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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Mixed Tin-Titanium Oxides by Atomic Layer Deposition on Planar Substrates: Physical and Electronic Structure

Ternary atomic layer deposition of Sn-doped TiO x (Sn:TiO x ) and Ti-doped SnO x (Ti:SnO x ) was performed using tetrakisdimethylamidotitanium (IV) and tin(IV) with water at 150 °C on (1 0 0) Si and quartz substrates. The physical structure of the films was investigated using scanning electron, atomic force, and Kelvin probe force microscopies as well as spectroscopic ellipsometry, Raman spectroscopy, and X-ray diffraction. All as-deposited films are amorphous and possessed flat and conformal topographies with low surface roughness (<1 nm). Annealed Sn:TiO x and Ti:SnO x crystallize into the tetragonal anatase TiO 2 and cassiterite SnO 2 structures, respectively, without large variations to the film topography. The chemical composition of the as-deposited and annealed films was investigated through X-ray photoelectron spectroscopy. While dopants are homogenously mixed, the ternary ALD to form Sn:TiO x and Ti:SnO x surprisingly does not follow the rule of mixtures, despite similarities in both the growth rates of binary SnO x and TiO x and the precursors used. This finding is also supported by spectroscopic ellipsometry. Here, we postulate the deviation from the rule of mixtures is due to non-ideal surface chemistry of the binary ALD of SnO x . The optical bandgap of all films was quantified by the Tauc analysis and the valence band position was investigated by valence band X-ray photoelectron spectroscopy. Intra-bandgap states above the valence band maximum were observed for as-deposited Ti:SnO x and Sn:TiO x and ascribed to 5 s states from Sn 2+ ions. The presence of these intra-bandgap states induces enhanced visible absorption and bathochromic shifts to the apparent bandgap for all as-deposited films. Annealing the films removes the intra-bandgap states and increases the optical bandgap. Simplified band diagrams created using the data available demonstrated that the conduction and valence band positions did not vary between the positions of pure TiO 2 or SnO 2 with monotonic increases to the dopant incorporation.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A fast, dense Chebyshev solver for electronic structure on GPUs

Matrix diagonalization is almost always involved in computing the density matrix needed in quantum chemistry calculations. In the case of modest matrix sizes (≲4000), performance of traditional dense diagonalization algorithms on modern GPUs is underwhelming compared to the peak performance of these devices. This motivates the exploration of alternative algorithms better suited to these types of architectures. We newly derive, and present in detail, an existing Chebyshev expansion algorithm whose number of required matrix multiplications scales with the square root of the number of terms in the expansion. Focusing on dense matrices of modest size, our implementation on GPUs results in large speed ups when compared to diagonalization. Additionally, we improve upon this existing method by capitalizing on the inherent task parallelism and concurrency in the algorithm. Furthermore, this improvement is implemented on GPUs by using CUDA and HIP streams via the MAGMA library and leads to a significant speed up over the serial-only approach for smaller (≲1000) matrix sizes. Finally, we apply our technique to a model system with a high density of states around the Fermi level, which typically presents significant challenges.

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

Single-Particle Insights Into the Electronic Structure and Enhanced Stability of CsPbBr 3 /FAPbBr 3 Core/Crown Nanoplatelets at the Nanometer Scale

Colloidal perovskite nanoplatelets (NPLs), despite their exceptional optoelectronic properties, face significant challenges due to their intrinsic instability and trap-assisted non-radiative recombination. Although many studies employing surface engineering, such as selecting ligands or coating to passivate defects, have demonstrated improved optoelectronic properties, these enhancements are typically inferred from bulk-ensemble measurements; the effects at the single-particle level remain elusive. Here, we conduct single-particle-level studies using scanning tunneling spectroscopy (STS) on a unique core–crown system, CsPbBr 3 @FAPbBr 3 NPLs, where the lateral surfaces of the CsPbBr 3 core are coated with an FAPbBr 3 crown. Experimental density-of-states (DOS) analysis reveals a 47% reduction in deep-trap states in core-crown NPLs compared to core-only NPLs, consistent with nearly two-fold enhancements in photoluminescence quantum yields. Progressive I–V sweep measurements demonstrate superior electrical stability in core-crown NPLs, preserving band structure with minimal degradation, while core-only NPLs exhibit rapid bandgap shrinkage and trap formation. Density functional theory (DFT) calculations indicate that FA incorporation distorts Pb octahedral lattice, widening the bandgap. This study elucidates how surface engineering modulates charge localization, passivates defects, and enhances stability at the single-particle level. Moreover, by uncovering bias-induced trap states in single unpassivated NPLs, this study establishes a precise, robust approach for characterizing emerging perovskite nanocrystals.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

Electronic structure of Li 1,2,3 +,0,– and nature of the bonding in Li 2,3 +,0,–

Abstract The current study of the small lithium molecules Li 2 +,0,− and Li 3 +,0,− focuses on the nature of the bonding in these molecules as well as their structures and energetics (bond energies, ionization energies, and electron affinities). Valence CASSCF (2s,2p) calculations incorporate nondynamical electron correlation in the calculations, while the corresponding multireference configuration interaction and coupled cluster calculations incorporate dynamical electron correlation. Treatment of nondynamical correlation is critical for properly describing the Li 2,3 +,0,− molecules as well as the Li − anion with dynamical correlation, in general, only fine‐tuning the predictions. All lithium molecules and ions are bound, with the Li 3 + and Li 2 + ions being the most strongly bound, followed by Li 3 − , Li 2 , Li 2 − and Li 3 . The minimum energy structures of Li 3 +,0,− are, respectively, an equilateral triangle, an isosceles triangle, and a linear structure. The results of SCGVB calculations are analyzed to obtain insights into the nature of the bonding in these molecules. An important finding of this work is that interstitial orbitals, a concept first put forward by McAdon and Goddard in 1985, play an essential role in the bonding of all lithium molecules considered here except for Li 2 . The interstitial orbitals found in the Li 3 +,0 molecules likely give rise to the non‐nuclear attractors/maxima observed in these molecules.

Chemistry↗
DOE OSTI2019

Analysis of superfast encoding performance for electronic structure simulations

In our recent work, we examined various fermion-to-qubit mappings in the context of quantum simulation including the original Bravyi-Kitaev superfast encoding (OSE) as well as a generalized version (GSE). We return to OSE and compare it against the Jordan-Wigner (JW) transform for quantum chemistry considering the number of qubits required, the Pauli weight of terms in the transformed Hamiltonians, and the L 1 norm of the Hamiltonian. We considered a test set of molecular systems known as the Atomization Energy 6 (AE6) as well as hydrogen lattices. Our results showed that the resource efficiency of OSE is strongly affected by the spatial locality of the underlying single-particle basis. We find that OSE is outperformed by JW when the orbitals in the underlying single-particle basis are highly overlapping, which limits its applicability to near-term quantum chemistry simulations utilizing standard basis sets. In contrast, when orbitals are overlapping with only few others, as is the case of hydrogen lattices with very tight orbitals, OSE fares comparatively better. Furthermore, our results illustrate the importance of choosing the right combination of basis sets and fermion-to-qubit mapping to get the most out of a quantum device when simulating physical systems.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Electronic structure in a transition metal dipnictide TaAs 2

Abstract The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently, T a A s 2 , a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the ( 2 ‾ 01 ) surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of T a A s 2 are trivial bulk bands. The absence of symmetry-protected surface state on the ( 2 ‾ 01 ) surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of T a A s 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

XAFS Spectroscopy Study of Microstructure and Electronic Structure of Heterosystems Containing Si/GeMn Quantum Dots

Using X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, atomic force microscopy, and Rutherford backscattering spectroscopy, the features of the microstructure and elemental composition of Si/GeMn magnetic systems obtained by molecular beam epitaxy and containing quantum dots are studied. Intense mixing of Ge and Si atoms is found in all samples. The degree of mixing (diffusion) correlates with the conditions of synthesis of Si/GeMn samples. For these systems, direct contacts of germanium atoms with manganese atoms are characterized and the presence of interstitial manganese with tetrahedral coordination and substitution of manganese for germanium and silicon in the lattice sites is found. The presence of stoichiometric phases Ge{sub 8}Mn{sub 11}, Ge{sub 3}Mn{sub 5} is not detected. The correlations of the Ge, Si, and Mn coordination numbers in the Ge environment are determined both with the Mn flux value (evaporator temperature) and with the temperature at which quantum dots are grown, as well as with other synthesis conditions. The manganese concentration in the samples is determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Ordering of Trotterization: Impact on Errors in Quantum Simulation of Electronic Structure

Trotter–Suzuki decompositions are frequently used in the quantum simulation of quantum chemistry. They transform the evolution operator into a form implementable on a quantum device, while incurring an error—the Trotter error. The Trotter error can be made arbitrarily small by increasing the Trotter number. However, this increases the length of the quantum circuits required, which may be impractical. It is therefore desirable to find methods of reducing the Trotter error through alternate means. The Trotter error is dependent on the order in which individual term unitaries are applied. Due to the factorial growth in the number of possible orderings with respect to the number of terms, finding an optimal strategy for ordering Trotter sequences is difficult. In this paper, we propose three ordering strategies, and assess their impact on the Trotter error incurred. Initially, we exhaustively examine the possible orderings for molecular hydrogen in a STO-3G basis. We demonstrate how the optimal ordering scheme depends on the compatibility graph of the Hamiltonian, and show how it varies with increasing bond length. We then use 44 molecular Hamiltonians to evaluate two strategies based on coloring their incompatibility graphs, while considering the properties of the obtained colorings. We find that the Trotter error for most systems involving heavy atoms, using a reference magnitude ordering, is less than 1 kcal/mol. Relative to this, the difference between ordering schemes can be substantial, being approximately on the order of millihartrees. The coloring-based ordering schemes are reasonably promising—particularly for systems involving heavy atoms—however further work is required to increase dependence on the magnitude of terms. Finally, we consider ordering strategies based on the norm of the Trotter error operator, including an iterative method for generating the new error operator terms added upon insertion of a term into an ordered Hamiltonian.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2018

Erratum to: “The Fluorescence and Electronic Structure of Phenyl-Substituted Tetraazachlorin Molecules” [Optics and Spectroscopy 117, 722 (2014)]

should read “porphyrazine” instead of “porphyrazin”;should read “intersystem crossing” instead of “intercombinational conversion”;should read “mirror-symmetrical” instead of “reflection-symmetric”;should read “fluorescence enhancement” instead of “fluorescence buildup.”Page 722, Abstract, lines 9–10 from the top, the sentence should read:This characteristic increases upon tetrapyrrole phenylation.Page 727, left column, lines 17–18 from the top, the sentence should read:The vibronic transition parts of the emission spectrum are diffuse, (…)Page 727, left column, lines 29–30 from the top, the sentence should read:Note that the Stokes shift in these cases is relatively large.Page 729, left column, lines 8–10 from the bottom, the sentence should read:For H{sub 2}Ph{sub 8}TAC{sup mp} and H{sub 2}Ph{sub 6}TAC{sup tm}, the enhancement of fluorescence is weaker than in the former case: on cooling, τ{sub F} is increased by a factor of 4–5 in MTHF and by a factor of only 2.6–3 in toluene.Page 729, right column, lines 9–10 from the footnote, the sentence should read:It is seen from Table 1 that our results agree reasonably well with previously reported data.Page 730, left column, lines 16–19 from the top, the sentence should read:The lifetime for the outgassed H{sub 2}TAP solution intoluene (τ{sub T}{sup 0}) was measured by A.S. Stasheuski as phosphorescence decay at λ{sub reg} = 1070 nm using the setup described in [29].Page 730, left column, lines 20–23 from the top, the sentence should read:The τ{sub T} value derived from the kinetics of the rise of singlet oxygen luminescence [26] for the nonoutgassed solution at λ{sub reg} = 1272 nm is 162 ns;Page 730, right column, lines 25–27 from the top, the sentence should read:Therefore, for H{sub 2}TAP (λ{sub F}{sup 00}≈ 620 nm), the internal conversion probability should not be high.Page 731, left column, the heading of Table 2 should read:Bond lengths in the porphine macrocycle (Å).Page 731, left column, lines 3–6 from the top, the sentence should read:For elucidating the mechanism of the deactivation of the S{sub 1} state, it was of essential interest to see whether the enhancement of H{sub 2}TAP fluorescence would be observed at 77 K.Page 733, left column, lines 2–3 from the top, the sentence should read:The corresponding difference for the H{sub 2}P–H{sub 2}C pair is 0.013 Å.Page 733, left column, lines 14–16 from the top, the sentence should read:The macrocycle undergoes additional contraction, and the symmetry of its conjugated system is very close to D{sub 2h}.Page 733, right column, line 21 from the top should read:Replacement of the methine bridges (…)Page 734, left column, lines 8–11 from the figure caption, the sentence should read:This is the main reason of the large bathochromic shift of the Soret band of H{sub 2}Ph{sub 8}TAP and MgPh{sub 8}TAP [16] relative to H{sub 2}TAP and MgTAP, respectively.Page 734, left column, lines 1–4 from the bottom, the sentence should read:The complete INDO/Sm calculation yields the corresponding quantitative characteristics, which are in good agreement with experimental data (see below).Page 740, left column, reference no. 25 should read:25. V. A. Galievsky, A. S. Stasheuski, V. V. Kiselyov, A. I. Shabusov, M. V. Belkov, and B. M. Dzhagarov, Instrum. Exp. Tech. 53, 568 (2010).25. V. A. Galievsky, A. S. Stasheuski, V. V. Kiselyov, A. I. Shabusov, M. V. Belkov, and B. M. Dzhagarov, Instrum. Exp. Tech. 53, 568 (2010).30. V. L. Ermolaev and E. B. Sveshnikova, Acta Phys. Pol. 34, 771 (1968).

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Synthesis, structure, electronic and thermal properties of sphalerite CuZn 2 InS 4

Quaternary chalcogenides continue to be of interest due to the variety of physical properties they possess, as well as their potential for different applications of interest. Investigations on materials with the sphalerite crystal structure have only recently begun. In this study we have synthesized sulfur-based sphalerite quaternary chalcogenides, including off-stoichiometric compositions, and investigated the temperature-dependent electronic, thermal and structural properties of these materials. Insulating to semiconducting transport is observed with stoichiometric variation, and analyses of heat capacity and thermal expansion revealed lattice anharmonicity that contributes to the low thermal conductivity these materials possess. Finally, we include similar analyses for CuZn 2 InSe 4 and compare these sphalerite quaternary chalcogenides to that of zinc blende binaries in order to fully understand the origin of the low thermal conductivity these quaternary chalcogenides possess.

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

Bidentate Lewis Base Ligand-Mediated Surface Stabilization and Modulation of the Electronic Structure of CsPbBr 3 Perovskite Nanocrystals

The desorption of conventional ligands from the surface of halide perovskite nanocrystals (NCs) often causes their structural instability and the deterioration of the optoelectronic properties. To address this challenge, we present an approach of using a bidentate Lewis base ligand, namely, 1,4-bis(diphenylphosphino)butane (DBPP), for the synthesis of CsPbBr 3 NCs. The phosphine group of DBPP has a strong interaction with PbBr 2 precursor, forming a highly crystalline intermediate complex during the reaction. In the presence of oleic acid, the uncoordinated phosphine group of DBPP is converted into the phosphonium cation, which strongly binds with the surface bromide of the formed CsPbBr 3 NCs through hydrogen bonding. Density functional theory calculations suggest that DBPP can strongly bind to the undercoordinated lead and surface bromide ions of CsPbBr 3 NCs through its unprotonated and protonated phosphine groups, respectively. The robust binding of DBPP with the surface of perovskite NCs helps to preserve their structural integrity under various environmental stresses. Moreover, the electron density and energy levels are regulated in DBPP-capped CsPbBr 3 NCs via the efficient transfer of electrons from the ligands to the NCs, resulting in their improved photocatalytic CO 2 reduction performance. Furthermore, our study highlights the potential of using bidentate ligands to stabilize the surface of perovskite NCs and modulate their optical and electronic properties.

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

Electronic, structural and magnetic properties of Mn (1+x) Pt (1-x) Sb

Electronic and magnetic properties of half-metallic Heusler alloys can be modified by tuning their chemical compositions. We have carried out a combined theoretical and experimental investigation of Mn (1+x) Pt (1-x) Sb (0 ≤ x ≤ 0.5) alloys. Our first-principles calculations indicate that the stoichiometric MnPtSb exhibits nearly half-metallic band structure, but a robust half-metallicity can be achieved in Mn-rich compositions Mn (1+x) Pt (1-x) Sb (0 ≤ x ≤ 0.5) with x = 0.25 and higher in their cubic structures. In addition, while MnPtSb exhibits ferromagnetic alignment, Mn (1+x) Pt (1-x) Sb are ferrimagnetic for all non-zero values of x. We have also synthesized cubic MnPtSb and Mn 1.25 Pt 0.75 Sb alloys using arc melting and annealing. The magnetic properties of these alloys are consistent with our theoretical predictions. Furthermore, these results indicate that the Mn-rich Mn (1+x) Pt (1-x) Sb alloys have potential for spin-transport-based devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nature of native atomic defects in ZrTe 5 and their impact on the low-energy electronic structure

Over the past decades, investigations of the anomalous low-energy electronic properties of ZrTe 5 have reached a wide array of conclusions. An open question is the growth method's impact on the stoichiometry of ZrTe 5 samples, especially given the very small density of states near its chemical potential. In this work, we report on high-resolution scanning tunneling microscopy and spectroscopy measurements performed on samples grown via different methods. Using density functional theory calculations, we identify the most prevalent types of atomic defects on the surface of ZrTe 5 , namely, Te vacancies and intercalated Zr atoms. Finally, we precisely quantify their density and outline their role as ionized defects in the anomalous resistivity of this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Socorro Electronic Structure Software

Socorro can be used to compute the ground-state electron density for a periodically repeated simulation cell in which the external potential is obtained from norm-conserving pseudopotentials or projector-augmented-wave functions. The ground-state electron density is computed by summation over one-electron orbitals which are obtained using the Kohn-Sham formulation of density-functional theory and which are expanded in a plane-wave basis. Various quantities can be computed from the ground-state solution, including atomic forces which can be used to optimize atom positions in the simulation cell and to perform molecular dynamics simulations.

Foiles, StephenM↗
DOE OSTI2017

Socorro Electronic Structure Software

Socorro can be used to compute the ground-state electron density for a periodically repeated simulation cell in which the external potential is obtained from norm-conserving pseudopotentials or projector-augmented-wave functions. The ground-state electron density is computed by summation over one-electron orbitals which are obtained using the Kohn-Sham formulation of density-functional theory and which are expanded in a plane-wave basis. Various quantities can be computed from the ground-state solution, including atomic forces which can be used to optimize atom positions in the simulation cell and to perform molecular dynamics simulations.

Foiles, Stephen M↗
DOE OSTI2018

Electronic and structural response to pressure in the hyperkagome-lattice Na 3 Ir 3 O 8

Here, the hyper-kagome lattice iridate Na 3 Ir 3 O 8 , closely related to spin liquid candidate Na 4 Ir 3 O 8 , is unusual in that spin-orbit interactions acting on the 1/3-filled J eff = 1/2 state lead to a semimetallic ground state, in contrast to the conventional insulating Mott state stabilized by S-O interactions in the 1/2-filled J eff = 1/2 state of other iridates including Na 4 Ir 3 O 8 . We have studied the evolution of crystal structure, electronic structure, and transport in Na 3 Ir 3 O 8 under high pressure using x-ray diffraction, x-ray absorption near edge structure, and electrical resistance measurements in a diamond anvil cell. The study was augmented by the use of ab initio calculations, which provided insight into pressure-induced changes in crystal and electronic structure. We found that Na 3 Ir 3 O 8 transforms from a semimetal to an insulator under pressure, with an estimated energy gap that increases to about 130 meV at P similar to 9 GPa. At approximately 10 GPa, a cubic-to-monoclinic structural transition takes place between two insulating phases. This structural transition features a sizable volume collapse and a high-pressure phase characterized by apparent dimerization of Ir-Ir distances, wide dispersion of Ir-O-Ir bond angles, and an increase in the occupation of 5 d states. Although the energy gap is reduced in the high-pressure phase, insulating behavior remains to the highest pressures similar to 1 Mbar. The strongly pressure-dependent insulating gap shows a positive correlation with the expectation value of the angular part of the S-O interaction, < L • S >, which may indicate an active role of S-O interactions in stabilizing the insulating state that emerges in the compressed Na 3 Ir 3 O 8 structure.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Vibrational spectra and electronic structure of 11-vertex boron-containing clusters: a comparative study of [B{sub 11}H{sub 11}]{sup 2–}, [CB{sub 10}H{sub 11}]{sup –}, and C{sub 2}B{sub 9}H{sub 11}

Vibrational spectroscopy studies of 11-vertex boron-containing clusters [B{sub 11}H{sub 11}]{sup 2–}(1), [2-CB{sub 10}H{sub 11}]{sup –} (2), and 2,3-C{sub 2}B{sub 9}H{sub 11} (3) revealed two low-frequency large-amplitude modes in their spectra and, therefore, structural nonrigidity of these polyhedra in contrast to related 12-, 10-, and 6-vertex closo-clusters. The nonrigidity originates from specific features of the electron density distribution. According to calculations within the framework of the 'Atoms in Molecule' (AIM) theory, these polyhedra are characterized by the lack of many bond paths localized on corresponding edges, in particular, between boron atoms with coordination numbers of 6 and 5 (6k and 5k). Consequently, these polyhedra are composed of not only trigonal faces, i.e., formally do not have the closo-structure that was traditionally ascribed to them based on formal count of (2n + 2) valence electrons. According to calculations of three-center delocalization indices, the absence of bond paths is not equivalent to the lack of interaction between corresponding atoms since this interaction occurs through open three-center bonds. The mechanism and reasons for signal averaging observed in {sup 11}B NMR spectra of compounds 1 and 2 in contrast to 3 are analyzed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗