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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Direct intercalation of MoS 2 and WS 2 thin films by vacuum filtration

In the development of next-generation electronics and energy devices, intercalation compounds of transition metal dichalcogenides (TMDCs) are gaining attention for their unique properties that result from synergistic interactions between guest species and host materials. Nowadays, intercalation compounds of MoS 2 and WS 2 are commonly prepared by a two-step process: (1) exfoliation to form single-layer and/or few-layer nanosheets and (2) restacking the nanosheets with the guest species by vigorously mixing the exfoliated suspension with the solution of guest species. While a wide variety of intercalation compounds have been synthesized using this approach, the intercalation process is often timeconsuming, and the product slurry limits material quality and impedes characterization and applications. Herein, we report a versatile method for preparing intercalated TMDCs in a thin-film morphology. Using this approach, we successfully prepared a range of existing intercalation compounds of MoS 2 and WS 2 (e.g., ferrocene and amine intercalated MoS 2 and WS 2 ). Additionally, by leveraging the versatility of this intercalation method, we report we intercalated phenazine and benzoquinone into MoS 2 and WS 2 for the first time.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Bandgap recovery of monolayer MoS 2 using defect engineering and chemical doping

Two-dimensional transition metal dichalcogenide materials have created avenues for exciting physics with unique electronic and photonic applications. Among these materials, molybdenum disulfide is the most known due to extensive research in understanding its electronic and optical properties. In this paper, we report on the successful growth and modification of monolayer MoS 2 (1L MoS 2 ) by controlling carrier concentration and manipulating bandgap in order to improve the efficiency of light emission. Atomic size MoS 2 vacancies were created using a Helium Ion Microscope, then the defect sites were doped with 2,3,5,6-tetrafluro7,7,8,8-tetracyanoquinodimethane (F4TCNQ). The carrier concentration in intrinsic (as-grown) and engineered 1L MoS 2 was calculated using Mass Action model. The results are in a good agreement with Raman and photoluminescence spectroscopy as well as Kelvin probe force microscopy characterizations.

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

Confinement effects on the solvation structure of solvated alkaline metal cations in a single-digit 1T-MoS 2 nanochannel: A first-principles study

Confinement plays an important role in determining ion transport in porous materials, which, in turn, may influence the performance of many energy storage and desalination devices. In this work, we combined density functional theory (DFT) with an implicit solvation model and ab initio molecular dynamics (AIMD) to investigate the effects of nanoconfinement on several solvated alkaline metal cations in a single-digit 1T-MoS 2 nanochannel. Our DFT calculations with a solvation model indicated that cations with stronger hydration energy introduce a higher number of co-intercalated water molecules into the channel, consistent with early experimental observation obtained for MXene (2D transition metal carbide) channels. The predicted optimal water numbers for the cations were then used for AIMD simulations that explicitly include the effects of the solvent. When compared with the cations in bulk solution, our simulations showed that the hydration structure and coordination number (CN) of the solvated cations confined in the MoS 2 channel can be significantly altered. Further, we found that larger cations with weaker hydration energy (K + , Rb + , and Cs + ) exhibited a distinctive CN decrease under confinement, while smaller cations (Li + and Na + ) retained a similar hydration shell as in the bulk solution. More specifically, the hydration shell of large cations (K + , Rb + , and Cs + ) in MoS 2 showed similar features of the coordination angle to the bulk, which suggests the partially broken hydration shell with no geometry change under confinement. Our simulations provided insights into the change of the hydration structure of alkaline metal cations under confinement, which may have important implications on their transport in the 1T-MoS 2 channel.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Toward alcohol synthesis from CO hydrogenation on Cu(111)-supported MoS 2 – predictions from DFT+KMC

In the quest for cheap and efficient catalysts for alcohol synthesis from syngas, a material of interest is single-layer MoS2 owing to its low cost, abundancy, and flexible structure. Because of the inertness of its basal plane, however, it is essential to find ways that make it catalytically active. Herein, by means of density functional theory based calculations of reaction pathways and activation energy barriers and accompanying kinetic Monte Carlo simulations, we show that while S vacancy row structures activate the MoS 2 basal plane, further enhancement of chemical activity and selectivity can be achieved by interfacing the MoS 2 layer with a metallic support. When defect-laden MoS2 is grown on Cu(111), there is not only an increase in the active region (surface area of active sites) but also charge transfer from Cu to MoS 2 , resulting in a shift of the Fermi level such that the frontier states (d orbitals of the exposed Mo atoms) appear close to it, making the MoS 2 /Cu(111) system ready for catalytic activity. Finally, our calculated thermodynamics of reaction pathways lead to the conclusion that the Cu(111) substrate promotes both methanol and ethanol as the products, while kinetic Monte Carlo simulations suggest a high selectivity toward the formation of ethanol.

2D materials↗
DOE OSTI2022

Charge transfer dynamics and interlayer exciton formation in MoS 2 /VOPc mixed dimensional heterojunction

Mixed-dimensional van der Waals heterojunctions involve interfacing materials with different dimensionalities, such as a 2D transition metal dichalcogenide and a 0D organic semiconductor. These heterojunctions have shown unique interfacial properties not found in either individual component. Here, we use femtosecond transient absorption to reveal photoinduced charge transfer and interlayer exciton formation in a mixed-dimensional type-II heterojunction between monolayer MoS 2 and vanadyl phthalocyanine (VOPc). Selective excitation of the MoS 2 exciton leads to hole transfer from the MoS 2 valence band to VOPc highest occupied molecular orbit in ~710 fs. On the contrary, selective photoexcitation of the VOPc layer leads to instantaneous electron transfer from its excited state to the conduction band of MoS 2 in less than 100 fs. Further, this light-initiated ultrafast separation of electrons and holes across the heterojunction interface leads to the formation of an interlayer exciton. These interlayer excitons formed across the interface lead to longer-lived charge-separated states of up to 2.5 ns, longer than in each individual layer of this heterojunction. Thus, the longer charge-separated state along with ultrafast charge transfer times provide promising results for photovoltaic and optoelectronic device applications.

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

Natural formation of linear defect structures in MoS 2

Near surface defects can significantly impact the quality of metallic interconnects and other interfaces necessary to create device structures incorporating two-dimensional materials. Furthermore, the impact of such defects can strongly depend on their organization. In this study, we present scanning tunneling microscopy images and tunneling spectroscopy of point and linear defects near the surface of natural MoS 2 . The point defects share similar structural and electronic characteristics and occur with comparable frequency as subsurface sulfur vacancies observed previously on natural MoS 2 . The linear defects observed here occur less frequently than the point defects but share the same depth profile and electronic structure. These data indicate that the linear defects are actually a one-dimensional organization of subsurface sulfur vacancies. Our density functional calculations agree with this assessment in that, for sufficient local defect concentrations, it is energetically more favorable for the defects to be organized in a linear fashion rather than as clusters or even isolated single point defects. Given these measurements were taken from naturally formed MoS 2 , this organization likely occurs during crystal formation. Considering the impact of one-dimensional organization on the local properties of layered materials, and the potential for them to be introduced purposefully during crystal formation, research into the formation mechanism and properties of these defects could enable new paths for defect engineering in MoS 2 -based systems.

2D materials↗
DOE OSTI2024

Surface dependence of electronic growth of Cu(111) on MoS 2

Scanning tunneling microscopy shows that copper deposited at room temperature onto a freshly exfoliated MoS 2 surface forms Cu(111) clusters with periodic preferred heights of 5, 8, and 11 atomic layers. These height intervals correlate with Fermi nesting regions along the necks of the bulk Cu Fermi surface, indicating a connection between physical and electronic structures. Density functional theory calculations of freestanding Cu(111) films support this as well, predicting a lower density of states at the Fermi level for these preferred heights. This is consistent with other noble metals deposited on MoS 2 that exhibit electronic growth, in which the metal films self-assemble as nanostructures minimizing quantum electronic energies. Here, we have discovered that it is critical for the metal deposition to begin on a clean MoS 2 surface. If copper is deposited onto an already Cu coated surface, even if the original film displays electronic growth, the resulting Cu film lacks quantization. Instead, the preferred heights of the Cu clusters simply increase linearly with the amount of Cu deposited upon the surface. We believe this is due to different bonding conditions during the initial stages of growth. Newly deposited copper would bond strongly to the already present copper clusters, rather than the weak bonding, which exists to the van der Waals terminated surface of MoS 2 . The stronger bonding with previously deposited clusters hinders additional Cu atoms from reaching their lowest quantum energy state. The interface characteristics of the van der Waals surface enable surface engineering of self-assembled structures to achieve different applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Tuning of the electronic and vibrational properties of epitaxial MoS 2 through He-ion beam modification

Abstract Atomically thin transition metal dichalcogenides (TMDs), like MoS 2 with high carrier mobilities and tunable electron dispersions, are unique active material candidates for next generation opto-electronic devices. Previous studies on ion irradiation show great potential applications when applied to two-dimensional (2D) materials, yet have been limited to micron size exfoliated flakes or smaller. To demonstrate the scalability of this method for industrial applications, we report the application of relatively low power (50 keV) 4 He + ion irradiation towards tuning the optoelectronic properties of an epitaxially grown continuous film of MoS 2 at the wafer scale, and demonstrate that precise manipulation of atomistic defects can be achieved in TMD films using ion implanters. The effect of 4 He + ion fluence on the PL and Raman signatures of the irradiated film provides new insights into the type and concentration of defects formed in the MoS 2 lattice, which are quantified through ion beam analysis. PL and Raman spectroscopy indicate that point defects are generated without causing disruption to the underlying lattice structure of the 2D films and hence, this technique can prove to be an effective way to achieve defect-mediated control over the opto-electronic properties of MoS 2 and other 2D materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2020

Metallicity of 2H-MoS 2 induced by Au hybridization

Recent studies have shown that MoS 2 can undergo a structural phase transition from the 2H to 1T' phase on Au substrates at moderate temperatures. In this study, we use ultraviolet and x-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopy, and scanning tunneling microscopy/spectroscopy to probe the impact of annealing exfoliated, monolayer MoS 2 on Au. Our results across multiple length scales indicate that 2H-MoS 2 becomes hybridized with Au upon thermal annealing without inducing the 1T' structural phase and the bandgap can be modulated to zero width depending on the degree of hybridization. These results can be used to control the resistance of metal-MoS 2 contacts at the atomic scale without introduction of defects or structural phase transitions and are important for devices such as memristors.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Dilute magnetic impurity-induced effective phonon magnetic moment in Fe-doped monolayer MoS 2

Realization of large effective phonon magnetic moment in monolayer MoS 2 has established an important route for exploring intriguing magnetic phenomena in a nonmagnetic material. The sizable coupling between the orbital transition and the circularly polarized phonon results in the large effective phonon magnetic moment. In this work, using magneto-Raman spectroscopy, we investigate substitutional doping of magnetic atoms as a tuning knob of the electronic and phononic properties of MoS 2 . We show that Fe-doping polarizes the spin of the conduction bands and introduces a localized Fe band underneath the conduction band. As a result, an additional orbital transition between the Mo 4d and Fe 3d states emerges, producing an orbital-phonon hybridized mode at 283 cm −1 . Our magnetic field dependent measurements demonstrate that this new mode carries 2.8 $\mu_{\mathrm{B}}$ effective phonon magnetic moment, which is comparable to that of the undoped MoS 2 . Moreover, even though a long-range magnetic order is absent in Fe-doped MoS 2 , the local magnetic moment of Fe modifies the nature of the spin fluctuation, producing monotonically increasing quasielastic scattering spectral weight as temperature decreases. Our results highlight two-dimensional dilute magnetic semiconductors synthesized by substitutional doping as a promising material platform to manipulate the phonon magnetic moment through orbital-phonon coupling.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Transient absorption measurements of excitonic dynamics in 3 R -MoS 2

The excitonic dynamics in MoS 2 monolayer, bilayer, and bulk flakes with different stacking orders, namely 3R and 2H, are investigated through transient absorption spectroscopy at room temperature. Samples are obtained by the mechanical exfoliation of bulk MoS 2 crystals with 3R and 2H stacking orders. Photoluminescence spectroscopy is performed to characterize and compare the samples. Time-resolved differential reflection measurements reveal that monolayer flakes exfoliated from the 3R and 2H crystals exhibit similar exciton lifetimes of a few picoseconds. However, the exciton lifetime in the 3R bilayer flake is approximately 85 ps, about twice as long as that in the 2H bilayer. This prolonged exciton lifetime is attributed to the reduction of the electron-hole wavefunction overlap in the 3R bilayer due to the built-in polarization. Similarly, in bulk 3R MoS 2 , the exciton lifetime is around 220 ps, approximately twice as long as those in bulk 2H MoS 2 . The extension of the exciton lifetime in 3R transition metal dichalcogenides could be advantageous for their applications in optoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Unusual flat and extended morphology of intercalated Cu under MoS 2

We report a general method was developed to intercalate metals under layered materials through a controlled density of sputtered defects. The method has already been applied to study a range of metals intercalated under graphite, and different types of morphologies were realized. In the current work, we extend the method to the study of intercalation under MoS 2 , noting that work on this system is rather limited. We use Cu as the prototype metal for comparison with Cu intercalation under graphite. Although the growth conditions needed for intercalation under graphite and MoS 2 are similar, the type of intercalated phases is very different. Each Cu island that nucleates on top of MoS 2 during Cu deposition provides material that is transferred below MoS 2 through sputtered defects under the island base; this transfer results in a uniform intercalated Cu “carpet” morphology that extends over the mesoscale. On the contrary, Cu intercalation under graphite results in well separated, compact islands formed by monomer detachment from small Cu islands on top and transfer below through defects far from the islands. The structural techniques (scanning electron microscopy and atomic force microscopy) and spectroscopic techniques (x-ray photoelectron spectroscopy and energy-dispersive spectroscopy) are used for the characterization of the intercalated Cu layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Interlayer coupling in two-dimensional MoS 2 and phosphorene bilayers: Benchmark quantum Monte Carlo study of interaction energies and quasiparticle band gaps

Using high-accuracy many-body quantum Monte Carlo (QMC) methods, we study the effect of interlayer coupling on the properties of two-dimensional freestanding bilayers (BLs) of MoS 2 and phosphorene. The properties of the two BL-materials are very different and largely determined by the interlayer interaction, which is purely van der Waals in MoS 2 and partially electronic/chemical in phosphorene, resulting in a modest layer-dependent property modulation in MoS 2 and strong modulation in phosphorene. Multireference and symmetry considerations are used to construct state-of-the-art accuracy QMC trial wave functions. We determine the quasiparticle band gaps for both materials, $Δ^{\textrm{qp}}_{Γ→\textrm{K}}$ = 2.45 ± 0.05 eV in BL-MoS 2 and $Δ^{\textrm{qp}}_{Γ→Γ}$=1.59 ±0.1eV in BL-phosphorene. In conclusion, these benchmark band-gap values make it possible to consolidate the interpretation of the widely scattered experimental and theory data.

Huang, Yongda [Slovak Academy of Sciences (SAS), B↗
DOE OSTI2023

Probing the Electronic and Opto-Electronic Properties of Multilayer MoS 2 Field-Effect Transistors at Low Temperatures

Transition metal dichalcogenides (TMDs)-based field-effect transistors (FETs) are being investigated vigorously for their promising applications in optoelectronics. Despite the high optical response reported in the literature, most of them are studied at room temperature. To extend the application of these materials in a photodetector, particularly at a low temperature, detailed understanding of the photo response behavior of these materials at low temperatures is crucial. Here we present a systematic investigation of temperature-dependent electronic and optoelectronic properties of few-layers MoS 2 FETs, synthesized using the mechanical exfoliation of bulk MoS 2 crystal, on the Si/SiO 2 substrate. Our MoS 2 FET show a room-temperature field-effect mobility μ FE ~40 cm 2 ·V -1 ·s -1 , which increases with decreasing temperature, stabilizing at 80 cm 2 ·V -1 ·s -1 below 100 K. The temperature-dependent (50 K < T < 300 K) photoconductivity measurements were investigated using a continuous laser source λ = 658 nm (E = 1.88 eV) over a broad range of effective illuminating laser intensity, P eff (0.02 μW < P eff < 0.6 μW). Photoconductivity measurements indicate a fractional power dependence of the steady-state photocurrent. The room-temperature photoresponsivity (R) obtained in these samples was found to be ~2 AW -1 , and it increases as a function of decreasing temperature, reaching a maximum at T = 75 K. The optoelectronic properties of MoS 2 at a low temperature give an insight into photocurrent generation mechanisms, which will help in altering/improving the performance of TMD-based devices for various applications.

2D materials↗
NASA NTRS1983

Radiation effects on MOS devices - dosimetry, annealing, irradiation sequence, and sources

This paper reports on some investigations of dosimetry, annealing, irradiation sequences, and radioactive sources, involved in the determination of radiation effects on MOS devices. Results show that agreement in the experimental and theoretical surface to average doses support the use of thermo-luminescent dosimeters (manganese activated calcium fluoride) in specifying the surface dose delivered to thin gate insulators of MOS devices. Annealing measurements indicate the existence of at least two energy levels,,s or a activation energies, for recovery of soft oxide MOS devices after irradiation by electrons, protons, and gammas. Damage sensitivities of MOS devices were found to be independent of combinations and sequences of radiation type or energies. Comparison of various gamma sources indicated a small dependence of damage sensitivity on the Cobalt facility, but a more significant dependence in the case of the Cesium source. These results were attributed to differences in the spectral content of the several sources.

Stassinopoulos, E. G.↗
NASA NTRS1984

MOSFET and MOS capacitor responses to ionizing radiation

The ionizing radiation responses of metal oxide semiconductor (MOS) field-effect transistors (FETs) and MOS capacitors are compared. It is shown that the radiation-induced threshold voltage shift correlates closely with the shift in the MOS capacitor inversion voltage. The radiation-induced interface-state density of the MOSFETs and MOS capacitors was determined by several techniques. It is shown that the presence of 'slow' states can interfere with the interface-state measurements.

Benedetto, J. M.↗
DOE OSTI2024

Atomic-Scale Dynamic Mechanisms of Embedded MoS 2 Wires

Nanowires composed of a 1:1 stoichiometry of transition metals and chalcogen ions can be fabricated from two-dimensional transition metal dichalcogenides (TMDs) by using electron beam irradiation. Wires fabricated through in situ experiments can be geometrically connected to TMD sheets in various ways, and their physical properties can vary accordingly. Understanding the structural transformation caused by electron beams is critical for designing wire-sheet structures for nanoelectronics. In this study, we report the behavior of nanowires formed inside a monolayer MoS 2 sheet by combining phase-contrast images and large-scale atomistic modeling. Here we investigate the effect of vacancies on the dynamic evolution of wires, such as rotations with different edge structures and breaking, by considering the interactions between MoS wires and MoS 2 nanosheets. The obtained insights can be applied to other monolayer TMDs to guide the behavior of TMD wires and fabricate favorable geometries for various applications.

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

Methanol carbonylation to acetaldehyde on Au particles supported by single-layer MoS 2 grown on silica

Homogenous single-layer MoS 2 films coated with sub-single layer amounts of gold are found to isolate the reaction of methanol with carbon monoxide, the fundamental step toward higher alcohols, from an array of possible surface reactions. Active surfaces were prepared from homogenous single-layer MoS 2 films coated with sub-single layer amounts of gold. These gold atoms formed clusters on the MoS2 surface. A gas mixture of carbon monoxide (CO) and methanol (CH 3 OH) was partially converted to acetaldehyde (CH 3 CHO) under mild process conditions (308 kPa and 393 K). This carbonylation of methanol to a C 2 species is a critical step toward the formation of higher alcohols. Density functional theory modeling of critical steps of the catalytic process identify a viable reaction pathway. Imaging and spectroscopic methods revealed that the single layer of MoS 2 facilitated formation of nanoscale gold islands, which appear to sinter through Ostwald ripening. Here, the formation of acetaldehyde by the catalytic carbonylation of methanol over supported gold clusters is an important step toward realizing controlled production of useful molecules from low carbon-count precursors.

2D catalyst↗