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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Charge Separation in Monolayer WSe 2 by Strain Engineering: Implications for Strain-Induced Diode Action

Strain-engineering band structure in transition-metal dichalcogenides (TMDC) is a promising avenue toward capabilities in optoelectronics. For example, controlling the flow of optically generated quasiparticles can be achieved by a localized strain field which reduces the bandgap and generates an energy-band gradient that funnels neutral excitons to the strain apex. It would be even more advantageous to mimic a diode’s internal field, where both conduction and valence bands bend in the same direction, to separate electrons and holes. This can be achieved if the strain in the TMDC layer lowers both the conduction band minimum as well as the valence band maximum during strain-induced band narrowing. Here, we have used density functional theory (DFT) calculations of monolayer WSe 2 electronic structure under biaxial strain to show that WSe 2 has this property. In this work, to test the band bending experimentally, we combined localized strain with electrostatic doping to follow photoluminescence from excitons and positive or negative trions. In unstrained WSe 2 , both positive and negative trion emissions dominate over excitons away from charge neutrality. In contrast, for strained areas, negative trions accumulate, while positive trion emission is near zero away from charge neutrality, indicating a lack of holes. Hence, strain bends both conduction and valence bands down, similarly to the band bending in a PN-diode depletion region, providing an opportunity to separate electrons and holes via localized strain.

2D materials↗
DOE OSTI2023

Quadrupolar excitons and hybridized interlayer Mott insulator in a trilayer moiré superlattice

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe 2 -WS 2 -WSe 2 trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe 2 valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe 2 layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe 2 under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Ultrafast pulse pumping of topological nanospaser

Here, we theoretically examine a topological nanospaser that is optically pumped using an ultra-fast circularly-polarized pulse. The spasing system consists of a silver nanospheroid, which supports surface plasmon (SP) excitations, and a transition metal dichalcogenide (TMDC) monolayer nanoflake. The silver nanospheroid screens the incoming pulse and creates a non-uniform spatial distribution of electron excitations in the TMDC nanoflake. These excitations decay into the localized SPs, which can be of two types with the corresponding magnetic quantum number ±1. The amount and the type of the generated SPs depend on the intensity of the optical pulse. For small pulse amplitude, only one plasmonic mode is predominantly generated, resulting in far-field elliptically polarized radiation. For large amplitude of the optical pulse, both plasmonic modes are generated in almost the same amount, resulting in linearly polarized far-field radiation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Ultrafast field-driven valley polarization of transition metal dichalcogenide quantum dots

Abstract We study theoretically the electron dynamics of transition metal dichalcogenide (TMDC) quantum dots (QDs) in the field of an ultrashort and ultrafast circularly polarized optical pulse. The QDs have the shape of a disk and their electron systems are described within an effective model with infinite mass boundary conditions. Similar to TMDC monolayers, a circularly polarized pulse generates ultrafast valley polarization of such QDs. The dependence of the valley polarization on the size of the dot is sensitive to the dot material and, for different materials, show both monotonic increase with the dot radius and nonmonotonic behavior with a local maximum at a finite dot radius.

Physics↗
DOE OSTI2020

Theoretical investigation of charge density wave instability in CuS 2

The existence of a charge density wave (CDW) in transition-metal dichalcogenide (TMDC) CuS 2 has remained undetermined since its first experimental synthesis nearly 50 years ago. Despite conflicting experimental literature regarding its low-temperature structure, there exists no theoretical investigation of the phonon properties and lattice stability of this material. By studying the first-principles electronic structure and phonon properties of CuS 2 at various electronic temperatures, we identify temperature-sensitive soft phonon modes which unveil a previously unreported Kohn anomaly at approximately 100 K. Variation of the electronic temperature shows the presence of two distinct phases, characterized at low electronic temperature by a 2 × 2 × 2 periodic charge modulation associated with the motion of the S 2 dimers. We find this is driven by a slight orbital occupation imbalance of the copper d and sulfur p orbitals, reminiscent of the Jahn-Teller effect in finite systems. Investigation of the Fermi surface presents a potential Fermi surface nesting vector related to the location of the Kohn anomaly and observed band splittings in the unfolded band structure. The combination of these results suggests a strong possibility of CDW order in CuS 2 . Further study of CuS 2 in monolayer form finds no evidence of a CDW phase, as the identified bulk periodic distortions cannot be realized in two dimensions. This behavior sets this material apart from other transition-metal dichalcogenide materials, which exhibit a charge density wave phase down to the two-dimensional limit. As CDW in TMDC materials is considered to compete with superconductivity, the lack of a CDW in monolayer CuS 2 suggests the possibility of enhanced superconductivity relative to other transition-metal dichalcogenides. Altogether, our work identifies CuS 2 as a previously unrealized candidate to study the interplay of superconductivity, CDW order, and dimensionality.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Spontaneous exciton dissociation in transition metal dichalcogenide monolayers

Since the seminal work on MoS 2 , photoexcitation in atomically thin transition metal dichalcogenides (TMDCs) has been assumed to result in excitons, with binding energies order of magnitude larger than thermal energy at room temperature. Here, we reexamine this foundational assumption and show that photoexcitation of TMDC monolayers can result in a substantial population of free charges. Performing ultrafast terahertz spectroscopy on large-area, single-crystal TMDC monolayers, we find that up to ~10% of excitons spontaneously dissociate into charge carriers with lifetimes exceeding 0.2 ns. Scanning tunneling microscopy reveals that photocarrier generation is intimately related to mid-gap defects, likely via trap-mediated Auger scattering. Only in state-of-the-art quality monolayers, with mid-gap trap densities as low as 10 9 cm –2 , does intrinsic exciton physics start to dominate the terahertz response. Our findings reveal the necessity of knowing the defect density in understanding photophysics of TMDCs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Topological nanospaser

We propose a nanospaser made of an achiral plasmonic–metal nanodisk and a two-dimensional chiral gain medium – a monolayer nanoflake of a transition-metal dichalcogenide (TMDC). When one valley of the TMDC is selectively pumped (e.g. by a circular-polarized radiation), the spaser (surface plasmon amplification by stimulated emission of radiation) generates a mode carrying a topological chiral charge that matches that of the gain valley. There is another, chirally mismatched, time-reversed mode with exactly the same frequency but the opposite topological charge; it is actively suppressed by the gain saturation and never generates, leading to a strong topological protection for the generating matched mode. This topological spaser is promising for use in nano-optics and nanospectroscopy in the near field especially in applications to biomolecules that are typically chiral. Another potential application is a chiral nanolabel for biomedical applications emitting in the far field an intense circularly polarized coherent radiation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Phase‐Dependent Band Gap Engineering in Alloys of Metal‐Semiconductor Transition Metal Dichalcogenides

Abstract Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)‐electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe 2 with metallic NbSe 2 , two structural phases of Mo 0.5 Nb 0.5 Se 2 , the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps in the range of 0.42–0.58 at 77 K. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions that result in an uncommon 4 × 4 supercell, rotated approximately 4° from the lattice. Density‐functional‐theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in 1T ‐Mo 0.5 Nb 0.5 Se 2 , but not in the 2H phase. This work expands the boundaries of alloy‐based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying.

Wang, Shuxi↗
DOE OSTI2022

Transition Metal Carbo-Chalcogenide “TMCC:” A New Family of 2D Materials

A new family of 2D transition metal carbo-chalcogenides (TMCCs) is reported, which can be considered a combination of two well-known families, TM carbides (MXenes) and TM dichalcogenides (TMDCs), at the atomic level. Single sheets are successfully obtained from multilayered Nb 2 S 2 C and Ta 2 S 2 C using electrochemical lithiation followed by sonication in water. The parent multilayered TMCCs are synthesized using a simple, scalable solid-state synthesis followed by a topochemical reaction. Superconductivity transition is observed at 7.55 K for Nb 2 S 2 C. The delaminated Nb 2 S 2 C outperforms both multilayered Nb 2 S 2 C and delaminated NbS 2 as an electrode material for Li-ion batteries. Ab initio calculations predict the elastic constant of TMCC to be over 50% higher than that of TMDC.

superconductors↗
DOE OSTI2022

Facile fabrication of 2D material multilayers and vdW heterostructures with multimodal microscopy and AFM characterization

Reliable transfer processes that enable manipulation of two-dimensional (2D) materials, e.g., transition metal dichalcogenides (TMDCs) and MXenes, from one substrate to another has been a necessity for successful device fabrication. Here, with both mechanical exfoliation and chemical vapor deposition (CVD) widely used, a versatile, clean, deterministic, and yet simple transfer technique is highly needed. To address such need, we developed a transfer method that takes advantage of wettability contrast between interfaces without the use of sacrificial layers or chemical processes. More importantly, a setup was developed to carry out this transfer method with high sample selectivity and fine control of the position and orientation of transferred TMDC crystals, a feature required for fabrication of the devices based on vertical 2D heterostructures. Using both exfoliated and CVD grown materials and subsequent atomic force microscopy (AFM), photoluminescence (PL), confocal Raman and tip enhanced Raman spectroscopy (TERS) characterization, we ascertained the quality of interfaces resulting from the transfer process while preserving excellent 2D material integrity. PL and TERS maps revealed nanometer-scale heterogeneities in the interfaces of fabricated heterostructures, which should enable further perfection of the transfer technique. TERS/TEPL information were employed to identify areas suitable for nanodevice fabrication, making the reported transfer and characterization methods ideal for making high quality assembly of 2D heterostructure more accessible, which should facilitate exploration of vertical 2D heterostructures for applications in electronics, batteries, solar cells, and twistronics.

2D material transfer↗
DOE OSTI2024

Solution-processed tungsten diselenide as an inorganic hole transport material for moisture-stable perovskite solar cells in the n-i-p architecture

Some of the obstacles to the commercialization of perovskite solar cells (PSCs) are their long-term moisture stability and material cost of the constituent layers, such as the commonly used spiro-OMeTAD hole transport layer (HTL). Replacing the spiro-OMeTAD with low-cost inorganic hole transport materials (HTMs) are important to further elevate the attractiveness of PSCs for commercialization. Perovskite-compatible, solution-exfoliated two-dimensional (2D) transition metal dichalcogenides (TMDCs) are being considered as viable candidates for inorganic HTMs. We consider one such TMDC, WSe 2 which was chemically exfoliated using dichlorobenzene (DCB), a perovskite-compatible solvent, as it was integrated with triple cation perovskite absorbers within the solar cell stack. Here, the WSe 2 HTL required heat treatment processes to be maintained below 100°C in order to preserve the integrity of the underlying perovskite; despite this lower temperature post treatment process, the structural morphology of the film revealed its dense and pinhole-free nature. Temperature-dependent transport studies conducted on the WSe 2 film provided evidence of its semiconducting character and its ability to extract holes well from the underlying triple-cation Cs 0.05 FA 0.79 MA 0.16 PbI 2.45 Br 0.55 absorber. The inorganic HTL offered better environmental stability in moisture-rich environments of up to 60% relative humidity, in comparison to spiro-OMeTAD HTL-based devices which degraded faster as a result of pinholes

14 SOLAR ENERGY↗
DOE OSTI2024

Structural Studies of Alloyed and Nanoparticle Transition Metal Dichalcogenides by Selenium-77 Solid-State Nuclear Magnetic Resonance Spectroscopy

Layered transition metal dichalcogenides (TMDCs) such as MoS 2 , MoSe 2 , and WSe 2 are under intense investigation because they are atomically thin semiconductors with photophysical properties that can be tuned by changing their composition or morphology. Mechanochemical processing has been proposed as a method to obtain alloyed TMDCs in the series Mo 1–x W x S y Se 2–y (x = 0–1; y = 0–2). However, elucidating the chemical transformations occurring at the atomic scale following mechanochemical processing can be challenging because the products are often amorphous or microcrystalline. To address this challenge, we probe TMDC mixing and alloying by using a combination of powder X-ray diffraction, Raman spectroscopy, diffuse reflectance spectroscopy, 77 Se solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and planewave density functional theory (DFT) calculations. The nature of the milling material and reaction atmosphere are shown to be essential factors in limiting the formation of undesired oxide byproducts. We demonstrate acquisition of 77 Se SSNMR spectra using different combinations of Carr-Purcell Meiboom-Gill acquisition (CPMG) pulse sequences, magic angle spinning (MAS), and MAS dynamic nuclear polarization. Further, the combination of SSNMR with the other characterization methods clearly demonstrates that high energy impact ball milling induces molecular level alloying of Mo, W and chalcogen atoms in the family Mo 1–x W x S y Se 2–y . Gauge including projector augmented wave DFT calculations yield accurate 77 Se chemical shift (CS) tensor components. 77 Se SSNMR spectroscopy was also applied to study the structure of WSe 2 nanocrystals intercalated with ethylenediamine. The intercalated WSe 2 nanocrystals exhibit a more positive isotropic 77 Se CS as compared to bulk WSe 2 , however, the 77 Se CS anisotropy is the same, confirming the WSe 2 layers have a similar structure as in their bulk counterparts.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Highly Active Rhenium-, Ruthenium-, and Iridium-Based Dichalcogenide Electrocatalysts for Oxygen Reduction and Oxygen Evolution Reactions in Aprotic Media

Transition metal dichalcogenides (TMDCs) have garnered much attention recently due to their remarkable performance for different electrochemical systems. In this study, we report on the synthesis and catalysis of less studied TMDC nanoflakes (NFs) with a design space comprised of three transition metals (rhenium, ruthenium, and iridium) and three chalcogens (sulfur, selenium, and tellurium) for the oxygen reduction and evolution reactions (ORR and OER) in an aprotic hybrid electrolyte containing 0.1 M lithium bis(trifluoromethanesulfonypimide salt in 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid and dimethyl sulfoxide. Our results indicate that among the tested catalysts, ReS2 exhibits the highest current density for both ORR and OER, beyond those of the state-of-the-art catalysts used in aprotic media with Li salts. We performed density functional calculations to provide a mechanistic understanding of the reactions in the ReS2 NFs/ionic liquid system. These novel bifunctional catalyst results could open a way for exploiting the unique properties of these materials in Li-O-2 batteries as well as other important electrochemical systems.

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

Hybridization of Excited Interlayer Excitons with Intralayer Excitons in Transition-Metal Dichalcogenide Heterostructures: Interplay of Orbital and Structural Symmetry

Hybridization between interlayer exciton (ILX) and intralayer excitons offers a powerful route to engineer light–matter interactions in transition-metal dichalcogenide (TMDC) heterostructures, yet the underlying mechanism and selection rules remain elusive. Here we probe exciton hybridization in MoTe2/MoSe2 heterobilayers under a tunable out-of-plane electric field and observe distinct avoided crossings in the electric-field dependent reflection spectra. We associate them with coupling between excited ILX states, including the 2p and 2s Rydberg states, and MoTe2 intralayer exciton. Supported by ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations, we identify the hybridizing states and establish optical selection rules based on the effective angular momentum of the exciton states. These findings highlight the role of excitonic binding in the observed hybridization, rather than that of simple band-mixing models, and provide a new pathway for brightening high-energy dark states.

Yao, Helen [Department of Materials Science and En↗
DOE OSTI2023

Nanoscale Periodic Trapping Sites for Interlayer Excitons Built by Deformable Molecular Crystal on 2D Crystal

The nanoscale moiré pattern formed at 2D transition-metal dichalcogenide crystal (TMDC) heterostructures provides periodic trapping sites for excitons, which is essential for realizing various exotic phases such as artificial exciton lattices, Bose–Einstein condensates, and exciton insulators. At organic molecule/TMDC heterostructures, similar periodic potentials can be formed via other degrees of freedom. Here, we utilize the structure deformability of a 2D molecular crystal as a degree of freedom to create a periodic nanoscale potential that can trap interlayer excitons (IXs). Specifically, two semiconducting molecules, PTCDI and PTCDA, which possess similar band gaps and ionization potentials but form different lattice structures on MoS 2 , are investigated. The PTCDI lattice on MoS 2 is distorted geometrically, which lifts the degeneracy of the two molecules within the crystal’s unit cell. The degeneracy lifting results in a spatial variation of the molecular orbital energy, with an amplitude and periodicity of ~0.2 eV and ~2 nm, respectively. On the other hand, no such energy variation is observed in PTCDA/MoS 2 , where the PTCDA lattice is much less distorted. Furthermore, the periodic variation in molecular orbital energies provides effective trapping sites for IXs. For IXs formed at PTCDI/MoS 2 , rapid spatial localization of the electron in the organic layer toward the interface is observed, which demonstrates the effectiveness of these interfacial IX traps.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Strong interaction between interlayer excitons and correlated electrons in WSe2/WS2 moiré superlattice

Heterobilayers of transition metal dichalcogenides (TMDCs) can form a moiré superlattice with flat minibands, which enables strong electron interaction and leads to various fascinating correlated states. These heterobilayers also host interlayer excitons in a type-II band alignment, in which optically excited electrons and holes reside on different layers but remain bound by the Coulomb interaction. Here we explore the unique setting of interlayer excitons interacting with strongly correlated electrons, and we show that the photoluminescence (PL) of interlayer excitons sensitively signals the onset of various correlated insulating states as the band filling is varied. When the system is in one of such states, the PL of interlayer excitons is relatively amplified at increased optical excitation power due to reduced mobility, and the valley polarization of interlayer excitons is enhanced. The moiré superlattice of the TMDC heterobilayer presents an exciting platform to engineer interlayer excitons through the periodic correlated electron states.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Programmable nanowrinkle-induced room-temperature exciton localization in monolayer WSe2

Abstract Localized states in two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been the subject of intense study, driven by potential applications in quantum information science. Despite the rapidly growing knowledge surrounding these emitters, their microscopic nature is still not fully understood, limiting their production and application. Motivated by this challenge, and by recent theoretical and experimental evidence showing that nanowrinkles generate strain-localized room-temperature emitters, we demonstrate a method to intentionally induce wrinkles with collections of stressors, showing that long-range wrinkle direction and position are controllable with patterned array design. Nano-photoluminescence (nano-PL) imaging combined with detailed strain modeling based on measured wrinkle topography establishes a correlation between wrinkle properties, particularly shear strain, and localized exciton emission. Beyond the array-induced wrinkles, nano-PL spatial maps further reveal that the strain environment around individual stressors is heterogeneous due to the presence of fine wrinkles that are less deterministic. At cryogenic temperatures, antibunched emission is observed, confirming that the nanocone-induced strain is sufficiently large for the formation of quantum emitters. At 300 K, detailed nanoscale hyperspectral images uncover a wide range of low-energy emission peaks originating from the fine wrinkles, and show that the states can be tightly confined to regions <10 nm, even in ambient conditions. These results establish a promising potential route towards realizing room temperature quantum emission in 2D TMDC systems.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Disentangling Oxygen and Water Vapor Effects on Optoelectronic Properties of Monolayer Tungsten Disulfide

By understanding how the environmental composition impacts the optoelectronic properties of transition metal dichalcogenide monolayers, we demonstrate that simple photoluminescence (PL) measurements of tungsten disulfide (WS 2 ) monolayers can differentiate relative humidity environments. In this paper, we examine the PL and photoconductivity of chemical vapor deposition grown WS 2 monolayers under three carefully controlled environments: inert gas (N 2 ), dry air (O 2 in N 2 ), and humid nitrogen (H 2 O vapor in N 2 ). The WS 2 PL is measured as a function of 532 nm laser power and exposure time and can be decomposed into the exciton, trion, and lower energy state(s) contributions. Under continuous illumination in either O 2 or H 2 O vapor environment, we find dramatic (and reversible) increases in PL intensity relative to the PL in an inert environment. The PL bathochromically shifts in an O 2 environment and is dominated by increased trion emission and diminished exciton emission. In contrast, the WS 2 PL increase in a H 2 O environment results from an overall increase in emission from all spectral components where the exciton contribution dominates. The drastic increases in PL are anticorrelated with corresponding decreases in photoconductivity, as measured by time-resolved microwave conductivity. The results suggest that both O 2 and H 2 O react photochemically with the WS 2 monolayer surface, modifying the optoelectronic properties, but do so via distinct pathways. Thus, we use these optoelectronic differences to differentiate the amount of humidity in the air, which we show with 0%, 40%, and 80% relative humidity environments. This deeper understanding of how ambient conditions impact WS 2 monolayers enables novel humidity sensors as well as a better understanding of the correlation between TMDC surface chemistry, light emission, and photoconductivity. Moreover, these WS 2 measurements highlight the importance of considering the impact of the local environment on reported results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗