Monolayer Tungsten Disulfide Research Articles

Salt-promoted growth of monolayer tungsten disulfide on hexagonal boron nitride using all chemical vapor deposition approach

Monolayer tungsten disulfide (WS2), an emerging two-dimensional (2D) direct-bandgap semiconductor, provides new prospects for next-generation electronic, optoelectronic and valleytronic devices. However, the physical properties of 2D WS2, are seriously perturbed by various extrinsic disorders to a large extent. Despite encapsulating exfoliated WS2 with hexagonal boron nitride (h-BN) flakes can effectively screen the surrounding disorders, its scalability is not applicable to large-area electronics. In this work, the construction of monolayer WS2 on h-BN is achieved with an all chemical vapor deposition (CVD) approach by using alkali metal salt to promote the growth of WS2. Detailed characterizations indicate that WS2 deposited on h-BN exhibits improved optical and electrical properties, as compared to those on SiO2/Si substrates. Furthermore, the influence of depositing WS2 on h-BN and salt-promoted growth behavior of WS2 is systematically investigated, highlighting the important role of salt in constructing the heterobilayers. This work paves a practical way for synthesis of 2D materials on h-BN to further probe their intrinsic properties and enable a plethora of potential applications.

Spatial variation in nanoscale wear behavior of chemical vapor deposited monolayer WS2

Two-dimensional (2D) materials are potential solid lubricants for a wide range of tribological applications. However, such atomically thin lubricants require detailed understanding of mechanisms underlying their tribological characteristics. In the present study, the nanoscale wear behavior of chemical vapor deposition (CVD) grown monolayer tungsten disulfide (WS2) was investigated using a combination of atomic force microscopy (AFM) and molecular dynamics (MD) simulations. The critical load to initiate wear in the interior region of the WS2 monolayer was found to be higher than the regions near the edge. Experimental findings also reveal significant variability in critical load values in both regions. The observed difference in wear behavior can be attributed to the presence of structural defects as elucidated from MD simulations. Our combined experimental and simulation study provides fundamental insights into the atomistic wear mechanism of monolayer WS2.

Optical Introduction and Manipulation of Plasmon–Exciton–Trion Coupling in a Si/WS2/Au Nanocavity

Strong plasmon-exciton coupling, which has potential applications in nanophotonics, plasmonics, and quantum electrodynamics, has been successfully demonstrated by using metallic nanocavities and two-dimensional materials. Dynamical control of plasmon-exciton coupling strength, especially by using optical methods, remains a big challenge although it is highly desirable. Here, we report the optical introduction and manipulation of plasmon-exciton-trion coupling realized in a dielectric-metal hybrid nanocavity, which is composed of a silicon (Si) nanoparticle and a thin gold (Au) film, with an embedded tungsten disulfide (WS2) monolayer. We employ scattering and photoluminescence spectra to characterize the coupling strength between plasmons and excitons in Si/WS2/Au nanocavities constructed by using Si nanoparticles with different diameters. We enhance the plasmon-exciton and plasmon-trion coupling strength by injecting excitons and trions into the WS2 monolayer with a 488 nm laser beam. It is revealed that the emission intensities of excitons and trions with respect to the reference WS2 monolayer can be modified through the change in the coupling strength induced by the laser light. Interestingly, the coupling strength between the plasmons and the excitons/trions can be manipulated from weak to strong coupling regime by simply increasing the laser power, which is clearly resolved in the scattering spectra of Si/WS2/Au nanocavities. More importantly, the plasmon-exciton-trion coupling induced by the laser light is confirmed by the energy exchange between excitons and trions. Our findings indicate the possibility for optically manipulating plasmon-exciton interaction and suggest the practical applications of dielectric-metal hybrid nanocavities in nanoscale plasmonic devices.

Electrical modulation of excitonic transition in monolayer tungsten disulfide on periodically poled ferroelectric substrates

Although the spectral investigation of monolayer two-dimensional transition metal dichalcogenide (1L-TMD) has been extensively performed for identifying modulation of excitonic transition, study of excitonic transition in ferroelectric/TMD hybrid structure is essential to understand and design high speed and low loss devices based on in-plane junctions. Here, we demonstrate electrical modulation of excitonic transition by transferring 1L-tungsten disulfide (WS2) onto a ferroelectric domain surface, a periodically poled lithium niobate (PPLN). We observe that exciton (A0) and trion (A−) emissions of 1L-WS2 are spatially modulated depending on the sign of the phase domain, such that A0 and A− emission is stronger at the positive (D+) and negative (D−) domain, respectively. In addition, different trends of excitonic transition of 1L-WS2 between D+ and D− domain are demonstrated by applying a back-gate bias voltage. The spatial modulation of electrical and optical characteristics of 1L-TMDs will help to design the homo- or hetero p-n junctions based on TMDs.

Electroluminescence of monolayer WS2 in a scanning tunneling microscope: Effect of bias polarity on spectral and angular distribution of emitted light

Inelastic electron tunneling in a scanning tunneling microscope is used to generate excitons in monolayer tungsten disulfide (${\mathrm{WS}}_{2}$). Excitonic electroluminescence is measured both at positive and negative sample bias. Using optical spectroscopy and Fourier-space optical microscopy, we show that the bias polarity of the tunnel junction determines the spectral and angular distribution of the emitted light. At positive sample bias, only emission from excitonic species featuring an in-plane transition dipole moment is detected. Based on the spectral distribution of the emitted light, we infer that the dominant contribution is from charged excitons, i.e., trions. At negative sample bias, additional contributions from lower-energy excitonic species are evidenced in the emission spectra and the angular distribution of the emitted light reveals a mixed character of in-plane and out-of-plane transition dipole moments.

Fabrication of near-invisible solar cell with monolayer WS2

Herein, we developed a near-invisible solar cell through a precise control of the contact barrier between an indium tin oxide (ITO) electrode and a monolayer tungsten disulfide (WS2), grown by chemical vapor deposition (CVD). The contact barrier between WS2 and ITO was controlled by coating various thin metals on top of ITO (Mx/ITO) and inserting a thin layer of WO3 between Mx/ITO and the monolayer WS2, which resulted in a drastic increase in the Schottky barrier height (up to 220 meV); this could increase the efficiency of the charge carrier separation in our Schottky-type solar cell. The power conversion efficiency (PCE) of the solar cell with the optimized electrode (WO3/Mx/ITO) was more than 1000 times that of a device using a normal ITO electrode. Large-scale fabrication of the solar cell was also investigated, which revealed that a simple size expansion with large WS2 crystals and parallel long electrodes could not improve the total power (PT) obtained from the complete device even with an increase in the device area; this can be explained by the percolation theory. This problem was addressed by reducing the aspect ratio (width/channel length) of the unit device structure to a value lower than a critical threshold. By repeating the experiments on this optimized unit device with an appropriate number of series and parallel connections, PT could be increased up to 420 pW from a 1-cm2 solar cell with a very high value (79%) of average visible transmission (AVT).

Directly Visualizing Photoinduced Renormalized Momentum-Forbidden Electronic Quantum States in an Atomically Thin Semiconductor.

Resolving the momentum degree of freedom of photoexcited charge carriers and exploring the excited-state physics in the hexagonal Brillouin zone of atomically thin semiconductors have recently attracted great interest for optoelectronic technologies. We demonstrate a combination of light-modulated scanning tunneling microscopy and the quasiparticle interference (QPI) technique to offer a directly accessible approach to reveal and quantify the unexplored momentum-forbidden electronic quantum states in transition metal dichalcogenide (TMD) monolayers. Our QPI results affirm the large spin-splitting energy at the spin-valley-coupled Q valleys in the conduction band (CB) of a tungsten disulfide monolayer. Furthermore, we also quantify the photoexcited carrier density-dependent band renormalization at the Q valleys. Our findings directly highlight the importance of the excited-state distribution at the Q valley in the band renormalization in TMDs and support the critical role of the CB Q valley in engineering the quantum electronic valley degree of freedom in TMD devices.

Optical nonlinearity goes ultrafast in 2D semiconductor-based nanocavities.

Hybrid systems of silver nanodisks strongly coupled to monolayer tungsten-disulfide (WS2) show giant room-temperature nonlinearity due to their deeply sub-wavelength localized nature, resulting in ultrafast modifications of nonlinear absorption in a solid-state system.

Etching-free high-throughput intersectional nanofabrication of diverse optical nanoantennas for nanoscale light manipulation

The capabilities to manipulate light-matter interaction at the nanoscale lie at the core of many promising photonic applications. Optical nanoantennas, made of metallic or dielectric materials, have seen a rapid development for their remarkable optical properties facilitating the coupling of electromagnetic waves with subwavelength entities. However, high-throughput and cost-effective fabrication of these nanoantennas is still a daunting challenge. In this work, we provide a versatile nanofabrication method capable of producing large scale optical nanoantennas with different shapes. It is developed from colloidal lithography with no dry etching required. Furthermore, both metallic and all-dielectric nanoantennas can be readily fabrication in a high-throughput fashion. Au and Si nanodisks were fabricated and employed to assemble heterostructures with monolayer tungsten disulfide. Strong coupling is observed in both systems between plasmon modes (Au nanodisks) or anapole modes (Si nanodisks) with excitons. We believe that this nanofabrication method could find a wide range of applications with the diverse optical nanoantennas it can engineer.

Interacting plexcitons for designed ultrafast optical nonlinearity in a monolayer semiconductor

Searching for ideal materials with strong effective optical nonlinear responses is a long-term task enabling remarkable breakthroughs in contemporary quantum and nonlinear optics. Polaritons, hybridized light-matter quasiparticles, are an appealing candidate to realize such nonlinearities. Here, we explore a class of peculiar polaritons, named plasmon–exciton polaritons (plexcitons), in a hybrid system composed of silver nanodisk arrays and monolayer tungsten-disulfide (WS2), which shows giant room-temperature nonlinearity due to their deep-subwavelength localized nature. Specifically, comprehensive ultrafast pump–probe measurements reveal that plexciton nonlinearity is dominated by the saturation and higher-order excitation-induced dephasing interactions, rather than the well-known exchange interaction in traditional microcavity polaritons. Furthermore, we demonstrate this giant nonlinearity can be exploited to manipulate the ultrafast nonlinear absorption properties of the solid-state system. Our findings suggest that plexcitons are intrinsically strongly interacting, thereby pioneering new horizons for practical implementations such as energy-efficient ultrafast all-optical switching and information processing.

Ultrafast Pump–Probe Microscopy on 2D Transition Metal Dichalcogenides

Although microscopic techniques have been used to characterize transition metal dichalcogenides (TMDs), direct observation of charge carrier dynamics distribution in TMDs with diverse shapes remains unexplored. Herein, ultrafast pump–probe microscopy (UPPM) is employed to reveal the carrier dynamics distribution in molybdenum disulfide (MoS2) and tungsten disulfide (WS2) monolayer of four shapes: triangular (t‐MoS2), curved triangular (c‐MoS2), triangular (t‐WS2), and hexagonal (h‐WS2). Monitoring the photon transmission T at 1.55 eV after pumping with a photon energy of 3.1 eV, a negative ΔT/T occurs in t‐MoS2 and c‐MoS2, while a positive ΔT/T is detected in t‐WS2 and h‐WS2 after 3–7 ps time evolution. This distinctive behavior is attributed to deep/shallow defects below the conduction band minimum (CBM) in MoS2 and WS2. Spatial‐independent ΔT/T is observed in t‐MoS2 and t‐WS2, while the ΔT/T in c‐MoS2 has a rapid decay of photoexcited carriers at the vertices and curved edges. Additionally, a threefold symmetry of ΔT/T is revealed in h‐WS2, attributed to the dissimilar occupation of defect states near the h‐WS2 CBM. This work paves the way for examining charge carrier dynamics of various shapes of TMDs and provides a unique microscopic method for studying the charge carrier dynamics in emerging TMDs heterostructures.

Control of trion-to-exciton conversion in monolayer WS2 by orbital angular momentum of light.

Controlling the density of exciton and trion quasiparticles in monolayer two-dimensional (2D) materials at room temperature by nondestructive techniques is highly desired for the development of future optoelectronic devices. Here, the effects of different orbital angular momentum (OAM) lights on monolayer tungsten disulfide at both room temperature and low temperatures are investigated, which reveal simultaneously enhanced exciton intensity and suppressed trion intensity in the photoluminescence spectra with increasing topological charge of the OAM light. In addition, the trion-to-exciton conversion efficiency is found to increase rapidly with the OAM light at low laser power and decrease with increasing power. Moreover, the trion binding energy and the concentration of unbound electrons are estimated, which shed light on how these quantities depend on OAM. A phenomenological model is proposed to account for the experimental data. These findings pave a way toward manipulating the exciton emission in 2D materials with OAM light for optoelectronic applications.

Band tuning in WS2 monolayer via substitutional doping

First-principles calculations are performed to investigate the band tuning (band structure, band gap and spin splitting) in tungsten disulfide (WS2) monolayer due to nitrogen and phosphorous substitutional doping with various concentrations. Our calculations show doping affects all the lattice parameters and reduces the symmetry point group from P 6¯ m2 to P3m1. Both nitrogen and phosphorus doping reduce the band gap, in some cases, transfer direct one to indirect and lead to apparent changes in the band structure. However, the changing trend for nitrogen occurs rapidly than that for phosphorus doping. Increasing dopant concentration decreases the spin splitting in the valence band and, contrary increases it remarkably in the conduction band almost in all concentrations, showing doping concentration can affect the spin-orbit coupling and hence the spin splitting in both valence and conduction band. This study clearly shows that dopant concentration can significantly tune the electronic properties of WS2 monolayer and provide a new way for tuning the spin splitting, which is essential in optoelectronic and spintronic devices.

Monolayer tungsten disulfide in photonic environment: Angle-resolved weak and strong light-matter coupling

Monolayer tungsten disulfide in photonic environment: Angle-resolved weak and strong light-matter coupling

Directing monolayer tungsten disulfide photoluminescence using a bent-plasmonic nanowire on a mirror cavity

Designing directional optical antennas without compromising the field enhancement requires specially designed optical cavities. Herein, we report on the experimental observations of directional photoluminescence emission from a monolayer tungsten disulfide using a bent-plasmonic nanowire on a mirror cavity. The geometry provides field enhancement and directivity to photoluminescence by sandwiching the monolayer between an extended cavity formed by dropcasting bent-silver nanowire on a gold mirror. We image the photoluminescence emission wavevectors using Fourier plane imaging technique. The cavity outcouples the emission in a narrow range of wavevectors with a radial and azimuthal spreading of only 11.0 $$^{\circ }$$ and 25.1 $$^{\circ }$$ , respectively. Furthermore, we performed three-dimensional finite difference time domain-based numerical calculations to corroborate and understand the experimental results. We envisage that the results presented here will be readily harnessed for on-chip coupling applications and in designing inelastic optical antennas.

Top-gate field-effect transistor based on monolayer WS2 with an ion-gel gate dielectric

A top-gate field-effect transistor (FET), based on monolayer (ML) tungsten disulfide (WS2), and with an ion-gel dielectric was developed. The high electrical contact resistance of the Schottky contacts at the n-type transition metal dichalcogenides/metal electrode interfaces often adversely affects the device performance. We report the contact resistance and Schottky barrier height of an FET with Au electrodes. The FET is based on ML WS2 that was synthesized using chemical vapour deposition and was assessed using the transfer-length method and low-temperature measurements. Raman and photoluminescence spectra were recorded to determine the optical properties of the WS2 layers. The ML WS2 FET with an ion-gel top gate dielectric exhibits n-type behaviour, with a mobility, on/off ratio of 1.97 cm2 V−1·s−1, 1.51 × 105, respectively.

Fe‐ and Co‐Doped Tungsten Disulfide Monolayers: 2D Ferromagnetic Half‐Metals at Room Temperature

2D ferromagnetic (FM) materials are rare at room temperature. In situ substitution of magnetic atoms may be a promising method to induce magnetism in nonmagnetic materials. Herein, the magnetism in Fe‐ and Co‐doped WS2 (Fe:WS2 and Co:WS2 ) monolayers by using density functional theory is studied. The Fe and Co atoms substitute the W atoms with a relatively large doping concentration (16.7%), respectively. Both doped monolayers are FM half‐metals with the out‐of‐plane easy magnetization axis. The estimated Curie temperature can be up to about 540 K in Fe:WS2 and 370 K in Co:WS2. Moreover, the magnetic anisotropic energy in Fe:WS2 can be larger than 16 meV cell−1 under −4% compressive strain. Although compressive strain leads to a higher Curie temperature in Co:WS2, it slightly reduces the Curie temperature in Fe:WS2 due to the enhanced anti‐FM coupling of next nearest‐neighboring exchange interaction. It is suggested by the calculations that both Fe:WS2 and Co:WS2 monolayers are 2D FM half‐metals at room temperature, which may be promising candidates in nanoscale spintronic devices.

Laser-Patterned Submicrometer Bi2Se3-WS2 Pixels with Tunable Circular Polarization at Room Temperature.

Characterizing and manipulating the circular polarization of light is central to numerous emerging technologies, including spintronics and quantum computing. Separately, monolayer tungsten disulfide (WS2) is a versatile material that has demonstrated promise in a variety of applications, including single photon emitters and valleytronics. Here, we demonstrate a method to tune the photoluminescence (PL) intensity (factor of ×161), peak position (38.4 meV range), circular polarization (39.4% range), and valley polarization of a Bi2Se3-WS2 2D heterostructure using a low-power laser (0.762 μW) in ambient conditions. Changes are spatially confined to the laser spot, enabling submicrometer (814 nm) features, and are long-term stable (>334 days). PL and valley polarization changes can be controllably reversed through laser exposure in a vacuum, allowing the material to be erased and reused. Atmospheric experiments and first-principles calculations indicate oxygen diffusion modulates the exciton radiative vs nonradiative recombination pathways, where oxygen absorption leads to brightening and desorption to darkening.

Greatly Enhanced Plasmon-Exciton Coupling in Si/WS2/Au Nanocavities.

A strong light-matter interaction is highly desirable from the viewpoint of both fundamental research and practical application. Here, we propose a dielectric-metal hybrid nanocavity composed of a silicon (Si) nanoparticle and a thin gold (Au) film and investigate numerically and experimentally the coupling between the plasmons supported by the nanocavity and the excitons in an embedded tungsten disulfide (WS2) monolayer. When a Si/WS2/Au nanocavity is excited by the surface plasmon polariton generated on the surface of the Au film, greatly enhanced plasmon-exciton coupling originating from the hybridization of the surface plasmon polariton, the mirror-image-induced magnetic dipole, and the exciton modes is clearly revealed in the angle- or size-resolved scattering spectra. A Rabi splitting as large as ∼240 meV is extracted by fitting the experimental data with a coupled harmonic oscillator model containing three oscillators. Our findings open new horizons for constructing nanoscale photonic devices by exploiting dielectric-metal hybrid nanocavities.

Non-classical nucleation in vapor\u2013liquid\u2013solid growth of monolayer WS2 revealed by in-situ monitoring chemical vapor deposition

The very early nucleation stage of a transition metal dichalcogenide (TMD) was directly observed with in-situ monitoring of chemical vapor deposition and automated image analysis. Unique nucleation dynamics, such as very large critical nuclei and slow to rapid growth transitions, were observed during the vapor–liquid–solid (VLS) growth of monolayer tungsten disulfide (WS2). This can be explained by two-step nucleation, also known as non-classical nucleation, in which metastable clusters are formed through the aggregation of droplets. Subsequently, nucleation of solid WS2 takes place inside the metastable cluster. Furthermore, the detailed nucleation dynamics was systematically investigated from a thermodynamic point of view, revealing that the incubation time of metastable cluster formation follows the traditional time–temperature transformation diagram. Quantitative phase field simulation, combined with Bayesian inference, was conducted to extract quantitative information on the growth dynamics and crystal anisotropy from in-situ images. A clear transition in growth dynamics and crystal anisotropy between the slow and rapid growth phases was quantitatively verified. This observation supports the existence of two-step nucleation in the VLS growth of WS2. Such detailed understanding of TMD nucleation dynamics can be useful for achieving perfect structure control of TMDs.