Monolayer WS2 Research Articles

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.

Vapor pressure-controllable molecular inorganic precursors for growth of monolayer WS2: Influence of precursor-substrate interaction on growth thermodynamics

A chemical route for the growth of centimeter-scale and homogeneous WS2 monolayer has been developed using the penta-coordinated inorganic W complex, WOCl4. The relatively low melting point of WOCl4 compared to other conventional W precursors enables the use of WOCl4 as a vapor phase precursor. Consequently, the precise control of partial pressure during chemical vapor deposition (CVD) allows the optimization of growth conditions, synthesizing the entirely covered homogeneous monolayer WS2 film. The contamination of amorphous carbon, which is considered a demerit of metal–organic CVD, is highly reduced because of the carbon-free compositional characteristic of WOCl4. Additionally, the growth behavior of WS2 with respect to the growth parameters is systematically studied by investigating the thermodynamics of the molecular precursors compared with MoS2 growth using MoOCl4. This approach enables us to understand the effect of the growth temperature and partial pressure on the optimum growth conditions of WS2 monolayer.

The adjustable and tunable narrowband optical absorber by insertion of the WSe2 and WS2 monolayer in symmetric and asymmetric defective photonic crystals

Narrowband optical absorption plays a key role in photonic sensors and photovoltaic elements. Here, we propose a design of creating narrowband absorbers based on the symmetric and asymmetric defective photonic crystals (SDPCs and ADPCs) consisting of defects in the form of DMD in which M indicates the WS2 or WSe2 monolayer and D represents the SiO2 layer. In this paper, the effect of the defect layer thickness, polarization, and incident angle of light on the number and wavelength of defect modes in both symmetric and asymmetric structures is investigated. The findings suggest that the wavelength of the defect modes can be adjusted by varying the thickness of layer D as well as the design of the wavelength and tuned by polarization and incident angle of light. In both SDPC and ADPC, the wavelength of the defect mode shows red shift with an increase in the thickness of layer D and exhibits blue shift with an increase in the angle of light in both TE and TM polarization. This narrowband absorber demonstrates great potential for being adopted in the detection of photodetectors, absorbent filters, and the like.

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.

Two-Dimensional Cs2AgBiBr6/WS2 Heterostructure-Based Photodetector with Boosted Detectivity via Interfacial Engineering.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers have been widely used for optoelectronic devices because of their ultrasensitivity to light detection acquired from their direct gap properties. However, the small cross-section of photon absorption in the atomically thin layer thickness significantly limits the generation of photocarriers, restricting their performance. Here, we integrate monolayer WS2 with 2D perovskites Cs2AgBiBr6, which serve as the light absorption layer, to greatly enhance the photosensitivity of WS2. The efficient charge transfer at the Cs2AgBiBr6/WS2 heterojunction is evidenced by the shortened photoluminescence (PL) decay time of Cs2AgBiBr6. Scanning photocurrent microscopy of Cs2AgBiBr6/WS2/graphene reveals that improved charge extraction from graphene leads to an enhanced photoresponse. The 2D Cs2AgBiBr6/WS2/graphene vertical heterostructure photodetector exhibits a high detectivity (D*) of 1.5 × 1013 Jones with a fast response time of 52.3 μs/53.6 μs and an on/off ratio of 1.02 × 104. It is worth noting that this 2D heterostructure photodetector can realize self-powered light detection behavior with an open-circuit voltage of ∼0.75 V. The results suggest that the 2D perovskites can effectively improve the TMDC layer-based photodetectors for low-power consumption photoelectrical applications.

Aggregation-Dependent Dielectric Permittivity in 2D Molecular Crystals.

The functionality of 2D molecular crystal-based devices crucially depends on their intrinsic properties, such as molecular energy levels, light absorption efficiency, and dielectric permittivity, which are highly sensitive to molecular aggregation. Here, it is demonstrated that the dielectric permittivity of the 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8 -BTBT) molecular crystals on monolayer WS2 substrates can be tuned from 4.62 in the wetting layer to 2.25 in the second layer. Its origin lies in the different molecular orientations in the wetting layer (lying-down) and in the subsequently stacked layers (standing-up), which lead to a positive Coulomb coupling (JCoup ) value (H-aggregation) and a negative JCoup value (J-aggregation), respectively. Polarized optical contrast spectroscopy reveals that the permittivity of C8 -BTBT is anisotropic, and its direction is related to the underlying substrate. The study offers guidelines for future manipulation of the permittivity of 2D molecular crystals, which may promote their applications toward various electronic and optoelectronic devices.

Strain-mediated ferromagnetism and low-field magnetic reversal in Co doped monolayer WS_2

Strain-mediated magnetism in 2D materials and dilute magnetic semiconductors hold multi-functional applications for future nano-electronics. Herein, First principles calculations are employed to study the influence of biaxial strain on the magnetic properties of Co-doped monolayer WS_2. The non-magnetic WS_2 shows ferromagnetic signature upon Co doping due to spin polarization, which is further improved at low compressive (-2 %) and tensile (+2 %) strains. From the PDOS and spin density analysis, the opposite magnetic ordering is found to be favourable under the application of compressive and tensile strains. The double exchange interaction and p-d hybridization mechanisms make Co-doped WS_2 a potential host for magnetism. More importantly, the competition between exchange and crystal field splittings, i.e. (Delta _{ex}>Delta _{cfs}), of the Co-atom play pivotal roles in deciding the values of the magnetic moments under applied strain. Micromagnetic simulation reveals, the ferromagnetic behavior calculated from DFT exhibits low-field magnetic reversal (190 Oe). Moreover, the spins of Co-doped WS_2 are slightly tilted from the easy axis orientations showing slanted ferromagnetic hysteresis loop. The ferromagnetic nature of Co-doped WS_2 suppresses beyond pm 2~% strain, which is reflected in terms of decrease in the coercivity in the micromagnetic simulation. The understanding of low-field magnetic reversal and spin orientations in Co-doped WS_2 may pave the way for next-generation spintronics and straintronics applications.

Polarized Raman study of large built-in strain in monolayer WS2 grown on Au/W substrate

Strain plays a crucial role in the energy landscape of atomically thin two-dimensional materials. Here, we report polarized Raman results of WS2 monolayers having either rough or wrinkled surface morphologies. For rough WS2, the E′ phonon intensity exhibits polar behavior that deviates from the Raman polarization selection rules. Furthermore, with the formation of wrinkles on the WS2 surface, the E′ phonon splits into E′− and E′+ phonons. Polar plots of the E′− and E′+ phonon intensities as a function of polarization angle θ show that both phonons have strong polar dependence with orthogonal characteristics in their polarized intensity profiles: the E′− phonon intensity is maximum when the E′+ phonon intensity is diminished, and vice versa. Our result demonstrates that built-in strain in low-dimensional materials can be quantitatively identified by polarized Raman studies and further provides a useful strategy to achieve better device performance for which strain engineering is required.

Enhanced Radiative Exciton Recombination in Monolayer WS2 on the hBN Substrate Competing with Nonradiative Exciton–Exciton Annihilation

Enhanced Radiative Exciton Recombination in Monolayer WS₂ on the hBN Substrate Competing with Nonradiative Exciton–Exciton Annihilation

Ultrafast modulation of valley dynamics in multiple WS2\u2009\u2212\u2009Ag gratings strong coupling system

Strong light-matter interactions in two-dimensional transition metal dichalcogenides (TMDCs) with robust spin-valley degrees of freedom open up the prospect of valleytronic devices. A thorough understanding on the dynamics of the valley polarizations in the strong coupling regime is urgently required. Here, multiple polarized TMDCs-SPPs hybrid systems were constructed by combining monolayer WS2 flakes to linear, circular, and spiral Ag gratings, resulting in linear and circular polarized modulation on the coherent hybrid states, respectively. Particularly, valley polaritons can be tailored asymmetrically by chiral strong coupling regime. Furthermore, the dynamics of the polarized polaritons were directly analyzed by transient absorption (TA) measurement. Both of the linear and circular polarization difference in the TA spectra can be retained for a remarkable long time, leading to a polarized PL even at room temperature. More importantly, in the chiral strong coupled WS2-spiral Ag grating devices, the mechanism of the asymmetrical valley-polarized PL (p σ+ = 14.9% and p σ- = 10.8%) is proved by the opposite valley polarization dynamics in the circularly polarized TA spectra. The multiple polarization modulation in monolayer TMDCs-SPPs strong coupling devices could provide a viable route toward multiple polarization polaritonic devices.

Experimental and theoretical research on CdS nanoparticles embedded in layered WS2 to construct type II heterostructure and improve the performance of photocatalytic degradation of pollutants

Due to its unique layered structure, tungsten disulfide can be used as an excellent co-catalyst in photocatalytic reactions. In this study, the lamellar WS2 was prepared by the liquid-phase exfoliation method, and the lamellar 2D WS2 has a large specific surface area and many active sites to promote the performance of the photocatalyst.The lamellar WS2 was embedded in CdS nanoparticles by hydrothermal synthesis. Compared with single CdS, the degradation efficiency of the obtained WS2/CdS composite material is 1.8 times higher than that of single CdS under visible light irradiation. Structural characterization shows that the enhanced photocatalytic activity of WS2/CdS composites is closely related to the unique structure of the heterojunction structure between WS2 and CdS that accelerates the transfer of photoexcited electrons. The two-dimensional sheet-like WS2 provides more reaction sites for the CdS attached to it, which improves the absorption of visible light and the migration rate of photogenerated carriers. At the same time, based on first-principles calculations, the charge transfer at the heterojunction interface between WS2 and CdS was obtained. The results show that a unique type II heterojunction is formed after the intercalation of lamellar WS2 in CdS nanoparticles due to the increased band gap of monolayer WS2. This unique structure makes WS2/CdS composite material promising to become a highly efficient multi-purpose photocatalyst.

Simultaneous control of plasmon–exciton and plasmon–trion couplings in an Au nanosphere and monolayer WS2 hybrid system

Simultaneous control of plasmon–exciton and plasmon–trion couplings is fundamentally interesting for tailoring the strong light–matter interaction at the nanoscale and is intriguing for developing high-efficiency optoelectronic and nonlinear photonic devices. Here, we integrate the monolayer WS2 with the Au nanosphere to take full advantages of both the strong excitonic effect and local field enhancement effect to realize strong resonance couplings between the dipolar plasmon mode and the exciton, as well as the trion, at room temperature. Interestingly, from the dark-field scattering spectrum, a transition from the dominated plasmon–exciton coupling to the plasmon–exciton–trion coupling in the hybrid system by simply increasing the radius of the nanosphere is revealed. This evolution of the scattering spectrum is further analyzed using the coupled-oscillator model to extract Rabi splittings of 89 and 48 meV for plasmon–exciton and plasmon–trion couplings, implying that the hybrid system enters the moderate coupling region. The moderate coupling imparts the hybrid system with a remarkable light-emitting capacity, rendering 1265- and 680-fold photoluminescence (PL) enhancement for the exciton and trion emissions, respectively. Our findings provide a facile way for the manipulation of excitonic quasiparticles in semiconductors at room temperature.

Deterministic and Scalable Generation of Exciton Emitters in 2D Semiconductor Nanodisks

AbstractIn recent cryogenic measurements, narrow photoluminescence (PL) peaks due to diverse quantum emitters have been found at random locations of monolayer transition metal dichalcogenides (TMDs), which impedes precise optoelectronic applications. Thus, it is of great importance to truly regulate these localized exciton emissions by deterministic spatial and spectral control. Here, such desired emission is primarily demonstrated in monolayer WS2 nanodisks. The size‐dependent PL studies indicate the clear evolution from the broad defect‐band emission to a set of spectrally isolated narrow peaks (linewidth of ≈ sub nm) at 4.2 K, which is associated with the prevailing effect of edge defects with the shrinkage of the disk diameter, providing a narrow emission energy range for bound excitons. When the disk diameter is reduced to 300 nm, more than 80% of emitter peaks are located between 610 and 616 nm, verifying the effective control of emission wavelength of these photon emitters. Furthermore, the strategy is extended to prepare scalable WS2 nanodisk arrays based on flakes of hundreds of µm, and size‐dependent narrow emissions of WSe2 nanodisks are testified. This work develops a defect‐engineering strategy to generate localized exciton emitters toward the promising TMD‐based optoelectronic applications.

Growth mode control of CVD-grown WS2 monolayer flakes via O2 pre-annealing for organic surfactant oxidation

Chemical vapor deposition (CVD) using liquid solution precursors is a promising method for synthesizing uniform and large-area monolayers of transition metal dichalcogenides. Organic surfactants, such as iodixanol, are crucial additives for the uniform coating of substrates with liquid solution precursors. However, during the heating process of CVD, carbons from the incompletely decomposed organic surfactant suppress crystal growth unfavorably. Here, we investigate the role of such residual carbons in the crystal growth behavior of WS2 monolayers. The crystal growth modes were precisely manipulated by residual carbon oxidation and the precursor etching rate, which was implemented by introducing O2 pre-annealing into the CVD process. Without O2 pre-annealing, flakes of limited size and multilayers were synthesized owing to the carbonization of surfactant, whereas sufficiently oxidized residual carbons via O2 pre-annealing allow for crystal growth with a large monolayer size. Moreover, the growth modes of the flakes in terms of triangles and hexagons can be systematically controlled by the O2 amount for pre-annealing because the remnant O2 compensates for the role of H2, which manipulates the etching rate of the residual W precursors on the substrate. Our results pave the way for controlling the growth modes of CVD-grown WS2 by introducing an optimized O2 pre-annealing process.

Characterization of high quality, monolayer WS2 domains via chemical vapor deposition technique

Characterization of high quality, monolayer WS2 domains via chemical vapor deposition technique

Controlling exciton distribution in WS2 monolayer on a photonic crystal

Transition metal dichalcogenides monolayers are promising candidates for novel optoelectronic devices because they exhibit a unique combination of atomic-scale thickness, direct band gap, high quantum yield and ease of integration properties, which make them intriguing for fundamental studies and applications. In this work, we manipulate the exciton distribution in the WS2 monolayer integrated with a photonic crystal at room temperature. By coupling with the optical modes of the photonic crystal, the excitons can distribute along a particular direction by around ∼10 μm. More importantly, the excitons distribute along the particular direction with locked linear polarization, the degree of polarization up to 60%. Our results pave the way to manipulate the polarization distribution and exciton distribution in the WS2 monolayer.

Direct measurement of biexcitons in monolayer WS2

The optical properties of atomically thin transition metal dichalcogenides are dominated by Coulomb bound quasi-particles, such as excitons, trions, and biexcitons. Due to the number and density of possible states, attributing different spectral peaks to the specific origin can be difficult. In particular, there has been much conjecture around the presence, binding energy and/or nature of biexcitons in these materials. In this work, we remove any ambiguity in identifying and separating the optically excited biexciton in monolayer WS2 using two-quantum multidimensional coherent spectroscopy (2Q-MDCS), a technique that directly and selectively probes doubly-excited states, such as biexcitons. The energy difference between the unbound two-exciton state and the biexciton is the fundamental definition of biexciton binding energy and is measured to be 26 ± 2 meV. Furthermore, resolving the biexciton peaks in 2Q-MDCS allows us to identify that the biexciton observed here is composed of two bright excitons in opposite valleys.

Inhomogeneous defect distribution of triangular WS2 monolayer revealed by surface-enhanced and tip-enhanced Raman and photoluminescence spectroscopy

Confocal optical microscopy and tip-enhanced optical microscopy are applied to characterize the defect distributions in chemical vapor deposition-grown WS2 monolayer triangles qualitatively and quantitatively. The presence of defects in individual monolayer WS2 triangles is revealed with diffraction-limited spatial resolution in their photoluminescence (PL) images, from which the inhomogeneous defect density distribution is calculated, showing an inverse relationship to the PL intensity. The defect-related surface-enhanced Raman spectroscopy (SERS) effect is investigated by depositing a thin copper phthalocyanine layer (5nm) as the probe molecule on the monolayer WS2 triangles surface. Higher SERS enhancement effects are observed at the defect-rich areas. Furthermore, tip-enhanced optical measurements are performed, which can reveal morphologically defected areas invisible in the confocal optical measurements. Furthermore, the area with high defect density appears brighter than the low-defected area in the tip-enhanced optical measurements, which are different from the observation in the confocal optical measurements. The underlying reasons are attributed to the near-field enhancement of the defect exciton emission induced by the optically excited tip and to an improved coupling efficiency between the tip-generated near-field with the altered dipole moment orientation at the local defect.

Enhanced Valley Polarization in WS2/LaMnO3 Heterostructure

AbstractMonolayer transition metal dichalcogenides have attracted great attention for potential applications in valleytronics. However, the valley polarization degree is usually not high because of the intervalley scattering. Here, a largely enhanced valley polarization up to 80% in monolayer WS2 under nonresonant excitation at 4.2 K is demonstrated using WS2/LaMnO3 thin film heterostructure, which is much higher than that for monolayer WS2 on SiO2/Si substrate with a valley polarization of 15%. Furthermore, the greatly enhanced valley polarization can be maintained to a high temperature of about 160 K with a valley polarization of 53%. The temperature dependence of valley polarization is strongly correlated with the thermomagnetic curve of LaMnO3, indicating an exciton–magnon coupling between WS2 and LaMnO3. A simple model is introduced to illustrate the underlying mechanisms. The coupling of WS2 and LaMnO3 is further confirmed with an observation of two interlayer excitons with opposite valley polarizations in the heterostructure, resulting from the spin‐orbit coupling induced splitting of the conduction bands in monolayer transition metal dichalcogenides. The results provide a pathway to control the valleytronic properties of transition metal dichalcogenides by means of ferromagnetic van der Waals engineering, paving a way to practical valleytronic applications.

Revealing the Competition between Defect‐Trapped Exciton and Band‐Edge Exciton Photoluminescence in Monolayer Hexagonal WS2

AbstractMonolayer transition‐metal dichalcogenides grown by chemical vapor deposition (CVD) always contain certain types of defects that dramatically affect their electronic and optical properties. For CVD‐grown hexagonal WS2 monolayer, complex photoluminescence (PL) patterns are commonly observed, but the defect‐related optical mechanisms are still not well understood. Here, by combining the optical and structural characterizations and ab initio calculations, the correlation between the patterned PL emission and the details of defects in CVD‐grown hexagonal WS2 monolayer are revealed. The temperature‐dependent PL spectra show the correlation between the defect‐trapped and band‐edge exciton emission. The high‐resolution scanning transmission electron microscopy identifies the positive correlation between the density of WSx‐vacancy and PL intensity. In the end, the ab initio calculations and molecule adsorption PL spectra show that the coexistence of p‐ and n‐doping effects, caused by the W and S complex vacancy, weakens the modulation of molecular adsorption on PL intensity. This work gives new insights into the origin of the inhomogeneous PL distribution in WS2 monolayer, which provides important guidance in the regulation of electronic and optical properties of transition‐metal dichalcogenides via defect engineering.