Multilayer WS2 Research Articles

Phase-Selective Synthesis of Rhombohedral WS2 Multilayers by Confined-Space Hybrid Metal-Organic Chemical Vapor Deposition.

Rhombohedral polytype transition metal dichalcogenide (TMDC) multilayers exhibit non-centrosymmetric interlayer stacking, which yields intriguing properties such as ferroelectricity, a large second-order susceptibility coefficient χ(2), giant valley coherence, and a bulk photovoltaic effect. These properties have spurred significant interest in developing phase-selective growth methods for multilayer rhombohedral TMDC films. Here, we report a confined-space, hybrid metal-organic chemical vapor deposition method that preferentially grows 3R-WS2 multilayer films with thickness up to 130 nm. We confirm the 3R stacking structure via polarization-resolved second-harmonic generation characterization and the 3-fold symmetry revealed by anisotropic H2O2 etching. The multilayer 3R WS2 shows a dendritic morphology, which is indicative of diffusion-limited growth. Multilayer regions with large, stepped terraces enable layer-resolved evaluation of the optical properties of 3R-WS2 via Raman, photoluminescence, and differential reflectance spectroscopy. These measurements confirm the interfacial quality and suggest ferroelectric modification of the exciton energies.

Opto-twistronic Hall effect in a three-dimensional spiral lattice.

Studies of moiré systems have explained the effect of superlattice modulations on their properties, demonstrating new correlated phases1. However, most experimental studies have focused on a few layers in two-dimensional systems. Extending twistronics to three dimensions, in which the twist extends into the third dimension, remains underexplored because of the challenges associated with the manual stacking of layers. Here we study three-dimensional twistronics using a self-assembled twisted spiral superlattice of multilayered WS2. Our findings show an opto-twistronic Hall effect driven by structural chirality and coherence length, modulated by the moiré potential of the spiral superlattice. This is an experimental manifestation of the noncommutative geometry of the system. We observe enhanced light-matter interactions and an altered dependence of the Hall coefficient on photon momentum. Our model suggests contributions from higher-order quantum geometric quantities to this observation, providing opportunities for designing quantum-materials-based optoelectronic lattices with large nonlinearities.

Effects of UV ozone treatment on the photoresponse of the Pt nanoparticles decorated WS2

Chemical vapor deposition (CVD)-grown and Pt nanoparticles (NPs) decorated multilayer WS2 is treated by ultraviolet (UV) ozone for the modification of the surface roughness to realize the enhanced hot carriers injection via the internal photoemission (IPE). The morphology, structure and photophysical properties as well as the dependence of the photoresponse on the UV ozone treatment are investigated. The UV ozone treatment demonstrates obvious effect on the WS2 surface roughness (Sa), which reaches the maximum around 14 nm after 2 min exposure and subsequently decrease with the extension of treatment duration. UV ozone treatment also leads to obviously diminished photoluminescence (PL) and shortened excitons lifetime for bare WS2 while its influence on the electronic structure is negligible. The diminished PL, shortened PL decaying lifetime and increased Sa are all originated from the formation of WOx due to the consumption of top WS2 layer by the severe oxidation, and such effects will be greatly alleviated when the fresh and undisturbed underlying WS2 layer appears with the treatment duration extended to 4 min. The 2 min UV ozone treated WS2/Pt exhibits optimal wide spectrum photoresponse (400–900 nm) with the near infrared (NIR) responsivity around 18 A/W at 750 nm, about 60 % higher compared to that of untreated one while the 4 min overtreated WS2/Pt demonstrates much deteriorated photoresponse. Such behavior can be ascribed to the enhanced hot electrons injection from Pt to WS2 due to the formation of rough Schottky contact as well as the separation of photo-excited electron-hole pairs by Schottky junction. Our study proves that the Sa is a key factor not only for the exciton dynamics but also for the hot electron injection from Pt to WS2.

Photoresponse dependence of WS2/Pt Schottky junction on the features of Pt nanoparticles

In this study, we delve into the photoresponsive characteristics of Schottky junctions fabricated through the deposition of Pt nanoparticles (NPs) onto multilayer WS2 sheets grown via chemical vapor deposition. Our analysis encompasses a comprehensive examination of the junction's morphology, structural attributes, and photophysical behavior. Of particular interest is the intricate relationship between the efficiency of hot electron injection and the specific properties of the Pt NPs, namely their density and size. Our findings reveal that Pt NPs effectively extinguish the photoluminescence of WS2, an outcome attributed to the efficient separation of photo-generated electron-hole pairs facilitated by the WS2/Pt Schottky junction. This process, however, does not significantly alter the electronic configuration of WS2. The modulation of exciton radiative decay is shown to be contingent upon the size and density of the Pt NPs, with an average size of 1.27 nm and density of 0.82/nm² yielding the most rapid exciton decay rate of 1.15 ns. This pronounced dependence of exciton dynamics on Pt NPs features can be attributed to the quenching effect exerted by densely packed Pt NPs, which also plays a pivotal role in the injection of hot electrons from Pt to WS2. The enhancement of internal photoemission is evidenced by a pronounced increase in near-infrared photoresponse within the 800–1000 nm range. Furthermore, the optimization of Pt NPs features leads to superior photoresponsive performance in the WS2/Pt Schottky junctions, manifesting as a responsivity of 280 A/W and a detectivity of 7.77 × 1012 Jones at a wavelength of 850 nm. This photoresponse's dependence on Pt NPs features underscores the dual mechanism governing photodetection in the WS2/Pt Schottky junction: a synergistic interplay between hot electron injection and the separation of photo-excited electron-hole pairs.

Excitonic States Beyond the Optical Spectrum in Epitaxially Grown Mono- And Multilayer WS2: A Spatially-Resolved EELS and DFT Study

Excitonic States Beyond the Optical Spectrum in Epitaxially Grown Mono- And Multilayer WS2: A Spatially-Resolved EELS and DFT Study

Weak interlayer interaction and enhanced electron mobility in multilayer WS2 achieved through a layer-by-layer wet transfer approach

Weak interlayer interaction and enhanced electron mobility in multilayer WS2 achieved through a layer-by-layer wet transfer approach

High Q Hybrid Mie-Plasmonic Resonances in van der Waals Nanoantennas on Gold Substrate.

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices; however, they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain low losses, while achieving stronger mode confinement. Here, we use a high refractive index multilayer transition-metal dichalcogenide WS2 exfoliated on gold to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-Perot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We measure the experimental quality factors of hybridized Mie-plasmonic (MP) modes to be up to 33 times that of standard Mie resonances in the nanoantennas on silica. We then tune the nanoantenna geometries to observe signatures of a supercavity mode with a further increased Q factor of over 260 in experiment. We show that this quasi-bound state in the continuum results from strong coupling between a Mie resonance and Fabry-Perot-plasmonic mode in the vicinity of the higher-order anapole condition. We further simulate WS2 nanoantennas on gold with a 5 nm thick hBN spacer in between. By placing a dipole within this spacer, we calculate the overall light extraction enhancement of over 107, resulting from the strong, subwavelength confinement of the incident light, a Purcell factor of over 700, and high directivity of the emitted light of up to 50%. We thus show that multilayer TMDs can be used to realize simple-to-fabricate, hybrid dielectric-on-metal nanophotonic devices granting access to high-Q, strongly confined, MP resonances, along with a large enhancement for emitters in the TMD-gold gap.

Robust Layer‐Dependent Valley Polarization and Valley Coherence in Spiral WS2 at Room Temperature

In the emerging field of valleytronics, it is aimed to coherently manipulate the valley pseudospin as an information‐bearing degree of freedom. The 2D transition‐metal dichalcogenides (TMDCs) provide a unique possibility to generate an excitonic valley pseudospin, opening the way to valley information. Although significant development of valley pseudospin in layered materials has been achieved recently, looking for new TMDCs featuring robust valley phenomenon at room temperature is still desirable for practical applications. Herein, the valley pseudospin of the spiral WS2 with different layer thicknesses at room temperature is investigated by both circular and linear polarization‐resolved photoluminescence spectroscopy. In the experimental results, it is demonstrated that the spiral WS2 emerges robust valley polarization and valley coherence, the degree of circular polarization, and linear polarization gradually increase with the lift of the layer thicknesses, reaching up to 0.91 for valley polarization and 0.94 for valley coherence, respectively. The robust layer‐dependent valley pseudospin may originate from the intrinsic broken inversion symmetry, due to the spiral structure of the multilayer WS2. The robust and near‐unity valley polarization and valley coherence at room temperature in the spiral WS2 may provide a new platform for optical manipulation of the valley pseudospin for further valleytronics applications.

Multilayer WS2 for low-power visible and near-infrared phototransistors

Mechanically exfoliated multilayer WS2 flakes are used as the channel of field effect transistors for low-power photodetection in the visible and near-infrared (NIR) spectral range. The electrical characterization as a function of the temperature reveals devices with n-type conduction and slightly different Schottky barriers at the drain and source contacts. The WS2 phototransistors can be operated in self-powered mode, yielding both a current and a voltage when exposed to light. The spectral photoresponse in the visible and the NIR ranges shows a high responsivity (4.5 μA/W) around 1250 nm, making the devices promising for telecommunication applications.

Anomalous layer-dependent photoluminescence spectra of supertwisted spiral WS2.

Twisted stacking of two-dimensional materials with broken inversion symmetry, such as spiral MoTe2 nanopyramids and supertwisted spiral WS2, emerge extremely strong second- and third-harmonic generation. Unlike well-studied nonlinear optical effects in these newly synthesized layered materials, photoluminescence (PL) spectra and exciton information involving their optoelectronic applications remain unknown. Here, we report layer- and power-dependent PL spectra of the supertwisted spiral WS2. The anomalous layer-dependent PL evolutions that PL intensity almost linearly increases with the rise of layer thickness have been determined. Furthermore, from the power-dependent spectra, we find the power exponents of the supertwisted spiral WS2 are smaller than 1, while those of the conventional multilayer WS2 are bigger than 1. These two abnormal phenomena indicate the enlarged interlayer spacing and the decoupling interlayer interaction in the supertwisted spiral WS2. These observations provide insight into PL features in the supertwisted spiral materials and may pave the way for further optoelectronic devices based on the twisted stacking materials.

Layer-Dependent Sensing Performance of WS2-Based Gas Sensors.

Two-dimensional (2D) materials, such as tungsten disulfide (WS2), have attracted considerable attention for their potential in gas sensing applications, primarily due to their distinctive electrical properties and layer-dependent characteristics. This research explores the impact of the number of WS2 layers on the ability to detect gases by examining the layer-dependent sensing performance of WS2-based gas sensors. We fabricated gas sensors based on WS2 in both monolayer and multilayer configurations and methodically evaluated their response to various gases, including NO2, CO, NH3, and CH4 at room temperature and 50 degrees Celsius. In contrast to the monolayer counterpart, the multilayer WS2 sensor exhibits enhanced gas sensing performance at higher temperatures. Furthermore, a comprehensive gas monitoring system was constructed employing these WS2-based sensors, integrated with additional electronic components. To facilitate user access to data and receive alerts, sensor data were transmitted to a cloud-based platform for processing and storage. This investigation not only advances our understanding of 2D WS2-based gas sensors but also underscores the importance of layer engineering in tailoring their sensing capabilities for diverse applications. Additionally, the development of a gas monitoring system employing 2D WS2 within this study holds significant promise for future implementation in intelligent, efficient, and cost-effective sensor technologies.

Spectroscopy of monolayer and multilayer tungsten disulfide under high pressure

Recently exfoliated monolayer and multilayered transition metal dichalcogenides have gathered significant interest based on their tunable bandgap and extremely high carrier mobility. We have investigated the Raman and photoluminescence spectra of monolayer and multilayer WS2 as a function of pressure. The Raman-inactive mode B1u, which is activated by structural disorder, was revealed at 6.7 GPa in monolayers, at 8.0 GPa in bilayers, and at 13.7 GPa in multilayers, respectively. With the enhancement of pressure-induced interlayer interaction, the crystal phase transition due to layer sliding like 2Hc to 2Ha occurs at 14.8 and 18.7 GPa in bilayers and multilayers, as evidenced by the split of E12g and B1u. The electronic phase transition of the monolayer is supposed to be a direct K-K bandgap changing to an indirect Λ-K bandgap at 2.6 GPa. These observations contribute to a better understanding of the impact of interlayer interactions on the modulation of WS2 energy bands and structure, as well as fundamental studies of two-dimensional layered materials, which can inform the development of device applications.

Sublimation-based wafer-scale monolayer WS2 formation via self-limited thinning of few-layer WS2.

Atomically-thin monolayer WS2 is a promising channel material for next-generation Moore's nanoelectronics owing to its high theoretical room temperature electron mobility and immunity to short channel effect. The high photoluminescence (PL) quantum yield of the monolayer WS2 also makes it highly promising for future high-performance optoelectronics. However, the difficulty in strictly growing monolayer WS2, due to its non-self-limiting growth mechanism, may hinder its industrial development because of the uncontrollable growth kinetics in attaining the high uniformity in thickness and property on the wafer-scale. In this study, we report a scalable process to achieve a 4 inch wafer-scale fully-covered strictly monolayer WS2 by applying the in situ self-limited thinning of multilayer WS2 formed by sulfurization of WOx films. Through a pulsed supply of sulfur precursor vapor under a continuous H2 flow, the self-limited thinning process can effectively trim down the overgrown multilayer WS2 to the monolayer limit without damaging the remaining bottom WS2 monolayer. Density functional theory (DFT) calculations reveal that the self-limited thinning arises from the thermodynamic instability of the WS2 top layers as opposed to a stable bottom monolayer WS2 on sapphire above a vacuum sublimation temperature of WS2. The self-limited thinning approach overcomes the intrinsic limitation of conventional vapor-based growth methods in preventing the 2nd layer WS2 domain nucleation/growth. It also offers additional advantages, such as scalability, simplicity, and possibility for batch processing, thus opening up a new avenue to develop a manufacturing-viable growth technology for the preparation of a strictly-monolayer WS2 on the wafer-scale.

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.

Controllable growth of wafer-scale two-dimensional WS2 with outstanding optoelectronic properties

As one of two-dimensional (2D) semiconductor materials, transition metal dichalcogenides (TMDs) have sparked enormous potential in next-generation electronics due to their unique and excellent physical, electronic and optical properties. Controllable growth of wafer-scale 2D TMDs is essential to realize the various high-end applications of TMDs, while it remains challenging. Herein, 2 inch 2D WS2 films were successfully synthesized by ambient pressure chemical vapor deposition based on substrate engineering and space-confined strategies. WS2 nucleation density can be effectively modulated depending on the annealing conditions of sapphire substrate. The thickness of WS2 films can be controllably fabricated by adjusting the space-confined height. Moreover, our strategies are demonstrated to be universal for the growth of other 2D TMD semiconductors. WS2-based photodetectors with different thicknesses were systematically investigated. Monolayer WS2 photodetector displays large responsivity of 0.355 A W−1 and high specific detectivity of 1.48 × 1011 Jones. Multilayer WS2 device exhibits negative self-powered photoresponse. Our work provides a new route for the synthesis of wafer-scale 2D TMD materials, paving the way for high performance integrated optoelectronic devices.

Large-area WS2 Deposited on Sapphire and Its In-Plane Raman and PL Spectral Distributions

Large-area and high-quality WS2 monolayer has been fabricated on the Al2O3 substrate. Three typical WS2 configurations were adopted to examine the in-plane spectral properties. For the triangle WS2 monolayer, the PL light region exhibits a large peak wavelength and could be deduced to be the relaxation of the compressed strain of the WS2 monolayer and the low defect density. For triangle WS2 monolayer with multilayer WS2 on center, combining the peak intensity and position results of PL and Raman spectra, the line traces near the side of the center triangle can be demonstrated to be the defects or dislocations due to the exist of the central multilayer WS2. For large-area WS2 monolayer with crystal domain, PL area integrated mapping shows a uniform light region across the whole surface, except the existing dark crystal domain boundary. The dark line traces could be attributed to compressed strain in the WS2 monolayer due to the formation of the WS2/Al2O3 hybrid structure. The in-plane PL and Raman spectra and mapping exhibited in this work reveal the distribution of stress and defects in this system and further clarify the effects of stress and defects on the optoelectronic properties of WS2 monolayer on Al2O3.

Bolstering functionality in multilayer and bilayer WS2 via focused laser micro-engraving

Bolstering functionality in multilayer and bilayer WS2 via focused laser micro-engraving

Conformal deposition of WS2 layered film by low-temperature metal-organic chemical vapor deposition

Large area multi-layer WS2 film has high potential as a channel material for MOSFETs in next-generation LSIs. State-of-the-art LSIs have complex three-dimensional (3D) structures such as vertical channels and multi-layer stacked channels surrounded by gate electrodes. To develop such structures, it is desirable to fabricate channel layers by CVD, which is suitable for conformal deposition along a substrate with a complicated 3D structure. In this study, we report on WS2 films deposited by Metal-Organic CVD using low-toxicity n-BuNC-W(CO)5 as a liquid tungsten precursor and (t-C4H9)2S2 for sulfur precursor. The deposited films have a roughly stoichiometric composition and are stable even after 60 d of shelf time in air atmosphere. A layered film along the 3D fin substrate parallel to the surface was fabricated on the entire structure.

Black Phosphorous-Based Highly Sensitive Surface Plasmonic Sensor for Detection of Formalin

In the present work, a highly sensitive prism-based surface plasmon resonance (SPR) sensor for the detection of formalin using multilayer black phosphorous (BP) is proposed. Formalin, which is widely used as a preservative, can cause a variety of diseases, making its detection crucial. The transfer matrix method is utilized to compare and analyze the proposed structures of the SPR-based prism sensor in the Kretschmann configuration. The effect of prism materials (BAK3, BK7, SF10, SF11, and 2S2G), metals (gold, silver, copper, and aluminum), and 2-D materials (graphene, MoS2, WS2, and BP) on the performance of the sensor for formalin detection has been analyzed and discussed. The analysis of the impact of employing multilayer 2-D materials is carried out. Optimization of the thickness of the 2-D material is performed to achieve a structure that gives better sensitivity. The sensitivity achieved for structures with multilayer graphene, MoS2, and WS2 are 172.82°/RIU, 195.97°/RIU, and 218.04°/RIU, respectively. A highly sensitive structure for formalin detection is proposed with BK7, silver, and nine layers of BP. A sensitivity of 289.4565°/RIU and a detection accuracy (DA) of 0.3125/° are obtained. The limit of detection of the proposed sensor for different concentrations of formalin lies in the range of 3.45– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.56~ \mu $ </tex-math></inline-formula> RIU.

Photocatalytic property of two dimensional heterostructure MoS2/WS2 for hydrogen evolution via water splitting; a first principles calculation

We investigated photocatalytic property of two dimensional heterostructure MoS2/WS2 by a first principles calculation. Firstly, we evaluated the energetic stability of different heterostructures by comparing combination energies of heterojunctions with those between adjacent layers in multilayer MoS2 or WS2 system after constructing nine heterostructures (MoS2)n/(WS2)m (n, m = 1, 2, 3). The subscripts n and m are the numbers of MoS2 or WS2 layers that are contained in the above heterostructures, respectively. The investigated heterostructures have all stable binding energy of about 0.2 eV. Secondly, we calculated band gap of every heterostructure in order to compare it with the oxidation/reduction level for water splitting. Among the considered nine structures, the heterostructure (MoS2)1/(WS2)1 is suitable for overall water splitting because it has a band gap of 1.70 eV and energy barrier of 0.21 eV for preventing recombination of electrons and holes generated by light irradiation. In addition, the heterostructure is also favorable to the evolution of hydrogen, as it has a low hydrogen desorption energy of 0.035 eV.