Composition-tunable transition metal dichalcogenide nanosheets via a scalable, solution-processable method.

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.

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.

Nonlinear Nanophotonics With 2D Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) nanosheets have shown extensive applications due to their excellent physical and chemical properties. However, the low light absorption efficiency limits their application in optoelectronics. By rolling up 2D TMDCs nanosheets, the one-dimensional (1D) TMDCs nanoscrolls are formed with spiral tubular structure, tunable interlayer spacing, and opening ends. Due to the increased thickness of the scroll structure, the light absorption is enhanced. Meanwhile, the rapid electron transportation is confined along the 1D structure. Therefore, the TMDCs nanoscrolls show improved optoelectronic performance compared to 2D nanosheets. In addition, the high specific surface area and active edge site from the bending strain of the basal plane make them promising materials for catalytic reaction. Thus, the TMDCs nanoscrolls have attracted intensive attention in recent years. In this review, the structure of TMDCs nanoscrolls is first demonstrated and followed by various preparation methods of the TMDCs nanoscrolls. Afterwards, the applications of TMDCs nanoscrolls in the fields of photodetection, hydrogen evolution reaction, and gas sensing are discussed.

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.

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.

As typical graphene analogues, two-dimensional (2D) transition-metal dichalcogenides (TMDs), including MoS2, WS2, etc., have attracted increasing research interest due to their semiconducting characteristics, sizable bandgap, large surface area, and outstanding performance in various applications such as electronics/optoelectronics, sensing, catalysis, and energy storage. For the widespread exploration of TMD nanosheets in fundamental studies and promising applications, a rapid pace of progress has recently been achieved in developing methodologies for reliable and scale-up production of atomically thin TMD nanosheets. In this chapter, we will present a comprehensive overview of different synthetic techniques of 2D TMDs and evaluate their relative merits and demerits. Particularly, the design principle, fabrication mechanism, typical process and special conditions will be emphasized to understand or follow the effective methods for obtaining required TMD nanosheets in single and few layers. Based on these synthetic techniques, further functionalization of TMD nanosheets with ligands and other nanocomponents are also discussed for gaining some specific properties. Based on our knowledge, we finally propose some possible challenges and potential opportunities in the exploitation of TMD nanosheets for novel applications.

Here, we explore a catalyst-free single-step growth strategy that results in high-quality self-assembled single-crystal vertical GaN nanowires (NWs) grown on a wide range of common and novel substrates (including GaN, Ga2O3, and monolayer two-dimensional (2D) transition-metal dichalcogenide (TMD)) within the same chamber and thus under identical conditions by pulsed laser deposition. High-resolution transmission electron microscopy and scanning transmission electron microscopy (HR-STEM) and grazing incidence X-ray diffraction measurements confirm the single-crystalline nature of the obtained NWs, whereas advanced optical and cathodoluminescence measurements provide evidence of their high optical quality. Further analyses reveal that the growth is initiated by an in situ polycrystalline layer formed between the NWs and substrates during growth, while as its thickness increases, the growth mode transforms into single-crystalline NW nucleation. HR-STEM and corresponding energy-dispersive X-ray compositional analyses indicate possible growth mechanisms. All samples exhibit strong band edge UV emission (with a negligible defect band) dominated by radiative recombination with a high optical efficiency (∼65%). As all NWs have similar structural and optical qualities irrespective of the substrate used, this strategy will open new horizons for developing III-nitride-based devices.

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.

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.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a first-rate candidate for post-silicon channels in field-effect transistors (FETs), particularly in the context of monolithic three-dimensional (M3D) integration. Despite the recent synthetic advances including industry-compatible wafer-scale production, they have predominantly been prepared on the ultra-flat, non-processed surfaces, limiting device architectures to outdated planar FETs and geometrically restricting their versatile applications. Integrating 2D semiconductors into 3D device architectures currently presents two significant challenges: i) stackablity, and ii) scalablility. While 2D material-based FETs have been primarily limited to traditional planar devices structures, their applications to vertically integrated 3D devices have been hindered by low-fidelity transfer method which is certainly challenging in conformally and controllably transferring atomically thin materials onto steep sidewalls. Here, we report a directly growth-and-fabrication approach enabling the dimensional transition from 2D planar FETs to a 3D vertical-channel FETs (VCFETs) through the atomically conformal direct growth of monolayer (ML) MoS2 onto the pre-fabricated VCFET structures (Fig. 1A). Leveraging the inherently anisotropic in-plane growth kinetics and precisely controlled supersaturation based on metal-organic chemical vapor deposition, we achieved 100% step coverage even with sub-1-nm channel thickness without compromised crystallinity along an exceptionally high aspect ratio exceeding 15,000 (Fig. 1B). We proved that monolayer MoS2 grown along the SiO2 vertical sidewall is a single crystal by comparing the angles of the selected-area electron diffraction (SAED) pattern in transmission electron microscope (TEM) at the three vertices of MoS2 (Fig. 1C). Fig. 1D shows a cross-sectional scanning transmission electron microscope (STEM) image of the VCFET with and spatial distribution of each element was also identified by energy dispersive X-ray spectroscopy (EDS) mapping images, and it convincingly suggests that atomically conformal monolayer MoS2 was indeed integrated without discontinuity and void even along a steep trench edge (Fig. 1D). Fig. 1E illustrates the unit device schematic of the monolayer MoS2 VCFET. The experimental and simulated transfer characteristic (I DS-V GS) of VCFET with vertical monolayer MoS2 well-coincide each other, showing the subthreshold swing (SS) of 77mV·dec−1 and the ON/OFF ratio of 108 under a drain-to-source bias (V DS) of 1V (Fig. 1F). Output characteristic (I DS-V DS) indicates the change in conductivity of the MoS2 channel by regulating the V G at steps of 1V from −2V to 5V (Fig. 1G). To evaluate device-to-device variation and statistic distribution of their performance, electrical characterization of the 7×7 VCFETs array was performed and their I DS-V GS curves under V DS = +1V exhibited the excellent reproducibility (Fig. 1H). Fig. 1I shows low OFF-state current density of 10−13A∙μm−1 at all devices in our monolayer MoS2 VCFET array, which is 10 times lower than the International Roadmap for Devices and Systems (IRDS) 2028 low-standby power device requirement (~10−12A∙μm−1). Sentaurus technology computer-aided design (TCAD) simulations for monolayer MoS2 VCFET were performed to investigate the electric field distribution of ON-state (V GS = +5V) and OFF-state (V GS = −2V), respectively (Fig. 1J-K). To demonstrate how electron concentration changes in monolayer MoS2 at the sidewall control the switching operation of the transistor, we fabricated a multi-gate VCFET (Fig. 1L). The additional terminals, a modulation gate (MG) and a back gate (BG), facilitate to form an effective gate length near the control gate (CG), enabling a switching behavior of the transistor. The MG screens the electric field toward the top of the trench from the CG, maintaining the electron concentration of the monolayer MoS2 on the top region of the trench and restricting switching operation to monolayer MoS2 on the vertical sidewall. I DS-V CG curve was obtained by applying the CG bias (V CG) and the BG bias (V BG) of +30V with the floated MG bias (V MG) (Fig. 1M). The fabricated multi-gate VCFET further validated the superior sidewall-gate controllability of the ultrathin monolayer MoS2 channel on the vertical sidewall, with aid of TCAD simulation. In the ON-state (V CG = +2.5V), the MG screens electric field generated by CG, allowing the electric field of CG to accumulate electrons effectively along the vertical monolayer MoS2 sidewall, as shown in Fig. 1N. in the OFF-state (V CG = −2.5V), the electric field contour plot reveals that the electric field from the CG does not reach the top region of trench due the MG, while forming a strong electric field near the CG, leading to deplete the MoS2 channel (Fig. 1O). Our work establishes a tailored pathway for the bespoke monolithic integration of ML semiconductors, positioning them as viable channels for high-density, high-performance computational device. Our integration strategy is undisputed pathway for M3D integration and usher in a new era of atomic-level fabric. Figure 1

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.