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At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Computational study on electrical properties of transition metal dichalcogenide field-effect transistors with strained channel

The performance limits of monolayer transition metal dichalcogenide (TMDC) field-effect transistors (FETs) with isotropic biaxial strain were examined with the “top-of-the-barrier” ballistic MOSFET model. Using a first-principle theory, we calculated the band structures and density of states of strained monolayer MoS{sub 2} and WS{sub 2}, and used the results in model calculations. Introducing strain moves the positions of the conduction band minimum and valence band maximum in k-space with resultant variation in the effective mass and population of carriers. Introducing 2% tensile strain into n-type MoS{sub 2} FETs decreases the electron effective mass and, at the same time, increases energy separation between the lower and the higher valleys in the conduction band, resulting in 26% improvement of the ON current up to 1260 A/m. Whereas compressive strain results in complicated effects, −2% strain also improves the ON current by 15%. These results suggest that introducing artificial strain is promising to improve TMDC FET performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Characterization and photocatalytic activity of Fe- and N-co-deposited TiO{sub 2} and first-principles study for electronic structure

Titanium dioxide (TiO{sub 2}), co-deposited with Fe and N, is first implanted with Fe by a metal plasma ion implantation (MPII) process and then annealed in N{sub 2} atmosphere at a temperature regime of 400-600 deg. C. First-principle calculations show that the (Fe, N) co-deposited TiO{sub 2} films produced additional band gap levels at the bottom of the conduction band (CB) and on the top of the valence band (VB). The (Fe, N) co-deposited TiO{sub 2} films were effective in both prohibiting electron-hole recombination and generating additional Fe-O and N-Ti-O impurity levels for the TiO{sub 2} band gap. The (Fe, N) co-deposited TiO{sub 2} has a narrower band gap of 1.97 eV than Fe-implanted TiO{sub 2} (3.14 eV) and N-doped TiO{sub 2} (2.16 eV). A significant reduction of TiO{sub 2} band gap energy from 3.22 to 1.97 eV was achieved, which resulted in the extension of photocatalytic activity of TiO{sub 2} from UV to Vis regime. The photocatalytic activity and removal rate were approximately two-fold higher than that of the Fe-implanted TiO{sub 2} under visible light irradiation. - Graphical abstract: The electronic properties of (Fe, N) co-deposited TiO{sub 2} films determined by theoretical calculations is graphically shown; Fe and N preferentially substitute the Ti and O site, to form impurity level. Highlights: > The MPII produces a simple and low cost process for the fabrication of visible light photocatalysts. > Both theoretical and experimental approaches are used to discuss the relationship between band structure and photocatalysis. > The Fe and N preferentially substitute the Ti and O site, which generate additional Fe-O and N-Ti-O impurity levels for the TiO{sub 2} band gap. > A significant reduction of TiO{sub 2} band gap energy from 3.22 to 1.97 eV was achieved.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Promotion of Pd nanoparticles by Fe and formation of a Pd 3 Fe intermetallic alloy for propane dehydrogenation

In this study, silica supported Pd (~2 nm), Pd 3 Fe (~2 nm) and Pd 3 Fe_large (~12 nm) catalysts were synthesized and tested for propane dehydrogenation at 510 °C. At the 10% conversion, Pd and Pd 3 Fe catalysts exhibited propylene selectivity of 45% and 94%, respectively. Moreover, the latter showed a turnover rate (TOR) of 0.2 s -1 , which is five times higher than that of the Pd catalyst (0.04 s -1 ). Pd K edge XAS, XRD, and CO adsorbed IR were used to characterize the geometric structure of the catalysts. By combined comparison of XRD and XAS spectra of Pd 3 Fe and Pd 3 Fe_large catalysts, we successfully identified the crystalline phase in the 2 nm Pd 3 Fe catalyst. The CO adsorbed IR suggests that the formation of Pd 3 Fe breaks the ensemble of Pd, which is responsible for the increase of selectivity. The Pd L edge XAS was used to characterize the electronic structure of the catalysts. The Pd 3 Fe catalyst exhibits an increase of the edge energy compared with the Pd catalyst, which indicates the change of d-band structure in the bimetallic catalyst. The change in the electronic structure is likely the reason for the increase in TOR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Two noncentrosymmetric polyphosphates featuring infinite one-dimensional (PO{sub 3}){sub ∞} chain, LiMP{sub 2}O{sub 6} (M = Rb, Cs): Synthesis, structure and optical properties

Highlights: • The crystal structure of two noncentrosymmetric LiMP{sub 2}O{sub 6} (M = Rb, Cs) is described in detail. • The linear and nonlinear optical properties for them were characterized firstly. • Band structures and density of states for them have been calculated firstly. - Abstract: Two noncentrosymmetric polyphosphates, namely LiCsP{sub 2}O{sub 6} and LiRbP{sub 2}O{sub 6} have been synthesized from the high temperature solution. The structural analysis suggests that both of them crystallize in orthorhombic space group Fdd2 (No.43), with lattice parameters: a = 18.517(4) Å, b = 19.037(4) Å, c = 13.011(3) Å, and Z = 32 for LiRbP{sub 2}O{sub 6,} while a = 19.009(13) Å, b = 19.402(13) Å, c = 13.180(9) Å, and Z = 32 for LiCsP{sub 2}O{sub 6}. In the structure, the PO{sub 4} polyhedra are connected through corner-sharing to form 1D infinite (PO{sub 3}) {sub ∞} chains stretching along the c axis, which are further linked by the LiO{sub 4} polyhedra to construct a 3D framework with two types of tunnels. And the Rb/Cs atoms are filled in or around the periphery of the tunnels to keep the charge equilibrium. The IR spectra verify their structural validity. In addition, other characterizations including the UV−vis−NIR diffuse reflectance spectra, as well as first-principles theoretical studies have been performed on the two compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Nuclear Data Sheets for A = 227

The evaluated spectroscopic data are presented for ten known nuclides of mass 227 (Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Np). For {sup 227}Po, {sup 227}At, {sup 227}Rn, {sup 227}Pa, {sup 227}U and {sup 227}Np nuclei, only the ground-state information is available. Their decay characteristics are mostly unknown. Levels in {sup 227}Fr are known only from the decay of {sup 227}Rn to {sup 227}Fr. This decay scheme at present cannot be normalized to deduce γ intensities per 100 decays due to lack of knowledge about multipolarities of many low-energy transitions. The levels in {sup 227}Ra, {sup 227}Ac and {sup 227}Th are known from several decays and reactions, including particle-transfer data for {sup 227}Ra and {sup 227}Ac. The decay scheme of {sup 227}Ra to {sup 227}Ac was last studied in 1971 using small Ge detectors. Improved γ-ray intensity data need to be obtained with a better γ-detection system. The datasets for {sup 227}Ac have undergone extensive revisions, including detailed data for 231Pa α decay from 1986BaYK report, and single-proton transfer data from 1986MaYU thesis. High-spin (J>13/2 or so) structures are known only for 227Th. Level lifetime data are quite scarce for all the nuclides in this mass chain, thus limiting the knowledge of reduced transition probabilities. Band structures for {sup 227}Fr, {sup 227}Ra, {sup 227}Ac and {sup 227}Th are known in detail, together with evidence of weak octupole deformation and consequent parity-doublet structures. This evaluation was carried out as part of a joint IAEA-ICTP workshop for Nuclear Structure and Decay Data, organized and hosted by the IAEA, Vienna and ICTP, Trieste, March 24–28, 2014. The evaluation work was coordinated by B. Singh (McMaster). This work supersedes previous A=227 evaluation (2001Br31) published by E. Browne which covered literature before May 2001.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Density Functional Theory Study of Controllable Optical Absorptions and Magneto-Optical Properties of Magnetic CrI 3 Nanoribbons: Implications for Compact 2D Magnetic Devices

A chromium triiodide (CrI 3 ) monolayer has an interesting ferromagnetic ground-state. In this work, we calculate band structures and magnetic moments of tensile-strained and bent zigzag CrI 3 nanoribbons with density functional theory. The edge iodine atoms form flat low-lying conduction bands and couple with chromium atoms ferromagnetically, while the non-edge iodine atoms weakly couple antiferromagnetically. Narrow CrI 3 nanoribbons have two locally stable magnetic moment orientations, namely out-of-plane and in-plane (along the nanoribbon periodic direction) configurations. This enables four magnetization states in CrI 3 nanoribbons, including two out-of-plane ones (up and down) and two in-plane ones (forward and backward along the nanoribbon periodical direction), increasing the operating controllability. Based on the one-dimensional Ising spin chain model, the spin correlation length of the narrow CrI 3 nanoribbon is estimated as about 10 Å at its estimated Curie temperature of 27 Kelvin, which is lower than the measured 45 Kelvin of the monolayer CrI 3 . The optical absorption and magneto-optical properties of CrI 3 nanoribbons are investigated with many-body perturbation GW-BSE (Bethe-Salpeter equation), including magnetic dichroism, Faraday and magneto-optical Kerr effects. Here, the low-energy dark excitons are mainly from transitions between electrons and holes with unlike spins and are non-Frenkel-like, while the bright excitons have mixed spin configurations. The intrinsic lifetime of excitons can be over one nanosecond, suitable for quantum information processes. Tensile strains and bending manifestly modulate the absorption spectra and magneto-optical properties of CrI 3 nanoribbons within a technologically important photon energy-range of ~1.0-2.0 eV. The CrI 3 nanoribbons can be used in 1D or 2D magnetic storage nanodevices, tunable magnetic optoelectronics, and spin-based quantum information controls.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The role of Anderson’s rule in determining electronic, optical and transport properties of transition metal dichalcogenide heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDs) MX 2 (M = Mo, W; X = S, Se, Te) possess unique properties and novel applications in optoelectronics, valleytronics and quantum computation. In this work, we performed first-principles calculations to investigate the electronic, optical and transport properties of the van der Waals (vdW) stacked MX 2 heterostructures formed by two individual MX 2 monolayers. Here, we found that the so-called Anderson's rule can effectively classify the band structures of heterostructures into three types: straddling, staggered and broken gap. The broken gap is gapless, while the other two types possess direct (straddling, staggered) or indirect (staggered) band gaps. The indirect band gaps are formed by the relatively higher energy level of Te-d orbitals or the interlayer couplings of M or X atoms. For a large part of the formed MX2 heterostructures, the conduction band maximum (CBM) and valence band minimum (VBM) reside in two separate monolayers, thus the electron–hole pairs are spatially separated, which may lead to bound excitons with extended lifetimes. The carrier mobilities, which depend on three competitive factors, i.e. elastic modulus, effective mass and deformation potential constant, show larger values for electrons of MX 2 heterostructures compared to their constituent monolayers. Finally, the calculated optical properties reveal strong absorption in the ultraviolet region.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Bootstrapping more QM systems

Abstract We test the bootstrap approach for determining the spectrum of one dimensional Hamiltonians. In this paper we focus on problems that have a two parameter search space in the bootstrap approach: the double well and a periodic potential associated with the Mathieu equation. For the double well, we compare the bootstrapped energies with contributions from perturbative and non-perturbative results, finding good agreement. For the periodic potentials, we notice that the bootstrap approach gives the band structure of the periodic potential, but it has trouble finding the quasi-momentum of the system. To make further progress on determining the dispersion relation of the bands, new techniques are needed.

Physics↗
DOE OSTI2021

Twisted bilayer graphene. I. Matrix elements, approximations, perturbation theory, and a k · p two-band model

We investigate the twisted bilayer graphene (TBG) model of Bistritzer and MacDonald (BM) [Bistritzer and MacDonald, Proc. Natl. Acad. Sci. 108, 12233 (2011)] to obtain an analytic understanding of its energetics and wave functions needed for many-body calculations. We provide an approximation scheme for the wave functions of the BM model, which first elucidates why the BM K M -point centered original calculation containing only four plane waves provides a good analytical value for the first magic angle (θ M ≈ 1°). The approximation scheme also elucidates why most of the many-body matrix elements in the Coulomb Hamiltonian projected to the active bands can be neglected. By applying our approximation scheme at the first magic angle to a Γ M -point centered model of six plane waves, we analytically understand the reason for the small Γ M -point gap between the active and passive bands in the isotropic limit w 0 = w 1 . Furthermore, we analytically calculate the group velocities of the passive bands in the isotropic limit, and show that they are almost doubly degenerate, even away from the Γ M point, where no symmetry forces them to be. Furthermore, moving away from the Γ M and K M points, we provide an explicit analytical perturbative understanding as to why the TBG bands are flat at the first magic angle, despite the first magic angle is defined by only requiring a vanishing K M -point Dirac velocity. We derive analytically a connected “magic manifold” w 1 = $2\sqrt{1 + w^{2}_{0}}$ $-\sqrt{2 + 3w^2_0}$, on which the bands remain extremely flat as w 0 is tuned between the isotropic (w 0 = w 1 ) and chiral (w 0 = 0) limits. We analytically show why going away from the isotropic limit by making w 0 less (but not larger) than w 1 increases the Γ M -point gap between the active and the passive bands. Finally, by perturbation theory, we provide an analytic Γ M point k ∙ p two-band model that reproduces the TBG band structure and eigenstates within a certain w 0 , w 1 parameter range. Further refinement of this model are discussed, which suggest a possible faithful representation of the TBG bands by a two-band Γ M point k ∙ p model in the full w 0 , w 1 parameter range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Critical role of magnetic moments in heavy-fermion materials: Revisiting mysteries of SmB 6

Heavy-fermion family exhibits fascinating and often puzzling properties due to the presence of open-shell f ions and the complexity of the associated charge, orbital, and spin degrees of freedom. SmB 6 is a prototypical heavy-fermion compound that is electrically insulating but yet it displays quantum oscillations, which are a telltale signature of the metallic state. Adding to the enigma is the possibility that SmB 6 is a topological Kondo insulator. Here, by treating the spin degree of freedom on an equal footing with other degrees of freedom using the parameter-free strongly-constrained and appropriately-normed (SCAN) density functional, we explore the ground-state electronic structure of SmB6. A number of competing magnetic phases lying very closely in energy are found, indicating the key role of spin fluctuations in the material. The computed band structure, crystal-field splittings in the f-electron complex, the heavy effective electron mass at the Fermi energy, and the large specific heat are all in good agreement with the corresponding experimental results. In particular, our predicted FS explains the experimentally observed bulk quantum oscillations as well as the low electrical conductivity of SmB 6 . The topological Kondo state of SmB6 is shown to be robust regardless of its magnetic configuration. The excellent performance of SCAN in heavy-fermion systems is explained in terms of its ability to treat self-interaction errors and symmetry breaking within the framework of the density functional theory. Our study provides a new approach for modeling heavy-fermion materials

36 MATERIALS SCIENCE↗
DOE OSTI2024

Nodal fermions in the strongly spin-orbit coupled pyrochlore-lattice compound RbBi2

We explore the topological electronic band structure of the pyrochlore lattice in the strong spin-orbit coupling regime. Using angle-resolved photoemission spectroscopy, first-principles calculations, and symmetry analysis, we have investigated the bulk electronic structure of RbBi2, which has a Bi pyrochlore network. We observe the presence of 3D massless Dirac fermions enforced by nonsymmorphic symmetry, as well as a 3D quadratic band crossing protected by cubic crystalline symmetry. Furthermore, we identify an additional 3D linear Dirac dispersion associated with band inversion protected by threefold rotation symmetry. These observations reveal the rich band topology of itinerant pyrochlore lattice systems in the strong SOC limit and serve as a starting point to explore correlated topological phases in geometrically frustrated lattices.

Oh, Dongjin↗
DOE OSTI2020

Synthesis, structural characterization, and electronic structure of the novel Zintl phase Ba 2 ZnP 2

The novel Zintl phase dibarium zinc diphosphide (Ba 2 ZnP 2 ) was synthesized for the first time. This was accom­plished using the Pb flux technique, which allowed for the growth of crystals of adequate size for structural determination via single-crystal X-ray diffraction methods. The Ba 2 ZnP 2 com­pound was determined to crystallize in a body-centered ortho­rhom­bic space group, Ibam (No. 72). Formally, this crystallographic arrangement belongs to the K 2 SiP 2 structure type. Therefore, the structure can be best described as infinite [ZnP 2 ] 4– polyanionic chains with divalent Ba 2+ cations located between the chains. All valence electrons are partitioned, which conforms to the Zintl–Klemm concept and suggests that Ba 2 ZnP 2 is a valence-precise com­position. In conclusion, the electronic band structure of this new com­pound, com­puted with the aid of the TB–LMTO–ASA code, shows that Ba 2 ZnP 2 is an intrinsic semiconductor with a band gap of ca 0.6 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Electronic and dielectric properties of MoS{sub 2}-MoX{sub 2} heterostructures

We present a comparative study of electronic and dielectric properties of MoS{sub 2}−MoX{sub 2} heteostructures (where X=S, Se, Te) within the framework of density functional theory (DFT). Electronic band structure, real and imaginary part of dielectric function, electron energy loss spectra and static dielectric constant have been calculated for each system and compared with one another. A systematic decrease/increase in band gap/static dielectric constant is observed as the X changes from S to Te. These results provide a physical basis for the potential applications of these heterostructures in optoelectronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Topological surface states of semimetal TaSb 2

Topological surface states, protected by the global symmetry of the materials, are the keys to understanding various novel electrical, magnetic, and optical properties. TaSb 2 is a newly discovered topological material with unique transport phenomena, including negative magnetoresistance and resistivity plateau, whose microscopic understanding is yet to be reached. In this study, we investigate the electronic band structure of TaSb 2 using angle-resolved photoemission spectroscopy and density functional theory. Our analyses reveal distinct bulk and surface states in TaSb 2 , providing direct evidence of its topological nature. Notably, surface states predominate the electronic contribution near the Fermi level, while bulk bands are mostly located at higher binding energies. Our study underlines the importance of systematic investigations into the electronic structures of topological materials, offering insights into their fundamental properties and potential applications in future technologies.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic properties and carrier mobilities of 6,6,12-graphyne nanoribbons

Structures, stabilities, electronic properties and carrier mobilities of 6,6,12-graphyne nanoribbons (GyNRs) with armchair and zigzag edges are investigated using the self-consistent field crystal orbital method based on density functional theory. It is found that the 1D GyNRs are more stable than the 2D 6,6,12-graphyne sheet in the view of the Gibbs free energy. The stabilities of these GyNRs decrease as their widths increase. The calculated band structures show that all these GyNRs are semiconductors and that dependence of band gaps on the ribbon width is different from different types of the GyNRs. The carrier mobility was calculated based on the deformation theory and effective mass approach. It is found that the carrier mobilities of these GyNRs can reach the order of 10{sup 5} cm{sup 2} V {sup –1}s{sup –1} at room temperature and are comparable to those of graphene NRs. Moreover, change of the mobilities with change of the ribbon width is quite different from different types of the GyNRs.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ab Initio Study of Electronic, Magnetic, and Thermoelectric Response of ZTi{sub 2}O{sub 4} (Z = Mg, Zn, and Cd) Through mBJ Potential

The electronic, magnetic, and thermoelectric properties of ZTi{sub 2}O{sub 4} (Z = Mg, Zn, and Cd) spinel oxides are analyzed by applying DFT approach. The band structure (BS) and density of states (DOS) illustrate a ferromagnetic state due to the evident exchange splitting leading to the energy shift in the spin-polarized sub-bands. Furthermore, ferromagnetic state stability is also justified through the computed crystal field, Jahn-Teller, and exchange energies. The exchange energy exceeding the crystal field energy reveals that ferromagnetism is mediated due to electrons, which is also consistent with the computed negative indirect exchange energy. Using BoltzTraP code, thermoelectric investigation illustrate that Mg, Zn, and Cd cations can tune the optimum power factor to intermediate and higher temperatures, which suggests potential energy harvesting applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Nitrogen: A promising doping strategy for high-performance ovonic threshold switching selectors

The Ovonic Threshold Switching (OTS) selector serves as an essential component in the development of three-dimensional high-density memory integration technology. Nevertheless, the state-of-the-art high-performance OTS materials usually contain toxic elements such as arsenic (As), posing significant risks to both environmental and human health. Nitrogen (N), which belongs to the same group as arsenic (As), has emerged as a highly promising alternative for As doping. However, the underlying mechanisms that govern N-based OTS materials have not yet been extensively investigated. In this study, we delve into the effects of N doping on the structural, bonding, and electronic properties of amorphous GeSe (a-GeNSe) by ab initio molecular dynamics simulations to bridge the knowledge gap. Our findings indicate that upon N doping in a-GeSe, the formation of robust Ge-N bonds, along with N-centered tetrahedral and triangular structures, resulting in the sluggish atomic movement that enhances the thermal stability and endurance of a-GeNSe. The OTS characteristics are significantly influenced by the material’s electronic band structure, and thus the relatively slow performance drift can be attributed to the stabilization of mid-gap states, a result of N doping which effectively slows down the aging process of chalcogenide glass. Moreover, the increased mobility gap in a-GeNSe raises the threshold voltage (V th ), making it more compatible with commercially available phase-change memory materials. Furthermore, our findings reveal the extensive impact of the N element on a typical OTS material and offer valuable perspectives for alternative doping strategies that could potentially supplant As practices.

36 MATERIALS SCIENCE↗