Ultrathin Transition Metal Dichalcogenides Research Articles

Large-Substrate-Terrace Confined Growth of Arrayed Ultrathin PtSe2 Ribbons on Step-Bunched Vicinal Au(001) Facets Toward Electrocatalytic Applications.

Ultrathin PtSe2 ribbons can host spin-polarized edge states and distinct edge electrocatalytic activity, emerging as a promising candidate for versatile applications in various fields. However, the direct synthesis is still challenging and the growth mechanism is still unclear. Herein, the arrayed growth of ultrathin PtSe2 ribbons on bunched vicinal Au(001) facets, via a facile chemical vapor deposition (CVD) route is reported. The ultrathin PtSe2 flakes can transform from traditional irregular shapes to desired ribbon shapes by increasing the height of bunched and unidirectionally oriented Au steps (with step height hstep) is found. This crossover, occurring at hstep ≈ 3.0nm, defines the tailored growth from step-flow to single-terrace-confined modes, as validated by density functional theory calculations of the different system energies. On the millimeter-scale single-crystal Au(001) films with aligned steps, the arrayed ultrathin PtSe2 ribbons with tunable width of ≈20-1000nm, which are then served as prototype electrocatalysts for hydrogen evolution reaction (HER) is achieved. This work should represent a huge leap in the direct synthesis and the mechanism exploration of arrayed ultrathin transition-metal dichalcogenides (TMDCs) ribbons, which should stimulate further explorations of the edge-related physical properties and practical applications.

Effective concentration ratio driven phase engineering of MBE-grown few-layer MoTe2.

The polymorphic nature of ultrathin transition metal dichalcogenide (TMDC) materials makes the phase engineering of these materials an interesting field of investigation. Understanding the phase-controlling behavior of different growth parameters is crucial for obtaining large-area growth of a desirable phase. Here, we report a detailed study on the effect of growth parameters for engineering different phases of few-layer MoTe2 on sapphire using molecular beam epitaxy (MBE). Our study shows that the 2H phase of MoTe2 is stabilized in a certain regime of the flux ratio and growth temperature, while on both the lower, as well as, the higher sides of this regime, the 1T' phase is favored. The combined effect of growth parameters is explained using the effective concentration ratio of Te and Mo at the growth surface, which is found to be the primary factor governing the phase selectivity in few-layer MoTe2. XPS and KPFM investigations show the contribution of excess carrier doping in driving the phase change. The effect of the sapphire substrate on the crystallinity and phase-dependent morphological features has also been studied. This knowledge of versatile and controlled phase engineering of few-layer MoTe2 paves the way for fabricating large-scale hetero-phase-based metal-semiconductor heterostructures for future electronic and optoelectronic device applications.

Extendable piezo/ferroelectricity in nonstoichiometric 2D transition metal dichalcogenides

Engineering piezo/ferroelectricity in two-dimensional materials holds significant implications for advancing the manufacture of state-of-the-art multifunctional materials. The inborn nonstoichiometric propensity of two-dimensional transition metal dichalcogenides provides a spiffy ready-available solution for breaking inversion centrosymmetry, thereby conducing to circumvent size effect challenges in conventional perovskite oxide ferroelectrics. Here, we show the extendable and ubiquitous piezo/ferroelectricity within nonstoichiometric two-dimensional transition metal dichalcogenides that are predominantly centrosymmetric during standard stoichiometric cases. The emerged piezo/ferroelectric traits are aroused from the sliding of van der Waals layers and displacement of interlayer metal atoms triggered by the Frankel defects of heterogeneous interlayer native metal atom intercalation. We demonstrate two-dimensional chromium selenides nanogenerator and iron tellurides ferroelectric multilevel memristors as two representative applications. This innovative approach to engineering piezo/ferroelectricity in ultrathin transition metal dichalcogenides may provide a potential avenue to consolidate piezo/ferroelectricity with featured two-dimensional materials to fabricate multifunctional materials and distinguished multiferroic.

Efficiency limit of transition metal dichalcogenide solar cells

Ultrathin transition metal dichalcogenide (TMD) films show great promise as absorber materials in high-specific-power (i.e., high-power-per-weight) solar cells, due to their high optical absorption, desirable band gaps, and self-passivated surfaces. However, the ultimate performance limits of TMD solar cells remain unknown today. Here, we establish the efficiency limits of multilayer (≥5 nm-thick) MoS2, MoSe2, WS2, and WSe2 solar cells under AM 1.5 G illumination as a function of TMD film thickness and material quality. We use an extended version of the detailed balance method which includes Auger and defect-assisted Shockley-Read-Hall recombination mechanisms in addition to radiative losses, calculated from measured optical absorption spectra. We demonstrate that single-junction solar cells with TMD films as thin as 50 nm could in practice achieve up to 25% power conversion efficiency with the currently available material quality, making them an excellent choice for high-specific-power photovoltaics.

Room temperature magnetron sputtering and laser annealing of ultrathin amorphous sulfur-rich MoSx films

Room temperature magnetron sputtering of an ultra-thin transition-metal dichalcogenide amorphous precursor and subsequent laser annealing-induced phase transformations were investigated for the synthesis of polycrystalline 2H-MoS2 semiconducting thin films. Radio frequency magnetron sputtering of a MoS3.6 target was used to produce amorphous precursors with S to Mo ratios greater than 2:1 on thermally annealed SiO2 substrates, glass, indium tin oxide coated glass, and amorphous boron-oxy-nitride. The influence of working pressure on the MoS2 and MoS3 interatomic bonding arrangements in amorphous MoSx films was explored with x-ray photoelectron spectroscopy. A 248 nm pulsed laser was used for annealing 1 cm2 sized areas of the amorphous precursor in an ambient atmosphere at fluences of 100–450 mJ for 1, 5, and 10 pulses to form a crystalline 2H-MoS2 phase. The role of laser fluence and substrate choice on phase transformation from the amorphous precursor to the 2H phase, as well as the competition between oxidation, elemental sulfur segregation, and ablation is discussed. Hall effect measurements confirmed p-type behavior of the produced semiconducting films and established mobility and charge carrier density characteristics of the laser-annealed films. The feasibility of scalable processing by combining sputtering from sulfur-rich targets and laser annealing for production of p-type 2H-MoS2 ultrathin films on various substrates is presented.

Valley-dependent vortex emission from exciton-polariton in non-centrosymmetric transition metal dichalcogenide metasurfaces.

Transition metal dichalcogenides (TMDs) have attracted great attention in valleytronics. Owing to the giant valley coherence at room temperature, valley pseudospin of TMDs open a new degree of freedom to encode and process binary information. The valley pseudospin only exists in non-centrosymmetric TMDs (e.g., monolayer or 3R-stacked multilayer), which is prohibited in conventional centrosymmetric 2H-stacked crystals. Here, we propose a general recipe to generate valley-dependent vortex beams by using a mix-dimensional TMD metasurface composed of nanostructured 2H-stacked TMD crystals and monolayer TMDs. Such an ultrathin TMD metasurface involves a momentum-space polarization vortex around bound states in the continuum (BICs), which can simultaneously achieve strong coupling (i.e., form exciton polaritons) and valley-locked vortex emission. Moreover, we report that a full 3R-stacked TMD metasurface can also reveal the strong-coupling regime with an anti-crossing pattern and a Rabi splitting of 95 meV. The Rabi splitting can be precisely controlled by geometrically shaping the TMD metasurface. Our results provide an ultra-compact TMD platform for controlling and structuring valley exciton polariton, in which the valley information is linked with the topological charge of vortex emission, which may advance valleytronic, polaritonic, and optoelectronic applications.

Ultra-thin grating coupler for guided exciton-polaritons in WS2 multilayers

Abstract An ultra-thin transition metal dichalcogenide (TMDC) layer can support guided exciton-polariton modes due to the strong coupling between excitons and photons. Herein, we report the guided mode resonance in an ultra-thin TMDC grating structure. Owing to the strong exciton resonances in TMDCs, a TMDC grating structure shows guided-mode resonance even at a thickness limit of ∼10 nm and is capable of realizing polaritonic dispersion in a monolithic grating structure. We investigated the polarization and thickness dependence of the optical dispersion relations of the tungsten disulfide (WS2) grating structure. In addition, we confirmed that the monolithic WS2 grating coupler can be used to couple the near-field guided exciton-polariton out into the far field. We believe that ultra-thin TMDC layers can facilitate sub-wavelength nanophotonic applications.

Optical propagation of exciton polaritons in ultrathin van der Waals microcrystals down to few monolayers

The exciton polariton is a kind of quasiparticles and provides a promising opportunity to explore fundamental quantum phenomena for photonic applications. Transition-metal dichalcogenide (TMD) materials provide the platform of nanophotonics that supports the propagative exciton polaritons even at room-temperature. Previously, real space studies on thin flakes of TMDs by scattering-type scanning nearfield optical microscopy (s-SNOM) were limited to waveguide thickness down to 30 nm. In this work, we present the nano-optical imaging of ordinary transverse electric modes of exciton polaritons in MoS2 and WSe2 down to a few atomic layers, measured by atomic force microscope-based s-SNOM. Surprisingly, the interference fringe patterns can be observed clearly at the prepared ultrathin TMD flakes with thickness down to ~3 nm (4 ML) and ~8 nm (12 ML) for MoS2 and WSe2, respectively, which breaks greatly the previous measurement limitation. The wavevectors stay around 1.6k0−1.7k0 constantly when the thickness approaching to a few MLs, instead of 1k0 according to the theory. These modes are supported by the nearly-freestanding TMD microflakes in the form of three-layer symmetric waveguide to confine the exciton polaritons. Our results provide in-depth understanding and open new avenues to explore the polaritonic devices operating at the near infrared region based on ultrathin TMD materials.

Extraordinary Nonlinear Optical Interaction from Strained Nanostructures in van der Waals CuInP2S6.

Local strain engineering and structural modification of 2D materials furnish benevolent control over their optoelectronic properties and provide an exciting approach to tune light-matter interaction in layered materials. Application of strain at the nanoscale is typically obtained through permanently deformed nanostructures such as nanowrinkles, which yield large band gap modulation, photoluminescence enhancement, and surface potential. Ultrathin transition metal dichalcogenides (TMDs) have been greatly analyzed for such purposes. Herein, we extend strain-induced nanoengineering to an emerging 2D material, CuInP2S6 (CIPS), and visualize extraordinary control over nonlinear light-matter interaction. Wrinkle nanostructures exhibit ∼160-fold enhancement in second harmonic generation (SHG) compared to unstrained regions, which is additionally influenced by a change in the dielectric environment. The SHG enhancement was significantly modulated by the percentage of applied strain which was numerically estimated. Furthermore, polarization-dependent SHG revealed quenching and enhancement in the parallel and perpendicular directions, respectively, due to the direction of the compressive vector. Our work provides an important advancement in controlling optoelectronic properties beyond TMDs for imminent applications in flexible electronics.

Room Temperature Micro-Photoluminescence Studies of Colloidal WS2 Nanosheets

Wet-chemical syntheses for quasi two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as promising methods for straightforward solution-processing of these materials. However, photoluminescence properties of colloidal TMDs are virtually unexplored due to the typically non-emitting synthesis products. In this work, we demonstrate room temperature micro-photoluminescence of delicate ultrathin colloidal WS2 nanosheets synthesized from WCl6 and elemental sulfur in oleic acid and oleylamine at 320 {\deg}C for the first time. Both, mono- and multilayer photoluminescence are observed, revealing comparable characteristics to exfoliated TMD monolayers and underpinning the high quality of colloidal WS2 nanosheets. In addition, a promising long-term air-stability of colloidal WS2 nanosheets is found and the control of photodegradation of the structures under laser excitation is identified as a challenge for further advancing nanosheet monolayers. Our results render colloidal TMDs as easily synthesized and highly promising 2D semiconductors with optical properties fully competitive with conventionally fabricated ultrathin TMDs.

Electronic states of multilayer VTe2 : Quasi-one-dimensional Fermi surface and implications for charge density waves

We have performed angle-resolved photoemission spectroscopy on epitaxial $\mathrm{V}{\mathrm{Te}}_{2}$ films to elucidate the relationship between the fermiology and charge density waves (CDWs). We found that a two-dimensional triangular pocket in 1 monolayer (ML) $\mathrm{V}{\mathrm{Te}}_{2}$ is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6 ML counterpart, likely associated with the $1T$-to-$1{T}^{\ensuremath{'}}$ structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6 ML at low temperature distinct from anisotropic pseudogap in 1 ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure calculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides.

Measurement of direct and indirect bandgaps in synthetic ultrathin MoS2 and WS2 films from photoconductivity spectra

Exploring the thickness-dependent electronic properties of ultrathin transition metal dichalcogenides is crucial for novel optoelectronic devices. Particularly important is experimental information regarding the bandgap width. This information is scarce and often inconsistent among the several measurement techniques that were employed for this task, such as optical absorption, scanning tunneling spectroscopy, and photoconductivity. Here, we present photoconductivity measurements in large-area synthetic MoS2 and WS2 films (one to five monolayers and the bulk crystal) grown on insulating layers (SiO2, Al2O3, or HfO2). The excitonic peaks of MoS2 and WS2 were detected in both the photocapacitor and traditional in-plane geometries. Their contribution to the photoconductivity is explained by the electric field-assisted dissociation mechanism. We have separated the excitonic and free carrier components in the photocurrent spectra and extracted the direct and indirect bandgaps using the Tauc plot, revealing their dependencies on the number of monolayers.

One-Step Hydrothermal Synthesis of Phase-Engineered MoS2/MoO3 Electrocatalysts for Hydrogen Evolution Reaction

The development of suitable approaches for the synthesis of ultrathin transition-metal dichalcogenide (TMD) catalysts is required to engineer phases, intercoupling between different phases, in-plane defects, and edges and hence maximize their catalytic performance for hydrogen production. In this work, we report a simple one-step hydrothermal approach for the synthesis of a three-dimensional (3D) network of self-assembled metallic MoS2/MoO3 nanosheets, using α-MoO3 and thiourea (TU) as the Mo and S precursors, respectively. A systematic structural/property relationship study, while varying the precursors’ molar concentration ratios (TU/MoO3) and reaction temperatures (TR), revealed a kinetically controlled regime, in hydrothermal synthesis, that enabled the formation of ultrathin branched MoS2/MoO3 nanosheets with the highest metallic content of ∼47 % in a reproducible manner. Importantly, the work established that in addition to the rich metallic MoS2 phase (1T), the electronically coupled interfaces between MoO3 and MoS2 nanodomains, profusion of active sites, and tuned electrical conductivity significantly contributed to hydrogen evolution reaction (HER)-catalytic activity, affording a low overpotential of 210 mV (with respect to the reversible hydrogen electrode) at a current density of 10 mA/cm2, a small Tafel slope of ∼50 mV/dec, and high stability. Overall, this work demonstrated a controllable one-step hydrothermal method for the rational design and synthesis of a 3D network of MoS2/MoO3 nanosheets with high 1T-MoS2 metallic yield, simultaneous incorporation of MoO3/MoS2 heterointerfaces, sulfur vacancies, and tuned electrical conductivity, which are highly beneficial for clean energy conversion applications that can potentially be expanded to other two-dimensional TMD materials.

Strongly enhanced electromechanical coupling in atomically thin transition metal dichalcogenides

Flexoelectricity in thin films has emerged as an effective electromechanical response owing to appealing scaling law and universal existence. However, current studies show limited out-of-plane converse flexoelectric effect (CFE) of ultra-thin transition metal dichalcogenides (TMDs) when compared to their conventional in-plane piezoresponse. Here, we report converse flexoresponse of atomically thin TMDs such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) which exceeds their intrinsic in-plane piezoresponses. Our piezoresponse force microscopy (PFM) measurements revealed strongly enhanced CFE of the atomically thin MoS2 and WSe2 than their bulk counterpart (∼700% enhancement in MoS2, ∼400% enhancement in WSe2). We observed an anomalous reduction in converse flexoresponse in the monolayer structure attributed to a puckering deformation. By inducing a built-in in-plane tension to reduce puckering, we estimated the CFE of monolayer WSe2 to be 8.14 pm/V, the highest among the atomically thin TMDs.

Liquid exfoliated biocompatible WS2@BSA nanosheets with enhanced theranostic capacity.

Ultrathin transition metal dichalcogenides (TMDs) seem to have a promising future in the field of theranostic agents due to their excellent near-infrared light absorption capacity and large specific surface area. Plenty of previous studies focused on the therapeutic effects of the materials, but were less concerned with the detailed studies of biocompatibility for clinical transformation. In this work, ultrathin WS2 nanosheets coated with bovine serum protein (BSA) (WS2@BSA NSs) were selected as experimental subjects with favorable biocompatibility to explore their potential as a theranostic agent. Firstly, ultrathin WS2 nanosheets were prepared by ultrasound-assisted exfoliation using n-methyl pyrrolidone (NMP) as the liquid phase, followed by coating with bovine serum protein. The physical and chemical properties of WS2@BSA NSs were investigated. Secondly, the biocompatibility experiments that are most relevant to clinical transformation were divided into cell level experiments and in vivo experiments with zebrafish as the model organism. Finally, to explore further applications for the diagnosis and treatment of tumors, the in vitro photothermal effect and the X-ray computed tomography (CT) imaging capability of WS2@BSA NSs were investigated. The obtained results were promising in terms of biocompatibility and theranostics, which suggested the potential of WS2@BSA NSs for use as a multifunctional theranostic agent in clinics.

Atomically thin defect-rich Ni-Se-S hybrid nanosheets as hydrogen evolution reaction electrocatalysts

Facile design of economic-effective hydrogen evolution reaction (HER) catalysts with non-noble materials are promising for the production of renewable chemical fuels. Two-dimensional (2D) ultrathin transition metal dichalcogenides (TMDs) materials with large specific surface area and abundant catalytic active sites can significantly enhance their catalytic activities. Herein, we design and synthesize an atomically thin Ni-Se-S based hybrid nanosheet (NiSe1.2S0.8) via a simple solvothermal method, the thickness of NiSe1.2S0.8 nanosheets is only about 1.1 nm. Benefiting from the ultrathin nanostructure and rich defects, the optimal NiSe1.2S0.8 exhibits good electrocatalytic activity with the overpotential of 144 mV at −10 mA·cm−2, a small Tafel slope of 59 mV·dec−1, and outstanding catalytic stability in acid electrolyte for HER. The theoretical results show that hybrid electrocatalyst by S incorporation possesses the optimal adsorption free energy of hydrogen (ΔGH*). This study provides a simple method to synthesize a highperformance multicomponent electrocatalysts with the ultrathin nanostructures and abundant defects.

Tunable Optoelectronic Properties of WS2 by Local Strain Engineering and Folding

AbstractLocal strain engineering is an exciting approach to tune optoelectronic properties of materials. Two‐dimensional (2D) materials such as 2D transition metal dichalcogenides (TMDs) are particularly well‐suited for this purpose due to their high flexibility and deformability. Local strain engineering in 2D TMDs is typically achieved via strained wrinkles. Wrinkles on thick TMD layers have been reported to show interesting photoluminescence enhancement due to bandgap modulation and funneling effect. However, the wrinkles in ultrathin TMDs are not investigated because they can easily fall down to form folds. Both wrinkles and folds are achieved simultaneously in 1–3L tungsten disulfide (WS2) using a new fabrication technique. A layer‐dependent reduction in surface potential is found for both folded layers and perfect packed layers due to the dominant interlayer screening effect. The strain produced from the wrinkles modulates the semiconductive junction properties significantly. Thermoionic modeling suggests that strained (1.6%) wrinkles can lower the Schottky barrier height (SBH) by 20%. Upon illumination, SBH reduces significantly due to photogenerated carriers. This is an important advance toward controlling the optoelectronic properties of 2D TMDs via strain engineering, with applications for electronics and optoelectronics devices.

Enhancement of light absorption in a WS2 monolayer using spacer and Au layers

Among ultrathin transition metal dichalcogenides, WS2 monolayers with a direct bandgap have attracted intensive attention because of their narrow optical absorption centered at 619 nm. However, these layers with stronger absorption are needed in applications such as photonic devices. In this paper, an increase in absorption is achieved through plasmonic coupling of a nearby Au thin layer directly or through a spacer to the WS2 monolayer. The optical properties of the designed structures are investigated by the transfer matrix method in the visible wavelength region. In the structure consisting of a WS2 monolayer with Au and spacer layers, the absorption at 619 nm was increased to 61%. Sharp optical absorption as high as 40% for a large range of incidence angles in both polarizations was retained. Our study provides useful information for the design of WS2 monolayers in photonic devices for practical applications.

Thermal transport properties of monolayer MoSe2 with defects.

Two-dimensional (2D) molybdenum diselenide (MoSe2) as one of the ultrathin transition metal dichalcogenides (TMDs) has attracted considerable attention because of its potential applications in thermoelectric and nano-electronic devices. Here, the thermal conductivity of monolayer MoSe2 and its responses to simulated size and defects are studied by nonequilibrium molecular dynamics simulations. With the increase of sample length, the thermal conductivity of monolayer MoSe2 nanoribbons exhibits an enhancement whereas it is insensitive to the width. At room temperature, the thermal conductivities of monolayer MoSe2 along armchair and zigzag directions are 17.758 and 18.932 W (m K)-1, respectively, which are consistent with previous results. The impact of defects on thermal conductivity has also been studied by varying the concentration of the vacancy from 0.1% to 0.5%. The results show that an increase of the defect concentration will greatly suppress the thermal conductivity. The 0.5% defect concentration with a Mo vacancy can result in a thermal conductivity reduction of ∼43%. Such a study would provide a good insight into the tunable thermal transport for potential applications of not only monolayer MoSe2, but also many other TMDs.

1T′-MoTe 2 -Based On-Chip Electrocatalytic Microdevice: A Platform to Unravel Oxidation-Dependent Electrocatalysis

Ultrathin transition metal dichalcogenides (TMDs) are of particular interest as low-cost alternatives to highly active electrocatalysts because of their high surface activation energy. However, the...