Real-space imaging of single-layer MoS2 by scanning tunneling microscopy

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Tuning the 1T'/2H phases in WxMo1-xSe2 nanosheets

Controlling materials' morphology, crystal phase and chemical composition at the atomic scale has become central in materials research. Wet chemistry approaches have great potential in directing the material crystallisation process to achieve tuneable chemical compositions as well as to target specific crystal phases. Herein, we report the compositional and crystal phase tuneability achieved in the quasi-binary W_xMo_1-xSe₂ system with chemical and crystal phase mixing down to the atomic level. A series of W_xMo_1-xSe₂ solid solutions in the form of nanoflowers with atomically thin petals were obtained via a direct colloidal reaction by systematically varying the ratios of transition metal precursors. We investigate the effect of selenium precursor on the morphology of the W_xMo_1-xSe₂ material and show how using elemental selenium can enable the formation of larger and distinct nanoflowers. While the synthesised materials are compositionally homogeneous, they exhibit crystal phase heterogeneity with the co-existing domains of the 1T' and 2H crystal phases, and with evidence of MoSe₂ in the metastable 1T' phase. We show at single atom level of resolution, that tungsten and molybdenum can be found in both the 1T' and 2H lattices. The formation of heterophase 1T'/2H W_xMo_1-xSe₂ electrocatalysts allowed for a considerable improvement in the activity for the acidic hydrogen evolution reaction (HER) compared to pristine, 1T'-dominated, WSe₂. This work can pave the way towards engineered functional nanomaterials where properties, such as electronic and catalytic, have to be controlled at the atomic scale.

Two-dimensional antiferromagnetic semiconductor T'-MoTeI from first principles

Two-dimensional intrinsic antiferromagnetic semiconductors are expected to stand out in the spintronic field. The present work finds the monolayer T'-MoTeI is intrinsically an antiferromagnetic semiconductor by using first-principles calculation. Firstly, the dimerized distortion of the Mo atoms causes T'-MoTeI to have dynamic stability, which is different from the small imaginary frequency in the phonon spectrum of T-MoTeI. Secondly, T'-MoTeI is an indirect-bandgap semiconductor with 1.35 eV. Finally, in the systematic study of strain effects, there are significant changes in the electronic structure as well as the bandgap, but the antiferromagnetic ground state is not affected. Monte Carlo simulations predict that the Néel temperature of T'-MoTeI is 95 K. The results suggest that the monolayer T'-MoTeI can be a potential candidate for spintronics applications.

Experimental and Theoretical Comparison of Potential-dependent Methylation on Chemically Exfoliated WS2 and MoS2

Reductant-activated functionalization is shown to enhance the methylation of chemically exfoliated MoS₂ (ceMoS₂) and ceWS₂ by introducing excess negative charge to facilitate a nucleophilic attack reaction. Relative to methylation in the absence of a reductant, the reaction produces a twofold increase in coverage of ceWS₂, from 25 to 52% coverage per WS₂. However, at every potential, the methyl coverage on ceWS₂ was ∼20% lower than that on ceMoS₂. We applied grand canonical density functional theory to show that at constant potential, more negative charge is present on 1T'-MoS₂ than on 1T'-WS₂, making methylation both thermodynamically and kinetically more favorable for 1T'-MoS₂ than 1T'-WS₂. This effect was moderated when the reactions were compared at constant charge, emphasizing the importance of comparing the reactivity of materials at nominally identical electrode potentials.

One order enhancement of charge carrier relaxation rate by tuning structural and optical properties in annealed cobalt doped MoS2 nanosheets

The effective manipulation of excitons is crucial for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.

Observation of topologically protected states at crystalline phase boundaries in single-layer WSe2

Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1T'-WSe2. We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1T'-WSe2 and a semiconducting domain of 1H-WSe2 in contiguous single layers. The QSHI nature of single-layer 1T'-WSe2 is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states.

Flexible modulation of electronic and magnetic properties of zigzag H-MoS2 nanoribbons by crack defects

The effects of crack defects on electronic and magnetic properties of zigzag MoS₂ nanoribbons are investigated systematically by first-principles calculations based on spin-polarized density functional theory. We find that not only the electronic and spin transport ability of zigzag MoS₂ nanoribbons can be enhanced significantly by the armchair crack defects, but also their magnetism could be modulated flexibly by crack defects. Our study suggests that the introduction of crack defect is a feasible way to modulate the electronic and magnetic properties of zigzag MoS₂ nanoribbons. We further propose that the crack defects may also provide a useful tool for improving the performance of devices.

Recent Advances in the Solution-Based Preparation of Two-Dimensional Layered Transition Metal Chalcogenide Nanostructures

The precise control in size/thickness, composition, crystal phases, doping, defects, and surface properties of two-dimensional (2D) layered transition metal chalcogenide (TMC) is important for the investigation of interwoven relationship between structures, functions, and practical applications. Of the multiple synthetic routes, solution-based top-down and bottom-up chemical methods have been uniquely important for their potential to control the size and composition at the molecular level in addition to their scalability, competitive production cost, and solution processability. Here, we introduce an overview of the recent advances in the solution-based preparation routes of 2D layered TMC nanostructures along with important scientific developments.

Insight into the structural and electronic nature of chemically exfoliated molybdenum disulfide nanosheets in aqueous dispersions

Chemical exfoliation of molybdenum disulfide (2H-MoS₂) for preparing high-yield single-layer sheets has attracted considerable attention in recent years. However, the stability and nature of the resulting nanosheets are poorly understood. Storing the dispersion in ambient air brings about the reoxidation of the nanosheets, releasing their residual negative charges into the environment. The reoxidation facilitates lateral fractures and destabilizes the dispersion. In-plane X-ray diffraction of the nanosheets indicates that they have a 1T structure with a 2D √3 × 1 rectangular cell as the intrinsic structure for chemically exfoliated MoS₂. We found that the 1T structure was preserved after reoxidation upon aging the dispersions in air, suggesting the formation of metastable neutral MoS₂. The changes in the chemical nature of the nanosheets can be monitored by X-ray diffraction of the restacked nanosheets. The restacked nanosheets, obtained by drying the freshly prepared dispersion, exhibited an expanded bilayer hydrate structure, accommodating Li ions. On the other hand, dried samples from the aged dispersions were substantially composed of a deintercalated phase and the bilayer hydrate. Upon prolonged aging, the former phase became predominant with total disappearance of the latter. This evolution suggests that the reoxidation occurred sheet by sheet with a direct restoration of the original oxidation states of the nanosheets, whereas the oxidation states of the nanosheets can be discrete at 4+ and (4 - δ)+.

The Coupled Straintronic-Photothermic Effect

We describe the coupled straintronic-photothermic effect where coupling between bandgap of the 2D layered semiconductor under localized strains, optical absorption and the photo-thermal effect results in a large chromatic mechanical response in TMD-nanocomposites. Under the irradiation of visible light (405 nm to 808 nm), such locally strained atomic thin films based on 2H-MoS2 embedded in an elastomer such as poly (dimethyl) siloxane matrix exhibited a large amplitude of photo-thermal actuation compared to their unstrained counterparts. Moreover, the locally strain engineered nanocomposites showed tunable mechanical response giving rise to higher mechanical stress at lower photon energies. Scanning photoluminescence spectroscopy revealed a change in bandgap of 30 meV between regions encompassing highly strained compared to the unstrained few layers. For 1.6% change in the bandgap, the macroscopic photo-thermal response increased by a factor of two. Millimeter scale bending actuators based on the locally strained 2H-MoS2 resulted in significantly enhanced photo-thermal actuation displacements compared to their unstrained counterparts at lower photon energies and operated up to 30 Hz. Almost 1 mN photo-activated force was obtained at 50 mW and provided long-term stability. This study demonstrates a new mechanism in TMD-nanocomposites that would be useful for developing broad range of transducers.

First-principles study on structural, thermal, mechanical and dynamic stability of T'-MoS2

Using first-principles density functional theory calculations, we investigate the structure, stability, optical modes and electronic band gap of a distorted tetragonal MoS₂ monolayer (T'-MoS₂). Our simulated scanning tunnel microscopy (STM) images of T'-MoS₂ are dramatically similar to those STM images which were identified as K _x (H₂O) _y MoS₂ from a previous experimental study. This similarity suggests that T'-MoS₂ might have already been experimentally observed, but due to being unexpected was misidentified. Furthermore, we verify the stability of T'-MoS₂ from the thermal, mechanical and dynamic aspects, by ab initio molecular dynamics simulation, elastic constants evaluation and phonon band structure calculation based on density functional perturbation theory, respectively. In addition, we calculate the eigenfrequencies and eigenvectors of the optical modes of T'-MoS₂ at [Formula: see text] point and distinguish their Raman and infrared activity by pointing out their irreducible representations using group theory. At the same time, we compare the Raman modes of T'-MoS₂ with those of H-MoS₂ and T-MoS₂. Our results provide useful guidance for further experimental identification and characterization of T'-MoS₂.

Chromatic Mechanical Response in 2-D Layered Transition Metal Dichalcogenide (TMDs) based Nanocomposites

The ability to convert photons of different wavelengths directly into mechanical motion is of significant interest in many energy conversion and reconfigurable technologies. Here, using few layer 2H-MoS₂ nanosheets, layer by layer process of nanocomposite fabrication, and strain engineering, we demonstrate a reversible and chromatic mechanical response in MoS₂-nanocomposites between 405 nm to 808 nm with large stress release. The chromatic mechanical response originates from the d orbitals and is related to the strength of the direct exciton resonance A and B of the few layer 2H-MoS₂ affecting optical absorption and subsequent mechanical response of the nanocomposite. Applying uniaxial tensile strains to the semiconducting few-layer 2H-MoS₂ crystals in the nanocomposite resulted in spatially varying energy levels inside the nanocomposite that enhanced the broadband optical absorption up to 2.3 eV and subsequent mechanical response. The unique photomechanical response in 2H-MoS₂ based nanocomposites is a result of the rich d electron physics not available to nanocomposites based on sp bonded graphene and carbon nanotubes, as well as nanocomposite based on metallic nanoparticles. The reversible strain dependent optical absorption suggest applications in broad range of energy conversion technologies that is not achievable using conventional thin film semiconductors.

Atomic-Scale Probing of the Dynamics of Sodium Transport and Intercalation-Induced Phase Transformations in MoS₂

For alkali-metal-ion batteries, probing the dynamic processes of ion transport in electrodes is critical to gain insights into understanding how the electrode functions and thus how we can improve it. Here, by using in situ high-resolution transmission electron microscopy, we probe the dynamics of Na transport in MoS2 nanostructures in real-time and compare the intercalation kinetics with previous lithium insertion. We find that Na intercalation follows the two-phase reaction mechanism, that is, trigonal prismatic 2H-MoS2 → octahedral 1T-NaMoS2, and the phase boundary is ∼2 nm thick. The velocity of the phase boundary at <10 nm/s is 1 order smaller than that of lithium diffusion, suggesting sluggish kinetics for sodium intercalation. The newly formed 1T-NaMoS2 contains a high density of defects and series superstructure domains with typical sizes of ∼3-5 nm. Our results provide valuable insights into finding suitable Na electrode materials and understanding the properties of transition metal dichalcogenide MoS2.

Single-Layer ReS₂: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials.

Understanding catalysis in a multiphasic two-dimensional transition metal dichalcogenide

Establishing processing–structure–property relationships for monolayer materials is crucial for a range of applications spanning optics, catalysis, electronics and energy. Presently, for molybdenum disulfide, a promising catalyst for artificial photosynthesis, considerable debate surrounds the structure/property relationships of its various allotropes. Here we unambiguously solve the structure of molybdenum disulfide monolayers using high-resolution transmission electron microscopy supported by density functional theory and show lithium intercalation to direct a preferential transformation of the basal plane from 2H (trigonal prismatic) to 1T′ (clustered Mo). These changes alter the energetics of molybdenum disulfide interactions with hydrogen (ΔG_H), and, with respect to catalysis, the 1T′ transformation renders the normally inert basal plane amenable towards hydrogen adsorption and hydrogen evolution. Indeed, we show basal plane activation of 1T′ molybdenum disulfide and a lowering of ΔG_H from +1.6 eV for 2H to +0.18 eV for 1T′, comparable to 2H molybdenum disulfide edges on Au(111), one of the most active hydrogen evolution catalysts known.

Establishing structure–property relationships for catalytic materials is essential for optimization of performance. Here, the authors solve the structure of molybdenum disulfide monolayers, and probe the role of lithium intercalation and the subsequent effects on catalytic hydrogen activation.

Probing the Dynamics of the Metallic-to-Semiconducting Structural Phase Transformation in MoS2 Crystals

We have investigated the phase transformation of bulk MoS2 crystals from the metastable metallic 1T/1T' phase to the thermodynamically stable semiconducting 2H phase. The metastable 1T/1T' material was prepared by Li intercalation and deintercalation. The thermally driven kinetics of the phase transformation were studied with in situ Raman and optical reflection spectroscopies and yield an activation energy of 400 ± 60 meV (38 ± 6 kJ/mol). We calculate the expected minimum energy pathways for these transformations using DFT methods. The experimental activation energy corresponds approximately to the theoretical barrier for a single formula unit, suggesting that nucleation of the phase transformation is quite local. We also report that femtosecond laser writing converts 1T/1T' to 2H in a single laser pass. The mechanisms for the phase transformation are discussed.

Phase engineering of transition metal dichalcogenides

The co-existence of 2H, 1T and 1T′ phases in monolayered TMDs.

Coherent atomic and electronic heterostructures of single-layer MoS2

Nanoscale heterostructures with quantum dots, nanowires, and nanosheets have opened up new routes toward advanced functionalities and implementation of novel electronic and photonic devices in reduced dimensions. Coherent and passivated heterointerfaces between electronically dissimilar materials can be typically achieved through composition or doping modulation as in GaAs/AlGaAs and Si/NiSi or heteroepitaxy of lattice matched but chemically distinct compounds. Here we report that single layers of chemically exfoliated MoS(2) consist of electronically dissimilar polymorphs that are lattice matched such that they form chemically homogeneous atomic and electronic heterostructures. High resolution scanning transmission electron microscope (STEM) imaging reveals the coexistence of metallic and semiconducting phases within the chemically homogeneous two-dimensional (2D) MoS(2) nanosheets. These results suggest potential for exploiting molecular scale electronic device designs in atomically thin 2D layers.

Photoluminescence from chemically exfoliated MoS2

A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chemically exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliated monolayers, indicating that their semiconducting properties are largely restored.

Optical properties of exfoliated MoS2 coaxial nanotubes - analogues of graphene

We report on the first exfoliation of MoS2 coaxial nanotubes. The single-layer flakes, as the result of exfoliation, represent the transition metal dichalcogenides' analogue of graphene. They show a very low degree of restacking in comparison with exfoliation of MoS2 plate-like crystals. MoS2 monolayers were investigated by means of electron and atomic force microscopies, showing their structure, and ultraviolet-visible spectrometry, revealing quantum confinement as the consequence of the nanoscale size in the z-direction.