Few-layer Transition Metal Dichalcogenides Research Articles

Polarization control of lasing from few-layer MoTe2 coupled with the optical metasurface supporting quasi-trapped modes

The development of technology for integrating optical metaresonators with two-dimensional and layered van der Waals (vdW) materials opens up broad prospects for the creation of subdiffraction concentrators of electromagnetic energy, surface-emitting lasers, laser displays, and highly efficient nonlinear converters. In this work, we develop a straightforward strategy for the design and fabrication of surface-emitting laser devices based on few-layer transition metal dichalcogenides placed on the dielectric metasurfaces in the regime of quasi-trapped mode excitation. We optimize the parameters of MoTe2 flake and Si metasurface to achieve a positive feedback and to observe the lasing, resulting from their integration, with the predicted characteristics. Promising potential for the development of vdW-metalaser platform is associated with the possibility of simple polarization control of lasing regimes by employing the features of the bianisotropic response of the metasurface's building blocks.

High-Performance Broadband Self-Driven Photodetector Based on MoS2/Cs2CuBr4 Heterojunction.

Few-layer transition metal dichalcogenides and perovskites are both promising materials in high-performance optoelectronic devices. Here, we developed a self-driven photodetector by creating a heterojunction between few-layer MoS2 and lead-free perovskite Cs2CuBr4. The detector shows a unique property of very high sensitivity in a broad spectral range of 400 to 800 nm with response speed in a millisecond order. Current-voltage characteristics of the heterojunction device show rectifying behavior, in contrast to the ohmic behavior of the MoS2-based device. The rectifying behavior is attributed to the type II band alignment of the MoS2/Cs2CuBr4 heterojunction. The device shows a broadband (400 to 800 nm) photodetection with very high responsivity reaching up to 2.8 × 104 A/W and detectivity of 1.6 × 1011 Jones at a bias voltage of 3 V. The detector can also operate in self-bias mode with sufficient response. The photocurrent, photoresponsivity, detectivity, and external quantum efficiency of the device are found to be dependent on the illumination power density. The response time of the device is found to be ∼32 and ∼79 ms during the rise and fall of the photocurrent. The work proposes a MoS2/Cs2CuBr4 heterostructure to be a promising candidate for cost-effective, high-performance photodetector.

Long lived photogenerated charge carriers in few-layer transition metal dichalcogenides obtained from liquid phase exfoliation.

Semiconducting transition metal dichalcogenides are important optoelectronic materials thanks to their intense light-matter interaction and wide selection of fabrication techniques, with potential applications in light harvesting and sensing. Crucially, these applications depend on the lifetimes and recombination dynamics of photogenerated charge carriers, which have primarily been studied in monolayers obtained from labour-intensive mechanical exfoliation or costly chemical vapour deposition. On the other hand, liquid phase exfoliation presents a high throughput and cost-effective method to produce dispersions of mono- and few-layer nanosheets. This approach allows for easy scalability and enables the subsequent processing and formation of macroscopic films directly from the liquid phase. Here, we use transient absorption spectroscopy and spatiotemporally resolved pump-probe microscopy to study the charge carrier dynamics in tiled nanosheet films of MoS2 and WS2 deposited from the liquid phase using an adaptation of the Langmuir-Schaefer technique. We find an efficient photogeneration of charge carriers with lifetimes of several nanoseconds, which we ascribe to stabilisation at nanosheet edges. These findings provide scope for photocatalytic and photodetector applications, where long-lived charge carriers are crucial, and suggest design strategies for photovoltaic devices.

Ultrafast Interlayer Charge Transfer Outcompeting Intralayer Valley Relaxation in Few-Layer 2D Heterostructures.

While 2D transition metal dichalcogenides (TMDs) feature interesting layer-tunable multivalley band structures, their preeminent role in determining the photoexcitation charge transfer dynamics in 2D heterostructures (HSs) is yet to be unraveled, as previous charge transfer studies on TMD HSs have been mostly focused on monolayers with a direct bandgap at the K valley. By ultrafast transient absorption spectroscopy and deliberately designed few-layer WSe2/WS2 HSs, we have observed an ultrafast interlayer electron transfer from photoexcited few-layer WSe2 to WS2, prior to intralayer relaxation to lower lying dark valleys. More interestingly, we have identified an unconventional ∼0.5 ps electron back-transfer process after the initial interlayer electron transfer in HSs with WSe2 layers ≥ 3, regenerating indirect intralayer excitons. The result reveals an ielectron and valley relaxation pathway mediated by interlayer charge transfer in 2D HSs, faster than intralayer relaxation. It also sheds light on the origin of generally observed robust ultrafast interlayer charge transfer in TMD HSs and provides guidance toward optoelectronic and valleytronic devices using few-layer TMDs.

Misfit Layer Compounds as Ultratunable Field Effect Transistors: From Charge Transfer Control to Emergent Superconductivity.

Misfit layer compounds are heterostructures composed of rocksalt units stacked with few-layer transition metal dichalcogenides. They host Ising superconductivity, charge density waves, and good thermoelectricity. The design of misfits' emergent properties is, however, hindered by the lack of a global understanding of the electronic transfer among the constituents. Here, by performing first-principles calculations, we unveil the mechanism controlling the charge transfer and demonstrate that rocksalt units are always donor and dichalcogenides acceptors. We show that misfits behave as a periodic arrangement of ultratunable field effect transistors where a charging as large as ≈6 × 1014 e- cm-2 can be reached and controlled efficiently by the La-Pb alloying in the rocksalt. Finally, we identify a strategy to design emergent superconductivity and demonstrate its applicability in (LaSe)1.27(SnSe2)2. Our work paves the way to the design synthesis of misfit compounds with tailored physical properties.

Ultrafast dynamics of bright and dark excitons in monolayer WSe2 and heterobilayer WSe2/MoS2

The energy landscape of optical excitations in mono- and few-layer transition metal dichalcogenides (TMDs) is dominated by optically bright and dark excitons. These excitons can be fully localized within a single TMD layer, or the electron- and the hole-component of the exciton can be charge-separated over multiple TMD layers. Such intra- or interlayer excitons have been characterized in detail using all-optical spectroscopies, and, more recently, photoemission spectroscopy. In addition, there are so-called hybrid excitons whose electron- and/or hole-component are delocalized over two or more TMD layers, and therefore provide a promising pathway to mediate charge-transfer processes across the TMD interface. Hence, an in-situ characterization of their energy landscape and dynamics is of vital interest. In this work, using femtosecond momentum microscopy combined with many-particle modeling, we quantitatively compare the dynamics of momentum-indirect intralayer excitons in monolayer WSe2 with the dynamics of momentum-indirect hybrid excitons in heterobilayer WSe2/MoS2, and draw three key conclusions: First, we find that the energy of hybrid excitons is reduced when compared to excitons with pure intralayer character. Second, we show that the momentum-indirect intralayer and hybrid excitons are formed via exciton-phonon scattering from optically excited bright excitons. And third, we demonstrate that the efficiency for phonon absorption and emission processes in the mono- and the heterobilayer is strongly dependent on the energy alignment of the intralayer and hybrid excitons with respect to the optically excited bright exciton. Overall, our work provides microscopic insights into exciton dynamics in TMD mono- and bilayers.

Toward Perfect Surfaces of Transition Metal Dichalcogenides with Ion Bombardment and Annealing Treatment.

Layered transition metal dichalcogenides (TMDs) are two-dimensional materials exhibiting a variety of unique features with great potential for electronic and optoelectronic applications. The performance of devices fabricated with mono or few-layer TMD materials, nevertheless, is significantly affected by surface defects in the TMD materials. Recent efforts have been focused on delicate control of growth conditions to reduce the defect density, whereas the preparation of a defect-free surface remains challenging. Here, we show a counterintuitive approach to decrease surface defects on layered TMDs: a two-step process including Ar ion bombardment and subsequent annealing. With this approach, the defects, mainly Te vacancies, on the as-cleaved PtTe2 and PdTe2 surfaces were decreased by more than 99%, giving a defect density <1.0 × 1010 cm-2, which cannot be achieved solely with annealing. We also attempt to propose a mechanism behind the processes.

Hybrid Heterostructures to Generate Long-Lived and Mobile Photocarriers in Graphene

We report the generation of long-lived and highly mobile photocarriers in hybrid van der Waals heterostructures that are formed by monolayer graphene, few-layer transition metal dichalcogenides, and the organic semiconductor F8ZnPc. Samples are fabricated by dry transfer of mechanically exfoliated MoS2 or WS2 few-layer flakes on a graphene film, followed by deposition of F8ZnPc. Transient absorption microscopy measurements are performed to study the photocarrier dynamics. In heterostructures of F8ZnPc/few-layer-MoS2/graphene, electrons excited in F8ZnPc can transfer to graphene and thus be separated from the holes that reside in F8ZnPc. By increasing the thickness of MoS2, these electrons acquire long recombination lifetimes of over 100 ps and a high mobility of 2800 cm2 V-1 s-1. Graphene doping with mobile holes is also demonstrated with WS2 as the middle layers. These artificial heterostructures can improve the performance of graphene-based optoelectronic devices.

Alloyed transition-metal dichalcogenides (Mo1−xWxSe2) through a hydrothermal synthesis route: Probing layer-number-dependent band energies and band-gap bowing via scanning tunneling spectroscopy

In this work, we form alloyed transition-metal dichalcogenides (TMDs), ${\mathrm{Mo}}_{1--x}{\mathrm{W}}_{x}{\mathrm{Se}}_{2}$, through a hydrothermal synthesis procedure. Due to the identical effective ionic radius of the cations, the common-anionic alloys did not experience any lattice strain. A complete series of the common-anionic TMD alloys could thereby be synthesized; their nanosheets were formed by a liquid exfoliation method. The electronic properties of the alloyed nanosheets in their monolayer, bilayer, and trilayer forms were recorded separately through scanning tunneling spectroscopy (STS) in an extremely localized manner. From the STS studies, which have a correspondence to the density of states of the semiconductors, we have deliberated on their electronic properties and commented on the band edges of the nanosheets in the alloy series. The transport gap of the alloys in their monolayer, bilayer, and trilayer forms exhibited band-gap bowing, and also a layer-number-dependent bowing coefficient. That is, the bowing phenomenon interestingly occurred only in the few-layered TMD alloys and diminished to finally vanish in thicker nanosheets. The conduction band has been found to be responsible for such a nonlinear behavior of the transport gap; the results could be explained in terms of the molecular orbitals which form the band. The results have established a coherent description of the band-gap tuning in atomically thin 2D TMD alloys.

Van der Waals Heterostructures With Built-In Mie Resonances For Polarization-Sensitive Photodetection.

Few-layer transition metal dichalcogenides (TMDs) and their combination as van der Waals heterostructures provide a promising platform for high-performance optoelectronic devices. However, the ultrathin thickness of TMD flakes limits efficient light trapping and absorption, which triggers the hybrid construction with optical resonant cavities for enhanced light absorption. The optical structure enriched photodetectors can also be wavelength- and polarization-sensitive but require complicated fabrication. Herein, a new-type TMD-based photodetector embedded with nanoslits is proposed to enhance light trapping. Taking ReS2 as an example, strong anisotropic Mie-type optical responses arising from the intrinsic in-plane anisotropy and nanoslit-enhanced anisotropy are discovered. Owing to the nanoslit-enhanced optical resonances and band engineering, excellent photodetection performances are demonstrated with high responsivity of 27 A W-1 and short rise/decay times of 3.7/3.7ms. More importantly, through controlling the angle between the nanoslit orientation and the polarization direction to excite different resonant modes, polarization-sensitive photodetectors with anisotropy ratios from 5.9 to 12.6 can be achieved, representing one of the most polarization-sensitive TMD-based photodetectors. The depth and orientation of nanoslits are demonstrated crucial for optimizing the anisotropy ratio. The findings bring an effective scheme to construct high-performance and polarization-sensitive photodetectors.

Electroreflectance spectroscopy of few-layer MoS2 : Issues related to A1s exciton subspecies, exciton binding energy, and inter-layer exciton

Optical spectra of few-layer transition metal dichalcogenide semiconductors reveal several transitions whose character and origins continue to be debated. We have studied hBN encapsulated few-layer MoS2 films using electroreflectance (ER) spectroscopy. Two strong features are seen in the reflectance spectrum of trilayer MoS2 around the ground state A–exciton transition. In ER, the corresponding features show opposite phase response to the applied voltage. Evidence from first principles ER line shape simulation and photoluminescence spectroscopy suggests that these two features are likely to be A1s exciton subspecies proposed earlier, whose energy depends on which layer the electron–hole pair is located in. The first excited state A2s exciton transition is also identifiable in ER. Through the two-dimensional hydrogenic exciton model, it enables an approximate estimation of the exciton binding energy Eb. The increase in Eb with decreasing film thickness, which originates from reduced dielectric screening, is phenomenologically analyzed through a film thickness and capping material dependent effective dielectric constant. Extending this idea, we show that the A1s exciton is mostly confined to a single S–Mo–S layer, as predicted by theory. An inter-layer (IL) exciton is expected in bilayer and thicker 2H-MoS2 films. However, we show that there can be bilayer films where the IL exciton is absent, which may be related to increased carrier concentration.

Valley-polarized hyperbolic exciton polaritons in few-layer two-dimensional semiconductors at visible frequencies

Polaritons are quasiparticles describing the coupling between light and matter. In two-dimensional materials, polaritonic phenomena are abundant and unique, owing to their low dimensionality and the extraordinary properties of the supported quasiparticles. In this work, we predict the existence of hyperbolic exciton polaritons (HEPs) in few-layer transition-metal dichalcogenides (TMDs) at visible frequencies. We show that hyperbolicity can be induced in the TMD under certain conditions owing to the resonant behavior of the supported excitons, leading to the existence of HEPs. We derive the HEPs' dispersion relation under these conditions and analyze their confinement and loss properties, incorporating nonlocal corrections stemming from the high momentum of the modes. In addition, we show that owing to the valley properties of TMDs, the HEPs are coupled to the valley degree of freedom, leading to a hyperbolic spin-valley Hall effect. Such highly confined and valley-polarized HEPs provide opportunities to control strong light-matter interaction at the atomic scale.

Surface ligand functionalized Few-layered MoSe2 nanosheets decorated CdS nanorods for spectacular rate of H2 production

The properties of few-layered transition metal dichalcogenides (TMDs) are extremely interesting in the category of two-dimensional (2D) materials due to their feasibility of band gap engineering, high carrier mobility, and the ability to tune carrier concentration, which makes its utilization in wide range application. In the current study, we report the conversion of MoSe2 multilayers into a few layers as well as tune its band gap and band potentials by surface organic ligand (benzylamine) functionalization. Further, this functionalized few-layered MoSe2 has been deposited on CdS nanorods and tested for photocatalytic H2 production through water splitting under solar similar light excitation. Consequently, the optimized BA-MoSe2/CdS composites generated efficient hydrogen (54.9 µmol∙h−1 g−1) production, which is 3 and 18 folds enhanced than the simple few-layered MoSe2/CdS and CdS, respectively. The decoration of a few layered BA-MoSe2 on CdS nanorods effectively alters the band potentials suitably for proton reduction, then, separated greater photo-induced chargers, thereby, improving electrons accommodation on the catalysts surface and transferring to the active sites. Particularly, these outcomes would give a potential prospect for prominent photocatalytic systems development owing to their spectacular photo-efficiency and economic feasibility.

A wide-angle X-ray scattering laboratory setup for tracking phase changes of thin films in a chemical vapor deposition chamber.

The few-layer transition metal dichalcogenides (TMD) are an attractive class of materials due to their unique and tunable electronic, optical, and chemical properties, controlled by the layer number, crystal orientation, grain size, and morphology. One of the most commonly used methods for synthesizing the few-layer TMD materials is the chemical vapor deposition (CVD) technique. Therefore, it is crucial to develop in situ inspection techniques to observe the growth of the few-layer TMD materials directly in the CVD chamber environment. We demonstrate such an in situ observation on the growth of the vertically aligned few-layer MoS2 in a one-zone CVD chamber using a laboratory table-top grazing-incidence wide-angle X-ray scattering (GIWAXS) setup. The advantages of using a microfocus X-ray source with focusing Montel optics and a single-photon counting 2D X-ray detector are discussed. Due to the position-sensitive 2D X-ray detector, the orientation of MoS2 layers can be easily distinguished. The performance of the GIWAXS setup is further improved by suppressing the background scattering using a guarding slit, an appropriately placed beamstop, and He gas in the CVD reactor. The layer growth can be monitored by tracking the width of the MoS2 diffraction peak in real time. The temporal evolution of the crystallization kinetics can be satisfactorily described by the Avrami model, employing the normalized diffraction peak area. In this way, the activation energy of the particular chemical reaction occurring in the CVD chamber can be determined.

Anomalous Dynamics of Defect-Assisted Phonon Recycling in Few-Layer Mo0.5W0.5S2.

Alloying has emerged as a new strategy to tune the function of 2D transition metal dichalcogenides (TMDCs). However, the lack of research on the electrical and structural properties of these alloys limits their practical applications. Here, femtosecond transient absorption spectroscopy with pump pulse tunability is performed to elucidate the ultrafast carrier dynamics in the few-layer Mo0.5W0.5S2 prepared by the liquid phase exfoliation method. An anomalous rebleaching of the ground state is observed at high pump fluence by 3.1 eV excitation. We ascribe this rebleaching of the ground state to the mechanism that the carriers trapped in the defect are thermally excited back to the untrapped exciton state due to the phonon recycling, which hinders the dissipation of nonradiative energy, through comparative experiments and global analysis. Our findings demonstrate a novel energy transfer channel assisted by defect in few-layer TMDCs which is critical for their advanced applications.

CVD growth and optical characterization of homo and heterobilayer TMDs

The stacking of few layers of transition metal dichalcogenides (TMDs) and their heterostructures allows us to create new structures, observe new physical phenomena, and envision new applications. Moreover, the twist angle in few-layer TMDs can significantly impact their electrical and optical properties. Therefore, controlling the TMD material and obtaining different stacking orientations when synthesizing TMDs via chemical vapor deposition (CVD) is a powerful tool, which can add functionality to TMD-based optoelectronic devices. Here, we report on the synthesis of few-layer MoS2 and WS2 crystals, as well as their heterobilayer structures with 0° and 60° twist angles between layers via CVD. Raman and photoluminescence spectroscopies demonstrate the quality, crystallinity, and layer count of our grown samples, while second harmonic generation shows that adjacent layers grow with 0° or 60° twist angles, corresponding to two different crystal phases. Our study based on TMDs with different and multiple stacking configurations provides an alternative route for the development of future optoelectronic and nonlinear optical devices.

Optical Properties of MoS2 , MoSe2 , WS2 , and WSe2 under External Electric Field

The combination of finite semiconductor band gap and ultrasmall thickness makes few-layer transition-metal dichalcogenides (TMDCs) promising materials for use in optoelectronics devices. Here, we use density-functional theory to investigate the photon-energy-dependent optical properties of two-layer (2L), three-layer, four-layer, and five-layer ${\mathrm{Mo}\mathrm{S}}_{2}$, ${\mathrm{Mo}\mathrm{Se}}_{2}$, ${\mathrm{WS}}_{2}$, and ${\mathrm{W}\mathrm{Se}}_{2}$ TMDCs. We find that the values of the dielectric constant and the refractive index always increase with the number of layers as well as with external electric field in the static region. However, in the visible region (400--600 nm), the values of the dielectric constant and the refractive index decrease with increasing external electric field. Among these TMDCs, ${\mathrm{Mo}\mathrm{Se}}_{2}$ shows the strongest response under electric field. We find that in contrast to a small electro-optic response in the weak-field limit, a large change in the refractive index is applied under large applied fields, with the increasing response obtained for thinner TMDC films. The magnitude and even the sign of the electro-optic response changes with the photon frequency and reaches the peak values in the 1.5--2.5 eV range. The position of the peak electro-optic response can be tuned by both changing the number of layers and the TMDC composition. 2L ${\mathrm{Mo}\mathrm{Se}}_{2}$ exhibits the highest response of 8.8 pm/V at 2.3 eV that is comparable to the response of typical oxide electro-optic ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_{3}$. Analysis of the sensitivity trends of these TMDC systems shows that compared to the changes in the band gap, the changes of the onset of strong absorption under electric field and change in the number of layers show better correlation with the changes in the electro-optic response. Our results provide guidance for the possible use of these materials in optical micro- or nanocavities for devices such as tunable optical filters and fast optical switches.

High-yield production of mono- or few-layer transition metal dichalcogenide nanosheets by an electrochemical lithium ion intercalation-based exfoliation method.

Transition metal dichalcogenide (TMD) nanomaterials, especially the mono- or few-layer ones, have received extensive research interest owing to their versatile properties, ranging from true metals (e.g., NbS2 and VSe2) and semimetals (e.g., WTe2 and TiSe2) to semiconductors (e.g., MoS2 and We2) and insulators (e.g., HfS2). Therefore, the reliable production of these nanomaterials with atomically thin thickness and laterally uniform dimension is essential for their promising applications in transistors, photodetectors, electroluminescent devices, catalysis, energy conversion, environment remediation, biosensing, bioimaging, and so on. Recently, the electrochemical lithium ion intercalation-based exfoliation method has emerged as a mature, efficient and promising strategy for the high-yield production of mono- or few-layer TMD nanosheets; monolayer MoS2 (yield of 92%), monolayer TaS2 (yield of 93%) and bilayer TiS2 (yield of 93%) with lateral dimensions of ~1 µm (refs. 1-3). This Protocol describes the details of experimental procedures for the high-yield synthesis of mono- or few-layer TMDs and other inorganic nanosheets such as MoS2, WS2, TiS2, TaS2, ZrS2, graphene, h-BN, NbSe2, WSe2, Sb2Se3 and Bi2Te3 by using the electrochemical lithium ion intercalation-based exfoliation method, which involves the electrochemical intercalation of lithium ions into layered inorganic crystals and a mild sonication process. The whole protocol takes 26-38 h for the successful production of ultrathin inorganic nanosheets.

Strong coupling of excitons in patterned few-layer WS2 with guided mode and bound state in the continuum.

The strong coupling of excitons in few-layer transition-metal dichalcogenide (TMDC) with guided mode resonance (GMR) and bound state in the continuum (BIC) is investigated. It is shown that the strong coupling between excitons and GMR or BIC can enable a large Rabi splitting, where up to 155 meV or 162 meV Rabi splitting could be realized through changing the grating period, respectively. The physical origins behind this behavior are revealed by studying the electric field distributions at resonance. In addition, such behaviors are further theoretically verified according to the coupled-oscillator model. Moreover, the effect of the geometric dimensions on the strong coupling is also studied, which can be employed to guide real fabrication. The results will provide a new route for realization of few-layer TMDC-based light-matter interactions and may pave the way toward novel, compact, few-layer TMDC-based polaritonic devices.

Self-Limiting Opto-Electrochemical Thinning of Transition-Metal Dichalcogenides.

Two-dimensional monolayer and few-layer transition-metal dichalcogenides (TMDs) are promising for advanced electronic and photonic applications due to their extraordinary optoelectronic and mechanical properties. However, it has remained challenging to produce high-quality TMD thin films with controlled thickness and desired micropatterns, which are essential for their practical implementation in functional devices. In this work, a self-limiting opto-electrochemical thinning (sOET) technique is developed for on-demand thinning and patterning of TMD flakes at high efficiency. Benefiting from optically enhanced electrochemical reactions, sOET features a low operational optical power density of down to 70 μW μm-2 to avoid photodamage and thermal damage to the thinned TMD flakes. Through selective optical excitation with different laser wavelengths based on the thickness-dependent band gaps of TMD materials, sOET enables precise control over the final thickness of TMD flakes. With the capability of thickness control and site-specific patterning, our sOET offers an effective route to fabricating high-quality TMD materials for a broad range of applications in nanoelectronics, nanomechanics, and nanophotonics.