Nonlinear Nanophotonics With 2D Transition Metal Dichalcogenides

As layered materials beyond graphene discovered after 2004, two-dimensional (2D) transition metal dichalcogenides (TMDs) are vital to fundamental research and practical applications, owing to their unique crystal structures and excellent properties, as well as diversity of the electronic band structures. 2D TMDs have played an important role in electronics, optoelectronics, energy storage and catalysis. To meet with the increasing requirements of programmable and function-integrated devices, property modulation could be concerned as one of the most essential strategies for 2D TMDs. In comparison with conventional external electrical field induction, strain regulation could be much more efficient. In detail, external induction through the electrical field leads to the electron delocalization along the field direction and then induces the transformation of band structures, but electrical field exhibits small modulation for monolayer TMDs. On the contrary, the strain regulation shows excellent tuning efficiency for continuous and reversible modulation. As a result, the strain regulation has become a commonly-used strategy for property tuning of 2D TMDs. On the basis of the lattice transformation, the strain regulation of 2D TMDs can lead to different overlapping of d and p orbits of metal and chalcogen atoms, and then affect the electronic structures of 2D TMDs. Therefore, it can be applied to electronics, optoelectronics, magnetic devices and piezoelectronics. The strategies of introducing strains to 2D TMDs are classified into lattice induction, local deformation, macroscopic regulation and so on. Lattice induction is attributable to the structural distortions and mismatches, including atomic defect induction and lattice mismatch induction. The former one demonstrates that the microenvironment affected by atomic vacancies and doping atoms can introduce strain to 2D TMDs. The latter one means that the lattice mismatches between two materials (between two different TMDs in a heterostructure or between a TMD and the substrate) can result in lattice distortion and then induce the strain. However, strain introduced through lattice induction is fixed and difficult to achieve the reversible modulation. Local deformation refers to the morphological transformation at the scale of several micrometers, which can be induced by the bubbles and wrinkles of 2D TMDs, external forces of tips, as well as patterned substrates. Generally, large but nonuniform strain can be introduced to 2D TMDs through the local deformation, which results in the funnel effects and then induces large property variation. Furthermore, macroscopic regulation introduces the strains to the lattices, including the deformation of flexible substrates (bend, tension and compression), external pressure (applied by the diamond anvil cell) and thermal expansion coefficient mismatch. Macroscopic regulation would be compatible with industrial manufacture to achieve strain regulation on an extremely large scale in the future. There are some other ways to achieve strain regulation such as the design of special stack structures and the induction of the external electrical field. After summarizing the methods of introducing strain to 2D TMDs, we presented the applications based on strain regulation, such as field effect transistors, flexible photodetectors and strain sensors. Finally, we pointed out the further development and challenges of strain regulation of 2D TMDs.

Two-dimensional (2D) transition metal dichalcogenide materials have attracted much attention in recent years due to their excellent electro-optical properties. FeS2, the ideal composition of iron pyrite, is a 2D transition metal dichalcogenide which has been potentially used in the electronic, optical, and chemical fields. On the other hand, the narrow band gap of FeS2 (≈0.96 eV) makes it very suitable and promising for the ultrafast application in near-infrared regimes. However, the potential application of FeS2 in laser technology has not been explored till now. Ultrashort pulse lasers have great applications in industry and science because of its stability, ease of operation, and portability. Passively mode-locked fiber lasers using 2D materials (such as MoS2, CuS2, and WS2) as saturable absorber are intensively investigated. Here, layered FeS2 has been characterized systematically. It is successfully applied in ultrafast photonics and plays a key component in the passively mode-locked laser for the first time. The single pulse can be obtained with 1.7-ps pulse duration, 1.89-nm spectral width, and fundamental repetition of 6.4 MHz at 1563 nm central wavelength. Through controlling the pump power, the evolution of the pulse train can be observed, which can be transformed from single pulse to bound states. Also, the harmonic mode-locked fiber laser is observed with the pump power high enough.

Since excitonic quasiparticles, including excitons, trions and charges, have a great influence on the photoelectric characteristics of two-dimensional (2D) transition metal dichalcogenides (TMDs), systematic explorations of the trion dynamics and charge photogeneration in 2D TMDs are important for their future optoelectronic applications. Here, broadband femtosecond transient absorption spectroscopic experiments are performed first to investigate the peak shifting and broadening kinetics in MoS2 nanosheets in solution prepared by liquid phase exfoliation (LPE-MoS2, ∼9 layers, 9L), which reveal that the binding energies for the A-, B-, and C-exciton states are ∼77 meV, ∼76 meV, and -70 meV (the energy difference between free charges and excitons; the negative sign for C-excitons means a spontaneous dissociation nature in band-nesting regions), respectively. Then, the trion dynamics and charge photogeneration in LPE-MoS2 nanosheets have been studied in detail, demonstrating that they are comparable to those in chemical vapor deposition grown MoS2 films (1L-, 3L- and 7L-MoS2). These experimental results suggest that LPE-TMD nanosheets also have the potential for use in charge-related optoelectronic devices based on 2D TMDs.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are attractive semiconductors for use in electronic, optoelectronic, and spintronic devices. This study examines how the electronic properties of 2D TMDs can be tuned for specific applications through a combination of alloying and applying strain. Group VIB TMDs (MoS2, MoSe2, WS2, and WSe2) are alloyed by mixing in the metal or chalcogen sublattices. Density functional theory is used to model the structures of the alloys at varying compositions and examine the electronic structure of the alloys under biaxial tensile and compressive strain. Alloying results in the continuous monotonic tuning of the direct bandgap between the limits of the pure components, with low bowing coefficients for all alloys. Applying strain results in a transition of the bandgap from direct to indirect at low values of tensile strain and higher values of compressive strain. Strain can also be used to increase or decrease the bandgap with low compressive strain or tensile strain, respectively. The shift rate, or the rate at which the bandgap changes with applied strain, changes monotonically with alloy composition. MoS2 is identified as the 2D TMD with the highest shift rate.

Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising materials for an array of applications, ranging from conventional field-effect transistors, photodetectors, and light-emitting diodes to their more recent use in quantum photonic technologies. Chemical functionalization of 2D TMDs with organic ligands and adlayers provides an additional means for customizing their electronic and optical properties. While many pathways have been reported for the chemical functionalization of 2D TMDs, their frequent reliance on solution-based methods results in limited control over adlayer thickness and coverage, thus hindering utility in high-performance applications. Here we describe the vapor-phase functionalization of a 2D TMD with carbene ligands, specifically tungsten disulfide (WS2) with N-heterocyclic carbenes (NHCs), resulting in molecularly smooth, thin, and uniform adlayers. Reacting NHCs with monolayer WS2 reduces the broad photoluminescence background observed at cryogenic temperatures by 58%, which facilitates the detection of single-photon emitters from strained monolayer WS2, as indicated by second order correlation values (g(2)) as low as 0.17 ± 0.07. Chemical characterization coupled with density functional theory calculations suggests that the NHC adlayer has a dual defect-passivation and doping effect on monolayer WS2 that results in enhanced single-photon emission. Overall, this study establishes vapor-phase carbene functionalization as a homogeneous surface modification scheme for tailoring the quantum emission properties of semiconducting 2D TMDs.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have demonstrated a very strong application potential. In order to realize it, the synthesis of stoichiometric 2D TMDCs on a large scale is crucial. Here, we consider a typical TMDC representative, MoS2, and present an approach for the fabrication of well-ordered crystalline films via the crystallization of a thin amorphous layer by annealing at 800 °C, which was investigated in terms of long-range and short-range orders. Strong preferential crystal growth of layered MoS2 along the ⟨002⟩ crystallographic plane from the as-deposited 3D amorphous phase is discussed together with the mechanism of the crystallization process disclosed by molecular dynamic simulations using the Vienna Ab initio Simulation Package. We believe that the obtained results may be generalized for other 2D materials. The proposed approach demonstrates a simple and efficient way to fabricate thin 2D TMDCs for applications in nano- and optoelectronic devices.

Atomic-scale symmetry breaking for out-of-plane piezoelectricity in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attraction due to their unique properties in novel physical phenomena, such as superconductors, Moiré superlattices, ferromagnetics, Weyl semimetals, which all require the high quality of 2D TMDs. Mechanical exfoliation (ME) as a top-down strategy shows great potential to obtain 2D TMDs with high quality and large scale. This paper reviews the theoretical and experimental details of this method. Subsequently, diverse modified ME methods are introduced. Significantly, the recent progress of the Au-assisted ME method is the highlight. Finally, this review will have an insight into their advantages and limitations, and point out a rational direction for the exfoliation of TMDs with high quality and large size.

Chemical vapor deposition (CVD) has shown great potential in synthesizing various high-quality two-dimensional (2D) transition metal dichalcogenides (TMDCs). However, the nondestruction transfer of these CVD-grown 2D TMDCs at a high yield remains a key challenge for applying these emerging materials in various aspects. To address this challenge, we designed a water-soluble transfer mediator consisting of two polymers, polyvinylpyrrolidone (PVP) and poly(vinyl alcohol) (PVA), which can form strong interactions with CVD-grown 2D TMDCs for the nondestruction transfer of these materials. With this mediator, we realized the physical transfer of CVD-grown MoS2 flakes and several other 2D TMDCs, including 2D alloys and heterostructures to a wide range of substrates at a high yield of >90% with well-retained properties as evidenced by various microscopic, spectroscopic, and electrical measurements. Field-effect transistors (FETs) made on thus-transferred CVD-grown MoS2 monolayers exhibited obviously higher mobility than those transferred by chemical method. We also constructed several artificial 2D crystals showing very strong interlayer coupling by the multiple transfer of CVD-grown 2D TMDCs monolayers with this approach. This transfer approach will make versatile CVD-grown 2D materials and their artificial stacks with pristine qualities easily accessible for both fundamental studies and practical applications.

Modulating the band gap of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is critical for their application in a wider spectral range. Alloying has been demonstrated as an effective method for regulating the band gap of 2D TMDC semiconductors. The fabrication of large-area 2D TMDC alloy films with centimeter-scale uniformity is fundamental to the application of integrated devices. Herein, we report a liquid-phase precursor one-step chemical vapor deposition (CVD) method for fabricating a MoxW1-xS2 alloy monolayer with a large size and an adjustable band gap. Good crystalline quality and high uniformity on a wafer scale enable the continuous adjustment of its band gap in the range of 1.8-2.0 eV. Density functional theory calculations provided a deep understanding of the Raman-active vibration modes of the MoxW1-xS2 alloy monolayer and the change in the conductivity of the alloy with photon energy. The synthesis of large-area MoxW1-xS2 alloy monolayers is a critical step toward the application of 2D layered semiconductors in practical optoelectronic devices.

In spite of having a large exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDs) are limited as light-emitting materials because the spectral weight of neutral excitons decreases exponentially with increasing the excitation density. That is, neutral excitons easily transfer to trions, and exciton-exciton annihilation (EEA) occurs due to the strengthening of exciton kinetic energy in the layered structure. In here, we come up with an isolated neutral exciton system, maintaining its high spectral weight when the carrier density increased, which is achieved via MoS2 clusters on a MoS2 trilayer directly synthesized by metal-organic chemical vapor deposition (MOCVD). While increasing the excitation density, trions are decomposed by spatial confinement at the saturation level of its full width at half maximum (FWHM), and simultaneously the spectral weight of neutral excitons restarts to increase. Furthermore, we reveal the causality relationship between trions and B excitons, providing a keen insight into organic interactions among radiative recombination processes in 2D TMDs.

Insight into exciton dynamics of two-dimensional (2D) transition metal dichalcogenides (TMDs) is critical for the optimization of their performance in photonic and optoelectronic devices. Although current researches have primarily concentrated on the near-resonant excitation scenario in 2D TMDs, the case of excitation energies resonating with high-energy excitons or higher energies has yet to be fully elucidated. Here, a comparative analysis is conducted between high-energy excitation (360 nm) and near-resonant excitation (515 nm) utilizing transient absorption spectroscopy to achieve a comprehensive understanding of the exciton dynamics within monolayer WS2. It has been observed that the high-energy C-exciton can be generated via an up-conversion process under 515 nm excitation, even the energy of which is less than that of the C-exciton. Furthermore, the capacity to efficiently occupy band-edge A-exciton states leads to longer lifetimes for both the C-excitons and the A-excitons under conditions of near-resonant excitation, accompanied by an augmented rate of radiative recombination. This study provides a paradigm for optimizing the performance of 2D TMDs-based devices by offering valuable insights into their exciton dynamics.

Understanding the ultrafast carrier dynamics and the mechanism of two-dimensional (2D) transition metal dichalcogenides (TMDs) is key to their applications in the field of optoelectronic devices. In this work, a single pulse pump probe method is introduced to detect the layer-dependent ultrafast carrier dynamics of monolayer and few-layer WS2 excited by a femtosecond pulse. Results show that the ultrafast carrier dynamics of the layered WS2 films can be divided into three stages: the fast photoexcitation phase with the characteristic time of 2–4 ps, the fast decay phase with the characteristic time of 4–20 ps, and the slow decay phase lasting several hundred picoseconds. Moreover, the layer dependency of the characteristic time of each stage has been observed, and the corresponding mechanism of free carrier dynamics has been discussed. It has been observed as well that the monolayer WS2 exhibits a unique rising time of carriers after photoexcitation. The proposed method can be expected to be an effective approach for studying the dynamics of the photoexcited carriers in 2D TMDs. Our results provide a comprehensive understanding of the photoexcited carrier dynamics of layered WS2, which is essential for its application in optoelectronics and photovoltaic devices.

SummaryAtomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted significant attention owing to their prosperity in material research. The inimitable features of TMDCs triggered the emerging applications in diverse areas. In this review, we focus on the tailored and engineering of the crystal lattice of TMDCs that finally enhance the efficiency of the material properties. We highlight several preparation techniques and recent advancements in compositional engineering of TMDCs structure. We summarize different approaches for TMDCs such as doping and alloying with different materials, alloying with other 2D metals, and scrutinize the technological potential of these methods. Beyond that, we also highlight the recent significant advancement in preparing 2D quasicrystals and alloying the 2D TMDCs with MAX phases. Finally, we highlight the future perspectives for crystal engineering in TMDC materials for structure stability, machine learning concept marge with materials, and their emerging applications.

The second-most common cause of death resulting from genetic mutations in DNA sequences is cancer. The difficulty in the field of anticancer research is the application of the traditional methods, which also affects normal cells. Mutations, genetic replication alterations, and chromosomal abnormalities have a direct impact on the effectiveness of anticancer drugs at different stages. Presently, therapeutic techniques utilize nanotechnology, transition metal dichalcogenides (TMDCs), and robotics. TMDCs are being increasingly employed in tumor therapy and biosensing applications due to their biocompatibility, adjustable bandgap, versatile functionality, exceptional photoelectric properties, and wide range of applications. This study reports the advancement of nanoplatforms based on TMDCs that are specifically engineered for responsive and intelligent cancer therapy. This article offers a thorough examination of the current challenges, future possibilities for theranostic applications using TMDCs, and recent progress in employing TMDCs for cancer therapy. Currently, there is significant interest in two-dimensional (2D) TMDCs nanomaterials as ultrathin unique physicochemical properties. These materials have attracted attention in various fields, including biomedicine. Due to their inherent ability to absorb near-infrared light and their exceptionally large surface area, significant efforts are being made to prepare multifunctional nanoplatforms based on 2D TMDCs.