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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Piezophototronic Effect in Single-Atomic-Layer MoS 2 for Strain-Gated Flexible Optoelectronics

Strain-gated flexible optoelectronics are reported based on monolayer MoS2. Utilizing the piezoelectric polarization created at metal-MoS2 interface to modulate the separation/transport of photogenerated carriers, the piezophototronic effect is applied to implement atomic-layer-thick phototransistor. Coupling between piezoelectricity and photogenerated carriers may enable the development of novel optoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Accumulation-Type Ohmic van der Waals Contacts to Nearly Intrinsic WSe 2 Nanosheet-Based Channels: Implications for Field-Effect Transistors

We report the fabrication of ohmic van der Waals (vdW) contacts to nearly intrinsic WSe 2 nanosheet-based channels in field-effect transistors (FETs) using degenerately p-doped MoS2 (p + -MoS2) as a contact metal. We demonstrate that accumulation-type ohmic contacts and the high device performance are achievable without electrostatically gating the drain/source contact regions despite the nearly intrinsic nature of WSe 2 . Back-gated WSe 2 FETs with p + -MoS 2 bottom contacts (which screen the back-gate electric field in the drain/source regions) exhibit linear output characteristics, a high on/off ratio of 10 8 , and a high two-terminal field-effect mobility up to ~200 cm2 V –1 s –1 at room temperature. Our theoretical modeling reveals that the p + -MoS 2 /WSe 2 vdW junction behaves like a metal/semiconductor ohmic contact signified by a vanishingly thin space-charge region of ~1 nm on the p + -MoS 2 side and a substantial accumulation layer of free holes on the WSe 2 side, which is further verified by additional temperature-dependent and dual-gated measurements of WSe 2 FETs. We attribute the formation of accumulation-type ohmic contacts free of a Schottky barrier to the near absence of Fermi-level pinning at the vdW interface and the work function of p + -MoS 2 being larger than the ionization energy of WSe 2 . This study represents an important step toward achieving low-resistance ohmic contacts to two-dimensional (2D) semiconductors by eliminating the Fermi-level pinning effects, which is expected to have significant implications for next-generation 2D semiconductor-based nanoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Enabling Quality Interfaces with Mask-Free Approach to Selective Growth of MoS 2 /Graphene Stacked Structures

The mask-free patterning technique is employed to fabricate arrays of MoS2 and WS2 structures on silicon and graphene substrates with quality interfaces. By depositing precursor inks with the AFM cantilevers and subsequent heat treatment in the CVD furnace, it is demonstrated that MoS2 and WS2 structures can be formed on graphene surfaces at predefined device architectures.

2D atomic crystals↗
DOE OSTI2016

Amorphous MoS 3 Infiltrated with Carbon Nanotubes as an Advanced Anode Material of Sodium-Ion Batteries with Large Gravimetric, Areal, and Volumetric Capacities

The search for earth-abundant and high-performance electrode materials for sodium-ion batteries represents an important challenge to current battery research. 2D transition metal dichalcogenides, particularly MoS2, have attracted increasing attention recently, but few of them so far have been able to meet expectations. In this study, it is demonstrated that another phase of molybdenum sulfide—amorphous chain-like MoS3—can be a better choice as the anode material of sodium-ion batteries. Highly compact MoS3 particles infiltrated with carbon nanotubes are prepared via the facile acid precipitation method in ethylene glycol. Compared to crystalline MoS2, the resultant amorphous MoS3 not only exhibits impressive gravimetric performance—featuring excellent specific capacity (≈615 mA h g-1), rate capability (235 mA h g-1 at 20 A g-1), and cycling stability but also shows exceptional volumetric capacity of ≈1000 mA h cm-3 and an areal capacity of >6.0 mA h cm-2 at very high areal loadings of active materials (up to 12 mg cm-2). The experimental results are supported by density functional theory simulations showing that the 1D chains of MoS3 can facilitate the adsorption and diffusion of Na+ ions. At last, it is demonstrated that the MoS3 anode can be paired with an Na3V2(PO4)3 cathode to afford full cells with great capacity and cycling performance.

Ye, Hualin↗
DOE OSTI2019

Highly Efficient Solar-Driven Carbon Dioxide Reduction on Molybdenum Disulfide Catalyst Using Choline Chloride-Based Electrolyte

Conversion of CO2 to energy-rich chemicals using renewable energy is of much interest to close the anthropogenic carbon cycle. However, the current photoelectrochemical systems are still far from being practically feasible. Here the successful demonstration of a continuous, energy efficient, and scalable solar-driven CO2 reduction process based on earth-abundant molybdenum disulfide (MoS2) catalyst, which works in synergy with an inexpensive hybrid electrolyte of choline chloride (a common food additive for livestock) and potassium hydroxide (KOH) is reported. The CO(2 )saturated hybrid electrolyte utilized in this study also acts as a buffer solution (pH approximate to 7.6) to adjust pH during the reactions. This study reveals that this system can efficiently convert CO(2 )to CO with solar-to-fuel and catalytic conversion efficiencies of 23% and 83%, respectively. Using density functional theory calculations, a new reaction mechanism in which the water molecules near the MoS(2 )cathode act as proton donors to facilitate the CO2 reduction process by MoS2 catalyst is proposed. This demonstration of a continuous, cost-effective, and energy efficient solar driven CO2 conversion process is a key step toward the industrialization of this technology.

flow cells↗
DOE OSTI2015

Dimeric [Mo₂S₁₂]²⁻ Cluster: A Molecular Analogue of MoS₂ Edges for Superior Hydrogen-Evolution Electrocatalysis

Proton reduction is one of the most fundamental and important reactions in nature. MoS2 edges have been identified as the active sites for hydrogen evolution reaction (HER) electrocatalysis. Designing molecular mimics of MoS2 edge sites is an attractive strategy to understand the underlying catalytic mechanism of different edge sites and improve their activities. Herein we report a dimeric molecular analogue [Mo₂S₁₂]²⁻, as the smallest unit possessing both the terminal and bridging disulfide ligands. Our electrochemical tests show that [Mo₂S₁₂]²⁻ is a superior heterogeneous HER catalyst under acidic conditions. Computations suggest that the bridging disulfide ligand of [Mo₂S₁₂]²⁻ exhibits a hydrogen adsorption free energy near zero (-0.05eV). This work helps shed light on the rational design of HER catalysts and biomimetics of hydrogen-evolving enzymes.

electrocatalysis↗
DOE OSTI2015

Can Ni phosphides become viable hydroprocessing catalysts?

We prepared higher surface area nickel phosphides than are normally found by reducing nickel phosphate. To do this, we hydrothermally synthesized Ni hydroxy phosphite precursors with low levels of molybdenum substitution. The molybdenum substitution increases the surface area of these precursors. During pretreatment in a sulfiding atmosphere (such as H2S/H2) dispersed islands of MoS2 segregate from the precursor and provide a pathway for H2 dissociation that allows reduction of the phosphite precursor to nickel phosphide at substantially lower temperatures than in the absence of MoS2. The results reported here show that to create nickel phosphides with comparable activity to conventional supported sulfide catalysts, one would have to synthesize the phosphide with surface areas exceeding 400 m2/g (i.e. with nanoparticles less than 30 Å in lateral dimension).

Hydrothermal↗
DOE OSTI2016

Highly efficient hydrogen evolution reaction using crystalline layered three-dimensional molybdenum disulfides grown on graphene film.

Electrochemistry is central to applications in the field of energy storage and generation. However, it has advanced far more slowly over the last two decades, mainly because of a lack of suitable and affordable catalysts. Here, we report the synthesis of highly crystalline layered three-dimensional (3D) molybdenum disulfide (MoS2) catalysts with bare Mo-edge atoms and demonstrate their remarkable performance for the hydrogen evolution reaction (HER). We found that Mo-edge-terminated 3D MoS2 directly grown on graphene film exhibits a remarkable exchange current density (18.2 mu A cm(-2)) and turnover frequency (>4 S-1) for HER. The obtained exchange current density is 15.2 and 2.3 times higher than that of MoS2/graphene and MoS2/Au catalysts, respectively, both with sulfided Mo-edge atoms. An easily scalable and robust growth process on a wide variety of substrates, along with prolonged stability, suggests that this material is a promising catalyst in energy-related applications.

Behranginia, Amirhossein↗
DOE OSTI2020

Gate-Tunable Semiconductor Heterojunctions from 2D/3D van der Waals Interfaces

Van der Waals (vdW) semiconductors are attractive for highly scaled devices and heterogeneous integration since they can be isolated into self-passivated, two-dimensional (2D) layers that enable superior electrostatic control. These attributes have led to numerous demonstrations of field-effect devices ranging from transistors to triodes. By exploiting the controlled, substitutional doping schemes in covalently-bonded, three-dimensional (3D) semiconductors and the passivated surfaces of 2D semiconductors, one can construct devices that can exceed performance metrics of “all-2D” vdW heterojunctions. Here, we demonstrate, 2D/3D semiconductor heterojunctions using MoS2 as the prototypical 2D semiconductor laid upon Si and GaN as the 3D semiconductor layers. By tuning the Fermi levels in MoS2, we demonstrate devices that concurrently exhibit over seven orders of magnitude modulation in rectification ratios and conductance. Our results further suggest that the interface quality does not necessarily affect Fermi-level tuning at the junction opening up possibilities for novel 2D/3D heterojunction device architectures

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Sono-Assisted Surface Energy Driven Assembly of 2D Materials on Flexible Polymer Substrates: A Green Assembly Method Using Water

The challenges in achieving a green and scalable integration of two-dimensional (2D) materials with flexible polymer substrates present a major barrier for the application of 2D materials, such as graphene, MoS2, and h-BN for flexible devices. Here, we create a sono-assisted surface energy driven assembly (SASEDA) method that can achieve foot-scale to micrometer-scale assembly of 2D materials, form a conductive network in as short as 10 s, and build hierarchical and hybrid flexible devices such as sensors, resistors, and capacitors by using water as the dispersion solvent. SASEDA highlights two counterintuitive innovations. First, we use an 'unfavorable' solvent (i.e., water) for both 2D materials (e.g., graphene, MoS2, and h-BN) and polymer substrates (e.g., polydimethylsiloxane) to drive the assembly process. Second, we use a weak sono-field (0.3 W/cm2) generated by a regular sonication bath cleaner to enhance the assembly efficiency and reorganize and unify the assembly network. This method and its principle pave the way toward affordable large-scale 2D material-based flexible devices.

2D material↗
DOE OSTI2025

All 2D Material Printed Diodes and Circuits on Paper for Sustainable Electronics

Sustainable electronics aim to reduce environmental impact by using ecofriendly materials, energy-efficient manufacturing, and recyclable components. However, existing approaches rely on complex, resource-intensive methods, rare metals, or nanomaterials with limited stability as well as plastic substrates, raising sustainability issues. Solution-processed two-dimensional (2D) materials offer a promising alternative: water-based and biocompatible conductive, semiconductive, and insulating 2D material inks can be produced with scalable techniques and are suitable for the fabrication of fully printed devices on low-cost and biodegradable paper substrates. However, 2D material only and fully printed diodes on paper have not yet been reported. Here, we demonstrate fully inkjet-printed 2D material-based diodes on paper using metal-insulator-semiconductor and metal-insulator-metal-semiconductor architectures. Water-based graphene and MoS2 inks, prepared by liquid-phase exfoliation, are used for the metallic and insulating films, while electrochemical exfoliation is used to produce the semiconducting MoS2 ink. The highest forward-to-reverse current ratio obtained is 330 (at ±2 V), while the forward current density is 1 mA/cm2 (at 1 V), making the diode performance comparable to the best solution-processed diodes reported so far. However, in contrast to previous works, fabrication occurs entirely at room temperature and ambient pressure, without using high-pressure sputtering, thermal evaporation, and any precious metal ink. The devices maintain stable performance under bending up to strain of 4% over 10,000 cycles. Finally, the diodes are successfully integrated with other 2D-material based electrical components to realize fully printed RC circuits, differentiators, integrators, and AC-to-DC converters onto paper, hence demonstrating the suitability of our approach for sustainable and disposable integrated circuits.

2D materials↗
DOE OSTI2019

Layer-Dependent Photoinduced Electron Transfer in 0D–2D Lead Sulfide/Cadmium Sulfide–Layered Molybdenum Disulfide Hybrids

We illustrate layer-dependent electron transfer between core/shell PbS/CdS quantum dots (QDs) and layered MoS 2 via energy bandgap engineering of both the donor (QDs) and the acceptor (MoS2) components. We do this by (i) changing the size of the QD or (ii) by changing the number of layers of MoS2 and each of these approaches alters the bandgap and/or the donor-acceptor separation distance, hence providing a mean of tuning the charge transfer rate. We find the charge transfer rate to be maximal for QDs of smallest size and for QDs combined with a 5-layer MoS 2 or thicker. We model this layer-dependent charge transfer rate with a theoretical model derived from Marcus theory previously applied to non-adiabatic electron transfer in weakly coupled systems by considering the QD transferring photogenerated electrons to non-interacting monolayers within a few layers MoS 2 .

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2019

Hierarchical Nanoassembly of MoS{sub 2}/Co{sub 9}S{sub 8}/Ni{sub 3}S{sub 2}/Ni as a Highly Efficient Electrocatalyst for Overall Water Splitting in a Wide pH Range.

The design of low-cost yet high-efficiency electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) over a wide pH range is highly challenging. We now report a hierarchical co-assembly of interacting MoS2 and Co9S8 nanosheets attached on Ni3S2 nanorod arrays which are supported on nickel foam (NF). This tiered structure endows high performance toward HER and OER over a very broad pH range. By adjusting the molar ratio of the Co:Mo precursors, we have created CoMoNiS-NF-xy composites (x:y means Co:Mo molar ratios ranging from 5:1 to 1:3) with controllable morphology and composition. The three-dimensional composites have an abundance of active sites capable of universal pH catalytic HER and OER activity. The CoMoNiS-NF-31 demonstrates the best electrocatalytic activity, giving ultralow overpotentials (113, 103, and 117 mV for HER and 166, 228, and 405 mV for OER) to achieve a current density of 10 mA cm(-2) in alkaline, acidic, and electrolytes, respectively. It also shows a remarkable balance between electrocatalytic activity and stability. Based on the distinguished catalytic performance of CoMoNiS-NF-31 toward HER and OER, we demonstrate a two-electrode electrolyzer performing water electrolysis over a wide pH range, with low cell voltages of 1.54, 1.45, and 1.80 V at 10 mA cm(-2) in alkaline, acidic, and neutral media, respectively. First-principles calculations suggest that the high OER activity arises from electron transfer from Co9S8 to MoS2 at the interface, which alters the binding energies of adsorbed species and decreases overpotentials. Our results demonstrate that hierarchical metal sulfides can serve as highly efficient all-pH (pH = 0-14) electrocatalysts for overall water splitting.

Yang, Yan↗
DOE OSTI2017

Rapid synthesis of transition metal dichalcogenide–carbon aerogel composites for supercapacitor electrodes

Transition metal dichalcogenide (TMD) materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase, which increases the conductivity of the network. However, freestanding, exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum (1.2 %), can fail during electrochemical operation due to poor mechanical properties, and often require pyrophoric chemicals to process. On the other hand, pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance, high conductivity, and a strong mechanical network of covalent chemical bonds. In this paper, we demonstrate the scalable, rapid nanomanufacturing of TMD (MoS2 and WS2) and carbon aerogel composites, favoring liquid-phase exfoliation to avoid pyrophoric chemicals. The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30 min. We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel, which maintains a high specific surface area up to 620 m2 g-1 with peak pore radii of 10 nm. While supercapacitor tests of the aerogels yield capacitances around 80 F g-1 at the lowest applied currents, the aerogels loaded with TMD’s exhibit volumetric capacitances up to 127% greater than the unloaded aerogels. In addition, the WS2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles, as well as markedly better rate capability and lower charge transfer resistance compared to their MoS2-loaded counterparts. We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide

The ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices are two-dimensional monolayer transition metal dichalcogenide semiconductors. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. We use the ‘Campanile’ nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Moreover, synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ~300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. In conclusion, the nanoscale structure–property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Electron–phonon interaction and ultrafast photoemission from doped monolayer MoS 2

We have examined the effect of electron–phonon coupling on photoluminescence and ultrafast response of electron doped monolayer MoS2, using a combination of density functional theory, time dependent density functional theory, and many-body theory. For small doping (~1–3%) of interest here, the electron–phonon coupling parameter is modest (~0.1–0.2) but its effect on the emissive properties and response of the system to femtosecond (fs) laser pulses is striking. We find an ultrafast (fs) relaxation of the electronic subsystem as well as a high fluence of visible light emission induced by electron phonon interaction. Together with high carrier mobility, these features of monolayer MoS2 may be relevant for optoelectronic technologies.

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

Research update: Spin transfer torques in permalloy on monolayer MoS 2

We observe current induced spin transfertorque resonance in permalloy (Py) grown on monolayer MoS 2 . By passing rf current through the Py/MoS 2 bilayer, field-like and damping-like torques are induced which excite the ferromagnetic resonance of Py. The signals are detected via a homodyne voltage from anisotropic magnetoresistance of Py. In comparison to other bilayer systems with strong spin-orbit torques, the monolayer MoS 2 cannot provide bulk spin Hall effects and thus indicates the purely interfacial nature of the spin transfer torques. Furthermore, our results indicate the potential of two-dimensional transition-metal dichalcogenide for the use of interfacial spin-orbitronics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2019

All-solid-state supercapacitors from natural lignin-based composite film by laser direct writing

Lignin as a renewable natural resource has been the aim of numerical interest in applications ranging from pitch to porous carbon material. In this work, a facile method is reported to transform lignin into porous conductive carbon structures and interdigitated circuits for supercapacitor devices using femtosecond laser direct writing. Comparative studies revealed that the laser irradiation induced tetrahedral amorphous carbon while only graphitic carbon was obtained through pyrolysis. At the same time, the composite membrane was easily prepared to further optimize the capacities by mixing functional materials (MoS 2 ) into the lignin/polyacrylonitrile (PAN) composite polymers. A MoS2 decorated porous carbon network material could be fabricated through focused femtosecond pulsed laser irradiation of the corresponding composite membranes. The microstructure and spectroscopic features of these laser induced hybrid carbon materials have been deeply investigated. The supercapacitor based on lignin/PAN has high areal specific capacitances of 6.7 mF cm –2 (0.9 F cm –3 ) at 10 mV s –1 . Moreover, doped microsupercapacitors with MoS2 demonstrated enhanced areal capacitances up to 16 mF cm –2 (2.2 F cm –3 ) and at 10 mV s –1 , respectively. The relatively high areal capacitances indicate that the introduced method is potential for innovative manufacturing energy storage devices based on natural lignin.

36 MATERIALS SCIENCE↗