Transition Metal Dichalcogenides Surface Research Articles

Heteroepitaxy in Organic/TMD Hybrids and Challenge to Achieve it for TMD Monolayers: The Case of Pentacene on WS2 and WSe2.

The intriguing photophysical properties of monolayer stacks of different transition-metal dichalcogenides (TMDs), revealing rich exciton physics including interfacial and moiré excitons, have recently prompted an extension of similar investigations to hybrid systems of TMDs and organic films, as the latter combine large photoabsorption cross sections with the ability to tailor energy levels by targeted synthesis. To go beyond single-molecule photoexcitations and exploit the excitonic signatures of organic solids, crystalline molecular films are required. Moreover, a defined registry on the substrate, ideally an epitaxy, is desirable to also achieve an excitonic coupling in momentum space. This poses a certain challenge as excitonic dipole moments of organic films are closely related to the molecular orientation and film structure, which critically depend on the support roughness. Using X-ray diffraction, optical polarization, and atomic force microscopy, we analyzed the structure of pentacene (PEN) multilayer films grown on WSe2(001) and WS2(001) and identified an epitaxial alignment. While (022)-oriented PEN films are formed on both substrates, their azimuthal orientations are quite different, showing an alignment of the molecular L-axis along the and directions. This intrinsic epitaxial PEN growth depends, however, sensitively on the substrates surface quality. While it occurs on exfoliated TMD single crystals and multilayer flakes, it is hardly found on exfoliated monolayers, which often exhibit bubbles and wrinkles. This enhances the surface roughness and results in (001)-oriented PEN films with upright molecular orientation but without any azimuthal alignment. However, monolayer flakes can be smoothed by AFM operated in contact mode or by transferring to ultrasmooth substrates such as hBN, which again yields epitaxial PEN films. As different PEN orientations result in different characteristic film morphologies (elongated mesa islands vs pyramidal dendrites), which can be easily distinguished by AFM or optical microscopy, this provides a simple means to judge the roughness of the used TMD surface.

Selective passivation of 2D TMD surface defects by atomic layer deposited Al2O3 to enhance recovery properties of gas sensor

The incomplete recovery of Transition Metal Dichalcogenides (TMD) based gas sensors hinders their reliability and scalability. The leading cause of incomplete recovery is the strong chemisorption of gas analytes, such as defects or grain boundaries on the active surface of 2D TMDs. Herein, we demonstrate an improvement in the recovery rate of TMD gas sensors by selectively passivating the TMD surface defects or vacancies with Al2O3 via atomic layer deposition. Scanning electron microscopy analysis confirms that the nucleation and growth of atomic-layer-deposited Al2O3 occur along the grain boundaries and defects of the 2D MoS2 and WS2, not covering the inert basal plane. In addition, the Raman, photoluminescence, and X-ray photoelectron spectroscopy data show lower surface defect densities and a slight n-doping effect of Al2O3. This unique selectively defect-passivated TMD gas sensor shows a 400 % response toward 10 ppm of NO2, along with an increase in the recovery rate from 74 to 96 %, even at room temperature, as the number of atomic layer deposition cycles increases. Also, the recovery rate of NH3, a reducing gas, shows an increase of more than 30 %. Thus, the method proposed here is a promising strategy for improving the recovery rate of 2D TMD gas sensors.

A Monolayer MoS2 FET with an EOT of 1.1nm Achieved by the Direct Formation of a High-κ Er2 O3 Insulator Through Thermal Evaporation.

Achieving the direct growth of an ultrathin gate insulator with high uniformity and high quality on monolayer transition metal dichalcogenides (TMDCs) remains a challenge due to the chemically inert surface of TMDCs. Although the main solution for this challenge is utilizing buffer layers before oxide is deposited on the atomic layer, this method drastically degrades the total capacitance of the gate stack. In this work, weconstructed a novel direct high-κ Er2 O3 deposition system based on thermal evaporation in a differential-pressure-type chamber. A uniform Er2 O3 layer with an equivalent oxide thickness of 1.1nm wasachieved as the gate insulator for top-gated MoS2 field-effect transistors (FETs). The top gate Er2 O3 insulator without the buffer layer on MoS2 exhibited a high dielectric constant that reached 18.0, which is comparable to that of bulk Er2 O3 and is the highest among thin insulators (< 10nm) on TMDCs to date. Furthermore, the Er2 O3 /MoS2 interface (Dit ≈ 6 × 1011 cm-2 eV-1 ) is confirmed to be clean and is comparable with that of the h-BN/MoS2 heterostructure. These results prove that high-quality dielectric properties with retained interface quality can be achieved by this novel deposition technique, facilitating the future development of 2D electronics.

Unveiling Unintentional Fluorine Doping in TMDs During the Reactive Ion Etching: Root Cause Analysis, Physical Insights, and Solution

Layered semiconductors, such as transition metal dichalcogenides (TMDs), gained popularity due to their unique properties favoring different applications. For all practical applications, it is essential to have the ability to pattern these layered semiconductors as per the desired dimensions, which are derived from device design or circuit design. Due to its anisotropic nature, plasma or reactive ion etching (RIE) has been the most used technique for patterning bulk semiconductors or thin films. This article unveils the challenges associated with the fluorine-based (SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and CHF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) plasma etching of TMDs. Subsequently, a detailed root cause analysis is presented with the solution to the problems thereof. We have discovered: 1) high affinity of resists leading to unwanted resist residues over the TMD surface and 2) unintentional doping induced by the fluorine plasma, due to fluorine occupation at the interstitial sites near sulfur (S) defects or its occupation of an S-defect site in the TMD. This was observed despite the surface being masked by the resist. Physical insights were developed using atomic force microscopy (AFM), Raman spectroscopy, electrical characterization, field-effect mobility and transfer length method (TLM)-based contact resistance comparison, and density functional theory (DFT)-based atomistic computations. Furthermore, these insights are used to demonstrate an improved plasma recipe, which mitigates these challenges and allows realization of transistors over a patterned channel while offering characteristic similar to an intrinsic/pristine layer. Finally, the improved plasma recipe, developed for MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , is also shown to be effective for MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers.

Passivation of Transition Metal Dichalcogenides Monolayers with a Surface‐Confined Atomically Thick Sulfur Layer

Surface passivation can eliminate the charge doping of monolayer transition metal dichalcogenides (TMDs) during the device fabrication, which is important for the large‐scale production of ultra‐stable materials and high‐performance devices. The uniformity and atomical thickness of the passivating layers with a low dielectric constant (κ) are essentials to preserving the intrinsic properties of monolayer TMDs in harsh environments. Herein, a surface‐confined mechanism is developed to encapsulate TMDs monolayers by atomically thin sulfur layers with high spatial homogeneity (named S–MX2). The bottom bilayer S atoms are strongly confined by the upper S monolayers when the low‐κ S reaches three layers on the surface of TMDs, which spontaneously renders the uniform distribution on a large scale. The intrinsic electrical and optical properties of monolayer S–MX2 are well maintained and show excellent long‐term stability under harsh environments. Herein this work, a way to eliminate surface doping of monolayer TMDs for their practical application in large‐area‐integrated circuits is provided.

Robust growth of two-dimensional metal dichalcogenides and their alloys by active chalcogen monomer supply

The precise precursor supply is a precondition for controllable growth of two-dimensional (2D) transition metal dichalcogenides (TMDs). Although great efforts have been devoted to modulating the transition metal supply, few effective methods of chalcogen feeding control were developed. Here we report a strategy of using active chalcogen monomer supply to grow high-quality TMDs in a robust and controllable manner, e.g., MoS2 monolayers perform representative photoluminescent circular helicity of ~92% and electronic mobility of ~42 cm2V−1s−1. Meanwhile, a uniform quaternary TMD alloy with three different anions, i.e., MoS2(1-x-y)Se2xTe2y, was accomplished. Our mechanism study revealed that the active chalcogen monomers can bind and diffuse freely on a TMD surface, which enables the effective nucleation, reaction, vacancy healing and alloy formation during the growth. Our work offers a degree of freedom for the controllable synthesis of 2D compounds and their alloys, benefiting the development of high-end devices with desired 2D materials.

Thermal stability of hafnium zirconium oxide on transition metal dichalcogenides

Ferroelectric oxides interfaced with transition metal dichalcogenides (TMDCs) offer a promising route toward ferroelectric-based devices due to lack of dangling bonds on the TMDC surface leading to a high-quality and abrupt ferroelectric/TMDC interface. In this work, the thermal stability of this interface is explored by first depositing hafnium zirconium oxide (HZO) directly on geological MoS2 and as-grown WSe2, followed by sequential annealing in ultra-high vacuum (UHV) over a range of temperatures (400–800 °C), and examining the interface through X-ray photoelectron spectroscopy (XPS). We show that the nucleation and stability of HZO grown through atomic layer deposition (ALD) varied depending on functionalization of the TMDC, and the deposition conditions can cause tungsten oxidation in WSe2. It was observed that HZO deposited on non-functionalized MoS2 was unstable and volatile upon annealing, while HZO on functionalized MoS2 was stable in the 400–800 °C range. The HZO/WSe2 interface was stable until 700 °C, after which Se began to evolve from the WSe2. In addition, there is evidence of oxygen vacancies in the HZO film being passivated at high temperatures. Lastly, X-ray diffraction (XRD) was used to confirm crystallization of the HZO within the temperature range studied.

(Invited) Non-Equilibrium Treatment of Molecular Dopants for 2D Materials

2D materials gather attention due to their unique and quantum-confined electronic structures applicable to optoelectronics and a platform for physics. One representative 2D material is transition metal dichalcogenides (TMDCs), which consist of a three-atomic-layered structure with a thickness of only ~0.7 nm. Representative TMDCs are MoS2 and WSe2, and both materials are semiconducting materials with the direct-band gap at the monolayer limit. To apply these unique materials for atomically thin electronics and quantum devices, controlling the carrier concentration is important. In this talk, I will present our recent results to control the carrier behaviors of TMDCs using molecules, with a particular focus on the molecules in non-equilibrium environments, starting with the preparation process of a dopant molecule in one (Ref.1). We found a method to prepare a strong electron dopant, benzyl viologen (BV), in a non-equilibrium manner. In the previous works, the BV molecule is synthesized using a strong reducing agent, sodium borohydride (NaBH4), in water. However, the method is inefficient because NaBH4 reacts vigorously with water at the same time. We replaced NaBH4 with indium metal and found a synthetic method to prepare BV dopant molecules under a non-equilibrium convection flow environment. The convection flow continuously synthesizes BV dopant molecules to a high concentration; furthermore, the method eliminates the purification process to use the dopant solution. Additionally, I will show a non-equilibrium dopant molecular assembly on TMDCs (Ref. 2 and 3). BV molecules usually interact immediately with the TMDC surface and assemble uniformly. Because of the hydrophobic character of the BV molecules, it is difficult to apply photoresists to make local carrier modulation at submicron scales. We overcome the issue by using the self-pattern formation process of the BV molecular films on the surface of TMDCs. By applying hydrophobic/hydrophilic repulsive interaction, the BV film destabilizes and shows pattern formation at scales of about ~200 nm periodicity. Our analysis found that the patterns induce pn and metal/semiconductor junctions on TMDCs. This non-equilibrium process would be useful to prepare electronic device structures via the spontaneous process, which is uniformly distributed at large scale. Keywords: Molecular dopant, Transition metal dichalcogenides, TMDCs, convection flow, molecular assembly, spontaneous pattern formation

Scanning Probe Lithography Patterning of Monolayer Semiconductors and Application in Quantifying Edge Recombination.

Scanning probe lithography is used to directly pattern monolayer transition metal dichalcogenides (TMDs) without the use of a sacrificial resist. Using an atomic-force microscope, a negatively biased tip is brought close to the TMD surface. By inducing a water bridge between the tip and the TMD surface, controllable oxidation is achieved at the sub-100 nm resolution. The oxidized flake is then submerged into water for selective oxide removal which leads to controllable patterning. In addition, by changing the oxidation time, thickness tunable patterning of multilayer TMDs is demonstrated. This resist-less process results in exposed edges, overcoming a barrier in traditional resist-based lithography and dry etch where polymeric byproduct layers are often formed at the edges. By patterning monolayers into geometric patterns of different dimensions and measuring the effective carrier lifetime, the non-radiative recombination velocity due to edge defects is extracted. Using this patterning technique, it is shown that selenide TMDs exhibit lower edge recombination velocity as compared to sulfide TMDs. The utility of scanning probe lithography towards understanding material-dependent edge recombination losses without significantly normalizing edge behaviors due to heavy defect generation, while allowing for eventual exploration of edge passivation schemes is highlighted, which is of profound interest for nanoscale electronics and optoelectronics.

Recent Advances in Interface Engineering of Transition-Metal Dichalcogenides with Organic Molecules and Polymers.

The interface engineering of two-dimensional (2D) transition-metal dichalcogenides (TMDs) has been regarded as a promising strategy to modulate their outstanding electrical and optoelectronic properties because of their inherent 2D nature and large surface-to-volume ratio. In particular, introducing organic molecules and polymers directly onto the surface of TMDs has been explored to passivate the surface defects or achieve better interfacial properties with neighboring surfaces efficiently, thus leading to great opportunities for the realization of high-performance TMD-based applications. This review provides recent progress in the interface engineering of TMDs with organic molecules and polymers corresponding to the modulation of their electrical and optoelectronic characteristics. Depending on the interfaces between the surface of TMDs and dielectric, conductive contacts or the ambient environment, we present various strategies to introduce an organic interlayer from materials to processing. In addition, the role of native defects on the surface of TMDs, such as adatoms or vacancies, in determining their electrical characteristics is also discussed in detail. Finally, the future challenges and opportunities associated with the interface engineering are highlighted.

(Invited) Tuning Optoelectronic Properties of Transition Metal Dichalcogenides for Hydrogen Generation

We tune the optical and electronic properties of 2D transition metal dichalcogenides (TMDCs) – MoS2, MoSe2, WS2, and WSe­2– to control the surface conductivity, energetics, and ultimately reactivity. The optoelectronic properties of monolayer and few-layered TMDCs are tuned by surface functionalization, environmental control, and phase engineering. These modifications have a direct impact on the surface reactivity for hydrogen generation. Specifically, we directly map the surface energetics, phase, and reactivity with scanning electrochemical microscopy, scanning Kelvin probe microscopy, confocal Raman/photoluminescence, and scanning transmission electron microscopy-electron energy loss spectroscopy. Moreover, we disentangle the influence of O2and H2O molecules on the surface of TMDCs, where these molecules impact the optoelectronic properties and can influence the potential applications of monolayer TMDCs in (photo)electrocatalysis.

Doping of Monolayer Transition-Metal Dichalcogenides via Physisorption of Aromatic Solvent Molecules.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) recently emerged as novel materials displaying a wide variety of physicochemical properties that render them unique scaffolds for high-performance (opto)electronics. The controlled physisorption of molecules on the TMD surface is a viable approach for tuning their optical and electronic properties. Solvents, made of small aromatic molecules, are frequently employed for the cleaning of the 2D materials or as a "dispersant" for their chemical functionalization with larger (macro)molecules, without considering their potential key effect in locally modifying the characteristics of 2D materials. In this work, we demonstrate how the electronic and optical properties of a mechanically exfoliated monolayer of MoS2 and WSe2 are modified when physically interacting with small aromatic molecules of common solvents. Low-temperature photoluminescence (PL) spectra recorded at 78 K revealed that physisorbed benzene derivatives could modulate the charge carrier density in monolayer TMDs, hence affecting the switching between a neutral exciton and trion (charged exciton). By combining experimental evidence with density functional theory calculations, we confirm that charge-transfer doping on TMDs depends not only on the difference in chemical potential between molecules and 2D materials but also on the thermodynamic stability of physisorption. Our results provide unambiguous evidences of the great potential of optical and electrical tuning of monolayer MoS2 and WSe2 by physisorption of small aromatic solvent molecules, which is highly relevant for both fundamental studies and device application purposes.

Direct observation of transition metal dichalcogenides in liquid with scanning tunneling microscopy

We present atomic-resolution images of TiSe $_2$ , MoTe $_2$ and TaS $_2$ single crystals in liquid condition using our home-built scanning tunneling microscopy (STM). By facilely cleaving of single crystals in liquid, we were able to keep the fresh surface not oxidized within a few hours. Using the high-stable home-built STM, we have obtained atomic resolution images of TiSe $_2$ accompanied with the single atom defects as well as the triangle defects in solution for the first time. Besides, the superstructure of MoTe $_2$ and hexagonal charge-density wave domain structure in nearly commensurate phase of TaS $_2$ were also obtained at room temperature (295 K). Our results provide a more efficient method in investigating the lively surface of transition metal dichalcogenides. Besides, the high stable liquid-phase STM will support the further investigations in liquid-phase catalysis or electrochemistry.

Room-temperature epitaxy of metal thin films on tungsten diselenide

The orientation of selected metals (Pd, Ni, Al, and Co) deposited on WSe2 by physical vapor deposition was examined using transmission electron microscopy and selected area electron diffraction. We discovered that Ni demonstrates room-temperature epitaxy, similarly to other face centered cubic (FCC) metals Au, Ag, and Cu. These epitaxial metals exhibit the following orientation relationship, where M stands for metal: M (111) || WSe2 (0001); M [22¯0] || WSe2 [112¯0]. Hexagonally close-packed Co, and FCC Pd and Al, were not epitaxial on deposition; however, Pd became epitaxial after annealing at 673 K for 5 h. To uncover critical variables for epitaxial growth, we correlated our experimental work and reports from the literature on Cu, Ag, and Au with density functional theory calculations of the energetics of metal atoms on the surface of WSe2 and thermodynamic calculations of metal-W-Se phase equilibria. Furthermore, we compared the findings to our previous work on metal/MoS2 systems to draw conclusions more generally applicable to epitaxial growth of metals on transition metal dichalcogenides (TMDs). We observed that epitaxy of metals on TMDs can occur when there is a match in crystallographic symmetry, even with a large lattice mismatch, and it is favored by metals exhibiting a low diffusion barrier on the TMD surface. However, reaction processes between the metal and WSe2 can prevent epitaxy even when the other factors are favorable, as occurred for Al/WSe2 with the formation of aluminum selenide, tungsten aluminide, and elemental tungsten. Consideration of crystallographic symmetry, surface diffusion barriers, and reactivity can be used to predict room-temperature epitaxy in other metal/TMD systems.

Strong Coupling in Si Nanoparticle Core - 2D WS2 Shell Structure

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) are attractive materials for optoelectronics. In this paper, a strong coupling regime in Si nanoparticle covered by single layer WS2 system at the magnetic optical Mie resonance is observed. This regime is achieved owing to the symmetry of the magnetic dipole Mie mode, polarized along the TMDC surface, and low dissipative losses of the Si nanoparticle. A Rabi splitting energy exceeding 110 meV is obtained.

Controllable, Wide-Ranging n-Doping and p-Doping of Monolayer Group 6 Transition-Metal Disulfides and Diselenides.

Developing processes to controllably dope transition-metal dichalcogenides (TMDs) is critical for optical and electrical applications. Here, molecular reductants and oxidants are introduced onto monolayer TMDs, specifically MoS2 , WS2 , MoSe2 , and WSe2 . Doping is achieved by exposing the TMD surface to solutions of pentamethylrhodocene dimer as the reductant (n-dopant) and "Magic Blue," [N(C6 H4 -p-Br)3 ]SbCl6 , as the oxidant (p-dopant). Current-voltage characteristics of field-effect transistors show that, regardless of their initial transport behavior, all four TMDs can be used in either p- or n-channel devices when appropriately doped. The extent of doping can be controlled by varying the concentration of dopant solutions and treatment time, and, in some cases, both nondegenerate and degenerate regimes are accessible. For all four TMD materials, the photoluminescence intensity; for all four materials the PL intensity is enhanced with p-doping but reduced with n-doping. Raman and X-ray photoelectron spectroscopy (XPS) also provide insight into the underlying physical mechanism by which the molecular dopants react with the monolayer. Estimates of changes of carrier density from electrical, PL, and XPS results are compared. Overall a simple and effective route to tailor the electrical and optical properties of TMDs is demonstrated.

Efficient Defect Healing of Transition Metal Dichalcogenides by Metallophthalocyanine.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention as alternatives to graphene with semiconducting band gaps. Mono- or few-layer TMDCs can be prepared by various methods, but regardless of the fabrication methods [such as mechanical exfoliation and chemical vapor deposition (CVD)], TMDCs contain many structural defects, which significantly affect their physical properties and limit their performance in applications. Metallophthalocyanines (MPcs) are organic semiconductors, and as dopants, they are capable of modulating the optical and electrical properties of other semiconducting materials. Here, we report that besides the ability to modulate the optoelectronic properties of 2D TMDCs, MPc molecules can be used to heal defects and improve the physicochemical properties of TMDCs. Doping of planar MPc molecules to TMDCs is achieved by a simple solution dip-coating method and results in a significant improvement in the optical properties and thermal responses of CVD-grown TMDCs, even comparable to those of mechanically exfoliated counterparts. Study of carrier dynamics shows that the adsorption of MPc on the TMDC surface leads to the complete suppression of the mid-gap defect-induced absorption in TMDCs. Furthermore, MPc molecules with a large lateral size are found to effectively reduce the point defects in mechanically exfoliated TMDCs introduced during the preparation process. Our results not only clarify the optoelectronic modulation mechanism of chemical doping but also offer a simple method to control the nanosized defects in 2D TMDCs.

MoS2 Functionalization with a Sub-nm Thin SiO2 Layer for Atomic Layer Deposition of High-κ Dielectrics

Several applications of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) in nanoelectronic devices require the deposition of ultrathin pinhole free high-κ dielectric films on 2D TMDs. However, deposition of nm-thin high-κ dielectric films on 2D TMDs remains challenging due to the inert TMD surface. Here, we demonstrate that the surface of a synthetic polycrystalline 2D MoS2 film is functionalized with SiO2 to enable the atomic layer deposition (ALD) of thin and continuous Al2O3 and HfO2 layers. The origins of nucleation, the growth mode, and layer coalescence process have been investigated by complementary physical characterization techniques, which can determine the chemical bonds, absolute amount, and surface coverage of the deposited material. SiO2 is prepared by oxidizing physical vapor deposited Si in air. The surface hydrophilicity of MoS2 significantly increases after SiO2 functionalization owing to the presence of surface hydroxyl groups. SiO2 layers with a Si content of...

A kinetic Monte Carlo simulation method of van der Waals epitaxy for atomistic nucleation-growth processes of transition metal dichalcogenides

Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.

Redox Exfoliation of Layered Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDs) have attracted considerable attention in a diverse array of applications due to the breadth of possible property suites relative to other low-dimensional nanomaterials (e.g., graphene, aluminosilicates). Here, we demonstrate an alternative methodology for the exfoliation of bulk crystallites of group V-VII layered TMDs under quiescent, benchtop conditions using mild redox chemistry. Anionic polyoxometalate species generated from edge sites adsorb to the TMD surface and create Coulombic repulsion that drives layer separation without the use of shear forces. This method is generalizable (MS2, MSe2, and MTe2) and effective in preparing high-concentration (>1 mg/mL) dispersions with narrow layer thickness distributions more rapidly and with safer reagents than alternative solution-based approaches. Finally, exfoliation of these TMDs is demonstrated in a range of solvent systems that were previously inaccessible due to large surface energy differences. These characteristics could be beneficial in the preparation of high-quality films and monoliths.