Monolayer Tungsten Disulfide Research Articles

Fifth-order optical nonlinear response of semiconducting 2D LTMD MoS2.

The effective fifth-order susceptibility, ${\chi}_{\rm eff}^{(5)}$, of two-dimensional (2D) semiconducting layered transition metal dichalcogenide (LTMD) molybdenum disulfide (${\rm MoS}_2$) is reported here for the first time, to the best of our knowledge. Using the $ Z $-scan technique with a laser operating at 800 nm, 1 kHz, 100 fs, we investigated the nonlinear behavior of ${\rm MoS}_2$ suspended in acetonitrile (concentration, 70 µg/ml). The effective nonlinear refractive index ${{n}_{4,{eff}}} = - ({7.6 \pm 0.5}) \times {10^{- 26}}\; {{\rm cm}^4}/{{\rm W}^2}$, proportional to ${\rm Re}{\chi}_{\rm eff}^{(5)}$, was measured for monolayer ${\rm MoS}_2$ nanoflakes, prepared by a modified redox exfoliation method. We also determined the value of the nonlinear refractive index ${{n}_2} = + ({4.8 \pm 0.5}) \times {10^{- 16}}\;{{\rm cm}^2}/{\rm W}$, which is related to the material's effective third-order optical susceptibility real part, ${Re\chi}_{\rm eff}^{(3)}$. For comparison, we also investigated the nonlinear response of tungsten disulfide (${\rm WS}_2$) monolayers, prepared by the same method and suspended in acetonitrile (concentration, 40 µg/ml), which only exhibited the third-order nonlinear effect in the same intensity range, up to ${120}\;{{{\rm GW}/{\rm cm}}^2}$. Nonlinear absorption was not observed in either ${\rm MoS}_2$ or ${\rm WS}_2$.

Modelling the optical control of electron spin dynamics in a quantum dot near a two-dimensional semiconductor

A quantum dot in the Voigt configuration is a basic system with important potential applications in quantum technologies. The spin states of this quantum dot can act as a prototype qubit. By applying laser pulses the dynamics of the quantum dot spin can be controlled and the necessary quantum gates can be achieved. An important problem is the initialization, i.e., the creation of one of the two electron spin states starting from the natural initial state of the system, which is an equal incoherent mixture of the two spin states. The initialization process can be achieved by proper interaction of the quantum dot with laser pulses. Also, it has been realized that the integration of the quantum dot with photonic structures that give preferential Purcell-enhanced decay rate towards the target spin state increases the fidelity of spin initialization. Here, we propose a new coupled quantum dot - nanophotonic structure that may give high initialization fidelity in short times by coupling the quantum dot with a tungsten disulfide (WS2) monolayer. For the modelling of the spin dynamics we combine quantum dynamics calculations with electromagnetic calculations. Specifically, we model the interaction of the quantum dot with the applied laser field with density matrix equations. Also, the spontaneous decay rates that enter in the density matrix equations are obtained by electromagnetic calculations based on the electromagnetic Green’s tensor, which is calculated with the scattering superposition method. We first show that the spontaneous decay rates for the quantum dot near a WS2 monolayer are enhanced by the Purcell effect and are anisotropic for quantum dot dipole moments parallel and perpendicular to the layer. We then use the most common method for initialization, optical pumping and show that a preferential Purcell-enhanced decay rate towards the target spin state increases the fidelity of spin initialization in short times, in comparison to the case that the quantum dot is placed in an isotropic photonic environment.

Synergistic vacancy defects and mechanical strain for the modulation of the mechanical, electronic and optical properties of monolayer tungsten disulfide.

Monolayer transition metal dichalcogenides (TMDs) are the potential candidate materials in nanoelectronic and optoelectronic applications due to their unique physical and chemical properties. Although both defect and strain greatly alter the structural, physical and chemical properties of monolayer TMDs, the defective monolayer TMDs under applied strain have not been adequately studied. In this paper, the synergistic effects of sulfur vacancy defects and mechanical strain on the mechanical, electronic and optical properties of monolayer tungsten disulfide (WS2) have been systematically studied using first principles density functional theory. The results indicate that the sulfur vacancy formation energy increases linearly with increasing sulfur vacancy concentration under different strains. The strain energy and stress of monolayer WS2 with different sulfur vacancy concentrations increase with increasing applied strain in the strain range of -10% to 10%. The band gap of monolayer WS2 decreases with increasing sulfur vacancy concentration under different strains. Moreover, compared with unstrained conditions, 5% compressive strain increases the band gap at a larger vacancy concentration and the case is just opposite at a smaller vacancy concentration, while 5% tensile strain decreases the band gap. The band gap of monolayer WS2 with different sulfur vacancy concentrations firstly increases and then shrinks with increasing applied strain under compressive strain, whereas it decreases monotonically under tensile strain in the strain range of -10% to 10%. In the visible-light wavelength region, the out-of-plane absorption coefficient under different strains increases with increasing sulfur vacancy concentration. Furthermore, 5% compressive strain enhances the absorption coefficient and 5% tensile strain decreases the absorption coefficient. Hence, the synergistic effects of sulfur vacancy defects and mechanical strain in monolayer TMDs can open new avenues for their applications in nanoelectronic and optoelectronic devices.

Strain-dependent exciton diffusion in transition metal dichalcogenides

Monolayers of transition metal dichalcogenides have a remarkable excitonic landscape with deeply bound bright and dark exciton states. Their properties are strongly affected by lattice distortions that can be created in a controlled way via strain. Here, we perform a joint theory-experiment study investigating exciton diffusion in strained tungsten disulfide (WS2) monolayers. We reveal a non-trivial and non-monotonic influence of strain. Lattice deformations give rise to different energy shifts for bright and dark excitons changing the excitonic landscape, the efficiency of intervalley scattering channels and the weight of single exciton species to the overall exciton diffusion. We predict a minimal diffusion coefficient in unstrained WS2 followed by a steep speed-up by a factor of 3 for tensile biaxial strain at about 0.6% strain—in excellent agreement with our experiments. The obtained microscopic insights on the impact of strain on exciton diffusion are applicable to a broad class of multi-valley 2D materials.

Strong exciton-photon coupling in large area MoSe2 and WSe2 heterostructures fabricated from two-dimensional materials grown by chemical vapor deposition

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities composed of large area transition metal dichalcogenide/hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide/hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices.

Monolayer Vanadium-Doped Tungsten Disulfide: A Room-Temperature Dilute Magnetic Semiconductor.

Dilute magnetic semiconductors (DMS), achieved through substitutional doping of spin‐polarized transition metals into semiconducting systems, enable experimental modulation of spin dynamics in ways that hold great promise for novel magneto–electric or magneto–optical devices, especially for two‐dimensional (2D) systems such as transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom. Practical applications of 2D magnetism will likely require room‐temperature operation, air stability, and (for magnetic semiconductors) the ability to achieve optimal doping levels without dopant aggregation. Here, room‐temperature ferromagnetic order obtained in semiconducting vanadium‐doped tungsten disulfide monolayers produced by a reliable single‐step film sulfidation method across an exceptionally wide range of vanadium concentrations, up to 12 at% with minimal dopant aggregation, is described. These monolayers develop p‐type transport as a function of vanadium incorporation and rapidly reach ambipolarity. Ferromagnetism peaks at an intermediate vanadium concentration of ~2 at% and decreases for higher concentrations, which is consistent with quenching due to orbital hybridization at closer vanadium–vanadium spacings, as supported by transmission electron microscopy, magnetometry, and first‐principles calculations. Room‐temperature 2D‐DMS provide a new component to expand the functional scope of van der Waals heterostructures and bring semiconducting magnetic 2D heterostructures into the realm of practical application.

Polaritons with a twist

Topological Systems The ability to design and fabricate optical systems with tunable topological features makes them especially attractive for developing analogs of topological condensed matter systems, which by themselves tend to be fixed or limited in their tunability. Liu et al. now show that the combination of a two-dimensional material with a photonic crystal can be used to develop an analogous quantum spin Hall system. The strong coupling between the monolayer tungsten disulfide excitons with a nontrivial hexagonal photonic crystal gives rise to helical topological polaritons observed at up to 200 kelvin. The topological polaritons can be actively tuned by temperature and may further be manipulated with electric or magnetic fields, thereby providing a flexible platform with which to explore exotic topological phenomena and new phases of quantum matter. Science , this issue p. [600][1] [1]: /lookup/doi/10.1126/science.abc4975

Directed Energy Transfer from Monolayer WS2 to Near-Infrared Emitting PbS-CdS Quantum Dots.

Heterostructures of two-dimensional (2D) transition metal dichalcogenides (TMDs) and inorganic semiconducting zero-dimensional (0D) quantum dots (QDs) offer useful charge and energy transfer pathways, which could form the basis of future optoelectronic devices. To date, most have focused on charge transfer and energy transfer from QDs to TMDs, that is, from 0D to 2D. Here, we present a study of the energy transfer process from a 2D to 0D material, specifically exploring energy transfer from monolayer tungsten disulfide (WS2) to near-infrared emitting lead sulfide–cadmium sulfide (PbS–CdS) QDs. The high absorption cross section of WS2 in the visible region combined with the potentially high photoluminescence (PL) efficiency of PbS QD systems makes this an interesting donor–acceptor system that can effectively use the WS2 as an antenna and the QD as a tunable emitter, in this case, downshifting the emission energy over hundreds of millielectron volts. We study the energy transfer process using photoluminescence excitation and PL microscopy and show that 58% of the QD PL arises due to energy transfer from the WS2. Time-resolved photoluminescence microscopy studies show that the energy transfer process is faster than the intrinsic PL quenching by trap states in the WS2, thus allowing for efficient energy transfer. Our results establish that QDs could be used as tunable and high PL efficiency emitters to modify the emission properties of TMDs. Such TMD-QD heterostructures could have applications in light-emitting technologies or artificial light-harvesting systems or be used to read out the state of TMD devices optically in various logic and computing applications.

Efficient Frequency Mixing of Guided Surface Waves by Atomically Thin Nonlinear Crystals.

Monolayer transition metal dichalcogenides possess considerable second-order nonlinear coefficients but a limited efficiency of frequency conversion due to the short interaction length with light under the typical direct illumination. Here, we demonstrate an efficient frequency mixing of the guided surface waves on a monolayer tungsten disulfide (WS2) by simultaneously lifting the temporal and spatial overlap of the guided wave and the nonlinear crystal. Three orders-of-magnitude enhancement of the conversion efficiency was achieved in the counter-propagating excitation configuration. Also, the frequency-mixing signals are highly collimated, with the emission direction and polarization controlled, respectively, by the pump frequencies and the rotation angle of WS2 relative to the propagation direction of the guided waves. These results indicate that the rules of nonlinear frequency conversion are applicable even when the crystal is scaled down to the ultimate single-layer limit. This study provides a versatile platform to enhance the nonlinear optical response of 2D materials and favor the scalable generation of a coherent light source and entangled photon pairs.

Synthesis of high-quality monolayer tungsten disulfide with chlorophylls and its application for enhancing bone regeneration

Due to the population explosion of the 21st century, nearly one billion people are over 64 years of age and bone fracture is one of the most frequent problems facing both sexes because of osteoporosis. However, difficulty in enhancing bone regeneration to repair bone fracture poses challenges and thus, a two-dimensional monolayer material (i.e. tungsten disulfide (WS2)) could be one of the candidates offering a possible solution to the problem. Here, we prepare high-quality monolayer WS2 thin sheets in a large quantity with the assistance of extracted chlorophyll molecules, the natural pigment used in photosynthesis, via a liquid-phase exfoliation method. Then, the exfoliated WS2 sheets were mixed with polycaprolactone (PCL)/calcium silicate (CS) to form a biocompatible WS2-based composite. The in vivo experiments show that the bone regeneration of the WS2-based composite was 120% superior to commercially available mineral trioxide aggregate (MTA) bone cement. Moreover, the mechanical properties of the WS2-based composite exhibited ~300% enhancement over PCL/CS, which is one of the most commonly used bone regeneration materials. Our findings highlight the prospects for the composite of WS2 towards the improvement of bone regeneration applications.

Generation of helical topological exciton-polaritons.

Topological photonics in strongly coupled light-matter systems offer the possibility for fabricating tunable optical devices that are robust against disorder and defects. Topological polaritons, i.e., hybrid exciton-photon quasiparticles, have been proposed to demonstrate scatter-free chiral propagation, but their experimental realization to date has been at deep cryogenic temperatures and under strong magnetic fields. We demonstrate helical topological polaritons up to 200 kelvin without external magnetic field in monolayer WS2 excitons coupled to a nontrivial photonic crystal protected by pseudo time-reversal symmetry. The helical nature of the topological polaritons, where polaritons with opposite helicities are transported to opposite directions, is verified. Topological helical polaritons provide a platform for developing robust and tunable polaritonic spintronic devices for classical and quantum information-processing applications.

Enhanced directional emission of monolayer tungsten disulfide (WS2) with robust linear polarization via one-dimensional photonic crystal (PhC) slab

Abstract Objectives Monolayer transition metal dichalcogenides (TMDCs) have been regarded as promising candidates for the future light-emitting devices. To date, though the modulation of emission intensity and directionality in monolayer TMDCs has received considerable scholarly attention, there has been no systematic investigation on the underlying critical polarization. The intensity, directionality and robust polarization are highly favorable and pivotal for the future on-chip optoelectronic emission devices based on TMDCs. Methods We explore the emission features of the monolayer TMDCs in the photonic crystal (PhC) platform at room temperature. A monolayer tungsten disulfide (WS2) is specifically integrated with a tailored PhC structure. Angle-resolved photoluminescence (PL), time-resolved PL and polarized PL measurements are carried out to study the enhanced emission and polarization properties. Results The photoluminescence (PL) of WS2 is greatly enhanced by over 300-fold, resulting from a ∼fivefold enhancement (from 1.5 to 7.2%) of the PL efficiency with accelerated spontaneous emission rates. Additionally, the overall polarized emission is obtained with the degree of linear polarization (DLP) up to 60%, which is independent of the excitation polarization. Moreover, two branched directional emissions with horizontal polarization are also achieved at a divergency angle of only 3.5°, accompanied by a surprising near-100% DLP at ±8° directions. Conclusions This comprehensive study sets out to assess the feasibility of the high-performance light emission device based on the monolayer TMDCs and PhC structures.

Rapid evaluation method for anisotropic growth of WS2 monolayers by combining machine learning algorithms and kinetic Monte Carlo simulation data

The growth of two-dimensional (2D) materials such as transition metal dichalcogenide (TMDC) monolayers is challenging and their growth mechanism has not been fully understood. It requires a large number of growth experiments and simulation calculations to reveal the growth mechanism. Also, a computational framework for combining the simulation results and the experimental results is essential. To speed up this time-consuming process for understanding the growth mechanism, a preliminary framework based on machine learning is herein proposed to analyze the simulation data and predict the growth trend. Specifically, the framework can be used for studying the anisotropic growth of tungsten disulfide (WS2) monolayers based on the data of kinetic Monte Carlo (kMC) simulations. Both an isotropic model with 91% accuracy and an anisotropic model with 90% accuracy are obtained. Moreover, both an undergrowth model with 98% accuracy and an overgrowth model with 99% accuracy are also obtained. This proposed framework can take both the experimental data and the simulation data as a single input data set, much speeding up the study of the growth mechanism.

Simultaneously enhanced linear and nonlinear photon generations from WS2 by using dielectric circular Bragg resonators

Abstract Monolayer transition metal dichalcogenides (TMDs) have emerged as a promising platform for chip-integrated optoelectronics and non-linear optics. Here, we demonstrate a two-dimensional (2D) monolayer tungsten disulfide (WS2) efficiently coupled to a dielectric circular Bragg resonator (CBR). The coupling of the WS2 and CBR leads to pronounced enhancements in both photoluminescence (PL) and second harmonic generation (SHG) by a factor of 34 and 5, respectively. Our work provides a powerful tool to enhance the interactions between light and the 2D materials, paving the way for efficient on-chip optoelectronic devices.

Effect of biaxial strain on thermal transport in WS2 monolayer from first principles calculations

Abstract Recently, tungsten disulfide (WS2) monolayer has emerged as a two-dimensional material due to its outstanding physical properties. In this work, the thermal transports in strained WS2 monolayer are studied systematically by combining the first principles calculations and the Boltzmann transport equation. It can be found that the lattice thermal conductivity (k) decreases from 262.78 to 190.66 W m−1 K−1 for biaxial tensile strain (from 0% to 8%) and from 262.78 to 217.40 W m−1 K−1 for biaxial compressive strain (from 0% to −4%), respectively. The softened transverse/longitudinal acoustic phonon mode and the stiffened flexural acoustic phonon mode in strained WS2 monolayer change the phonon group velocities. We attribute the reduction of k in tensilely strained WS2 monolayer to the declining phonon group velocity, heat capacity and phonon lifetime of acoustic phonon modes. The comprehensive competition among the increasing phonon group velocity, heat capacity and the decreasing phonon lifetime suppresses the lattice thermal conductivity of compressively strained WS2 monolayer. Furthermore, the increasing phase space proves that the enhancing anharmonic phonon scattering, which results in the suppression of the k in tensilely strained WS2 monolayer. Additionally, the critical phonon mean free path (MFP) under different biaxial strain levels is calculated to demonstrate the contribution of the phonons with different MFPs to the total k of WS2 monolayer. This work provides a phonon behavior analysis of the thermal conduction in WS2 monolayer and paves the path for potential applications of WS2-based devices.

Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling.

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal-insulator-semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS2 ). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS2 , which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS2 , followed by formation and radiative recombination of excitons in WS2 . The photocarrier transfer rate can be described by Fowler-Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.

Reconstructing Local Profile of Exciton-Emission Wavelengths across a WS2 Bubble beyond the Diffraction Limit.

Air bubbles formed between layers of two-dimensional (2D) materials not only are unavoidable but also emerge as an important means of engineering their excitonic emission properties, especially as controllable quantum light sources. Measuring the actual spatially resolved optical properties across such bubbles is important for understanding excitonic physics and for device applications; however, such a measurement is challenging due to nanoscale features involved which require spatial resolution beyond the diffraction limit. Additional complexity is the involvement of multiple physical effects such as mechanical strain and dielectric environment that are difficult to disentangle. In this paper, we demonstrate an effective approach combining micro-photoluminescence measurement, atomic force microscope profile mapping, and a theoretical strain model. We succeeded in reconstructing the actual spatial profiles of the emission wavelengths beyond the diffraction limit for bubbles formed by a monolayer tungsten disulfide on boron nitride. The agreements and consistency among various approaches established the validity of our approach. In addition, our approach allows us to disentangle the effects of strain and dielectric environment and provides a general and reliable method to determine the true magnitude of wavelength changes due to the individual effects across bubbles. Importantly, we found that micro-optical measurement underestimates the red and blue shifts by almost 5 times. Our results provide important insights into strain and screening-dependent optical properties of 2D materials on the nanometer scale and contribute significantly to our understanding of excitonic emission physics as well as potential applications of bubbles in optoelectronic devices.

Substrate dependent terahertz response of monolayer WS2

We investigate experimentally the terahertz (THz) optoelectronic properties of monolayer (ML) tungsten disulfide (WS2) placed on different substrates using THz time-domain spectroscopy (TDS). We find that the THz optical response of n-type ML WS2 depends sensitively on the choice of the substrate. This dependence is found to be a consequence of substrate induced charge transfer, extra scattering centers, and electronic localization. Through fitting the experimental results with the Drude–Smith formula, we can determine the key sample parameters (e.g., the electronic relaxation time, electron density, and electronic localization factor) of ML WS2 on different substrates. The temperature dependence of these parameters is examined. Our results show that the THz TDS technique is an efficient non-contact method that can be utilized to characterize and investigate the optoelectronic properties of nano-devices based on ML WS2.

Hybrid Polariton Devices: Ultrafast Response of a Hybrid Device Based on Strongly Coupled Monolayer WS2 and Photonic Crystals: The Effect of Photoinduced Coulombic Screening (Laser Photonics Rev. 14(4)/2020)

In article number 1900419, Lei Shi, Tian Jiang, and co-workers fabricate a hybrid polariton device in which strong coupling interactions occur between a photonic crystal (PhC) slab and monolayer tungsten disulfide (WS2). Moreover, transient dynamics studies show that the nonequilibrium coulombic screening effect in WS2 can explicitly influence the ultrafast response of this polariton device. These results are expected to deepen the understanding of photophysics in polariton and permit the active tunability of light-matter interactions.

Chiral Second-Harmonic Generation from Monolayer WS2/Aluminum Plasmonic Vortex Metalens.

Two-dimensional spiral plasmonic structures have emerged as a versatile approach to generate near-field vortex fields with tunable topological charges. We demonstrate here a far-field approach to observe the chiral second-harmonic generation (SHG) at designated visible wavelengths from a single plasmonic vortex metalens. This metalens comprises an Archimedean spiral slit fabricated on atomically flat aluminum epitaxial film, which allows for precise tuning of plasmonic resonances and subsequent transfer of two-dimensional materials on top of the spiral slit. The nonlinear optical measurements show a giant SHG circular dichroism. Furthermore, we have achieved an enhanced chiral SHG conversion efficiency (about an order of magnitude greater than the bare aluminum lens) from monolayer tungsten disulfide (WS2)/aluminum metalens, which is designed at the C-exciton resonance of WS2. Since the C-exciton is not a valley exciton, the enhanced chiral SHG in this hybrid system originates from the plasmonic vortex field-enhanced SHG under the optical spin-orbit interaction.