MoS2 Lattice Research Articles

In Situ Atomic-Scale Studies of the Formation of Epitaxial Pt Nanocrystals on Monolayer Molybdenum Disulfide.

Pt-nanocrystal:MoS2 hybrid materials have promising catalytic properties for hydrogen evolution, and understanding their detailed structures at the atomic scale is crucial to further development. Here, we use an in situ heating holder in an aberration-corrected transmission electron microscope to study the formation of Pt nanocrystals directly on the surface of monolayer MoS2 from a precursor on heating to 800 °C. Isolated single Pt atoms and small nanoclusters are observed after in situ heating, with two types of preferential alignment between the Pt nanocrystals and the underlying monolayer MoS2. Strain effects and thickness variations of the ultrasmall Pt nanocrystal supported on MoS2 are studied, revealing that single atomic planes are formed from a nonlayered face-centered cubic bulk Pt configuration with a lattice expansion of 7-10% compared to that of bulk Pt. The Pt nanocrystals are surrounded by an amorphous carbon layer and in some cases have etched the local surrounding MoS2 material after heating. Electron beam irradiation also initiates Pt nanocrystal etching of the local MoS2, and we study this process in real time at atomic resolution. These results show that the presence of carbon around the Pt nanocrystals does not affect their epitaxial relationship with the MoS2 lattice. Single Pt atoms within the carbon layer are also immobilized at high temperature. These results provide important insights into the formation of Pt:MoS2 hybrid materials.

Epitaxial Templating of Two-Dimensional Metal Chloride Nanocrystals on Monolayer Molybdenum Disulfide.

We demonstrate the formation of ionic metal chloride (CuCl) two-dimensional (2D) nanocrystals epitaxially templated on the surface of monolayer molybdenum disulfide (MoS2). These 2D CuCl nanocrystals are single atomic planes from a nonlayered bulk CuCl structure. They are stabilized as a 2D monolayer on the surface of the MoS2 through interactions with the uniform periodic surface of the MoS2. The heterostructure 2D system is studied at the atomic level using aberration-corrected transmission electron microscopy at 80 kV. Dynamics of discrete rotations of the CuCl nanocrystals are observed, maintaining two types of preferential alignments to the MoS2 lattice, confirming that the strong interlayer interactions drive the stable CuCl structure. Strain maps are produced from displacement maps and used to track real-time variations of local atomic bonding and defect production. Density functional theory calculations interpret the formation of two types of energetically advantageous commensurate superlattices via strong chemical bonds at interfaces and predict their corresponding electronic structures. These results show how vertical heterostructured 2D nanoscale systems can be formed beyond the simple assembly of preformed layered materials and provide indications about how different 2D components and their interfacial coupling mode could influence the overall property of the heterostructures.

Atomic Structure and Dynamics of Single Platinum Atom Interactions with Monolayer MoS2.

We have studied atomic level interactions between single Pt atoms and the surface of monolayer MoS2 using aberration-corrected annular dark field scanning transmission electron microscopy at an accelerating voltage of 60 kV. Strong contrast from single Pt atoms on the atomically resolved monolayer MoS2 lattice enables their exact position to be determined with respect to the MoS2 lattice, revealing stable binding sites. In regions of MoS2 free from surface contamination, the Pt atoms are localized in S vacancy sites and exhibit dynamic hopping to nearby vacancy sites driven by the energy supplied by the electron beam. However, in areas of MoS2 contaminated with carbon surface layers, the Pt atoms appear at various positions with respect to the underlying MoS2 lattice, including on top of Mo and in off-axis positions. These variations are due to the Pt bonding with the surrounding amorphous carbon layer, which disrupts the intrinsic Pt-MoS2 interactions, leading to more varied positions. Density functional theory (DFT) calculations reveal that Pt atoms on the surface of MoS2 have a small barrier for migration and are stabilized when bound to either a single or double sulfur vacancies. DFT calculations have been used to understand how the catalytic activity of the MoS2 basal plane for hydrogen evolution reaction is influenced by Pt dopants by variation of the hydrogen adsorption free energy. This strong dependence of catalytic effect on interfacial configurations is shown to be common for a series of dopants, which may provide a means to create and optimize reaction centers.

Atomic structure and formation mechanism of sub-nanometer pores in 2D monolayer MoS2.

We use electron-beam nanofabrication to create sub-nanometer (sub-nm) pores in 2D monolayer MoS2 with fine control over the pore size down to 0.6 nm, corresponding to the loss of a single Mo atom and surrounding S atoms. The sub-nm pores are created in situ with 1 nm spatial precision in the MoS2 lattice by control of the angstrom sized probe in an aberration corrected scanning transmission electron microscope with real time tracking of the pore creation. Dynamics of the sub-nm pore creation are captured at the atomic scale and reveal the mechanism of nanopore formation at accelerating voltages of 60 and 80 kV to be due to displacing a Mo atom from the lattice site onto the surface of the MoS2. This process is enabled by the destabilization of the Mo bonding from localized electron beam induced S atom loss. DFT calculations confirm the energetic advantage of having the ejected Mo atom attach on the sheet surface rather than being expelled into vacuum, and indicate sensitivity of the nanopore potential as a function of the adsorption position of the ejected Mo atom. These results provide detailed atomic level insights into the initial process of single Mo loss that underpins the nucleation of a nanopore and explains the formation mechanism of sub-nm pores in MoS2.

Energetics and kinetics of phase transition between a 2H and a 1T MoS2 monolayer-a theoretical study.

Phase transitions between semiconducting 2H and metallic 1T (or 1T') molybdenum disulphides (MoS2) are explored comprehensively by first-principles calculations. The nucleation of a 1T (or 1T') nucleus in a 2H MoS2 lattice, the formation of the 2H-1T (1T') interfaces and the kinetics of interface propagation during phase transition were thoroughly investigated in this study. It was found that, among various potential interface structures between the two phases, Mo-rich and two S-rich zigzag (ZZ) boundaries are energetically more preferable than others. Therefore, triangular or hexagonal 1T MoS2 domains with the three types of ZZ boundaries are expected to be the equilibrium structures of the 1T nucleus in a 2H lattice. Further exploration reveals a very high nucleation energy of the 1T phase which suggests that the nucleation may start from a defect site or the edge of the 2H phase. Besides, the kinetics of 2H-1T (1T') boundary propagation show that the growth rate of the ZZ boundaries is significantly slower than the armchair (AC) ones'. Therefore, the ZZ-boundary dominated domains are expected to be observed during the growth of the 1T phase while the rounded domains with various high-index edges are expected to be seen throughout shrinkage. This study reveals both the energetics and the kinetics of the phase transition of transition metal dichalcogenide materials and also sheds light on other 2D materials.

Pressure and temperature-dependent Raman spectra of MoS2 film

Molybdenum disulfide (MoS2), a relatively new and exciting two-dimensional graphene-like material, has been attracting more and more attentions from the researchers due to its unique structural and fascinating properties. The potential application of MoS2 under high-pressure and low-temperature is expected, while the related research is few at present. In this paper, quadrilayer MoS2 was synthesized by chemical vapor deposition, and its structural properties under different pressures (0–20.7 GPa) and temperatures (10–300 K) were investigated via the Raman spectra. We find that the lattice of quadrilayer MoS2 is not damaged and the quadrilayer MoS2 exhibits good semiconductive properties under large variable pressures from atmospheric to 20.7 GPa, which is much different to its bulk and single crystalline phases. In addition, the lattice structures of the quadrilayer MoS2 are stable in 10–300 K, and the Grüneisen parameters of E12g and A1g modes are smaller than that of bulk. This study indicates that quadrilayer MoS2 has a better prospect in high-pressure and low-temperature environment.

Nanostructured MoS2/Graphene Composite As Superior Anode Material for Li-Ion Batteries

To meet rising demands for energy density of lithium ion batteries (LIBs), a novel nanostructured MoS2/graphene (MoS2/G) composite with MoS2 nanoplates (NPs) perpendicularly grown on graphene sheets was prepared by a facile hydrothermal method. The direct coupling of edge Mo of MoS2 lattice with oxygen of functional groups from graphene oxide (forming C-O-Mo bond) is achieved by adjusting the synthesis parameters, which is beneficial to the electron transport and structural stability of MoS2/G electrode, and thus to the improvement of rate performance and long-cycle life of the electrode. The graphene sheets can enhance the electrical conductivity of the composite and meanwhile act not only as a substrate to disperse active MoS2 NPs homogeneously but also a cushion to accommodate the volume change during cycling. As an anode material for LIBs, MoS2/G manifests a high specific capacity (1077 mAh g-1 at 100 mA g-1 after 150 cycles), excellent rate capability and a stable cycling performance (907 mAh g-1 at 1000 mA g-1 after 400 cycles). The prepared MoS2/G composite is a promising anode material for next generation high performance LIBs.

Cobalt-doped edge-rich MoS2/nitrogenated graphene composite as an electrocatalyst for hydrogen evolution reaction

Co-doped MoS2/nitrogenated graphene composite has been successfully fabricated through a facile hydrothermal method and is investigated as an electrocatalyst for hydrogen evolution reaction (HER). The results reveal that Co2+ ions have been successfully incorporated into MoS2 lattice which can give rise to structural strain and dislocation. As a result, the Co-doped MoS2 nanosheets are characteristic of few layers (3–6 layers) and abundant exposed active edges. Meanwhile, Co-doped MoS2 nanosheets are well implanted into the nitrogen-doped graphene networks to form robust composite. When evaluated as a catalyst for HER, the obtained composite demonstrates superior electrocatalytic activities towards HER including low onset potential (∼90mV), large cathodic current density and small Tafel slope of 59mV dec−1. It is believed that the desirable catalytic properties can be credited to the unique microstructures including few-layer features, plentiful exposed active sites as well as intimate integration with highly electroconductive nitrogenated graphene.

Remote Plasma Oxidation and Atomic Layer Etching of MoS2

Exfoliated molybdenum disulfide (MoS2) is shown to chemically oxidize in a layered manner upon exposure to a remote O2 plasma. X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and atomic force microscopy (AFM) are employed to characterize the surface chemistry, structure, and topography of the oxidation process and indicate that the oxidation mainly occurs on the topmost layer without altering the chemical composition of underlying layer. The formation of S-O bonds upon short, remote plasma exposure pins the surface Fermi level to the conduction band edge, while the MoOx formation at high temperature modulates the Fermi level toward the valence band through band alignment. A uniform coverage of monolayer amorphous MoO3 is obtained after 5 min or longer remote O2 plasma exposure at 200 °C, and the MoO3 can be completely removed by annealing at 500 °C, leaving a clean ordered MoS2 lattice structure as verified by XPS, LEED, AFM, and scanning tunneling microscopy. This work shows that a remote O2 plasma can be useful for both surface functionalization and a controlled thinning method for MoS2 device fabrication processes.

Single step, bulk synthesis of engineered MoS2 quantum dots for multifunctional electrocatalysis

Bi- or tri- functional catalysts based on atomic layers are receiving tremendous scientific attention due to their importance in various energy technologies. Recent studies on molybdenum disulphide (MoS2) nanosheets revealed that controlling the edge states and doping/modifying with suitable elements are highly important in tuning the catalytic activities of MoS2. Here we report a bulk, single step method to synthesize metal modified MoS2 quantum dots (QDs). Three elements, namely Fe, Mg and Li, are chosen to study the effects of dopants in the catalytic activities of MoS2. Fe and Mg are found to act like dopants in the MoS2 lattice forming respective doped MoS2 QDs, while Li formed an intercalated MoS2 QD. The efficacy and tunability of these luminescent doped QDs towards various electrocatalytic activities (hydrogen evolution reaction, oxygen evolution reaction and oxygen reduction action) are reported here.

Detailed Atomic Reconstruction of Extended Line Defects in Monolayer MoS2.

We study the detailed bond reconstructions that occur in S vacancies within monolayer MoS2 using a combination of aberration-corrected transmission electron microscopy, density functional theory (DFT), and multislice image simulations. Removal of a single S atom causes little perturbation to the surrounding MoS2 lattice, whereas the loss of two S atoms from the same atomic column causes a measurable local contraction. Aggregation of S vacancies into linear line defects along the zigzag direction results in larger lattice compression that is more pronounced as the length of the line defect increases. For the case of two rows of S line vacancies, we find two different types of S atom reconstructions with different amounts of lattice compression. Increasing the width of line defects leads to nanoscale regions of reconstructed MoS2 that are shown by DFT to behave as metallic channels. These results provide important insights into how defect structures could be used for creating metallic tracks within semiconducting monolayer MoS2 films for future applications in electronics and optoelectronics.

Threshold Dependence of Deep- and Near-subwavelength Ripples Formation on Natural MoS2 Induced by Femtosecond Laser.

Deep sub-wavelength ripples (DSRs) and near sub-wavelength ripples (NSRs) with uniform periods of ~160 nm and ~660 nm generated at the MoS2-vacuum interface is reported for the first time by the processing of femtosecond laser (800 nm, 120 fs, 1 kHz) in this paper. The DSRs and NSRs formation fluence thresholds are experimentally determined as 160 mJ/cm2 and 192 mJ/cm2, respectively. In addition, the ripple period is insensitive to the pulse number. Moreover, Raman analyses show that the MoS2 lattice in the irradiated area does not exhibit oxidation at room environment and the crystalline representation is well preserved in NSRs region. We attribute our result to the joint interactions of the spallation and sublimation of layered MoS2 together with the laser induced surface plasmon polaritons and propose an explanation to the threshold dependence of the ripple period. Our study provides some insights for ultrafast laser-matter interactions and indicates a simple effective method for future nano-fabrication of MoS2.

Chirality and vacancy effect on phonon dispersion of MoS2 with strain

With the first principles theory calculations, we reveal the strain, chirality and defect effects to the mechanical properties, phonon dispersion, and structural stability of monolayer MoS2. Phonon dispersion relation is calculated based on the lattice dynamics theory. Chirality effect is trivial to the Young's modulus at zero strain but significant to the ultimate strength and Poisson's ratio, regardless of the defects. A phonon band gap of 49 cm−1 exists in the perfect MoS2 lattice, and reduces to 30.6 cm−1 in the MoS2 lattice with two sulfur missing vacancy. The phonon band gap reduces with the increasing of tensile strain, regardless of chirality. The failure mode is investigated near the ultimate tensile strength of both the perfect MoS2 lattice and the MoS2 lattice with two sulfur missing vacancy. Regardless of the defect, elastic instability and structural instability are identified for the MoS2 in zigzag direction tension test and armchair direction tension test, respectively. Based on the weakly linear component in the flextural acoustic mode of phonon dispersion, we evaluate the intrinsic critical compressive strain of buckling to be ϵC=−2.0%±0.5% for the perfect MoS2 lattice and demonstrate it numerically from first principles theory calculation.

Electrochemical Reaction in Single Layer MoS2: Nanopores Opened Atom by Atom.

Ultrathin nanopore membranes based on 2D materials have demonstrated ultimate resolution toward DNA sequencing. Among them, molybdenum disulfide (MoS2) shows long-term stability as well as superior sensitivity enabling high throughput performance. The traditional method of fabricating nanopores with nanometer precision is based on the use of focused electron beams in transmission electron microscope (TEM). This nanopore fabrication process is time-consuming, expensive, not scalable, and hard to control below 1 nm. Here, we exploited the electrochemical activity of MoS2 and developed a convenient and scalable method to controllably make nanopores in single-layer MoS2 with subnanometer precision using electrochemical reaction (ECR). The electrochemical reaction on the surface of single-layer MoS2 is initiated at the location of defects or single atom vacancy, followed by the successive removals of individual atoms or unit cells from single-layer MoS2 lattice and finally formation of a nanopore. Step-like features in the ionic current through the growing nanopore provide direct feedback on the nanopore size inferred from a widely used conductance vs pore size model. Furthermore, DNA translocations can be detected in situ when as-fabricated MoS2 nanopores are used. The atomic resolution and accessibility of this approach paves the way for mass production of nanopores in 2D membranes for potential solid-state nanopore sequencing.

Interlayer-expanded molybdenum disulfide nanocomposites for electrochemical magnesium storage.

Mg rechargeable batteries (MgRBs) represent a safe and high-energy battery technology but suffer from the lack of suitable cathode materials due to the slow solid-state diffusion of the highly polarizing divalent Mg ion. Previous methods improve performance at the cost of incompatibility with anode/electrolyte and drastic decrease in volumetric energy density. Herein we report interlayer expansion as a general and effective atomic-level lattice engineering approach to transform inactive intercalation hosts into efficient Mg storage materials without introducing adverse side effects. As a proof-of-concept we have combined theory, synthesis, electrochemical measurement, and kinetic analysis to improve Mg diffusion behavior in MoS2, which is a poor Mg transporting material in its pristine form. First-principles simulations suggest that expanded interlayer spacing allows for fast Mg diffusion because of weakened Mg-host interactions. Experimentally, the expansion was realized by inserting a controlled amount of poly(ethylene oxide) into the lattice of MoS2 to increase the interlayer distance from 0.62 nm to up to 1.45 nm. The expansion boosts Mg diffusivity by 2 orders of magnitude, effectively enabling the otherwise barely active MoS2 to approach its theoretical storage capacity as well as to achieve one of the highest rate capabilities among Mg-intercalation materials. The interlayer expansion approach can be leveraged to a wide range of host materials for the storage of various ions, leading to novel intercalation chemistry and opening up new opportunities for the development of advanced materials for next-generation energy storage.

Photothermoelectric and photovoltaic effects both present in MoS2.

As a finite-energy-bandgap alternative to graphene, semiconducting molybdenum disulfide (MoS2) has recently attracted extensive interest for energy and sensor applications. In particular for broad-spectral photodetectors, multilayer MoS2 is more appealing than its monolayer counterpart. However, little is understood regarding the physics underlying the photoresponse of multilayer MoS2. Here, we employ scanning photocurrent microscopy to identify the nature of photocurrent generated in multilayer MoS2 transistors. The generation and transport of photocurrent in multilayer MoS2 are found to differ from those in other low-dimensional materials that only contribute with either photovoltaic effect (PVE) or photothermoelectric effect (PTE). In multilayer MoS2, the PVE at the MoS2-metal interface dominates in the accumulation regime whereas the hot-carrier-assisted PTE prevails in the depletion regime. Besides, the anomalously large Seebeck coefficient observed in multilayer MoS2, which has also been reported by others, is caused by hot photo-excited carriers that are not in thermal equilibrium with the MoS2 lattice.

Unravelling orientation distribution and merging behavior of monolayer MoS2 domains on sapphire.

Monolayer MoS2 prepared by chemical vapor deposition (CVD) has a highly polycrystalline nature largely because of the coalescence of misoriented domains, which severely hinders its future applications. Identifying and even controlling the orientations of individual domains and understanding their merging behavior therefore hold fundamental significance. In this work, by using single-crystalline sapphire (0001) substrates, we designed the CVD growth of monolayer MoS2 triangles and their polycrystalline aggregates for such purposes. The obtained triangular MoS2 domains on sapphire were found to distributively align in two directions, which, as supported by density functional theory calculations, should be attributed to the relatively small fluctuations of the interface binding energy around the two primary orientations. Using dark-field transmission electron microscopy, we further imaged the grain boundaries of the aggregating domains and determined their prevalent armchair crystallographic orientations with respect to the adjacent MoS2 lattice. The coalescence of individual triangular flakes governed by unique kinetic processes is proposed for the polycrystal formation. These findings are expected to shed light on the controlled MoS2 growth toward predefined domain orientation and large domain size, thus enabling its versatile applications in next-generation nanoelectronics and optoelectronics.

Epitaxial Growth of PbSe Quantum Dots on MoS2 Nanosheets and their Near‐Infrared Photoresponse

A facile one‐pot synthesis of hybrid materials consisting of PbSe quantum dots (QDs) that grow epitaxially on MoS2 nanoflakes resulting in three equivalent orientation variants of the PbSe QDs with respect to the MoS2 lattice is demonstrated. The epitaxial growth and cross‐sectional high‐resolution transmission electron microscopy (HRTEM) investigations verify a direct and linker‐free contact between the quantum dots and the transition metal dichalcogenide (TMD) nanoflakes, while maintaining surface passivation of the PbSe with oleic acid ligands on the outside. Solution‐processed photodetectors based on PbSe‐MoS2 hybrids exhibit stable photoconduction when illuminated with near‐IR light (wavelength > 1200 nm) without any laborious ligand‐exchange steps. Flexible devices fabricated on polyethylene terephthalate (PET) substrates show excellent stability upon repeated bending. These hybrid materials are air‐stable and solution‐processable at low temperatures and thus promising for low‐cost flexible near‐IR photodetectors.

Effect of point and grain boundary defects on the mechanical behavior of monolayer MoS2 under tension via atomistic simulations

Atomistic simulation is used to study the structure and energy of defects in monolayer MoS2 and the role of defects on the mechanical properties of monolayer MoS2. First, energy minimization is used to study the structure and energy of monosulfur vacancies positioned within the bottom S layer of the MoS2 lattice, and 60° symmetric tilt grain boundaries along the zigzag and armchair directions, with comparison to experimental observations and density functional theory calculations. Second, molecular dynamics simulations are used to subject suspended defect-containing MoS2 membranes to a state of multiaxial tension. A phase transformation is observed in the defect-containing membranes, similar to prior work in the literature. For monolayer MoS2 membranes with point defects, groups of monosulfur vacancies promote stress-concentration points, allowing failure to initiate away from the center of the membrane. For monolayer MoS2 membranes with grain boundaries, failure initiates at the grain boundary and it is found that the breaking force for the membrane is independent of grain boundary energy.

The Role of Lead (Pb) in the High Temperature Formation of MoS2 Nanotubes

Recent studies have clearly indicated the favorable effect of lead as a growth promoter for MX2 (M = Mo, W; X = S, Se) nanotubes using MX2 powder as a precursor material. The experimental work indicated that the lead atoms are not stable in the molybdenum oxide lattice ion high concentration. The initial lead concentration in the oxide nanowhiskers (Pb:Mo ratio = 0.28) is reduced by one order of magnitude after one year in the drawer. The initial Pb concentration in the MoS2 nanotubes lattice (produced by solar ablation) is appreciably smaller (Pb:Mo ratio for the primary samples is 0.12) and is further reduced with time and annealing at 810 °C, without consuming the nanotubes. In order to elucidate the composition of these nanotubes in greater detail; the Pb-“modified” MX2 compounds were studied by means of DFT calculations and additional experimental work. The calculations indicate that Pb doping as well as Pb intercalation of MoS2 lead to the destabilization of the system; and therefore a high Pb content within the MoS2 lattice cannot be expected in the final products. Furthermore; substitutional doping (PbMo) leads to p-type semiconducting character; while intercalation of MoS2 by Pb atoms (Pby/MoS2) should cause n-type semiconducting behavior. This study not only sheds light on the role of added lead to the growth of the nanotubes and their role as electron donors; but furthermore could pave the way to a large scale synthesis of the MoS2 nanotubes.