Ultrathin Two‐Dimensional Multinary Layered Metal Chalcogenide Nanomaterials

Ultrathin two‐dimensional (2D) layered transition metal dichalcogenides (TMDs), such as MoS₂, WS₂, TiS₂, TaS₂, ReS₂, MoSe₂ and WSe₂, have attracted considerable attention over the past six years owing to their unique properties and great potential in a wide range of applications. Aiming to achieve tunable properties and optimal application performances, great effort is devoted to the exploration of 2D multinary layered metal chalcogenide nanomaterials, which include ternary metal chalcogenides with well‐defined crystal structures, alloyed TMDs, heteroatom‐doped TMDs and 2D metal chalcogenide heteronanostructures. These novel 2D multinary layered metal chalcogenide nanomaterials exhibit some unique properties compared to 2D binary TMD counterparts, thus holding great promise in various potential applications including electronics/optoelectronics, catalysis, sensors, biomedicine, and energy storage and conversion with enhanced performances. This article focuses on the state‐of‐art progress on the preparation, characterization and applications of ultrathin 2D multinary layered metal chalcogenide nanomaterials.     

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(C₆H₄‐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.     

Synthesis of In‐Plane Artificial Lattices of Monolayer Multijunctions

Recently, monolayers of van der Waals materials, including transition metal dichalcogenides (TMDs), are considered ideal building blocks for constructing 2D artificial lattices and heterostructures. Heterostructures with multijunctions of more than two monolayer TMDs are intriguing for exploring new physics and materials properties. Obtaining in‐plane heterojunctions of monolayer TMDs with atomically sharp interfaces is very significant for fundamental research and applications. Currently, multistep synthesis for more than two monolayer TMDs remains a challenge because decomposition or compositional alloying is thermodynamically favored at the high growth temperature. Here, a multistep chemical vapor deposition (CVD) synthesis of the in‐plane multijunctions of monolayer TMDs is presented. A low growth temperature synthesis is developed to avoid compositional fluctuations of as‐grown TMDs, defects formations, and interfacial alloying for high heterointerface quality and thermal stability of monolayer TMDs. With optimized parameters, atomically sharp interfaces are successfully achieved in the synthesis of in‐plane artificial lattices of the WS₂/WSe₂/MoS₂ at reduced growth temperatures. Growth behaviors as well as the heterointerface quality are carefully studied in varying growth parameters. Highly oriented strain patterns are found in the second harmonic generation imaging of the TMD multijunctions, suggesting that the in‐plane heteroepitaxial growth may induce distortion for unique material symmetry.     

Epitaxial Growth of Rectangle Shape MoS2 with Highly Aligned Orientation on Twofold Symmetry a‐Plane Sapphire

Research on transition metal dichalcogenides (TMDs) has been accelerated by the development of large‐scale synthesis based on chemical vapor deposition (CVD) growth. However, in most cases, CVD‐grown TMDs are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs), which seriously degrade their electrical and photoelectrical properties. Here, the epitaxial growth of highly aligned MoS₂ grains is reported on a twofold symmetry a‐plane sapphire substrate. The obtained MoS₂ grains have an unusual rectangle shape with perfect orientation alignment along the [1‐100] crystallographic direction of a‐plane sapphire. It is found that the growth temperature plays a key role in its orientation alignment and morphology evolution, and high temperature is beneficial to the initial MoS₂ seeds rotate to the favorable orientation configurations. In addition, the photoluminescence quenching of the well‐aligned MoS₂ grains indicates a strong MoS₂?substrate interaction which induces the anisotropic growth of MoS₂, and thus brings the formation of rectangle shape grains. Moreover, the well‐aligned MoS₂ grains splice together without GB formation, and thus that has negligible effect on its electrical transport properties. The progress achieved in this work could promote the controlled synthesis of large‐area TMDs single crystal film and the scalable fabrication of high‐performance electronic devices.     

3D Anisotropic Thermal Conductivity of Exfoliated Rhenium Disulfide

ReS₂ represents a different class of 2D materials, which is characterized by low symmetry having 1D metallic chains within the planes and extremely weak interlayer bonding. Here, the thermal conductivity of single‐crystalline ReS₂ in a distorted 1T phase is determined at room temperature for the in‐plane directions parallel and perpendicular to the Re‐chains, and the through‐plane direction using time‐domain thermoreflectance. ReS₂ is prepared in the form of flakes having thicknesses of 60–450 nm by micromechanical exfoliation, and their crystalline orientations are identified by polarized Raman spectroscopy. The in‐plane thermal conductivity is higher along the Re‐chains, (70 ± 18) W m^?1 K^?1, as compared to transverse to the chains, (50 ± 13) W m^?1 K^?1. As expected from the weak interlayer bonding, the through‐plane thermal conductivity is the lowest observed to date for 2D materials, (0.55 ± 0.07) W m^?1 K^?1, resulting in a remarkably high anisotropy of (130 ± 40) and (90 ± 30) for the two in‐plane directions. The thermal conductivity and interface thermal conductance of ReS₂ are discussed relative to the other 2D materials.     

Probing Effective Out‐of‐Plane Piezoelectricity in van der Waals Layered Materials Induced by Flexoelectricity

Many van der Waals layered 2D materials, such as h‐BN, transition metal dichalcogenides (TMDs), and group‐III monochalcogenides, have been predicted to possess piezoelectric and mechanically flexible natures, which greatly motivates potential applications in piezotronic devices and nanogenerators. However, only intrinsic in‐plane piezoelectricity exists in these 2D materials and the piezoelectric effect is confined in odd‐layers of TMDs. The present work is intent on combining the free‐standing design and piezoresponse force microscopy techniques to obtain and directly quantify the effective out‐of‐plane electromechanical coupling induced by strain gradient on atomically thin MoS₂ and InSe flakes. Conspicuous piezoresponse and the measured piezoelectric coefficient with respect to the number of layers or thickness are systematically illustrated for both MoS₂ and InSe flakes. Note that the promising effective piezoelectric coefficient (d^eff₃₃) of about 21.9 pm V^?1 is observed on few‐layered InSe. The out‐of‐plane piezoresponse arises from the net dipole moment along the normal direction of the curvature membrane induced by strain gradient. This work not only provides a feasible and flexible method to acquire and quantify the out‐of‐plane electromechanical coupling on van der Waals layered materials, but also paves the way to understand and tune the flexoelectric effect of 2D systems.     

Properties of RBaCuFeO5+δ (R = Y, La, Pr, Nd, Sm-Lu)

The structural properties, thermal stability, thermal expansion, and electrical properties of the RBaCuFeO_5+δ (R = Y, La, Pr, Nd, Sm-Lu) layered ferrocuprates have been studied systematically. The linear thermal expansion coefficients of the RBaCuFeO_5+δ phases are ? 13 × 10^?6 and ? 13 × 10^?6 K^?1 for R = La-Eu and R = Gd-Lu, respectively. Their electrical conductivity varies widely, from σ = 2.0 × 10¹ S/cm for LaBaCuFeO_5.47 to σ = 8.4 × 10^?8 S/cm for TmBaCuFeO_5.06 at 300 K. The lattice parameters, thermal expansion, and conductivity of RBaCuFeO_5+δ vary nonmonotonically with the ionic radius of R³⁺ (atomic number of the rare earth), which is due to the fact that the 4f electrons of the rare-earth ions play a significant role in determining the structural and transport properties of these phases.     

Discovery of Superconductivity in 2M WS2 with Possible Topological Surface States

Recently the metastable 1T′‐type VIB‐group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS₂ dubbed “2M” WS₂, is constructed from 1T′ WS₂ monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS₂ with a transition temperature T_c of 8.8 K, which is the highest among TMDs not subject to any fine‐tuning process. Furthermore, the electronic structure of 2M WS₂ is found by Shubnikov–de Haas oscillations and first‐principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z₂, are predicted through first‐principles calculations. These findings reveal that the new 2M WS₂ might be an interesting topological superconductor candidate from the VIB‐group transition metal dichalcogenides.     

Highly Anisotropic Sb2Se3 Nanosheets: Gentle Exfoliation from the Bulk Precursors Possessing 1D Crystal Structure

2D materials, particularly those bearing in‐plane anisotropic optical and electrical properties such as black phosphorus and ReS₂, have spurred great research interest very recently as promising building blocks for future electronics. However, current progress is limited to layered compounds that feature atomic arrangement asymmetry within the covalently bonded planes. Herein, a series of highly anisotropic nanosheets (Sb₂Se₃, Sb₂S₃, Bi₂S₃, and Sb₂(S, Se)₃), which are composed of 1D covalently linked ribbons stacked together via van der Waals force, is introduced as a new member to the anisotropic 2D material family. These unique anisotropic nanosheets are successfully fabricated from their polymer‐like bulk counterparts through a gentle water freezing‐thawing approach. Angle‐resolved polarized Raman spectroscopy characterization confirms the strong in‐plane asymmetry of Sb₂Se₃ nanosheets, and photodetection study reveals their high responsivity and anisotropic in‐plane transport. This work can enlighten the synthesis and application of new anisotropic 2D nanosheets that can be potentially applied for future electronic and optoelectronic devices.     

Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V2VI3 or Pn2Ch3 (Pn = Pnictogen,Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui‐chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase‐change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Bi₂S₃, Sb₂Te₃, Sb₂Se₃, Sb₂S₃, and β‐As₂Te₃) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui‐chalcogenides (Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and β‐As₂Te₃) show an unconventional bond‐breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal‐like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui‐chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum‐chemistry‐based bonding analysis employing charge partitioning, which reveals that the four sesqui‐chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.     

Investigation of Transport Properties of Some Superconductor Nickel-Based Antiperovskite XNNi<Subscript>3</Subscript> (X = Mg,Al, Cu,Zn, Ga,Ag, Cd,In, Sn,Sb, Pt and Pb)

We report, in this work, a theoretical study of electronic and transport (thermoelectric) properties of some superconductor nickel-based antiperovskite XNNi₃ (X = Mg, Al, Cu, Zn, Ga, Ag, Cd, In, Sn, Sb, Pt and Pb) using first-principles calculations with the full-potential linearized augmented plane wave (FPLAPW) method based on the density functional theory (DFT) as implemented in the WIEN2k package. Electronic properties are calculated and show that the studied materials are of metallic type which is in good agreement with experimental data. The Seebeck coefficient, thermal conductivity, electrical conductivity and figure of merit were reported. The results obtained show that the zinc (Zn) and silver (Ag) materials are characterized by a high value of the figure of merit at room temperature (300 K) which is respectively 0.86 and 1.02 in a p-type region. In the case of the transition metals, the maximum values of S increase in going from the Pt atom to the Zn atom and then decrease for the Cd atom. Furthermore, the Wiedemann–Franz law which states that the ratio of thermal to electrical conductivity for metals is constant is well verified in this work. The electric conductivity values are almost invariant with the temperature except for the case of MgNNi₃ and AgNNi₃ compounds in which it increases with T slightly. So, the superconducting materials based on silver and zinc are the best for the thermoelectric applications at room temperature due to the very important value of the factor of merit and the Seebeck coefficient obtained.     

Thermal stresses in surface-coated Fe-3%Si sheet

The aim of the paper is to present a model of thermal stresses in a layered plane system, and consequently to verify the model validity by comparing calculated thermal stresses with measured thermal stresses. The paper deals thus with thermal stresses in the Fe-3%Si sheet for magnetic applications, having a surface coating of the 2MgO·TiO₂, ZrO₂, TiO₂, Al₂O₃, 2MgO·SiO₂, MgO·Al₂O₃oxides. The thermal stresses, originating during a cooling process as a consequence of the difference in thermal expansion coefficients between the Fe-3%Si sheet and the surface coating, degrade hysteresis losses of the Fe-3%Si sheet. Magnetic properties of the presented coating–Fe-3%Si systems, as the magnetic induction and the hysteresis losses before and after coating formation, are presented. The theoretical background, including the model of the thermal stresses in anisotropic and isotropic two-layered plane systems, consequently transformated to the three-layered plane system, is presented. Calculated thermal stresses are compared with those of a tension measurement of the presented coating—Fe-3%Si systems, numerical equality of the calculated and measured thermal stresses is observed.     

Recent Developments in Controlled Vapor‐Phase Growth of 2D Group 6 Transition Metal Dichalcogenides

An overview of recent developments in controlled vapor‐phase growth of 2D transition metal dichalcogenide (2D TMD) films is presented. Investigations of thin‐film formation mechanisms and strategies for realizing 2D TMD films with less‐defective large domains are of central importance because single‐crystal‐like 2D TMDs exhibit the most beneficial electronic and optoelectronic properties. The focus is on the role of the various growth parameters, including strategies for efficiently delivering the precursors, the selection and preparation of the substrate surface as a growth assistant, and the introduction of growth promoters (e.g., organic molecules and alkali metal halides) to facilitate the layered growth of (Mo, W)(S, Se, Te)₂ atomic crystals on inert substrates. Critical factors governing the thermodynamic and kinetic factors related to chemical reaction pathways and the growth mechanism are reviewed. With modification of classical nucleation theory, strategies for designing and growing various vertical/lateral TMD‐based heterostructures are discussed. Then, several pioneering techniques for facile observation of structural defects in TMDs, which substantially degrade the properties of macroscale TMDs, are introduced. Technical challenges to be overcome and future research directions in the vapor‐phase growth of 2D TMDs for heterojunction devices are discussed in light of recent advances in the field.     

Adjustable Intermolecular Interactions Allowing 2D Transition Metal Dichalcogenides with Prolonged Scavenging Activity for Reactive Oxygen Species

There is an increasing demand for control over the dimensions and functions of transition metal dichalcogenides (TMDs) in aqueous solution toward biological and medical applications. Herein, an approach for the exfoliation and functionalization of TMDs in water via modulation of the hydrophobic interaction between poly(ε‐caprolactone)‐b‐poly(ethylene glycol) (PCL‐b‐PEG) and the basal planes of TMDs is reported. Decreasing the hydrophobic PCL length of PCL‐b‐PEG from 5000 g mol^?1 (PCL₅₀₀₀) to 460 g mol^?1 (PCL₄₆₀) significantly increases the exfoliation efficiency of TMD nanosheets because the polymer–TMD hydrophobic interaction becomes dominant over the polymer–polymer interaction. The TMD nanosheets exfoliated by PCL₄₆₀‐b‐PEG₅₀₀₀ (460‐WS₂, 460‐WSe₂, 460‐MoS₂, and 460‐MoSe₂) show excellent and prolonged scavenging activity for reactive oxygen species (ROS), but each type of TMD displays a different scavenging tendency against hydroxyl, superoxide, and 2,2′‐azino‐bis(3‐ethylbenzothiazoline‐6‐sulfonic acid) radicals. A mechanistic study based on electron paramagnetic resonance spectroscopy and density functional theory simulations suggests that radical‐mediated oxidation of TMDs and hydrogen transfer from the oxidized TMDs to radicals are crucial steps for ROS scavenging by TMD nanosheets. As‐prepared 460‐TMDs are able to effectively scavenge ROS in HaCaT human keratinocytes, and also exhibit excellent biocompatibility.     

An Ordered Ni6‐Ring Superstructure Enables a Highly Stable Sodium Oxide Cathode

Sodium‐based layered oxides are among the leading cathode candidates for sodium‐ion batteries, toward potential grid energy storage, having large specific capacity, good ionic conductivity, and feasible synthesis. Despite their excellent prospects, the performance of layered intercalation materials is affected by both a phase transition induced by the gliding of the transition metal slabs and air‐exposure degradation within the Na layers. Here, this problem is significantly mitigated by selecting two ions with very different M? O bond energies to construct a highly ordered Ni₆‐ring superstructure within the transition metal layers in a model compound (NaNi_2/3Sb_1/3O₂). By virtue of substitution of 1/3 nickel with antimony in NaNiO₂, the existence of these ordered Ni₆‐rings with super‐exchange interaction to form a symmetric atomic configuration and degenerate electronic orbital in layered oxides can not only largely enhance their air stability and thermal stability, but also increase the redox potential and simplify the phase‐transition process during battery cycling. The findings reveal that the ordered Ni₆‐ring superstructure is beneficial for constructing highly stable layered cathodes and calls for new paradigms for better design of layered materials.     

Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2

Here, the hydrogen evolution reaction (HER) activities at the edge and basal‐plane sites of monolayer molybdenum disulfide (MoS₂) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected‐area lithography. Reaction windows are opened by e‐beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)‐covered monolayer MoS₂ triangles. The HER properties of MoS₂ edge sites are obtained by subtraction of the activity of the basal‐plane sites from results containing both basal‐plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10^?4 s^?1, found for 1T′‐, 2H‐MoS₂ edges, and 1T′‐, 2H‐MoS₂ basal planes, respectively. Edge sites of both 2H‐ and 1T′‐MoS₂ are proved to have comparable activities to platinum (≈1–10 s^?1). When fitted into the HER volcano plot, the MoS₂ active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition‐metal dichalcogenides (TMDs) and metal catalysts.     

Integrated Freestanding Two‐dimensional Transition Metal Dichalcogenides

This paper reports on the integration of freestanding transition metal dichalcogenides (TMDs). Monolayer (1‐L) MoS₂, WS₂, and WSe₂ as representative TMDs are transferred on ZnO nanorods (NRs), used here as nanostructured substrates. The photoluminescence (PL) spectra of 1‐L TMDs on NRs show a giant PL intensity enhancement, compared with those of 1‐L TMDs on SiO₂. The strong increases in Raman and PL intensities, along with the characteristic peak shifts, confirm the absence of stress in the TMDs on NRs. In depth analysis of the PL emission also reveals that the ratio between the exciton and trion peak intensity is almost not modified after transfer. The latter shows that the effect of charge transfer between the 1‐L TMDs and ZnO NRs is here negligible. Furthermore, confocal PL and Raman spectroscopy reveal a fairly consistent distribution of PL and Raman intensities. These observations are in agreement with a very limited points contact between the support and the 1‐L TMDs. The entire process reported here is scalable and may pave the way for the development of very efficient ultrathin optoelectronics.     

Preparation of High‐Percentage 1T‐Phase Transition Metal Dichalcogenide Nanodots for Electrochemical Hydrogen Evolution

Nanostructured transition metal dichalcogenides (TMDs) are proven to be efficient and robust earth‐abundant electrocatalysts to potentially replace precious platinum‐based catalysts for the hydrogen evolution reaction (HER). However, the catalytic efficiency of reported TMD catalysts is still limited by their low‐density active sites, low conductivity, and/or uncleaned surface. Herein, a general and facile method is reported for high‐yield, large‐scale production of water‐dispersed, ultrasmall‐sized, high‐percentage 1T‐phase, single‐layer TMD nanodots with high‐density active edge sites and clean surface, including MoS₂, WS₂, MoSe₂, Mo_0.5W_0.5S₂, and MoSSe, which exhibit much enhanced electrochemical HER performances as compared to their corresponding nanosheets. Impressively, the obtained MoSSe nanodots achieve a low overpotential of ?140 mV at current density of 10 mA cm^?2, a Tafel slope of 40 mV dec^?1, and excellent long‐term durability. The experimental and theoretical results suggest that the excellent catalytic activity of MoSSe nanodots is attributed to the high‐density active edge sites, high‐percentage metallic 1T phase, alloying effect and basal‐plane Se‐vacancy. This work provides a universal and effective way toward the synthesis of TMD nanostructures with abundant active sites for electrocatalysis, which can also be used for other applications such as batteries, sensors, and bioimaging.     

Controlling Photoluminescence Enhancement and Energy Transfer in WS2:hBN:WS2 Vertical Stacks by Precise Interlayer Distances

2D semiconducting transition metal dichalcogenides (TMDs) are endowed with fascinating optical properties especially in their monolayer limit. Insulating hBN films possessing customizable thickness can act as a separation barrier to dictate the interactions between TMDs. In this work, vertical layered heterostructures (VLHs) of WS₂:hBN:WS₂ are fabricated utilizing chemical vapor deposition (CVD)‐grown materials, and the optical performance is evaluated through photoluminescence (PL) spectroscopy. Apart from the prohibited indirect optical transition due to the insertion of hBN spacers, the variation in the doping level of WS₂ drives energy transfer to arise from the layer with lower quantum efficiency to the other layer with higher quantum efficiency, whereby the total PL yield of the heterosystem is increased and the stack exhibits a higher PL intensity compared to the sum of those in the two WS₂ constituents. Such doping effects originate from the interfaces that WS₂ monolayers reside on and interact with. The electron density in the WS₂ is also controlled and subsequent modulation of PL in the heterostructure is demonstrated by applying back‐gated voltages. Other influential factors include the strain in WS₂ and temperature. Being able to tune the energy transfer in the VLHs may expand the development of photonic applications in 2D systems.     

Thermal conductivity of single crystals of M1−x M x ′F2 (M = Ca, Sr; M′ = Mn, Co) isovalent solid solutions

The thermal conductivity of single crystals of the Ca_0.99Co_0.01F₂, Ca_0.97Co_0.03F₂, Sr_0.99Mn_0.01F₂, Sr_0.989Mn_0.01Co_0.001F₂, and Sr_0.995Co_0.005F₂ solid solutions has been measured in the temperature range 50–300 K. The results demonstrate that doping of CaF₂ and SrF₂ crystals with small amounts of isovalent transition metal impurities is accompanied by a significant reduction in thermal conductivity.