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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Observation of Single-Electron Transport and Charging on Individual Point Defects in Atomically Thin WSe 2

Defects significantly impact the properties of 2D semiconductors, providing in-gap quantum states that can serve as a natural platform for single-electron operations, localization sites for excitons to serve as single-photon sources, and potentially quantum spin memories. To date, however, a microscopic observation of such a single-electron transport (SET) behavior has been rarely reported on single defects of 2D semiconductors. In this study, we report a SET and charge-state transition on individual point-defect states buried in the bilayer WSe 2 using scanning tunneling microscopy. SET characteristics of their states is evidenced by both the Coulomb staircase and the saturation behavior of the transport current, consistent with the SET model. Furthermore, we demonstrate that, through the local field of a scanning tunneling microscope tip, it is possible to successively charge the defects by single electrons, suggested by both the ring structures and charging peaks in dI/dV measurements. Our results provide new insights into the quantum nature of these defect-bound states, as well as the possibility of using their single-electron behavior and response for applications in quantum information and local-field sensing.

2D semiconductor↗
DOE OSTI2022

Efficacy of Stabilizing Calcium Battery Electrolytes through Salt-Directed Coordination Change

Achieving practical, high-energy-density calcium batteries requires controlling the stability of Ca 2+ electrolytes during calcium metal cycling. Because of the highly reactive nature of calcium, most typical electrolyte constituents are unstable, leading to electrode passivation and low Coulombic efficiency. Among various commercially available salts, calcium bis(trifluoromethylsulfonyl)imide (Ca(TFSI) 2 ) is attractive because of its oxidative stability and high solubility in a variety of solvents. However, this salt does not allow for calcium metal plating, and it has been proposed that TFSI – instability induced by Ca 2+ coordination is to blame. In this work, we test the ability of strongly coordinating Ca 2+ cosalts such as halides and borohydrides to displace TFSI – from the first coordination shell of Ca 2+ and thereby stabilize TFSI-based electrolytes to enable calcium plating. Through spectroscopic analysis, we find that the effectiveness of these cosalts at displacing the TFSI – anion is dependent on the solvent’s coordination strength toward Ca 2+ . Surprisingly, electrochemical calcium deposition behavior is not correlated to the population of bound or free TFSI – . Instead, the nature of the coordination interaction between Ca 2+ and the cosalt anion is more important for determining stability. Our findings indicate that TFSI – anions are inherently unstable during calcium deposition even in the nominally free state. Furthermore, strategies aimed at eliminating the interactions of these anions with the electrode surface via interface/interphase design are required.

25 ENERGY STORAGE↗
DOE OSTI2022

Room-Temperature Calcium Plating and Stripping Using a Perfluoroalkoxyaluminate Anion Electrolyte

Multivalent ion chemistries, like Ca 2+ , used in energy storage boast the potential to utilize solid metallic anodes and provide high volumetric energy density. The promise of such systems relies on the ability to incorporate high-voltage cathode materials while sustaining robust plating and stripping at the anode. These processes are dependent on an electrochemically stable electrolyte for ion transport and storage. In this work, we report an addition to the limited pool of nonaqueous electrolytes capable of plating and stripping calcium at room temperature. The synthesis and structural properties of calcium tetrakis(perfluoro-tert-butoxy) aluminate (Ca[TPFA] 2 ) are described along with the electrochemical performance during plating and stripping (up to 55% Coulombic efficiency) and a proof of concept for full cell viability with a CuS cathode material. X-ray experiments were used to study both the bulk solvation structure around calcium and possible decomposition pathways of the weakly coordinating TPFA anion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Role of Coatings as Artificial Solid Electrolyte Interphases on Lithium Metal Self-Discharge

Artificial solid electrolyte interphases have provided a path to improved cycle life for high energy density, next-generation anodes like lithium metal. Although long cycle life is necessary for widespread implementation, understanding and mitigating the effects of aging and self-discharge are also required. In this report we investigate several coating materials and their role in calendar life aging of lithium. We find that the oxide coatings are electronically passivating whereas the LiF coating slows charge transfer kinetics. Furthermore, the Coulombic loss during self-discharge measurements improves with the oxide layers and worsens with the LiF layer. It is found that none of the coatings create a continuous conformal, electronically passivating layer on top of the deposited lithium nor are they likely to distribute evenly through a porous deposit, suggesting that none of the materials are acting as an artificial solid electrolyte interphase. Instead, they likely alter performance through modulating lithium nucleation and growth.

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

Failure Process During Fast Charging of Lithium Metal Batteries with Weakly Solvating Fluoroether Electrolytes

While improving the lithium metal (Li) Coulombic efficiency has been a focus for electrolyte design, the performance under high current densities is less studied yet highly relevant for practical applications. Here, we evaluate the charge-rate-dependent cycling stability using three types of weakly solvating fluoroether electrolytes. Although good cycle life was achieved in all three electrolytes under low current densities, they all exhibited a soft shorting behavior above various threshold current densities (between 2 and 5.2 mA cm –2 ). In this study, we attributed the current-dependent electrode morphology to both Li growth and residual solid electrolyte interface (rSEI) growth processes. In early cycles, Li morphology guided the formation of rSEI structures. In later cycles, the rSEI structure partially impacted Li growth. Under low current densities, the rSEI was inhomogeneous with large voids for subsequent bulky lithium growth. Under high current densities, the rSEI became more dense, which aggravated the high-surface/volume-ratio Li growth through and on the top of the rSEI. Among the three weakly solvating fluoroether electrolytes, the ones with lower ionic conductivity were observed to short within fewer cycles and at lower charge current densities. Our work suggests that fast ion transport in electrolytes may be a desirable feature for the stable operation at >1C charging in high-energy-density lithium metal batteries.

25 ENERGY STORAGE↗
DOE OSTI2025

Evolution and Degradation Patterns of Electrochemical Cells Based on the Analysis of Interfacial Phenomena at Li Metal Anode/Electrolyte Interfaces

In this work, we report the results of a theoretical–computational analysis of the solid electrolyte interphase (SEI) growth and degradation dynamics occurring in lithium metal batteries during cycling. We use ab initio-kinetic Monte Carlo simulations to generate a synthetic data set, which is analyzed by machine learning methods. We aim to determine: (i) how modifications in interfacial interaction energies between solid electrolyte interphase (SEI) blocks and between Li ions and SEI facets impact the Coulombic efficiency (CE) of the battery and (ii) what factors, including reactions, microscopic transport, and other interfacial events, may lead to cell performance “failure” during prolonged charge and discharge cycles, signaled as a sharp decay in the CE over cycling. The demonstration of our approach is done on a cell including a Li metal surface interfacing with a previously introduced state-of-the-art electrolyte, and the idea can be applied to any electrochemical system. Outcomes include the identification of the leading chemical, physical, and structural variables causing cell failure and relating them to the electrolyte formulation, thus paving the way to future more refined analysis and electrolyte design.

batteries↗
DOE OSTI2020

Auger Recombination Lifetime Scaling for Type I and Quasi-Type II Core/Shell Quantum Dots

Having already achieved near-unity quantum yields, with promising properties for light-emitting diode, lasing, and charge separation applications, colloidal core/shell quantum dots have great technological potential. The shell thickness and band alignment of the shell and core materials are known to influence the efficiency of these devices. In many such applications, improving the efficiency requires a deep understanding of multiexcitonic states. In this paper, we elucidate the shell thickness and band alignment dependencies of the biexciton Auger recombination lifetime for quasi-type II CdSe/CdS and type I CdSe/ZnS core/shell quantum dots. We find that the biexciton Auger recombination lifetime increases with the total nanocrystal volume for quasi-type II CdSe/CdS core/shell quantum dots and is independent of the shell thickness for type I CdSe/ZnS core/shell quantum dots. To perform these calculations and compute Auger recombination lifetimes, we developed a low-scaling approach based on the stochastic resolution of identity. The numerical approach provided a framework for studying the scaling of the biexciton Auger recombination lifetimes in terms of the shell thickness dependencies of the exciton radii, Coulomb couplings, and density of final states in quasi-type II CdSe/CdS and type I CdSe/ZnS core/shell quantum dots.

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

Efficient Intermolecular Energy Exchange and Soft-ionization of Water at Nanoplatelet Interfaces

X-ray, energetic photon and electron irradiation can ionize and electronically excite target atoms and molecules. These excitations undergo complicated relaxation and energy transfer processes that ultimately determine the manifold of system responses to the deposited excess energy. In weakly bound gas- and solution-phase samples, intermolecular Coulomb decay (ICD) and electron-transfer mediated decay (ETMD) can occur with neighboring atoms or molecules leading to efficient transfer of the excess energy to the surroundings. In ionic solids such as metal-oxides, intra- and inter-atomic Auger decay produces localized final states that lead to lattice damage and typically removal of cations from the substrate. The relative importance of Auger stimulated damage (ASD) versus ICD and ETMD in micro-solvated nanoparticle interfaces is not known. Though ASD is generally expected, essentially no lattice damage and very efficient ICD and ETMD resulting from ionization and electronic excitation of micro-solvated boehmite (AlOOH) nanoplatelets has been observed. Efficient energy transfer and soft ionization of interfacial molecules is likely a general phenomenon at gas-oxyhydroxide nanoparticle interfaces where the density of states of the ionized chemisorbed species significantly overlap the core hole states of the solid.

Jones, Brant M.↗
DOE OSTI2021

Spectroscopic Determination of Key Energy Scales for the Base Hamiltonian of Chromium Trihalides

The van der Waals (vdW) chromium trihalides (CrX 3 ) exhibit field-tunable, two-dimensional magnetic orders that vary with the halogen species and the number of layers. Their magnetic ground states with proximity in energies are sensitive to the degree of ligand-metal (p-d) hybridization and relevant modulations in the Cr d-orbital interactions. We use soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS) spectroscopy at Cr L -edge along with the atomic multiplet simulations to determine the key energy scales such as the crystal field 10 Dq and interorbital Coulomb interactions under different ligand metal charge transfer (LMCT) in CrX 3 (X= Cl, Br, and I). Through this systematic study, we show that our approach compared to the literature has yielded a set of more reliably determined parameters for establishing a base Hamiltonian for CrX 3 .

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

Dielectric Screening Modulates Semiconductor Nanoplatelet Excitons

The influence of external dielectric environments is well understood for 2D semiconductor materials but overlooked for colloidally grown II–VI nanoplatelets (NPLs). In this work, we synthesize MX (M = Cd, Hg; X = Se, Te) NPLs of varying thicknesses and apply the Elliott model to extract exciton binding energies—reporting values in good agreement with prior methods and extending to less studied cadmium telluride and mercury chalcogenide NPLs. Here, we find that the exciton binding energy is modulated both by the relative effect of internal vs external dielectric and by the thickness of the semiconductor material. An analytical model shows dielectric screening increases the exciton binding energy relative to the bulk by distorting the Coulombic potential across the NPL surface. We further confirm this effect by decreasing and recovering the exciton binding energy of HgTe NPLs through washing in polarizable solvents. Our results illustrate NPLs are colloidal analogues of van der Waals 2D semiconductors and point to surface modification as an approach to control photophysics and device properties.

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

Photoelectron Spectroscopy and Theoretical Investigations of Gaseous Doubly Deprotonated 2’-Deoxynucleoside 5’-Monophosphate Dianions

This work reports a systematic photoelectron spectroscopy and theoretical investigation on the electronic and geometric structures of four doubly deprotonated 2’-deoxynucleoside 5’-monophosphate dianions, the smallest quintessential DNA building block. These dianions are intrinsically stable with their adiabatic/vertical detachment energies (ADE/VDE) ranging from 0.85/1.07 (A), 1.05/1.30 (G) to 1.20/1.50 (C) and 1.80/2.10 eV (T). The repulsive Coulomb barrier against electron detachment is 2.0 eV for purines and 2.5 eV for pyrimidines. Dianions are deprotonated at the phosphate group and the amino group of a nucleobase. The calculated ADE/VDE values, as well as the simulated spectrum, agree well with the experimental results. The ?-type HOMO orbital resides on the nucleobase moiety for each dianion. This spatial distribution of HOMO suggests that the most loosely bound electron is detached along the direction perpendicular to the nucleobase. When combined with the previous results this work makes complete the depiction of basic building blocks of DNA at the molecular level.

Yuan, Qinqin↗
DOE OSTI2021

Distinct Chemistries Explain Decoupling of Slip and Wettability in Atomically Smooth Aqueous Interfaces

Despite essentially identical crystallography and equilibrium structuring of water, nanoscopic channels composed of hexagonal boron nitride and graphite exhibit an order-of-magnitude difference in fluid slip. We investigate this difference using molecular dynamics simulations, demonstrating that its origin is in the distinct chemistries of the two materials. In particular, the presence of polar bonds in hexagonal boron nitride, absent in graphite, leads to Coulombic interactions between the polar water molecules and the wall. We demonstrate that this interaction is manifested in a large typical lateral force experienced by a layer of oriented hydrogen atoms in the vicinity of the wall, leading to the enhanced friction in hexagonal boron nitride. The fluid adhesion to the wall is dominated by dispersive forces in both materials, leading to similar wettabilities. Finally, our results rationalize recent observations that the difference in frictional characteristics of graphite and hexagonal boron nitride cannot be explained on the basis of the minor differences in their wettabilities.

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

Isolated [B 2 (CN) 6 ] 2– : Small Yet Exceptionally Stable Nonmetal Dianion

Here, we report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B 2 (CN) 6 ] 2– in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) —a remarkably high value of electronic stability and a ~2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN) 3 ] •– is energetically favorable. Quantum chemical calculations demonstrate a $D_{3d}$ staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Birth of the Hydrated Electron via Charge-Transfer-to-Solvent Excitation of Aqueous Iodide

A primary means to generate hydrated electrons in laboratory experiments is excitation to the charge-transfer-to-solvent (CTTS) state of a solute such as I – (aq), but this initial step in the genesis of e – (aq) has never been simulated directly using ab initio molecular dynamics. Here, we report the first such simulations, combining ground- and excited-state simulations of I – (aq) with a detailed analysis of fluctuations in the Coulomb potential experienced by the nascent solvated electron. What emerges is a two-step picture of the evolution of e – (aq) starting from the CTTS state: I – (aq) + hν → I –* (aq) → I • (aq) + e – (aq). Notably, the equilibrated ground state of e – (aq) evolves from I –* (aq) without any nonadiabatic transitions, simply as a result of solvent reorganization. The methodology used here should be applicable to other photochemical electron transfer processes in solution, an important class of problems directly relevant to photocatalysis and energy transfer.

Carter-Fenk, Kevin↗
DOE OSTI2024

Design Principles and Routes for Calcium Alkoxyaluminate Electrolytes

Multivalent-ion battery technologies are increasingly attractive options for meeting diverse energy storage needs. Calcium ion batteries (CIB) are particularly appealing candidates for their earthly abundance, high theoretical volumetric energy density, and relative safety advantages. Here, at present, only a few Ca-ion electrolyte systems are reported to reversibly plate at room temperature: for example, aluminates and borates, including Ca[TPFA] 2 , where [TPFA] - = [Al(OC(CF 3 ) 3 ) 4 ] - and Ca[B(hfip) 4 ] 2 , [B(hfip) 4 ] 2 – = [B(OCH(CF 3 ) 2 ) 4 ] - . Analyzing the structure of these salts reveals a common theme: the prevalent use of a weakly coordinating anion (WCA) consisting of a tetracoordinate aluminum/boron (Al/B) center with fluorinated alkoxides. Leveraging the concept of theory-aided design, we report an innovative, one-pot synthesis of two new calcium-ion electrolyte salts (Ca[Al(tftb) 4 ] 2 , Ca[Al(hftb) 4 ] 2 ) and two reported salts (Ca[Al(hfip) 4 ] 2 and Ca[TPFA] 2 ) where hfip = (-OCH(CF 3 ) 2 ), tftb = (-OC(CF 3 )(Me) 2 ), hftb = (-OC(CF 3 ) 2 (Me)), [TPFA] - = [Al(OC(CF 3 ) 3 ) 4 ] - . We also reveal the dependence of Coulombic efficiency on their inherent propensity for cation–anion coordination.

25 ENERGY STORAGE↗
DOE OSTI2025

Complexing Agent-Assisted Membraneless Zinc–Iodine Aqueous Batteries

A membrane is required for conventional zinc–iodine aqueous batteries, since soluble polyiodides cross over to the anode side and react with zinc metal spontaneously. Making the battery membraneless increases ion transport and reduces its cost and overall footprint. In this paper, a membraneless Zn–I 2 aqueous battery is demonstrated, employing a complexing agent, 1-butyl-1-methylpyrrolidinium iodide (MBPI), to promote the formation of I 5 – -containing, phase-separated polyiodides upon charging, to minimize self-discharge and suppress Zn dendrite growth. With an additional 0.3 M MBPI in 4 M ZnI 2 electrolyte, the membraneless battery achieved 65 cycles with >85% Coulombic efficiency, whereas the MBPI-free control failed immediately. Additionally, a volumetric capacity of 14.3 Ah L –1 was achieved, surpassing those of most membraneless batteries reported to date regardless of redox chemistry, and underscores the potential of complexing agents in simplifying the architecture of conventional Zn–I 2 flow batteries.

25 ENERGY STORAGE↗
DOE OSTI2026

Electron–Ion Covariance Reveals Geometry-Resolved Inner-Shell Spectra in CS 2 Photodissociation

The chemical shifts of inner-shell atomic orbitals are highly sensitive to chemical bonding and molecular structure. In evolving systems, however, the spectra of distinct molecular species and geometries overlap, obscuring the underlying chemical dynamics. Here we demonstrate the use of electron–ion covariance analysis to combine the structural sensitivity of Coulomb explosion imaging with inner-shell spectroscopy, yielding geometry-specific spectra of transient and product species. We apply this approach to the excited state dynamics of CS 2 probed by ionization above the S 2p edge. Electron–ion covariance with time- and momentum-selected S + and S 2+ ions isolates distinct S 2p photoelectron spectra for ground-state CS 2 , bent photoexcited CS 2 , the CS photoproduct, and bare atomic sulfur ─ species whose spectra overlap strongly in the channel-averaged measurement. Clear chemical shifts are observed in the covariance photoelectron spectrum for each of these species, all of which are consistent with high-level calculations. Here, by extracting the atomic S contribution to the photoelectron spectrum in a finely time-resolved manner we can disentangle this contribution to the overall time-resolved photoelectron spectrum as the photodissociation proceeds. These results demonstrate the promise of electron–ion covariance as a general approach to geometry-resolved inner-shell spectroscopy, opening a route to tracking structural evolution through chemical shifts in complex photoexcited molecules.

Ionization↗
DOE OSTI2019

Defects in Highly Anisotropic Transition-Metal Dichalcogenide PdSe 2

The atomic and electronic structures of the pristine PdSe 2 as well as various intrinsic vacancy defects in PdSe 2 are studied comprehensively by combining scanning tunneling microscopy, spectroscopy and density functional theory calculations. Other than the topmost Se atoms, sublayer Pd atoms, the intrinsic Pd and Se vacancy-defects are identified. Both V Se and V Pd defects induce defect states near Fermi level. As a result, the vacancy defects can be negatively charged by tip gating effect. At negative sample bias, the screened Coulomb interaction between STM tip and the charged vacancies creates disk-like protrusion around V Pd and crater-like features around V Se . The magnification effect of the long-range charge localization at the charged defect site makes sublayer defects as deep as 1nm visible even in STM images. In conclusion, this result proves that by gating the probe, scanning probe microscopy can be used as an easy tool for characterizing sublayer defects in a nondestructive way.

36 MATERIALS SCIENCE↗