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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Ligand-Driven Electrochemical Tuning of Co 6 Se 8 Chevrel Clusters

Molecular “Chevrel-type” clusters of the formula Co 6 Se 8 L 6 (L = neutral ligand) are a well-studied class of clusters due to their utility as molecular analogues to the Chevrel extended solid phase and their application as subunits in hierarchical materials. However, their solution and optical properties remain relatively underexplored. Aiming to develop the fundamental relationships between the molecular and electronic structures of these clusters and their electrochemical and photophysical properties, this work reports the preparation of a series of Co 6 Se 8 (P­(C 6 H 4 R) 3 ) 6 -type clusters with R = Cl (1), F (2), H (3), CH 3 (4), and OCH 3 (5) via a stepwise synthetic approach. Solution and solid-state experimental characterization and density functional theory calculations reveal that the Co 6 Se 8 cores of 1–5 maintain consistent electronic and structural properties despite the variation of the triarylphosphine ligand para-substituent Hammett parameters (σ p ). However, cyclic voltammetry measurements indicate that the electron transfer energetics of 1–5 are strongly influenced by ligand substitution, with the E 1/2 of a given redox event spanning ∼0.5 V depending on the triarylphosphine ligand’s σ p . In conclusion, these findings support the characterization of Co 6 Se 8 clusters as atomically precise nanoclusters with both the structural robustness and the electrochemical tunability needed to act as components in larger charge transfer assemblies.

Wheaton, Amelia M. [Argonne National Laboratory (A↗
DOE OSTI2022

Atomic-Scale Insights into the Lateral and Vertical Epitaxial Growth in Two-Dimensional Pd 2 Se 3 –MoS 2 Heterostructures

Two-dimensional (2D) materials form heterostructures in both the lateral and vertical directions when two different materials are interfaced, but with totally different bonding mechanisms of covalent in-plane to van der Waal’s layered interactions. Understanding how the competition between lateral and vertical forces influences the epitaxial growth is important for future materials development of complex mixed layered heterostructures. In this study, we use atomic-resolution annular dark-field scanning transmission electron microscopy to study the detailed atomic arrangements at mixed 2D heterostructure interfaces composed of two semiconductors with distinctly different crystal symmetry and elemental composition, Pd 2 Se 3 :MoS 2 , in order to understand the role of different chemical bonds on the resultant epitaxy. Pd 2 Se 3 is grown off the step edge in bilayer MoS 2 , and the vertical and lateral epitaxial relationships of the Pd 2 Se 3 –MoS 2 heterostructures are investigated. We find that the similarity of geometry at the interface with one metal (Pd or Mo) atoms bonded with two chalcogens (S or Se) are the crucial factors to make the atomically stitched lateral junction of 2D heterostructures. In addition, the vertical van der Waal interactions that are normally dominant in layered materials can be overcome by in-plane forces if the interfacial atomic stitching is high in quality and low in defect density. This knowledge should help guide the approaches for improving the epitaxy in mixed 2D heterostructures and seamless stitching of in-plane 2D heterostructures with various complex monolayer structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Competition between Hydrogen and Chalcogen Bonding in Homodimers of Chalcogen Hydrides (H 2 X) 2 , X = O, S, Se, Te

The structural and chemical bonding motifs manifested by the competition between hydrogen and chalcogen bonding in the homodimers of chalcogen hydrides (H 2 X) 2 , where X = O, S, Se, Te have been characterized using high-level electronic structure calculations and analysis of the electron density based on Quasi-atomic orbital (QUAO) and the Symmetry-adapted perturbation theory (SAPT) methods. The QUAO analysis clearly identifies a three-center interaction responsible for either hydrogen or chalcogen bonds: in the former, the σ-bond connecting the donor and hydrogen atom participating in the hydrogen bond interacts with the lone pair on the nucleophile acceptor via the hydrogen atom, while in the latter this same σ-bond interacts with the nucleophile lone pair via the donor chalcogen. The number of minimum energy structures increase dramatically from one for (H 2 O) 2 , three for (H 2 S) 2 , four for (H 2 Se) 2 , and finally six for (H 2 Te) 2 . Furthermore, the emergence of the chalcogen-bonded arrangements appears for (H 2 S) 2 with their subsequent energetic stabilization over the hydrogen-bonded minima manifesting in (H 2 Se) 2 and (H 2 Te) 2 . In particular, one of the (H 2 S) 2 , two of the (H 2 Se) 2 , and three of the (H 2 Te) 2 dimers are chalcogen bonded. Induction plays a small but important role in stabilizing hydrogen over chalcogen-bonded structures, while dispersion is more important for chalcogen bonds.

Chemical structure↗
DOE OSTI2025

Enantioselective Se lattices for stable chiroptoelectronic processing media

Chiroptoelectronic devices are crucial for applications in quantum computing, spin optical communications, and magnetic recording. However, the limited efficiency and low stability of conventional circularly polarized light (CPL)-sensing materials have restricted their broader use. Here, we introduce atomic chiral Se nanorod (NRs) films as broadband CPL detectors, leveraging the intrinsic chirality and stability of Se nanocrystals. We also perform incident circular polarization (ICP)-Raman optical activity (ROA) to explore the chiroptical activity of the large-area films. The Se NRs thin films detected CPL across a broad range from ultraviolet (UV) to short-wavelength infrared (SWIR), with a responsivity dissymmetry factor of up to 0.4, maintaining high stability under ambient conditions for longer than 13 months. CPL-sensitive Se NRs with intrinsic chirality have potential applications in chiral photonic synapses, chiral spin devices, and CPL-sensitive photocatalysts. ICP-ROA mapping also advances the analysis of 2D chiral materials.

Nanophotonics and plasmonics↗
DOE OSTI2021

Transport anomalies in the layered compound BaPt 4 Se 6

We report a layered ternary selenide BaPt 4 Se 6 featuring sesqui-selenide Pt 2 Se 3 layers sandwiched by Ba atoms. The Pt 2 Se 3 layers in this compound can be derived from the Dirac-semimetal PtSe 2 phase with Se vacancies that form a honeycomb structure. This structure results in a Pt (VI) and Pt (II) mixed-valence compound with both PtSe 6 octahedra and PtSe 4 square net coordination configurations. Temperature-dependent electrical transport measurements suggest two distinct anomalies: a resistivity crossover, mimic to the metal-insulator (M-I) transition at ~150 K, and a resistivity plateau at temperatures below 10 K. The resistivity crossover is not associated with any structural, magnetic, or charge order modulated phase transitions. Magnetoresistivity, Hall, and heat capacity measurements concurrently suggest an existing hidden state below 5 K in this system. Angle-resolved photoemission spectroscopy measurements reveal a metallic state and no dramatic reconstruction of the electronic structure up to 200 K.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Magnetic structures and excitations in sawtooth olivine chalcogenides Mn 2 SiX 4 (X = S, Se)

The Mn lattice in olivine chalcogenide Mn 2 SiX 4 (X = S, Se) compounds forms a sawtooth, which is of special interest in magnetism owing to the possibility of realizing flat bands in magnon spectra, a key component in magnonics. In this work, we investigate the Mn 2 SiX 4 olivines using magnetic susceptibility, and X-ray and neutron diffraction. We have determined the average and local crystal structures of Mn 2 SiS 4 and Mn 2 SiSe 4 using synchrotron X-ray, neutron diffraction, and X-ray total scattering data followed by Rietveld and pair distribution function analyses. It is found from the pair distribution function analysis that the Mn triangle that constitutes the sawtooth is isosceles in Mn 2 SiS 4 and Mn 2 SiSe 4 . The temperature evolution of magnetic susceptibility of Mn 2 SiS 4 and Mn 2 SiSe 4 shows anomalies below 83 K and 70 K, respectively, associated with magnetic ordering. From the neutron powder diffraction measurements the magnetic space groups of Mn 2 SiS 4 and Mn 2 SiSe 4 are found to be Pnma and Pnm'a', respectively. Here, we find that the Mn spins adopt a ferromagnetic alignment on the sawtooth in both Mn 2 SiS 4 and Mn 2 SiSe 4 but along different crystallographic directions for the S and the Se compounds. From the temperature evolution of Mn magnetic moments obtained from refining neutron diffraction data, the transition temperatures are accurately determined as T N (S) = 83(2) K and T N (Se) = 70.0(5) K. Broad diffuse magnetic peaks are observed in both the compounds, and are prominently seen close to T N , suggesting the presence of a short-range magnetic order. The magnetic excitations studied using inelastic neutron scattering reveal a magnon excitation with an energy corresponding to approximately 4.5 meV in both S and Se compounds. Spin correlations are observed to persist up to 125 K much above the ordering temperature and we suggest the possibility of short-range spin correlations responsible for this.

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

Room temperature chemical transformation of SnSe to Ag 2 Se nanocrystals via cation exchange

Atomic-scale control of the chemical composition of semiconductor nanocrystals through a cation exchange reaction affords greater tunability in the design of multifunctional semiconductor composite nanocrystals. Here, we report a facile route to SnSe–Ag 2 Se composite nanocrystals using cation exchange at room temperature. Starting from freshly synthesized SnSe nanorods, we leverage the strong distortion of the Sn 2+ octahedral coordination in SnSe and the hard–soft acid–base (HSAB) principle, to promote the exchange of an Sn 2+ ion with two Ag + ions in methanol leading to Ag 2 Se/SnSe nanocomposites. The morphology and chemistry of the nanocrystals evolve from nanorods with SnSe@Ag 2 Se (core@shell) structures for SnSe-rich composites to nanorods with a random distribution of Sn 2+ and Ag + ions for nearly equimolar composites, and finally to irregular fragmented nanocrystals for Ag 2 Se-rich composites. A mechanistic understanding of the observed morphology evolution is discussed using the change in the cation coordination from octahedral (Sn 2+ ) to tetrahedral (Ag + ) geometry and the accompanying expansion of the hcp Se 2– sublattice. Interestingly, the synthesized composite nanocrystals exhibit an optical band gap value tunable within a wide energy range by increasing the Ag 2 Se/SnSe ratio. This work provides a useful and facile strategy to modify the optical behavior of semiconductor nanomaterials, which can be leveraged for the design of better optical and/or photovoltaic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Influence of tungsten doping on nonradiative electron–hole recombination in monolayer MoSe 2 with Se vacancies

Two-dimensional transition metal dichalcogenides (TMDs) are receiving significant attention due to their excellent electronic and optoelectronic properties. The material quality is greatly affected by defects that are inevitably generated during material synthesis. Focusing on chalcogenide vacancies, which constitute the most common defect, we use the state-of-the-art simulation methodology developed in our group to demonstrate that W doping of MoSe 2 with Se vacancies reduces charge carrier losses by two mechanisms. First, W doping makes the formation of double Se vacancies unfavorable, while it is favorable in undoped MoSe 2 . Second, if a Se vacancy is present, the charge carrier lifetimes are extended in the W-doped MoSe 2 . Combining ab initio real-time time-dependent density functional theory with nonadiabatic molecular dynamics, the simulations show that the nonradiative carrier losses in the presence of Se vacancies proceed by sub-10 ps electron trapping and relaxation down the manifold of trap states, followed by a 100 ps recombination of trapped electrons with free holes. The electron–vibrational energy exchange is driven by both in-plane and out-of-plane vibrational motions of the MoSe 2 layer. Additionally, the atomistic studies advance our understanding of the influence of defects on charge carrier properties in TMDs and guide improvements of material quality and development of TMD applications.

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

Se diffusion in CdTe thin films for photovoltaics

Manipulating CdSeTe bandgrading to enhance photocurrent and carrier lifetime is an essential part of high-performance CdTe photovoltaics (PVs). Here, we examine Se diffusion kinetics in single-crystal and polycrystalline CdTe during deposition, thermal annealing, and CdCl 2 treatments. Se distributions are determined by dynamic secondary-ion-mass spectroscopy and Auger electron spectroscopy depth profiling and coupled with electron backscatter diffraction images of the crystalline structure. Effective bulk and grain boundary diffusion coefficients are determined by analytical models and discussed in the context of processing and film morphology. Se is found to diffuse in CdTe at much higher rates during CdCl 2 treatments than with thermal processing alone. GB diffusion also occurs at a significantly faster rate than bulk diffusion. As a result of these two effects, the near interface bulk and GB Se diffusion during CdCl 2 treatments dominates the bandgrading profiles in CdTe PVs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Impact of the Annealing Temperature on the Local Se Distribution and Optical Properties in CdSexTe1-x Solar Cells by Multimodal X-ray Microscopy

The introduction of Se in CdTe solar-cell absorbers has led to multiple beneficial effects in the devices, including an increased short-circuit current and an extended charge-carrier lifetime. A critical manufacturing step of CdSexTe1-x solar cells is CdCl2 annealing at high temperatures, during which Se diffuses through the absorber layer. Understanding the local Se distribution in the absorber and its impact on optical properties of the material is key for the further development of this thin-film solar cell technology. In a multimodal x-ray scanning microscopy approach, we investigated the impact of the annealing temperature of CdSexTe1-x solar cells on the Se distribution and on optical characteristics of the material. For this purpose, we exploited an upgraded x-ray excited optical luminescence (XEOL) detection unit that enables simultaneous spectrally and temporally resolved XEOL mapping at synchrotron facilities that are ideal for studying thin-film solar-cell materials with hard x-rays featuring high penetration depth and lateral resolution.

14 SOLAR ENERGY↗
DOE OSTI2026

Block-Type Antiferromagnetism in Single Chain Quasi-One-Dimensional K 3 ⁢Fe 2 ⁢Se 4

One-dimensional (1D) structures provide a unique platform to study the correlated quantum interactions and phase transitions such as unconventional magnetism and superconducting states. Here, we report that iron chalcogenide K 3 ⁢Fe 2 ⁢Se 4 exhibits an unusual block-type canted antiferromagnetic (AFM) order with a clear single chain quasi-1D structure, which is structurally different from the two-leg ladder BaFe 2 ⁢Se 3 , through both experimental measurements and density matrix renormalization group (DMRG) calculations. The narrow bandgap semiconductor K 3 ⁢Fe 2 ⁢Se 4 has a quasi-1D edge-shared FeSe4 tetrahedra chain structure and orders antiferromagnetically below 110 K. The magnetic moments couple antiferromagnetically along the quasi-1D chain direction of the 𝑏 axis and form an up-down-down-up (↑−↓−↓−↑)–like spin structure with a commensurate propagation vector 𝒌=⁢(0,0,0), where block-type spin ↑−↑ or ↓−↓ coupling are between the longer Fe-Fe bonds of the quasi-1D chain. DMRG results show that block antiferromagnetic state is stable in K 3 ⁢Fe 2 ⁢Se 4 and reveal that the block-ordered arrangement of Fe 2.5+ ions spins arise from the competition between ferromagnetic and AFM interaction in the presence of strong electronic correlation. Our research results not only report the discovery of a clear block-type canted antiferromagnetic structure in a real quasi-1D chain material but also provide a theoretical approach to understand the block-type antiferromagnetism in quasi-1D iron chalcogenides.

antiferromagnetism↗
DOE OSTI2020

Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb 0.25 Bi 2 Se 3

Recently, the niobium (Nb) doped topological insulator Bi 2 Se 3 , in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi 2 Se 3 layers, has been shown to exhibit both superconductivity with $T_c$ ≃ 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth $λ_{\text{eff}}(T)$ in the layered topological superconductor candidate Nb 0.25 Bi 2 Se 3 . In this work the exponential temperature dependence of $λ^{–2}_{\text{eff}}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature $T_c$ = 2.9 K and the gap to $T_c$ ratio 2Δ /$k_BT_c$ = 3.95(19). We also reveal that the ratio Tc/λ–2eff is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field μSR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of Nb 0.25 Bi 2 Se 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Electronic origin of half-metal to semiconductor transition and colossal magnetoresistance in spinel HgCr 2 Se 4

Half metals are ferromagnets hosting spin-polarized conducting carriers and are crucial for spintronics applications. The chromium spinel HgCr 2 Se 4 represents a unique type of half-metal, which features a half-metal to semiconductor transition (HMST) and exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Here, using angle-resolved photoemission spectroscopy, we find that the Fermi surface of n-type HgCr 2 Se 4 (n–HgCr 2 Se 4 ) consists of a single electron pocket which moves above the Fermi level (E F ) upon the FM-PM transition, leading to the HMST. Such a Lifshitz transition manifests a giant band splitting which originates from the exchange interaction unveiled with a specific chemical nonstoichiometry. The exchange band splitting and the chemical nonstoichiometry are two key ingredients to the HMST and CMR, consistent with our ab initio calculation. Our findings provide spectroscopic evidence of the electronic origin of the anomalous properties of HgCr 2 Se 4 , which address the unique phase transition in half-metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Fermi surface topology and magnetotransport properties of superconducting Pd 3 Bi 2 Se 2

Pd 3 Bi 2 Se 2 is a rare realization of a superconducting metal with a non-zero topological invariant. Here, in this study, we report the growth of high-quality single crystals of layered Pd 3 Bi 2 Se 2 with a superconducting transition at T c ≈ 0.80 K and upper critical fields of ~10 mT and ~5 mT for the in plane and out-of-plane directions, respectively. Our density functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level all displaying clear three dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd 3 Bi 2 Se 2 is evident in magneto-transport measurements, yielding a sign changing Hall resistivity at low temperatures. The magnetoresistance is non-saturating and follows Kohler’s scaling rule. We interpret the magneto-transport data in terms of open orbits that are revealed in the DFT calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: F α = (150 ± 26) T, F β = (293 ± 10) T, F γ = (375 ± 20) T and F η = (1017 ± 12) T, with the low frequency dominating the spectrum. Through the measurement of angular dependent dHvA oscillations and DFT calculations we identify the F α frequency with an approximately ellipsoidal electron pocket centered on the L 2 point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass m* = (0.11 ± 0.02)m 0 and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd 3 Bi 2 Se 2 as a potential candidate for exploring topological superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Highly tunable band inversion in AB 2 X 4 (A=Ge, Sn, Pb; B=As, Sb, Bi; X=Se, Te) compounds

Topological materials have been discovered so far largely by searching for existing compounds in crystallographic databases, but there are potentially new topological materials with desirable features that have not been synthesized. One of the desirable features is high tunability resulting from the band inversion with a very small direct band gap, which can be tuned by changes in pressure or strain to induce a topological phase transition. Here, using density-functional theory (DFT) calculations, we have studied the septuple layered AB 2 X 4 series compounds, where A=(Ge, Sn and Pb), B=(As, Sb and Bi), and X=(Se and Te). With the DFT thermodynamic stability validated by the already-reported compounds in these series, we predict stable Se compounds, which are not found in crystallographic database. Among them, we find that GeBi 2 Se 4 and GeSb 2 Se 4 having a small direct band gap at the Z point are very close to a strong topological insulator, which can be tuned by a moderate pressure to induce the band inversion. Importantly, the topological features with the small direct band gap are well isolated in both momentum and energy windows, which offers high tunability for studying the topological phase transition.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Band structure of Bi surfaces formed on Bi 2 Se 3 upon exposure to air

Bi 2 Se 3 has been the focus of intense interest over the past decade due to its topological properties. Bi surfaces are known to form on Bi 2 Se 3 upon exposure to atmosphere, but their electronic structure has not been investigated. Here, we report band structure measurements of such Bi surfaces using angle-resolved photoemission spectroscopy. Measured spectra can be well explained by the band structure of a single bilayer of Bi on Bi 2 Se 3 , and show that Bi surfaces consistently dominate the photoemission signal for air exposure times of at least 1 hour. These results demonstrate that atmospheric effects should be taken into consideration when identifying two-dimensional transport channels, and when designing surface-sensitive measurements of Bi 2 Se 3 , ideally limiting air exposure to no more than a few minutes.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Solid-phase heteroepitaxy of oriented Sb 2 Se 3 on GaAs for birefringent thin films

Here, we investigate the amorphous-to-crystalline transformation of antimony selenide (Sb 2 Se 3 ) on UHV-prepared GaAs (001) substrates. In the bulk orthorhombic form, Sb 2 Se 3 is a layered quasi-1D semiconductor with highly anisotropic properties of interest for optical and electronic devices. We find that an amorphous layer deposited by molecular beam epitaxy annealed at or above 230 °C yields a textured-epitaxial structure among some randomly oriented domains. The textured-epitaxial Sb 2 Se 3 grains are oriented with the covalently bonded “1D axis” constrained in-plane to GaAs [110] and with multiple van der Waals (hk0) orientations out-of-plane. The same texture was achieved exclusively without randomly oriented grains using continuous-wave laser radiation, highlighting the use of thermal and optical methods to yield anisotropic crystalline Sb 2 Se 3 films directly from the amorphous phase. Polarized reflectance and polarized microscopy confirm the unique state of in-plane birefringence in the crystallized thin film. Overall, we show that solid-phase heteroepitaxy provides additional pathways to the integration of low-symmetry chalcogenide semiconductors for demanding applications where the inherent anisotropy needs to be preserved.

Xiao, Kelly [Stanford Univ., CA (United States)] (↗