Crystalline MoS2 Research Articles

Uniform and delicate adjustment of plasmonic coupling upon evolved MoS2 crystalline shields on individual gold nanoparticles for photocatalytic water splitting

With the guided epitaxial growth of multiple MoS2 crystalline shields directly on individual gold nanoparticles and delicate control of shield thickness, the initiated self-assembly of hybrid nanoparticles in solutions results in various magnitudes of irradiation-stimulated plasmonic coupling. The theoretically derived medium ranges of separation distances for the optimization of plasmonic coupling has been experimentally verified and evaluated eventually, which is found critically subject to nanoparticles sizes and initiated tunneling currents. In addition to the enhancement of Raman scattering, the interfacial transition of hot electrons from Au cores to MoS2 shields has been clarified also to critically depend on the reached magnitudes of plasmonic coupling. Surprisingly, the activated plasmonic coupling is accompanied with the increase of work functions of hybrid nanoparticles, and capable to transfer Schottky contact to Ohmic contact with water molecules, revealed as a new mechanism to facilitate interfacial electronic transition. Furthermore, the plasmonic coupling is analyzed analogous to the result of mutual polarization between hybrid nanoparticles, which is also intensified with the created local electric fields and thus nanoparticle sizes. With activated plasmonic coupling subject to the thickness of MoS2 shields and nanoparticle sizes, the efficiency of hydrogen production upon photocatalytic water splitting by MoS2 shields is able to reach the record value of 35 mmol/g·hr, at least one-order-of-magnitude higher than other reported results. Accordingly, the adjustment of plasmonic coupling and thus nanoparticle polarization has been experimentally clarified as a promising strategy able to promote photocatalytic reactions in solutions.

Amorphous mesoporous sulfur-rich 1T/2H-MoS2 nanospheres as high-capacity cathode materials for advanced magnesium ion batteries

Two-dimensional layered-structure MoS2 has been explored as the promising cathode materials for magnesium ion batteries benefited by its wide layer distance and satisfactory capacity. However, such strong interaction between Mg2+ and MoS2 brings about the sluggish diffusion kinetics and irreversible electrochemical reaction, leading to a decreased specific capacity and poor rate capability. How to adopt its structure engineering and interface engineering to boost the Mg2+ diffusion kinetics and enhance the electrochemical reversibility of MoS2 is still a huge challenge. Herein, the amorphous sulfur-rich 1T/2H-MoS2 mesoporous nanospheres (a-MoS2+x, 0<x<1) have been developed via a facile one-pot hydrothermal route. Acted as a cathode material for magnesium ion batteries, owing to the cooperate effects of inherent rich active sites, high conductivity and wide layer distance induced by the coexistence of sulfur-rich, 1T phase and nearly amorphous features, the as-prepared a-MoS2+x demonstrates an ultra-high discharge capacity (438.7 mAh g−1 for 50 cycles at 0.1 A g−1), promising rate capability (97.6 mAh g−1 at 1.0 A g−1), and good cyclic stability (110 cycles at 0.5 A g−1), superior to the crystalline MoS2 (c-MoS2) and other previously reported MoS2 counterparts. Besides, the kinetics results indicate that the a-MoS2+x manifests good charge transfer and diffusion kinetics, and its entire charge storage is a combination of surface-controlled capacitance behavior and diffusion-controlled battery behavior. Moreover, some ex-situ Raman, XPS and HRTEM characterization techniques are employed to disclose the Mg2+ storage mechanism. Such remarkable magnesium storage properties of a-MoS2+x demonstrates its promising application as cathode for magnesium ion batteries.

(Invited) Low Temperature Atomic Layer Deposition Processes for Large-Area Synthesis of 2D Transition Metal Dichalcogenides

2D materials have been the focus of intense research in the last decade due to their unique physical properties. This presentation will highlight our recent progress on the large-area synthesis of two-dimensional transition metal chalcogenides for nanoelectronics using advanced atomic layer deposition cycle schemes. First, how we can use advanced cycle schemes to deposit wafer-scale polycrystalline MoS2 thin films at very low temperatures down to 100 °C. We have identified the critical role of hydrogen during the plasma step in controlling the composition and properties of molybdenum sulfide films. By increasing the H2/H2S ratio or adding an extra hydrogen plasma step to our ALD process, we are able to deposit pure polycrystalline MoS2 films at temperatures as low as 100 °C. To the best of our knowledge, this represents the lowest temperature for crystalline MoS2 films prepared by any chemical gas-phase method.[1]The most dominant methods for preparing MoS2 via ALD is to alternately expose a substrate to a metalorganic precursor and a hydrogen sulfide (H2S) or a plasma containing H2S. H2S is a corrosive, toxic, and flammable gas that is heavier than air, which makes it hazardous and expensive to store, install, and transport. Alternative sulfur precursors in the solid or liquid phase would be beneficial in terms of cost and safety and would require the installation of no additional hardware for most ALD reactors. In the second half of this contribution, the widely researched ALD process using bis(tert-butylimido)bis(dimethylamino)molybdenum(IV) ((tBuN)2(NMe2)2Mo) and H2S plasma is compared to a novel ALD process using (tBuN)2(NMe2)2Mo, hydrogen plasma, and di-tert-butyl disulfide (TBDS), which is an inexpensive, liquid-phase sulfur precursor.[1] M. Mattinen et al., Chem. Mater. (2022), 34, 5104

Growth of Monolayer MoS2 Flakes via Close Proximity Re-Evaporation.

We report a two-step growth process of MoS2 nanoflakes using a low-pressure chemical vapor deposition technique. In the first step, a MoS2 layer was synthesized on a c-plane sapphire substrate. This layer was subsequently re-evaporated at a higher temperature to form mono- or few-layer MoS2 flakes. As a result, the close proximity re-evaporation enabled the growth of pristine MoS2 nanoflakes. Atomic force microscopy analysis confirmed the synthesis of nanoclusters/nanoflakes with lateral dimensions of over 10 μm and a flake height of approximately 1.3 nm, demonstrating bi-layer MoS2, whereas transmission electron microscopy analysis revealed triangular MoS2 nanoflakes, with a diffraction pattern proving the presence of single crystalline hexagonal MoS2. Raman data revealed the typical modes of high-quality MoS2 nanoflakes. Finally, we presented the photocurrent dependence of a MoS2-based photoresist under illumination with light-emitting diode of 405 nm wavelength. The measured current-voltage dependence across various luminous flux outlined the sensitivity of MoS2 to polarized light and thus opens further opportunities for applications in high-performance photodetectors with polarization sensitivity.

Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction.

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

Synthesis of Wrinkled MoS2 Thin Films Using a Two-Step Method Consisting of Magnetron Sputtering and Sulfurization in a Confined Space

Considering the increasing need for sustainable and economical energy storage solutions, the integration of layered materials such as MoS2 into these systems represents an important step toward enhancing energy sustainability and efficiency. Exploring environmentally responsible fabrication techniques, this study assesses wrinkled MoS2 thin films synthesized from distinct Mo and MoS2 targets, followed by sulfurization conducted in a graphite box. We utilized magnetron sputtering to deposit precursor Mo and MoS2 films on Si substrates, achieving thicknesses below 20 nm. This novel approach decreases sulfur by up to tenfold during sulfurization due to the confined space technique, contributing also to avoiding the formation of toxic gases such as SO2 or the necessity of using H2S, aligning with sustainable materials development. Thinner MoS2 layers were obtained post-sulfurization from the MoS2 precursors, as shown by X-ray reflectometry. Raman spectroscopy and grazing X-ray diffraction analyses confirmed the amorphous nature of the as-deposited films. Post-sulfurization, both types of films exhibited crystalline hexagonal MoS2 phases, with the sulfurized Mo showing a polycrystalline nature with a (100) orientation and sulfurized MoS2 displaying a (00L) preferred orientation. The X-ray photoelectron spectroscopy results supported a Mo:S ratio of 1:2 on the surface of the films obtained using the MoS2 precursor films, confirming the stoichiometry obtained by means of energy dispersive X-ray spectroscopy. Scanning electron microscopy and atomic force microscopy images revealed micrometer-sized clusters potentially formed during rapid cooling post-sulfurization, with an increased average roughness. These results open the way for the further exploration of wrinkled MoS2 thin films in advanced energy storage technologies.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides.

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

Post-annealing in ultra-high vacuum or nitrogen plasma for MoS2 thin films deposited by magnetron sputtering

The thin films of amorphous molybdenum disulfide were deposited at room temperature by magnetron sputtering technique. Post-annealing process in ultra-high vacuum (∼10−8 Pa) or nitrogen plasma environments at the temperatures of 300, 400, 500, and 700 °C have been first proposed to enhance the microstructure and optical properties of MoS2 thin films. The phase transformation of MoS2 thin films from amorphous to polycrystalline was characterized by in situ reflection high-energy electron diffraction during the post-annealing process. The microstructure of MoS2 thin films was also analyzed by Raman spectrum and X-ray diffractometer after the post-annealing process. In addition, the thermal analysis of the differential scanning calorimeter and optical measurement of photoreflectance confirmed the phase transformation of MoS2 thin films. The analysis of photoreflectance also estimated the exciton transition at the bandgap energy of 2.038 eV at 0 K, attributed by the crystalline MoS2 film annealed at 700 °C in ultra-high vacuum. The surface chemical composition of MoS2 thin films has been identified by X-ray photoelectron spectroscopy, but the desulfurization of MoS2 was observed after post-annealing in ultra-high vacuum. Moreover, the preferred orientation of (004) plane in the MoS2 films was performed as the increase in post-annealing temperature.

Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

Comparative study on gas-sensing properties of amorphous MoS2 nanoparticles and crystalline MoS2 nanoflowers

High-performance gas sensors for the field of gas sensing have drawn a lot of attention to molybdenum sulfide materials, but there is little research on the influence of crystallization state of MoS2 on gas sensitivity performance. In this study, amorphous MoS2 and crystalline MoS2 were prepared by the hydrothermal method. Then, the gas-sensing characteristics of the as-synthesized amorphous MoS2 and crystalline MoS2 toward NO2 gas were thoroughly investigated and compared. According to the results, with an excellent response value of 1.99 toward 5 ppm NO2 at room temperature, the crystalline MoS2 gas sensor outperformed the amorphous one, while that of the amorphous MoS2 under the same condition was 1.25. However, Four times faster recovery periods were observed in the amorphous MoS2 sample compared to the crystalline MoS2 sample. At the same time, the recovery level of amorphous MoS2 is high and can be fully restored. We discovered that the primary cause of the strong recovery performance of amorphous MoS2 is the presence of localized states that facilitate the easy transition of electrons from the conduction band to the valence band, allowing for effective recombination with the holes. Additionally, the surface of the material absorbs a significant amount of oxygen, which will occupy more of the NO2 adsorption site and hasten the desorption of NO2. This work may aid in the development of gas sensors based on amorphous molybdenum sulfide.

(Invited) Advanced Atomic Layer Deposition Cycle Schemes for Large-Area Synthesis of 2D Transition Metal Dichalcogenides

2D materials have been the focus of intense research in the last decade due to their unique physical properties. This presentation will highlight our recent progress on the large-area synthesis of two-dimensional transition metal chalcogenides for nanoelectronics using advanced atomic layer deposition cycle schemes. First, how we can use advanced cycle schemes to deposit wafer-scale polycrystalline MoS2 thin films at very low temperatures down to 100 °C. We have identified the critical role of hydrogen during the plasma step in controlling the composition and properties of molybdenum sulfide films. By increasing the H2/H2S ratio or adding an extra hydrogen plasma step to our ALD process, we are able to deposit pure polycrystalline MoS2 films at temperatures as low as 100 °C. To the best of our knowledge, this represents the lowest temperature for crystalline MoS2 films prepared by any chemical gas-phase method.[1]ALD-grown 2D films tend to exhibit a high density of out-of-plane 3D structures in addition to 2D horizontal layers. While the out-of-plane 3D structures are ideal for catalysis applications, the presence of such 3D structures can hinder charge transport, which hampers device applications. In this presentation I will show how we used mechanistic insight obtained by HRTEM to tune the shape and density of the 3D structures during plasma-enhanced ALD using advanced atomic layer deposition schems. The obtained morphology control was further confirmed by electrical measurements.[2]Earlier [3] we have shown that ALD is an excellent technique to make MoxW1-xS2 alloys with precise control over the alloy composition. Here, I will focus on how (plasma-enhanced) atomic layer deposition can aid in synthesizing doped 2DTMCs with precise control over the doping concentration by using elaborate ALD dosing schemes.[1] M. Mattinen et al., Chem. Mater. (2022), 34, 5104[2] S. Balasubramanyam, et al. ACS Appl. Mater. Interfaces (2020), 12, 3873[3] J. J. P. M. Schulpen et al. 2D Mater. (2022), 9, 025016

Amorphous bimetallic polysulfide for all-solid-state batteries with superior capacity and low-temperature tolerance

Transition-metal sulfides with high capacity and low cost have been widely acknowledged as promising cathode candidate for all-solid-state batteries (ASSBs). However, the solid-solid conversion reaction mechanism based on ordered crystal structures and the high concentration of metal element result in sluggish reaction kinetics and sacrificed specific capacity. Herein, an amorphous bimetallic polysulfide (Mo0.5Ti0.5S4) is designed to create abundant reaction sites, improve diffusion kinetics and attain sulfur-equivalent capacity. As a result of the decreased elastic modulus through the introduction of Ti, the amorphization of crystalline MoS2 is accelerated during ball milling. Compared with its counterpart (MoS4, 757 mAh g−1), Mo0.5Ti0.5S4 ASSB delivers a much higher reversible capacity (914 mAh g−1), thanks to the abundant reaction sites and suppressed S/Li2S separation from amorphous Mo-Sx matrix. Moreover, due to the improved diffusion kinetics and pseudocapacitive contribution, the capacity retention of Mo0.5Ti0.5S4 at 4 C current rate increases from 47.2 % to 65.8 %. In addition, Mo0.5Ti0.5S4 shows excellent low-temperature performances, including good long-term cycle stability and rate capability (with a capacity retention of 50.5 % at 0.5 C) at −20 ℃, and increased capacity retention (from 35.1 % to 50.7 %) at −40 ℃. Therefore, Mo0.5Ti0.5S4 with bimetallic amorphous structure and enriched sulfur anions has enormous potential on high-rate and low-temperature ASSB applications.

Ultrafast synthesis of amorphous molybdenum sulfide by magnetic induction heating for hydrogen evolution reaction

Molybdenum sulfides have emerged as viable alternatives to noble-metal catalysts for green hydrogen production via the hydrogen evolution reaction (HER). Herein, magnetic induction heating (MIH) is exploited for the rapid preparation of carbon-supported MoSx nanocomposites. The sample prepared at 200 A for 10 s shows an amorphous Mo3S7Cly-like structure and a low overpotential (η10) of −184 mV at 10 mA cm−2 in acidic media, whereas samples prepared at higher induction currents display a largely crystalline MoS2 structure and drastically lower HER performance. This is due to the formation of dimeric Mo6S14 moieties in amorphous MoSx that is facilitated by the loss of Cl residues during electrochemical reaction and enhanced H adsorption at both the S and Mo sites, in comparison to crystalline MoS2, as shown in First-principles calculations. These results show that MIH may be used as a powerful tool in the preparation of nonequilibrium structures as high-performance electrocatalysts.

Dodecylation of molybdenum disulfide and its electrochemical properties for hydrogen evolution reaction

Functionalization of MoS2 was achieved by treatment in a strongly reducing sodium naphthalene solution. Dodecyl was grafted onto MoS2 nanosheets using alkyl sulphates as electrophiles to obtain dodecylated MoS2 without affecting the MoS2 crystalline structure. Superior electrocatalytic properties are obtained for dodecylated MoS2. The polarisation curve of this nanomaterial remained constant even after 1000 consecutive cycles. This route provides a new pathway for covalent functionalization of MoS2 and might find a variety of applications, such as electrocatalysts.

Probing Crystallinity and Grain Structure of 2D Materials and 2D‐Like Van der Waals Heterostructures by Low‐Voltage Electron Diffraction

4D scanning transmission electron microscopy (4D‐STEM) is a powerful method for characterizing electron‐transparent samples with down to sub‐Ångstrom spatial resolution. 4D‐STEM can reveal local crystallinity, orientation, grain size, strain, and many more sample properties by rastering a convergent electron beam over a sample area and acquiring a transmission diffraction pattern (DP) at each scan position. These patterns are rich in information about the atomic structure of the probed volume, making this technique a potent tool to characterize even inhomogeneous samples. 4D‐STEM can also be used in scanning electron microscopes (SEMs) by placing an electron‐sensitive camera below the sample. 4D‐STEM‐in‐SEMs is ideally suited to characterize 2D materials and 2D‐like van der Waals heterostructures (vdWH) due to their inherent thickness of a few nanometers. The lower accelerating voltage of SEMs leads to strong scattering even from monolayers. The large field of view and down to sub‐nm spatial resolution of SEMs are ideal to map properties of the different constituents of 2D‐like vdWH by probing their combined sample volume. A unique 4D‐STEM‐in‐SEM system is applied to reveal the single crystallinity of MoS2 exfoliated with gold‐mediation as well as the crystal orientation and coverage of both components of a C60/MoS2 vdWH are determined.

Engineering interfacial band hole extraction on chemical-vapor-deposited MoS2/CdS core-shell heterojunction photoanode: The junction thickness effects on photoelectrochemical performance

Heterojunction fabrication is a promising strategy that can greatly boost the charge carrier separation and improve the solar-to-hydrogen conversion efficiency of photoelectrochemical (PEC) cells. However, such technology still suffers from limited contact interfaces. In this study, the chemical vapor deposition (CVD) technique was for the first time used to construct the CdS/MoS2 heterojunction photoanode with a unique core-shell nanoarchitecture, in which a continuous crystalline MoS2 nanosheet layer was grown directly on one-dimensional (1D) oriented CdS nanorods (NRs) in a plane-to-plane stacking fashion. The optimization of junction thickness with adjustable MoS2 loading from mono to a few layers was achieved by experimental parameters variation. Systematic characterizations show that the MoS2 shell plays a dual role as an optical absorption booster for more photo-exciton generation and a surface passivator of trap states. Meanwhile, the formed heterojunction helps regulate the unidirectional charge migration for a significantly suppressed electron-hole recombination process, which synergistically contributes to higher quantum yield and efficiency. As a result, the optimized CdS/MoS2 heterojunction photoanode with 3-layered MoS2 wrapping exhibits the highest photocurrent density and photoconversion efficiency, over a two-fold increase, compared to those of pristine CdS and the previously reported CdS/MoS2 heterojunctions. Moreover, due to the rapid hole extraction from CdS and transferred surface oxidation sites, the present CdS/MoS2 heterostructure demonstrates better corrosion resistance and higher photostability. The present work is expected to provide a versatile platform for exploiting the CVD technique to develop other MoS2-based heterojunction photoelectrodes with extensive PEC applications.

An Amorphous Molybdenum Polysulfide Cathode for Rechargeable Magnesium Batteries.

Rechargeable magnesium batteries (RMBs) attract research interest owing to the low cost and high reliability, but the design of cathode materials is the major difficulty of their development. The bivalent magnesium cation suffers from a strong interaction with the anion and is difficult to intercalate into traditional magnesium intercalation cathodes. Herein, an amorphous molybdenum polysulfide (a-MoSx ) is synthesized via a simple one-step solvothermal reaction and used as the cathode material for RMBs. The a-MoSx cathode provides a high capacity (185 mAh g-1 ) and a good rate performance (50 mAh g-1 at 1000 mA g-1 ), which are much superior compared with crystalline MoS2 and demonstrate the privilege of amorphous RMB cathodes. A mechanism study demonstrates both of molybdenum and sulfur undergo redox reactions and contribute to the capacity. Further optimizations indicate low-temperature synthesis would favor the magnesium storage performance of a-MoSx .

In situ growth MoS2 quantum dots as promising interface materials for silicon solar cells

The carrier selectivity at interface is one key factor to improve the performance of silicon solar cells. Here, Molybdenum disulfide (MoS2) quantum dots were successfully synthesized by one-step in situ plasma enhanced atom layer deposition. A transmission electron microscope was employed to investigate nano-size scaled as well as growth mechanism of the crystalline MoS2 quantum dots. MoS2 quantum dots with additional molybdenum-oxygen (Mo–O) bonding were prepared by co-reactant plasma processing, which exhibited photo-generated carrier selectivity as promising interface materials for silicon heterojunction solar cells. Our results indicated their carrier selectivity of electron-blocking and hole-transporting at interface was attributable to unique quantum effects and tunneling effects. A photovoltaic conversion efficiency 23% was achieved by a sample silicon solar cell combined with MoS2 quantum dots interface materials. Because the interface engineering is well-defined, and the synthesis is low-cost, this bottom-up strategy provides a potential advance in design and construction of innovative and highly efficient photovoltaic devices.

Synthesis of Large-Area Crystalline MoS2 by Sputter Deposition and Pulsed Laser Annealing

The wafer-scale synthesis of layered transitional metal dichalcogenides presenting good crystal quality and homogeneous coverage is a challenge for the development of next-generation electronic devices. This work explores a fairly unconventional growth method based on a two-step process consisting in sputter deposition of stochiometric MoS2 on Si/SiO2 substrates followed by nanosecond UV (248 nm) pulsed laser annealing. Large-scale 2H-MoS2 multi-layer films were successfully synthetized in a N2-rich atmosphere thanks to a fine-tuning of the laser annealing parameters by varying the number of laser pulses and their energy density. The identification of the optimal process led to the success in achieving a (002)-oriented nanocrystalline MoS2 film without performing post-sulfurization. It is noteworthy that the spatial and temporal confinement of laser annealing keeps the Si/SiO2 substrate temperature well below the back-end-of-line temperature limit of Si CMOS technology (770 K). The synthesis method described here can speed up the integration of large-area 2D materials with Si-based devices, paving the way for many important applications.

Unleash sodium storage potential of MoS2 nanosheets: Generating favorable kinetics from optimal crystallinity and elaborate structure

Metal oxides with high crystallinities have been reported to exhibit better ion storage performances, which demonstrates the significance of controlling the crystallinity as a facile and economical method to synthesize high-performance electrode materials. However, the relationship between crystallinity and charge storage property remains unclear for transition metal disulfides (TMDs). In this work, we employ a series of MoS2-based electrode materials with different crystallinities as a case to study the trend in sodium ion (Na+) storage characteristics of TMDs with varying crystallinity. Structure characterizations prove that the crystallinity of MoS2 improves with the increase in calcination temperature. Electrochemical tests and kinetic analyses indicate that MoS2-based electrode material with higher crystallinity shows better high-rate Na+ storage performance and stronger pseudocapacitive response. Finally, we carry out density functional theory calculations to study Na+ diffusion behaviors in MoS2 with different crystallinities, and we prove that MoS2 with higher crystallinity possesses better Na+ diffusion kinetics. The conclusions of this work provide beneficial guidance for the rational design of TMDs toward superior high-rate ion storage.