Monolayer WS2 Research Articles

Area-Selective Growth of Two-Dimensional Mono- And Bilayer WS2 for Field Effect Transistors

Herein, we propose a novel approach for area-selective tunable growth of uniform monolayer or bilayer WS2 on dielectric substrates through in situ conversion of a predeposited W metal pad to WOx initially and then to WS2 mono- and bilayers. Compared with the various transfer methods that have been used previously for multilayer stacking, this direct-growth method has the advantages of producing cleaner interfaces and the capability of growing tunable layers on target substrates, thereby making it more suitable for manufacturing processes. The WS2 bilayer displayed uniform optical properties, with the atomic arrangement between layers having an AA stacking order that are supposed to have higher mobility. We adopted these WS2 monolayers and bilayers in field-effect transistors. Accordingly, this approach for highly area-selective growth of transition metal dichalcogenide monolayers and bilayers with metal pads and their in situ conversion appears to provide effective platforms for further device applications.

Strongly enhanced light-matter coupling of monolayer WS2 from a bound state in the continuum.

Exciton-polaritons derived from the strong light-matter interaction of an optical bound state in the continuum with an excitonic resonance can inherit an ultralong radiative lifetime and significant nonlinearities, but their realization in two-dimensional semiconductors remains challenging at room temperature. Here we show strong light-matter interaction enhancement and large exciton-polariton nonlinearities at room temperature by coupling monolayer tungsten disulfide excitons to a topologically protected bound state in the continuum moulded by a one-dimensional photonic crystal, and optimizing for the electric-field strength at the monolayer position through Bloch surface wave confinement. By a structured optimization approach, the coupling with the active material is maximized here in a fully open architecture, allowing to achieve a 100 meV photonic bandgap with the bound state in the continuum in a local energy minimum and a Rabi splitting of 70 meV, which results in very high cooperativity. Our architecture paves the way to a class of polariton devices based on topologically protected and highly interacting bound states in the continuum.

Engineering of Vacancy Defects in WS2 Monolayer by Rare‐Earth (Er, Tm, Lu) Doping: A First‐Principles Study

Vacancy defects in two‐dimensional (2D) materials are critical to their properties. The substitutional doping in vacancy defects by rare‐earth atoms results in diverse material versatilities, but the effects and the corresponding mechanisms are far from being answered. Herein, the electronic and optical properties of vacancy defects and lanthanide rare‐earth (RE = Er, Tm, Lu) doping in WS2 monolayers are studied based on density functional theory with Hubbard correction (DFT + U). The formation energies of defective and doped systems are calculated under W‐rich and S‐rich conditions, respectively. The bandgap of WS2 with one W‐defect in a 4 × 4 × 1 supercell changes from direct to indirect, while S‐defect does not change the bandgap type of the supercell. When RE atoms are doped in the W‐defect, the bandgap returns to direct. WS2 supercells with intrinsic defects and Lu doped in [W] defect show the nonmagnetic property, while Er‐ and Tm‐doped supercells show obvious magnetic properties. The imaginary part of dielectric function and absorption coefficients increase drastically from visible to near‐infrared as RE atoms substitute W sites, and the bandgap shrinks due to the rise of the valance band maximum. This work proves RE doping in 2D WS2 is an effective technique for bandgap and carrier engineering.

Giant Nonlinear Optical Response via Coherent Stacking ofIn-Plane Ferroelectric Layers.

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generationwasobserved to be 100 times more intense than monolayer WS2 . This work demonstratesin-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

Hydrogen Sensing Mechanism of WS2 Gas Sensors Analyzed with DFT and NAP-XPS.

Nanostructured tungsten disulfide (WS2) is one of the most promising candidates for being used as active nanomaterial in chemiresistive gas sensors, as it responds to hydrogen gas at room temperature. This study analyzes the hydrogen sensing mechanism of a nanostructured WS2 layer using near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT). The W 4f and S 2p NAP-XPS spectra suggest that hydrogen makes physisorption on the WS2 active surface at room temperature and chemisorption on tungsten atoms at temperatures above 150 °C. DFT calculations show that a hydrogen molecule physically adsorbs on the defect-free WS2 monolayer, while it splits and makes chemical bonds with the nearest tungsten atoms on the sulfur point defect. The hydrogen adsorption on the sulfur defect causes a large charge transfer from the WS2 monolayer to the adsorbed hydrogen. In addition, it decreases the intensity of the in-gap state, which is generated by the sulfur point defect. Furthermore, the calculations explain the increase in the resistance of the gas sensor when hydrogen interacts with the WS2 active layer.

Routing the Exciton Emissions of WS2 Monolayer with the High-Order Plasmon Modes of Ag Nanorods.

Locally routing the exciton emissions in two-dimensional (2D) transition-metal dichalcogenides along different directions at the nanophotonic interface is of great interest in exploiting the promising 2D excitonic systems for functional nano-optical components. However, such control has remained elusive. Herein we report on a facile plasmonic approach for electrically controlled spatial modulation of the exciton emissions in a WS2 monolayer. The emission routing is enabled by the resonance coupling between the WS2 excitons and the multipole plasmon modes in individual silver nanorods placed on a WS2 monolayer. Different from prior demonstrations, the routing effect can be modulated by the doping level of the WS2 monolayer, enabling electrical control. Our work takes advantage of the high-quality plasmon modes supported by simple rod-shaped metal nanocrystals for the angularly resolved manipulation of 2D exciton emissions. Active control is achieved, which offers great opportunities for the development of nanoscale light sources and nanophotonic devices.

Tunable Couplings of Photons with Bright and Dark Excitons in Monolayer Semiconductors on Plasmonic-Nanosphere-on-Mirror Cavities

Tunable exciton–photon couplings are of great importance for cavity quantum electrodynamics (QED), polariton chemistry, optical computing, and bosonic lasing. In this regard, two-dimensional (2D) excitons in transition metal dichalcogenides (TMDs) have attracted tremendous interest─the huge oscillator strengths endow sensitive responses to optical modes, and their excitonic properties can be actively tuned by external stimuli. However, tunable coupling with spin-forbidden dark excitons in monolayer TMDs has rarely been demonstrated, and how the bright/dark-exciton–photon couplings coexist in one system is still a mystery. Herein, we utilize waveguide and antenna modes in a gold nanosphere-on-mirror cavity to match the in-plane and out-of-plane dipole moments of bright and dark excitons in monolayer WS2, respectively, and simultaneously. Strong bright-exciton–photon couplings with a coupling strength of up to 60 meV are observed at room temperature. We show that the waveguide mode can be tuned in wavelengths by controlling the spacer thickness and demonstrate the anti-crossing feature of polaritons. Next, by increasing the excitation angle, we dynamically enlarge the relative contribution from the antenna mode coupled with dark excitons and suppress the waveguide mode coupled with bright states. Moreover, a tunable bright exciton linewidth is achieved as a result of suppressed homogeneous broadening caused by the dominant dark exciton decays under strong dark-exciton–photon couplings. This is not only proof of tunable couplings with dark excitons but also an insightful finding that reveals the novel interactions between bright- and dark-exciton–photon hybrids in a single optical cavity, opening new possibilities in tunable QED.

WS2 Monolayer in Fabry–Perot Cavity Support for Plasmonic Fano Resonance

WS2 Monolayer in Fabry–Perot Cavity Support for Plasmonic Fano Resonance

Direct Spectroscopic Observation of Cross-Plane Heat Transfer in a Two-Dimensional Van der Waals Heterostructure

Two-dimensional (2D) transition metal chalcogenides (TMDs) have drawn significant attention in recent years due to their extraordinary optical and electronic properties. As heat transfer plays an important role in device performance, various methods such as optothermal Raman spectroscopy and time-domain thermoreflectance have been developed to measure the thermal conductivity and interfacial thermal conductance in 2D van der Waals (vdW) heterostructures. Here, we employ the vibrational-pump-visible-probe (VPVP) spectroscopy to directly visualize the heat transfer process in a heterostructure of multilayer h-BN and monolayer WS2. Following an impulsive vibrational excitation of h-BN in the mid-infrared, we probe the heat transfer from h-BN through WS2 and finally to the substrate from the subpicosecond to the submicrosecond timescale. The interfacial thermal conductance of the h-BN/WS2 and WS2/SiO2 interfaces is obtained by corroborating the experiments with heat transfer calculations based on the Fourier’s law of heat conduction. Our study demonstrates an alternative, time-resolved optical method to measure cross-plane heat dissipation and opens up a new pathway to investigate the interlayer electron–phonon and phonon–phonon interactions in vdW heterostructures.

Suppression of substrate-induced charge doping in hBN-encapsulated monolayer WS2

Strong coupling between a two-dimensional material and substrate is problematic in that interlayer charge transfer at the interface often complicates optoelectronic device applications. Such substrate-induced charge doping effects are particularly large for two-dimensional materials on SiO2/Si substrate. Here, we report a Raman study of monolayer WS2 encapsulated with hexagonal boron nitride (hBN) to unveil substrate-related charge doping phenomena under the influence of laser irradiation. Raman correlation analysis between the E′ and A1′ phonon frequencies of WS2 reveals that the encapsulation of WS2 with hBN leads to a screening of electron transfer from SiO2 to WS2. Further, photo-induced charge doping in WS2 is completely suppressed by hBN encapsulation. Our results demonstrate that understanding the photo-induced charge doping effects in two-dimensional heterostructures is useful in characterizing the role of hBN encapsulation and, thus, shed light on design strategies for efficient two-dimensional optoelectronic applications where a precise control of charge doping is required.

Temperature Dependence of Excitonic Auger Recombination in Excitonic-Complex-Free Monolayer WS2 by Considering Auger Broadening and Generation Efficiency.

Monolayer transition metal dichalcogenides (TMDs) have been extensively studied for their optoelectronic properties and applications. However, even at moderate exciton densities, their light-emitting capability is severely limited by Auger-type exciton-exciton annihilation (EEA). Previous work on EEA used oversimplified models in the presence of excitonic complexes, resulting in seriously underestimated values for the Auger coefficient. In this work, we transferred monolayer WS2 on a gold substrate with hBN encapsulation, where excitons persist as the main species at 3-300 K via metal proximity. We numerically solved the rate equation for excitons to accurately determine the Auger coefficient as a function of temperature by considering laser pulse width and spatially inhomogeneous exciton distribution. We found that the Auger coefficient consists of temperature-dependent and independent terms, consistent with a theoretical model for direct and exchange processes, respectively. We believe that our results provide a guide for enhancing the luminescence quantum yield of TMDs.

Self-Limiting Growth of Monolayer Tungsten Disulfide Nanoribbons on Tungsten Oxide Nanowires

Transition metal dichalcogenides (TMDCs) are promising two-dimensional (2D) materials for next-generation optoelectronic devices; they can also provide opportunities for further advances in physics. Structuring 2D TMDC sheets as nanoribbons has tremendous potential for electronic state modification. However, a bottom-up synthesis of long TMDC nanoribbons with high monolayer selectivity on a large scale has not yet been reported yet. In this study, we successfully synthesized long WxOy nanowires and grew monolayer WS2 nanoribbons on their surface. The supply of source atoms from a vapor-solid bilayer and chemical reaction at the atomic-scale interface promoted a self-limiting growth process. The developed method exhibited a high monolayer selection yield on a large scale and enabled the growth of long (∼100 μm) WS2 nanoribbons with electronic properties characterized by optical spectroscopy and electrical transport measurements. The produced nanoribbons were isolated from WxOy nanowires by mechanical exfoliation and used as channels for field-effect transistors. The findings of this study can be used in future optoelectronic device applications and advanced physics research.

Photoluminescence modulation and charge transfer dynamics in R6G-decorated monolayer WS2

Photoluminescence modulation and charge transfer dynamics in R6G-decorated monolayer WS2

First principle study of electronic and optical properties of WS2(1-x)Se2x obtained by isoelectronic Se substitution on S-site of monolayer WS2

Two-dimensional semiconducting transition metal dichalcogenides have recently grabbed attention among the researchers, due to their extraordinary electrical, optical and thermal properties. Also, they have shown their suitability for application in digital electronics, photovoltaic cells, Thermoelectric generators and so on. Still, it is desired to precisely control the material properties to expand their scope of application and revamp the device performance. The substitutional doping technique is one of the widely explored processes, which is conventionally used to modulate the material properties. In the present work, the substitutional doping of selenium at the sulfur site within the WS2 sheet, is used indigenously to improve the electrical and optical response of the material. In doing so, electrical and optical properties of such doped materials are studied by employing density functional theory and linearized Boltzmann transport equation by considering the relaxation time approximation. The study has shown an almost linear variation of electronic band gap energy with the doping percentages. Also, the electron and hole mobility were found to be tailored due to the doping. Notably, hole mobility has shown a noticeable improvement at or near 60 percent of selenium doping. In the last section of the current work, the optical properties are extracted for the doped structures. The WS2 monolayer has shown the highest absorption peak near 3 eV. But for the doped materials, absorption peaks are relatively smaller and left shifted in the energy axis due to their low band gap energies.

Resolidified Chalcogen Precursors for High-Quality 2D Semiconductor Growth.

Two-dimensional (2D) semiconductors including transition metal dichalcogenides (TMDCs) have gained attention in optoelectronics for their extraordinary properties. However, the large amount and locally distributed lattice defects affect the optical properties of 2D TMDCs, and the defects originate from unstable factors in the synthesis process. In this work, we develop a method of pre-melting and resolidification of chalcogen precursors (sulfur and selenium), namely resolidified chalcogen, as precursor for the chemical vapor deposition growth of TMDCs with ultrahigh quality and uniformity. Taking WS2 as an example, the monolayer WS2 shows uniform fluorescence intensity and a small full-width at half-maximum of photoluminescence peak at low temperatures with an average value of 13.6±1.9 meV. The defect densities at the interior and edge region are both low and comparable, i.e., (9±3)×1012 cm-2 and (10±4)×1012 cm-2 , indicating its high structural quality and uniformity. This method is universal in growing high quality monolayer MoS2 , WSe2 , MoSe2 , and will benefit their applications.

Resolidified Chalcogen Precursors for High‐Quality 2D Semiconductor Growth

AbstractTwo‐dimensional (2D) semiconductors including transition metal dichalcogenides (TMDCs) have gained attention in optoelectronics for their extraordinary properties. However, the large amount and locally distributed lattice defects affect the optical properties of 2D TMDCs, and the defects originate from unstable factors in the synthesis process. In this work, we develop a method of pre‐melting and resolidification of chalcogen precursors (sulfur and selenium), namely resolidified chalcogen, as precursor for the chemical vapor deposition growth of TMDCs with ultrahigh quality and uniformity. Taking WS2 as an example, the monolayer WS2 shows uniform fluorescence intensity and a small full‐width at half‐maximum of photoluminescence peak at low temperatures with an average value of 13.6±1.9 meV. The defect densities at the interior and edge region are both low and comparable, i.e., (9±3)×1012 cm−2 and (10±4)×1012 cm−2, indicating its high structural quality and uniformity. This method is universal in growing high quality monolayer MoS2, WSe2, MoSe2, and will benefit their applications.

Silver nanowire electrodes for transparent light emitting devices based on WS2 monolayers

Transition metal dichalcogenide (TMDC) monolayers with their direct band gap in the visible to near-infrared spectral range have emerged over the past years as highly promising semiconducting materials for optoelectronic applications. Progress in scalable fabrication methods for TMDCs like metal-organic chemical vapor deposition (MOCVD) and the ambition to exploit specific material properties, such as mechanical flexibility or high transparency, highlight the importance of suitable device concepts and processing techniques. In this work, we make use of the high transparency of TMDC monolayers to fabricate transparent light-emitting devices (LEDs). MOCVD-grown WS2 is embedded as the active material in a scalable vertical device architecture and combined with a silver nanowire (AgNW) network as a transparent top electrode. The AgNW network was deposited onto the device by a spin-coating process, providing contacts with a sheet resistance below 10 Ω sq−1 and a transmittance of nearly 80%. As an electron transport layer we employed a continuous 40 nm thick zinc oxide (ZnO) layer, which was grown by atmospheric pressure spatial atomic layer deposition (AP-SALD), a precise tool for scalable deposition of oxides with defined thickness. With this, LEDs with an average transmittance over 60% in the visible spectral range, emissive areas of several mm2 and a turn-on voltage of around 3 V are obtained.

Understanding Dopant–Host Interactions on Electronic Structures and Optical Properties in Ce‐Doped WS2 Monolayers

AbstractSubstitutional lanthanide doping of 2D transition metal dichalcogenides (TMDs) is expected to be a promising strategy to engineer optical, electronic, and optoelectronic properties of TMDs. Understanding the interactions between lanthanide dopants and 2D TMDs host is one of the key problems to be resolved for their profound research studies. Herein, the interactions between Ce dopants and monolayer WS2 in a physical vapor deposition grown Ce‐doped WS2 monolayer are studied by combining scanning tunneling microscopy with optical characterizations with high spatial and temporal resolution. It is found that the highly anisotropic crystal field can effectively split the energy levels of the Ce dopants’ f orbital. The electrons in the split energy levels can bind the holes in the valence band maximum of the Ce‐doped WS2, forming optical bright excitons. These excitons collide with the free A excitons when increasing the pump fluences, reducing the A exciton's lifetime. This study may be beneficial for the design and fabrication of optical, electronic, and optoelectronic devices based on lanthanide‐doped TMDs.

γ-Ray-Induced Surface-Charge Redistribution and Change of the Surface Morphology in Monolayer WS2

γ-Ray-Induced Surface-Charge Redistribution and Change of the Surface Morphology in Monolayer WS₂

Regulation on the electrical and optical properties of two-dimensional stacked graphene quantum dots/gold quantum dots/tungsten sulfide heterostructures

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), like WS2, have been particularly promising in the application of solar energy conversion. Nevertheless, how to enhance the absorption capacity and photogenerated dcharge transfer is yet challenging. Herein, 2D stacked heterostructures were firstly constructed by 2D Au quantum dots (QDs), graphene quantum dots (GQDs) and WS2 monolayer. Compared with WS2 monolayer, the absorption abilities of the Au QDs/WS2 heterostructures in visible-near infrared light region of 450–2300 nm increase evidently. Simultaneously, charge redistribution occurs at the WS2–Au interfaces and generates the built-in electric field with the direction from Au QDs to the WS2 monolayer. This built-in electric field is conducive to boosting photo-induced charge separation and transfer. Notably, more obvious charge transfer and stronger built-in electric field can be achieved in WS2–Au6 interface after introducing the GQDs. Therefore, this work provides unique insights for designing new WS2-based heterostructures to improve the solar energy conversion.