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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Frustration and Atomic Ordering in a Monolayer Semiconductor Alloy

Frustrated interactions can lead to short-range ordering arising from incompatible interactions of fundamental physical quantities with the underlying lattice. The simplest example is the triangular lattice of spins with antiferromagnetic interactions, where the nearest-neighbor spin-spin interactions cannot simultaneously be energy minimized. In this work, we show that engineering frustrated interactions is a possible route for controlling structural and electronic phenomena in semiconductor alloys. Using aberration-corrected scanning transmission electron microscopy in conjunction with density functional theory calculations, we demonstrate atomic ordering in a two-dimensional semiconductor alloy as a result of the competition between geometrical constraints and nearest-neighbor interactions. Statistical analyses uncover the presence of short-range ordering in the lattice. In addition, we show how the induced ordering can be used as another degree of freedom to considerably modify the band gap of monolayer semiconductor alloys.

2-dimensional systems↗
DOE OSTI2013

Theoretical study of Al{sub n}V{sup +} clusters and their interaction with Ar

Recently, it has been experimentally elucidated whether a V impurity in Al{sub n}V{sup +} clusters occupies an external or an internal site by studying their interaction with argon as a function of cluster size [S. M. Lang, P. Claes, S. Neukermans, and E. Janssens, J. Am. Soc. Mass Spectrom. 22, 1508 (2011)]. In the work presented here we studied, by means of density functional theoretic calculations, the structural and electronic properties of Al{sub n}V{sup +} clusters with n = 14–21 atoms, as well as the adsorption of a single Ar atom on them. For n < 17 the lowest energy structure of Al{sub n}V{sup +} is related to that of the pure Al {sub n+1}{sup +} cluster with the V atom substituting a surface Al atom. For n ⩾ 17 the V impurity becomes embedded in the cluster, in agreement with the experimental results, and the clusters adopt a fcc-like structure instead of the icosahedral-like skeleton of pure Al {sub n+1}{sup +}. We have studied the binding energy per atom, the second energy difference, and the V and Al atom separation energies, in comparison with those of pure Al {sub n+1}{sup +}. We also studied the adsorption of atomic Ar on endohedral and exohedral V doped clusters. The optimized Ar adsorption geometries are formed with Ar on top of a surface atom (V for n < 17, and Al for n ⩾ 17) without noticeable structural distortion of the host cluster. At the critical size (n = 17) of the exohedral-endohedral transition, the calculated Ar adsorption energy exhibits a drop and the Ar-cluster distance increases drastically, indicating that Ar becomes physisorbed rather than chemisorbed. All these results confirm the assumptions made by the experimentalists when interpreting their measurements.

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

Accelerated Discovery of Cost-Effective Photoabsorber Materials for Near-Infrared (λ = 1600 nm) Photodetector Applications

Current infrared sensing devices are based on costly materials with relatively few viable alternatives known. To identify promising candidate materials for infrared photodetection, we have developed a high-throughput screening methodology based on high-accuracy r 2 SCAN and HSE calculations in density functional theory. Using this method, we identify ten already synthesized materials between the inverse perovskite family, the barium silver pnictide family, the alkaline pnictide family, and ZnSnAs 2 as top candidates. Among these, ZnSnAs 2 emerges as the most promising candidate due to its experimentally verified band gap of 0.74 eV at 0 K and its cost-effective synthesis through Bridgman growth. BaAgP also shows potential with an HSE-calculated band gap of 0.64 eV, although further experimental validation is required. Lastly, we discover an additional material, Ca 3 BiP, which has not been previously synthesized, but exhibits a promising optical spectra and a band gap of 0.56 eV. The method applied in this work is sufficiently general to screen wider bandgap materials in high-throughput and now extended to narrow-band gap materials.

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

Copper adparticle enabled selective electrosynthesis of n-propanol

Here, the electrochemical reduction of carbon monoxide is a promising approach for the renewable production of carbon-based fuels and chemicals. Copper shows activity toward multi-carbon products from CO reduction, with reaction selectivity favoring two-carbon products; however, efficient conversion of CO to higher carbon products such as n-propanol, a liquid fuel, has yet to be achieved. We hypothesize that copper adparticles, possessing a high density of under-coordinated atoms, could serve as preferential sites for n-propanol formation. Density functional theory calculations suggest that copper adparticles increase CO binding energy and stabilize two-carbon intermediates, facilitating coupling between adsorbed *CO and two-carbon intermediates to form three-carbon products. We form adparticle-covered catalysts in-situ by mediating catalyst growth with strong CO chemisorption. The new catalysts exhibit an n-propanol Faradaic efficiency of 23% from CO reduction at an n-propanol partial current density of 11 mA cm –2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Enantioselectivity of (321) chiral noble metal surfaces: A density functional theory study of lactate adsorption

The adsorption of the chiral molecule lactate on the intrinsically chiral noble metal surfaces Pt(321), Au(321), and Ag(321) is studied by density functional theory calculations. We use the oPBE-vdW functional which includes van der Waals forces on an ab initio level. It is shown that the molecule binds via its carboxyl and the hydroxyl oxygen atoms to the surface. The binding energy is larger on Pt(321) and Ag(321) than on Au(321). An analysis of the contributions to the binding energy of the different molecular functional groups reveals that the deprotonated carboxyl group contributes most to the binding energy, with a much smaller contribution of the hydroxyl group. The Pt(321) surface shows considerable enantioselectivity of 0.06 eV. On Au(321) and Ag(321) it is much smaller if not vanishing. The chiral selectivity of the Pt(321) surface can be explained by two factors. First, it derives from the difference in van der Waals attraction of L- and D-lactate to the surface that we trace to differences in the binding energy of the methyl group. Second, the multi-point binding pattern for lactate on the Pt(321) surface is sterically more sensitive to surface chirality and also leads to large binding energy contributions of the hydroxyl group. We also calculate the charge transfer to the molecule and the work function to gauge changes in electronic structure of the adsorbed molecule. The work function is lowered by 0.8 eV on Pt(321) with much smaller changes on Au(321) and Ag(321)

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

Detection of Cu{sub 2}Zn{sub 5}SnSe{sub 8} and Cu{sub 2}Zn{sub 6}SnSe{sub 9} phases in co-evaporated Cu{sub 2}ZnSnSe{sub 4} thin-films

Cu{sub 2}ZnSnSe{sub 4} thin-films for photovoltaic applications are investigated using combined atom probe tomography and ab initio density functional theory. The atom probe studies reveal nano-sized grains of Cu{sub 2}Zn{sub 5}SnSe{sub 8} and Cu{sub 2}Zn{sub 6}SnSe{sub 9} composition, which cannot be assigned to any known phase reported in the literature. Both phases are considered to be metastable, as density functional theory calculations yield positive energy differences with respect to the decomposition into Cu{sub 2}ZnSnSe{sub 4} and ZnSe. Among the conceivable crystal structures for both phases, a distorted zinc-blende structure shows the lowest energy, which is a few tens of meV below the energy of a wurtzite structure. A band gap of 1.1 eV is calculated for both the Cu{sub 2}Zn{sub 5}SnSe{sub 8} and Cu{sub 2}Zn{sub 6}SnSe{sub 9} phases. Possible effects of these phases on solar cell performance are discussed.

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

Synthesis and Characterization of Pt–Ag Alloy Nanocages with Enhanced Activity and Durability toward Oxygen Reduction

Engineering the elemental composition of metal nanocrystals offers an effective strategy for the development of catalysts or electrocatalysts with greatly enhanced activity. Herein, we report the synthesis of Pt–Ag alloy nanocages with an outer edge length of 18 nm and a wall thickness of about 3 nm. Such nanocages with a composition of Pt 19 Ag 81 could be readily prepared in one step through the galvanic replacement reaction between Ag nanocubes and a Pt(II) precursor. Here, after 10 000 cycles of potential cycling in the range of 0.60–1.0 V as in an accelerated durability test, the composition of the nanocages changed to Pt 56 Ag 44 , together with a specific activity of 1.23 mA cm –2 toward oxygen reduction, which was 3.3 times that of a state-of-the-art commercial Pt/C catalyst (0.37 mA cm –2 ) prior to durability testing. Density functional theory calculations attributed the increased activity to the stabilization of the transition state for breaking the O–O bond in molecular oxygen. Even after 30 000 cycles of potential cycling, the mass activity of the nanocages only dropped from 0.64 to 0.33 A mg –1 Pt , which was still about two times that of the pristine Pt/C catalyst (0.19 A mg –1 Pt ).

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Controlled Crystalline Surface of Hydroxyapatite on Methane Oxidation Reactions

Hydroxyapatite (HAP, Ca 10 (PO 4 ) 6 (OH) 2 ) has a hexagonal prismatic structure that exposes two crystalline surfaces: prism-faceted a- and basal-faceted c-surfaces. Here, the predominant exposure of c-surface was controlled, and its influences in methane oxidation reactions (combustion and oxidative coupling over HAP and lead-substituted HAP (Pb-HAP), respectively) were studied. The c-surface exposure was realized by crystal orientation in HAP-based catalyst film, which was created by an electrochemical deposition of HAP seeds on a titanium substrate, followed by hydrothermal crystallization and peeling off of the crystalline films from the substrate. In comparison to a-surface that is prevalently exposed in unoriented HAP-based catalysts, the c-surface (i.e., (002) crystalline plane) of HAP-based catalysts exhibited up to 47-fold enhancement in areal rate in both reactions. The distinct catalytic activity between these two crystalline surfaces is attributed to the preferential formation of oxide ions and vacancies on c-surfaces. The oxide ions and vacancies in turns function as actives sites for promoting methane activation and complete oxidation into CO 2 . Density functional theory calculations confirmed the close relationship between different catalytic activities in c-surface of oriented and a-surface of unoriented HAP through the tendency of vacancy formation. Without the presence of vacancies, the methyl or methylene group after methane activation forms ethane or ethylene via coupling. The present study explored the effects of HAP crystal orientation in methane oxidation reactions, which revealed distinct catalytic behaviors of crystal surfaces in HAP-based materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

First principles study of the magnetic properties of LaOMnAs

Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here, a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the ab plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effect of surface-bound sulfide on oxygen reduction reaction on Pt: Breaking the scaling relationship and mechanistic insights

A combined in situ electrochemical attenuated total reflection-surface enhanced IR absorption spectroscopy, microkinetic simulation, and density functional theory calculation study shows that not only can the adsorbed sulfide disproportionally affect the surface binding of OOH* (E OOH *) vs OH* (E OH *), i.e., breaking the original scaling relationship of pure metals (Ir, Pd, Pt, Au), to enhance oxygen reduction reaction (ORR) activity but can also be used as a reaction pathway alternating species to help deepen our mechanistic understanding of ORR.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Edge effects on the electronic properties of phosphorene nanoribbons

Two dimensional few-layer black phosphorus crystal structures have recently been fabricated and have demonstrated great potential in electronic applications. In this work, we employed first principles density functional theory calculations to study the edge and quantum confinement effects on the electronic properties of the phosphorene nanoribbons (PNR). Different edge functionalization groups, such as H, F, Cl, OH, O, S, and Se, in addition to a pristine case were studied for a series of ribbon widths up to 3.5 nm. It was found that the armchair-PNRs (APNRs) are semiconductors for all edge groups considered in this work. However, the zigzag-PNRs (ZPNRs) show either semiconductor or metallic behavior in dependence on their edge chemical species. Family 1 edges (i.e., H, F, Cl, OH) form saturated bonds with P atoms in the APNRs and ZPNRs, and the edge states keep far away from the band gap. However, Family 2 edges (pristine, O, S, Se) form weak unsaturated bonds with the p{sub z} orbital of the phosphorus atoms and bring edge states within the band gap of the ribbons. For the ZPNRs, the edge states of Family 2 are present around the Fermi level within the band gap, which close up the band gap of the ZPNRs. For the APNRs, these edge states are located at the bottom of the conduction band and result in a reduced band gap.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Isotope tracer investigation and ab-initio simulation of anisotropic hydrogen transport and possible multi-hydrogen centers in tin dioxide

Hydrogen as an impurity in single crystals of tin dioxide was investigated through diffusivity and vibrational-mode analyses performed using isotope tracers and density functional theory calculations. It was found that hydrogen diffusion along the 〈001〉 axis is very fast, even at relatively low temperatures (400 °C), but is considerably slower within the (001) plane. Using transitional state calculations, this diffusion behavior was determined to be the result of anisotropy in the migration barrier for interstitial hydrogen (H{sub i}). In addition, the two distinct vibrational modes observed in the optical spectrum were identified as the O-H stretching modes of Hi and the substitutional hydrogen at the tin sites.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Charged nanoparticle attraction in multivalent salt solution: A classical-fluids density functional theory and molecular dynamics study

Here, negatively charged nanoparticles (NPs) in 1:1, 1:2, and 1:3 electrolyte solutions are studied in a primitive ion model using molecular dynamics (MD) simulations and classical density functional theory (DFT). We determine the conditions for attractive interactions between the like-charged NPs. Ion density profiles and NP–NP interaction free energies are compared between the two methods and are found to be in qualitative agreement. The NP interaction free energy is purely repulsive for monovalent counterions, but can be attractive for divalent and trivalent counterions. Using DFT, the NP interaction free energy for different NP diameters and charges is calculated. The depth and location of the minimum in the interaction depend strongly on the NPs’ charge. For certain parameters, the depth of the attractive well can reach 8–10 k B T, indicating that kinetic arrest and aggregation of the NPs due to electrostatic interactions is possible. Rich behavior arises from the geometric constraints of counterion packing at the NP surface. Layering of counterions around the NPs is observed and, as secondary counterion layers form the minimum of the NP–NP interaction free energy shifts to larger separation, and the depth of the free energy minimum varies dramatically. We find that attractive interactions occur with and without NP overcharging.

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

Charge Density Wave and Narrow Energy Gap at Room Temperature in 2D Pb 3– x Sb 1+ x S 4 Te 2-δ with Square Te Sheets

We report a new two-dimensional compound Pb 3– x Sb 1+ x S 4 Te 2-δ has a charge density wave (CDW) at room temperature. The CDW is incommensurate with q-vector of 0.248(6)a* + 0.246(8)b* + 0.387(9)c* for x = 0.29(2) and d = 0.37(3) due to positional and occupational long range ordering of Te atoms in the sheets. The modulated structure was refined from the single crystal X-ray diffraction data with a superspace group P$\bar{1}$(αβγ)0 using (3 + 1)-dimensional crystallography. The resistivity increases with decreasing temperature, suggesting semiconducting behavior. The transition temperature (T CDW ) of the CDW is ~ 345 K above which the Te square sheets become disordered with no q-vector. Lastly, first-principles density functional theory calculations on the undistorted structure and an approximate commensurate supercell reveal that the gap is due to the structure modulation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural defects induced by Fe-ion implantation in TiO{sub 2}

X-ray photoelectron spectroscopy and resonant x-ray emission spectroscopy measurements of pellet and thin film forms of TiO{sub 2} with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO{sub 2} pellet sample induces heterovalent cation substitution (Fe{sup 2+} → Ti{sup 4+}) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe{sup 3+} are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory calculations of formation energies for different configurations of structural defects in the implanted TiO{sub 2}:Fe system. According to our calculations, the clustering of Fe-atoms in TiO{sub 2}:Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different surface to volume ratios.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has D{sub T} = 1.369 cm{sup −1}, E{sub T} = 0.093 cm{sup −1}, and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows D{sub Q} = − 0.306 cm{sup −1}, E{sub Q} = 0.0137 cm{sup −1}, and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has D{sub S} = − 0.203 cm{sup −1}, E{sub S} = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the “heavy atom effect.” This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

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

The Bonding Nature and Adhesion of Polyacrylic Acid Coating on Li-Metal for Li Dendrite Prevention

The success of polyacrylic acid (PAA) to suppress Li dendrite growth suggests that the mechanical properties of polymer-based coatings, including the modulus, toughness, and interfacial adhesion are important design criteria. However, the measurement of the adhesion of thin PAA, as well as other polymer coatings to the reactive Li-metal anode surface is limited experimentally and challenging computationally. In this paper, a strategy was proposed to estimate the adhesion and delamination of the PAA(polymer)/Li interface, based on the bonding nature at the simpler PAA (oligomer)/Li interfaces using density functional theory calculations. It has been shown that the carboxylic acid groups in PAA reacted strongly with metallic Li, which significantly enhances the interfacial adhesion through the Li–O bonds formation, Li ionization and its incorporation into PAA, and –H or –OH termination of Li after decomposition of the COOH functional group. During delamination, it was found that the most likely PAA delamination route involved breaking partial Li–O bonds and lifting some ionized Li atoms from the Li-metal, especially for the Li atoms that showed a charge closer to +1 or are bonded with two O atoms from PAA. Based on the average bonding energies from PAA(oligomer)/Li interface delamination calculations, the work of separation, W sep , of the PAA(polymer)/Li interface was estimated to be ~1.0 (J/m 2 ). The high W sep of PAA (polymer)/Li was comparable with the Li 2 O/Li interface and higher than Li 2 CO 3 /Li and LiF/Li interfaces. This order correlated well with the areal density of Li–O bonds, which can serve as a descriptor for the interfacial adhesion. Furthermore, this computational approach can be applied to other interfaces with polymer-based coatings.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗