.5 CdSb 6.5 Se 12 (n = 7) and crystallized in the monoclinic space group C2/m. The pavonite structure consists of two alternating slabs, a thinner slab composed of pairs of monocapped trigonal prisms separated by octahedra and a thicker slab with a galena-like structure motif. In the general formula, n corresponds to the number of octahedra along the diagonal direction of the galena-like slab. The complex compositions, mixed occupancies of the cations, and quasi-two-dimensional structures endow compounds of this family with extremely low thermal conductivity. Here, the charge and thermal transport properties of CdPb 2 Bi 4 S 9 and CdAg 2 Bi 6 Se 11 were characterized from 300 to 810 K, and both materials exhibited n-type semiconductor behaviors and ultralow lattice thermal conductivities of less than 0.35 W·m –1 ·K –1 for CdAg 2 Bi 6 Se 11 and 0.73 W·m –1 ·K –1 for CdPb 2 Bi 4 S 9 in the measured temperature range. Density functional theory calculations revealed the origins of this low lattice thermal conductivity to be a combination of the low Debye temperature, small phonon velocities, and large Grüneisen parameters. Thermoelectric properties were measured, and the highest ZT values of 0.53 and 0.95 for undoped CdPb 2 Bi 4 S 9 and CdAg 2 Bi 6 Se 11 , respectively, were attained at 775 K.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag 2 SrSiS 4 and Ag 2 SrGeS 4

Quaternary chalcogenide materials have long been a source of semiconductors for optoelectronic applications. Recent studies on the I2-II-IV-X4 (I = Ag, Cu, Li; II = Ba, Sr, Eu, Pb; IV = Si, Ge, Sn; X = S, Se) materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of anti-site defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag- and Sr-containing compounds, Ag 2 SrSiS 4 and Ag 2 SrGeS 4 , each found to form in the tetragonal I4¯2m structure of Ag 2 BaGeS 4 . During the synthesis targeting the title compounds, two additional materials, Ag 2 Sr 3 Si 2 S 8 and Ag 2 Sr 3 Ge 2 S 8 , have also been identified. Furthermore, these cubic compounds represent impurity phases during the synthesis of Ag 2 SrSiS 4 and Ag 2 SrGeS 4 . We show through hybrid density functional theory calculations that Ag 2 SrSiS 4 and Ag 2 SrGeS 4 have highly dispersive band edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag 2 SrSiS 4 and Ag 2 SrGeS 4 powders show frequency doubling capabilities in the near-infrared range.

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

Mercury Chalcogenide Nanoplatelet–Quantum Dot Heterostructures as a New Class of Continuously Tunable Bright Shortwave Infrared Emitters

Despite broad applications in imaging, energy conversion, and telecommunications, few nanoscale moieties emit light efficiently in the shortwave infrared (SWIR, 1000–2000 nm or 1.24–0.62 eV). We report quantum-confined mercury chalcogenide (HgX, where X = Se or Te) nanoplatelets (NPLs) can be induced to emit bright (QY > 30%) and tunable (900–1500+ nm) infrared emission from attached quantum dot (QD) “defect” states. Here, we demonstrate near unity energy transfer from NPL to these QDs, which completely quench NPL emission and emit with a high QY through the SWIR. This QD defect emission is kinetically tunable, enabling controlled midgap emission from NPLs. Spectrally resolved photoluminescence demonstrates energy-dependent lifetimes, with radiative rates 10–20 times faster than those of their PbX analogues in the same spectral window. Coupled with their high quantum yield, midgap emission HgX dots on HgX NPLs provide a potential platform for novel optoelectronics in the SWIR.

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

Design of Heterogeneous Chalcogenide Nanostructures with Pressure-Tunable Gaps and without Electronic Trap States

Heterogeneous nanostructures, such as quantum dots (QDs) embedded in solid matrices or core–shell nanoparticles, are promising platforms for a wide variety of applications, including phosphors with increased quantum yield, photocatalysis, and solar energy conversion. However, characterizing and controlling their interfacial morphology and defects, which greatly influence their electronic properties, have proven difficult in numerous cases. Here we carried out atomistic calculations on chalcogenide nanostructured materials, i.e., PbSe QDs in CdSe matrices and CdSe embedded in PbSe, and we established how interfacial and core structures affect their electronic properties. In particular, we showed that defects present at interfaces of PbSe nanoparticles and CdSe matrices give rise to detrimental intragap states, degrading the performance of photovoltaic devices. Instead, the electronic gaps of the inverted system (CdSe dots in PbSe) are clean, indicating that this material has superior electronic properties for solar applications. In addition, our calculations predicted that the core structure of CdSe and in turn its band gap may be tuned by applying pressure to the PbSe matrix, providing a means to engineering the properties of new functional materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Metal–metal chalcogenide molecular precursors to binary, ternary, and quaternary metal chalcogenide thin films for electronic devices

We found that bulk metals and metal chalcogenides dissolve in primary amine–dithiol solvent mixtures at ambient conditions. Thin-films of CuS, SnS, ZnS, Cu 2 Sn(Sx,Se 1-x ) 3 , and Cu 2 ZnSn(SxSe 1-x ) 4 (0 ≤ x ≤ 1) were deposited using the as-dissolved solutions. Furthermore, Cu 2 ZnSn(SxSe 1-x ) 4 solar cells with efficiencies of 6.84% and 7.02% under AM1.5 illumination were fabricated from two example solution precursors, respectively.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Phase Separation in the Iron Chalcogenide Superconductor Fe1+yTexSe1−x

We present direct evidence for phase separation and chemical inhomogeneity in Fe{sub 1+y}Te{sub x}Se{sub 1-x} single crystals from scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) data. In STEM, images recorded using a high-angle annular dark field (HAADF) detector show characteristic nanometer-scale patterns of phase separation from the Z-dependent image contrast. The separation was observed both in non-superconducting samples with excess iron and in superconducting samples. Using EELS, we determined {approx}20% (or lower) fluctuation in Te concentration from the average local compositions by integrating the intensity of the Te-M{sub 4,5} edge. The energy-loss near-edge structure (ELNES) of the Fe-L{sub 2,3} edge changes with varying composition, especially the L{sub 3}/L{sub 2} white-line intensity ratio, which is sensitive to the d-state occupancy of the Fe. The results suggest a miscibility gap in the Fe{sub 1+y}Te{sub x}Se{sub 1-x} system and changes in the d-electron states at the nanometer scale from the separated phases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Stable Carbon-Selenium Bonds for Enhanced Performance in Tremella -Like 2D Chalcogenide Battery Anode [Stable Carbon-Selenium Bonds for Enhanced Performance in Tremella -Like Two-Dimensional Chalcogenide Battery Anode]

Two-dimensional cobalt selenide based on conversion reaction has attracted much attention due to its open layered structure and high specific capacity. However, effectively suppressing the fast capacity fade caused by the irreversible Se dissolution and serious volume change during the cycling process is still a challenges. Herein, we tune the concentration of dispersion liquid under supercritical condition to induce the CoSe crystal to grow along the graphene oxide (GO), and finally obtain the Tremella -like CoSe-rGO hybrid. The nature of epitaxial growth leads to the formation of stable C–Se bonds, which maintains favorable conductive connection between CoSe and reduced GO (rGO) as well as enhances the mechanical strength of active material to suppress Se dissolution and volume expansion during Na/Li intercalation and deintercalation. The unique microstructural merits of the hybrid result in superior sodium/lithium storage performance (400.8 mAh g -1 at 1 A g -1 after 100 cycles for NIBs and 769.6 mAh g -1 at 2 A g -1 after 500 cycles for LIBs). Moreover, the transmission X-ray microscopy technique is firstly used to directly observe the Se segregation in cobalt selenide and it being suppressed by the C–Se bonds.

25 ENERGY STORAGE↗
DOE OSTI2018

Quaternary Chalcogenide Semiconductors with 2D Structures: Rb 2 ZnBi 2 Se 5 and Cs 6 Cd 2 Bi 8 Te 17

In this work, two new layered compounds Rb 2 ZnBi 2 Se 5 and Cs 6 Cd 2 Bi 8 Te 17 are described. Rb 2 ZnBi 2 Se 5 crystallizes in the orthorhombic space group Pnma, with lattice parameters of a = 15.6509(17) Å, b = 4.218(8) Å, and c = 18.653(3) Å. Cs 6 Cd 2 Bi 8 Te 17 crystallizes in the monoclinic C2/m space group, with a = 28.646(6) Å, b = 4.4634(9) Å, c = 21.164(4) Å, and β = 107.65(3)°. The two structures are different and composed of anionic layers which are formed by inter connecting of BiQ 6 octahedra (Q = Se or Te) and MQ 4 (M = Zn or Cd) tetrahedra. The space between the layers hosts alkali metal as counter cations. The rubidium atoms of Rb 2 ZnBi 2 Se 5 structure can be exchanged with other cations (Cd 2+ , Pb 2+ and Zn 2+ ) in aqueous solutions forming new phases. Rb 2 ZnBi 2 Se 5 is an n-type semiconductor and exhibits an indirect band gap energy of 1.0 eV. Rb 2 ZnBi 2 Se 5 is a congruently melting compound (mp ~644 °C). The thermal conductivity of this semiconductor is very low with 0.38 W· m -1 ·K -1 at 873 K. Density functional theory (DFT) calculations suggest that the low lattice thermal conductivity of Rb 2 ZnBi 2 Se 5 is attributed to heavy Bi atom induced slow phonon velocities and large Gruneisen parameters especially in the a and c directions. The thermoelectric properties of Rb 2 ZnBi 2 Se 5 were characterized with the highest ZT value of ~0.25 at 839 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗