Search NASA⌕ Search

SEARCH · Search NASA

Results for “charge tunable”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Ten-valley excitonic complexes in charge-tunable monolayer WSe 2

Excitons dominate the optical response of two-dimensional (2D) semiconductors. Strong interactions produce peculiar excitonic complexes, which provide a testing ground for exciton and quantum many-body theories. Here, we report a hitherto unobserved many-body exciton that emerges upon filling both the K and Q valleys of WSe 2 . We optically probe the exciton landscape using charge-tunable devices with unusually thin dielectrics that facilitate doping up to several 10 13 cm −2 . We observe the emergence of the thermodynamically stable complex when 10 valleys are electrostatically filled. We gain insight into its physics using magneto-optical measurements. Our results are well-described by a model where the number of distinguishable Fermi seas interacting with the photoexcited electron-hole pair defines the complex’s behavior. In addition to expanding the repertoire of excitons in 2D semiconductors, this complex could probe the limit of exciton models and answer open questions about screened Coulomb interactions in 2D semiconductors.

M exciton↗
DOE OSTI2023

Direct experimental evidence of tunable charge transfer at the LaNiO 3 / CaMnO 3 ferromagnetic interface

Interfacial charge transfer in oxide heterostructures gives rise to a rich variety of electronic and magnetic phenomena. Designing heterostructures where one of the thin-film components exhibits a metal-insulator transition opens a promising avenue for controlling such phenomena both statically and dynamically. In this work, we utilize a combination of depth-resolved soft x-ray standing-wave and hard x-ray photoelectron spectroscopies in conjunction with polarization-dependent x-ray absorption spectroscopy to investigate the effects of the metal-insulator transition in LaNiO 3 on the electronic and magnetic states at the LaNiO 3 /CaMnO 3 interface. Here, we report a direct observation of the reduced effective valence state of the interfacial Mn cations in the metallic superlattice with an above-critical LaNiO 3 thickness (6 unit cells, u.c.) facilitated by the charge transfer of itinerant Ni 3d e g electrons into the interfacial CaMnO 3 layer. Conversely, in an insulating superlattice with a below-critical LaNiO 3 thickness of 2 u.c., a homogeneous effective valence state of Mn is observed throughout the CaMnO 3 layers due to the blockage of charge transfer across the interface. The ability to switch and tune interfacial charge transfer enables precise control of the emergent ferromagnetic state at the LaNiO 3 /CaMnO 3 interface and, thus, has far-reaching consequences on the future strategies for the design of next-generation spintronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Electronic structures, charge transfer, and charge order in twisted transition metal dichalcogenide bilayers

Moiré superlattices of transition metal dichalcogenide (TMD) bilayers have been shown to host correlated electronic states, which arise from the interplay of long wavelength moiré potential and long-range Coulomb interaction. Here, we theoretically investigate structural relaxation and single-particle electronic structure of twisted TMD homobilayer. From the large-scale density functional theory calculation and continuum model with layer degrees of freedom, we find that the out-of-plane gating field creates a tunable charge transfer gap at the Dirac point between the first and second moiré valence bands. We further study the charge orders at the fractional band fillings. In the flat band limit, we find from Monte Carlo simulations a series of charge-ordered insulating states at various fillings $n = \frac{1}{4}, \frac{1}{3}, \frac{1}{2}, \frac{2}{3}, 1$. Further, we predict that gating field induces a phase transition between different electron crystals at fixed filling $n = \frac{1}{2}$ or $\frac{2}{3}$. At half-filling $\textit{n} = 1$, the ground state is a Mott insulator with electronically driven ferroelectricity. This work demonstrates that TMD homobilayer provides a powerful platform for the investigation of tunable charge transfer insulator and charge orders.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Tunable Localized Charge Transfer Excitons in Nanoplatelet–2D Chalcogenide van der Waals Heterostructures

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D–2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate–2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe 2 and WSe 2 monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHs were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Finally, our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.

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

Switching charge states in quasi-2D molecular conductors

Abstract 2D molecular entities build next-generation electronic devices, where abundant elements of organic molecules are attractive due to the modern synthetic and stimuli control through chemical, conformational, and electronic modifications in electronics. Despite its promising potential, the insufficient control over charge states and electronic stabilities must be overcome in molecular electronic devices. Here, we show the reversible switching of modulated charge states in an exfoliatable 2D-layered molecular conductor based on bis(ethylenedithio)tetrathiafulvalene molecular dimers. The multiple stimuli application of cooling rate, current, voltage, and laser irradiation in a concurrent manner facilitates the controllable manipulation of charge crystal, glass, liquid, and metal phases. The four orders of magnitude switching of electric resistance are triggered by stimuli-responsive charge distribution among molecular dimers. The tunable charge transport in 2D molecular conductors reveals the kinetic process of charge configurations under stimuli, promising to add electric functions in molecular circuitry.

Huang, Yulong (ORCID:0000000296160134)↗
DOE OSTI2023

Measurement of the conduction band spin-orbit splitting in $\mathrm{WSe_2}$ and $\mathrm{WS_2}$ monolayers

We have investigated charge tunable devices based on $\mathrm{WSe_2}$ and $\mathrm{WS_2}$monolayers encapsulated in hexagonal boron nitride. In addition to the well-known radiative recombination of neutral and charged excitons, stationary photoluminescence measurements highlight a weaker-intensity optical transition that appears when the monolayer is electrostatically doped with electrons. We have carried out a detailed characterization of this photoluminescence line based on its doping dependence, its variation as a function of the optical excitation power, its polarization characteristics, and its g factor measured under longitudinal magnetic field. We interpret this luminescence peak as an impurity-assisted radiative recombination of the intervalley negatively charged exciton (triplet trion). In addition to the standard direct recombination, the triplet trion has a second recombination channel triggered by elastic scattering off the short-range impurity potential. The energy difference between the emitted photons from these two possible recombination processes of the same triplet trion is simply the single-particle conduction band spin-orbit splitting Δ c . We measure Δ c = 12 ± 0.5 meV for the $\mathrm{WSe_2}$ monolayer and Δ c = 12 ± 1 meV for the $\mathrm{WS_2}$ monolayer. Here, these results demonstrate the importance of second-order exciton recombination processes in transition-metal dichalcogenide structures.

2-dimensional systems↗
DOE OSTI2026

Modeling Single-Crystal Battery Materials: From Fundamental Understanding to Performance Evaluation

The performance of rechargeable batteries is fundamentally influenced by the physicochemical properties and microstructural features of their key material components. Recent experimental advancements have highlighted the potential of single-crystal (SC) morphologies to address inherent limitations of polycrystalline (PC) electrodes and solid-state electrolytes, offering tunable charge transport kinetics and improved cell cycling performance. Here, this review examines how state-of-the-art computational modeling, from atomistic and mesoscale to continuum-level approaches, including machine learning methodologies, has been utilized to investigate the critical factors governing the electrochemical behavior of SC battery materials. We explore how predictive modeling can elucidate the processing–structure–property–performance relationships of SC cathodes, anodes, and solid-state electrolytes, with a focus on unique SC characteristics such as crystallographic anisotropy, size effects, and facet-dependent properties. Additionally, we identify limitations in commonly used modeling techniques and discuss strategies to address these challenges. By integrating high-fidelity simulations with experimental insights, this review aims to outline a clear path for the rational design and optimization of SC battery components, paving the way for accelerated advancements in energy storage technologies.

Materials science↗
DOE OSTI2025

Encapsulation of Monolayer 2D Materials Using Kinetic Energy-Controlled Pulsed Laser Deposition

The integration of monolayer (ML) two-dimensional (2D) materials into next-generation microelectronics, optoelectronics, and sensors is hindered by their sensitivity to environmental exposure. Deposition of additional layers for encapsulation or growth on ML 2D materials by versatile but energetic plasma techniques such as pulsed laser deposition (PLD) has not been considered at the monolayer level because of potential damage caused by hyperthermal species with kinetic energies (KEs) exceeding the threshold displacement energy (TDE) of the ML. Here, we describe a general strategy to understand and mitigate damage during PLD by reducing the incident KE of ablated species below the TDE of the 2D monolayer using background gas collisions. Ion flux diagnostics, combined with in situ Raman spectroscopy of monolayer graphene during PLD of amorphous boron nitride (a-BN) as a dielectric encapsulation layer, show that damage is primarily correlated with fast ions that penetrate the background gas in accordance with Beer’s Law and are often overlooked in ICCD imaging due to the dominance of the bright, delayed plasma luminescence. Significantly, if fast ions are eliminated and a ∼2 nm-thick a-BN layer is “soft landed”, the monolayer graphene is effectively protected from damage by high KE species in the boron nitride plasma plume. Deposited a-BN films display a characteristic dielectric constant of 3.6 at 100 kHz and tunable charge injection properties. Our results enable PLD as a viable option for encapsulation and thin film growth onto ML 2D materials, with implications for both fundamental research and device integration.

2D materials↗
DOE OSTI2025

Breakdown of the Static Dielectric Screening Approximation of Coulomb Interactions in Atomically Thin Semiconductors

Coulomb interactions in atomically thin materials are remarkably sensitive to variations in the dielectric screening of the environment, which can be used to control exotic quantum many-body phases and engineer exciton potential landscapes. For decades, static or frequency-independent approximations of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, have been sufficient. These approximations were first applied to quantum wells and were more recently extended with initial success to layered transition metal dichalcogenides (TMDs). Here, we use charge-tunable exciton resonances to investigate screening effects in TMD monolayers embedded in materials with low-frequency dielectric constants ranging from 4 to more than 1000, a range of 2 orders of magnitude larger than in previous studies. In contrast to the redshift predicted by static models, we observe a blueshift of the exciton resonance exceeding 30 meV in higher dielectric constant environments. We explain our observations by introducing a dynamical screening model based on a solution to the Bethe-Salpeter equation (BSE). When dynamical effects are strong, we find that the exciton binding energy remains mostly controlled by the low-frequency dielectric response, while the exciton self-energy is dominated by the high-frequency one. Our results supplant the understanding of screening in layered materials and their heterostructures, introduce a knob to tune selected many-body effects, and reshape the framework for detecting and controlling correlated quantum many-body states and designing optoelectronic and quantum devices.

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

Energy Shifts and Broadening of Excitonic Resonances in Electrostatically Doped Semiconductors

Tuning the density of resident electrons or holes in semiconductors through electrostatic doping provides crucial insight into the composition of the many excitonic complexes that are routinely observed as absorption or photoluminescence resonances in optical studies. Moreover, we can change the way these resonances shift and broaden in energy by controlling the quantum numbers (e.g., spin and valley) of the resident carriers with applied magnetic fields and doping levels, as well as by selecting the quantum numbers of the photoexcited or recombining electron-hole (𝑒−ℎ) pair through optical polarization. Here, we discuss the roles of distinguishability and optimality of excitonic complexes, showing them to be key ingredients that determine the energy shifts and broadening of optical resonances in charge-tunable semiconductors. A distinguishable 𝑒−ℎ pair means that the electron and hole undergoing photoexcitation or recombination have quantum numbers that are not shared by any of the resident carriers. An optimal excitonic complex refers to a complex whose particles come with all available quantum numbers of the resident carriers. Based on the carrier density, magnetic field, and light polarization, all optical resonances may be classified as either distinct or indistinct depending on the distinguishability of the 𝑒−ℎ pair, and the underlying excitonic complex can be classified as either optimal or suboptimal. The universality of these classifications, inherited from the fundamental Pauli exclusion principle, allows us to understand how optical resonances shift in energy and whether they should broaden as doping is increased. This understanding is supported by conclusive evidence that the broadening and decay of optical resonances cannot be simply attributed to enhanced screening when resident carriers are added to a semiconductor. Finally, applying the classification scheme in either monolayer or moiré heterobilayer semiconductor systems, we relate the energy shift and amplitude of the neutral-exciton resonance to the compressibility of the resident carrier gas.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Emergence of composite many-body exciton states in WS 2 and MoSe 2 monolayers

When doped with a high density of mobile charge carriers, monolayer transition-metal dichalcogenide (TMD) semiconductors can host new types of composite many-particle exciton states that do not exist in conventional semiconductors. Further, such multiparticle bound states arise when a photoexcited electron-hole pair couples not to just a single Fermi sea that is quantum-mechanically distinguishable (as in the case of conventional charged excitons or trions), but rather couples simultaneously to multiple Fermi seas, each having distinct spin and valley quantum numbers. Composite six-particle “hexciton” states were recently identified in electron-doped $WSe$ $2$ monolayers, but under suitable conditions they should also form in all other members of the monolayer TMD family. Here we present spectroscopic evidence demonstrating the emergence of many-body hexcitons in charge-tunable $WS$ $2$ monolayers (at the A-exciton) and $MoSe$ $2$ monolayers (at the B-exciton). The roles of distinguishability and carrier screening on the stability of hexcitons are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Tuning the critical Li intercalation concentrations for MoX 2 (X = S, Se, or Te) bilayer phase transitions

Transition metal dichalcogenides (TMDs), such as MoS 2 , are known to undergo a structural phase transformation as well as a change in the electronic conductivity upon Li intercalation. These properties make them candidates for charge-tunable ion-insertion materials that could be used in electro-chemical devices. In this work we study the phase stability and electronic structure of H and T’ Li-intercalated MoX 2 bilayers with X=S, Se, or Te. Here, using first-principles calculations in combination with classical and machine learning modeling approaches, we find that the H phase is more stable at low Li concentration for all X, and the critical Li concentration at which the T’→H transition occurs decreases with increasing mass of X. Furthermore the relative free energy of the two phases becomes less sensitive to Li insertion with increasing atomic mass of the chalcogen atom X. While the electronic conductivity increases with increasing ion concentration at low concentrations, we do not observe a (positive) conductivity jump at the phase transition from H to T’.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tuning the critical Li intercalation concentrations for MoX 2 bilayer phase transitions using classical and machine learning approaches

Transition metal dichalcogenides (TMDs) such as MoX 2 are known to undergo a structural phase transformation as well as a change in the electronic conductivity upon Li intercalation. These properties make them candidates for charge tunable ion-insertion materials that could be used in electro-chemical devices for neuromorphic computing applications. In this work we study the phase stability and electronic structure of Li-intercalated bilayer MoX 2 with X=S, Se or Te. Using first-principles calculations in combination with classical and machine learning modeling approaches we find that the energy needed to stabilize the conductive phase decreases with increasing atomic mass of the chalcogen atom X. A similar decreasing trend is found in the threshold Li concentration where the structural phase transition takes place. While the electronic conductivity increases with increasing ion concentration at low concentrations, we do not observe a conductivity jump at the phase transition point.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Graphene-polyelectrolyte multilayer membranes with tunable structure and internal charge

One great advantage of graphene-polyelectrolyte multilayer (GPM) membranes is their tunable structure and internal charge for improved separation performance. In this study, we synthesized GO-dominant GPM membrane with internal negatively-charged domains, polyethyleneimine (PEI)-dominant GPM membrane with internal positively-charged domains and charge-balanced dense/loose GPM membranes by simply adjusting the ionic strength and pH of the GO and PEI solutions used in layer-by-layer membrane synthesis. A combined system of quartz crystal microbalance with dissipation (QCM-D) and ellipsometry was used to analyze the mass deposition, film thickness, and layer density of the GPM membranes. The performance of the GPM membranes were compared in terms of both permeability and selectivity to determine the optimal membrane structure and synthesis strategy. One effective strategy to improve the GPM membrane permeability-selectivity tradeoff is to assemble charge-balanced dense membranes under weak electrostatic interactions. This balanced membrane exhibits the highest MgCl 2 selectivity (~86%). Another effective strategy for improved cation removal is to create PEI-dominant membranes that provide internal positively-charged barrier to enhance cation selectivity without sacrificing water permeability. Finally, these findings shine lights on the development of a systematic approach to push the boundary of permeability-selectivity tradeoff for GPM membranes.

42 ENGINEERING↗