TMDCs and related vdW-heterostructures: a gap analysis of hybrid synthesis and the realization of optimal properties for practical applications, current challenges, and future prospects

The exploration of hybridizing transition metal dichalcogenide (TMD) nanosheets with other materials as a unique approach for engineering their properties has attracted considerable attention from the scientific community for both basic studies and numerous potential applications. Among the various kinds of functional materials in hand, the utilization of intrinsically conducting polymers (CPs) in the construction of advanced hybrid composites with TMD nanosheets is considered as a fascinating approach. In this review, we aim at providing a survey of the literature on recent progress in composites based on 2D TMD and CPs. In this regard, we first discuss the different synthetic strategies used for the fabrication of two-dimensional transition metal dichalcogenide/conducting polymer (2D TMD/CP) composites in detail. Subsequently, we demonstrate the state-of-the-art advances in the utilization of these novel composites in promising applications such as energy storage, sensing devices, hydrogen production and so on. Finally, we also highlight some perspectives on the major challenges and future directions in this field of research.

Graphene and transition metal dichalcogenide nanosheets as charge transport layers for solution processed solar cells

Owing to the specific two-dimensional (2D) confinement of electron motion and the absence of interlayer perturbation, 2D semiconductors possess unique optoelectronic properties and has become a research hot-spot in recent years. Whereas the electronic and luminescent properties of 2D transition metal dichalcogenide (TMDC) nanosheets, say, MoS2, MoSe2, WS2, etc., have been generating much research interest, the ultrafast nonlinear optical (NLO) properties remain largely unexplored. Realized that the sizable and thickness-dependent bandgap offers TMDCs a huge potential in the development of photonic devices with high performance and unique functions, we studied extensively the ultrafast NLO property of a range of TMDC nanosheets. 2D TMDC nanosheets with high-quality layered nanosheets were prepared using liquid-phase exfoliation technique. Ultrafast saturable absorption, two-photon absorption, ultrafast nonlinear photoluminescence were observed from the 2D nanostructures. The exciting results open up the door to 2D photonic nano-devices, such as optical switches, pulse shaping devices, mode-lockers, optical limiters, etc., capable of ultrafast response and broadband tunability.

Transition metal dichalcogenide (TMD) nanosheets have evoked enormous research enthusiasm and have shown increased potentials in the biomedical field. However, a great challenge lies in high-throughput, large-scale, and eco-friendly preparation of TMD nanosheet dispersions with high quality. Herein, we report a universal polyphenol-assisted strategy to facilely exfoliate various TMDs into monolayer or few-layer nanosheets. By optimizing the exfoliation condition of molybdenum disulfide (MoS2), the yield and concentration of as-exfoliated nanosheets are up to 60.5% and 1.21 mg/mL, respectively. This is the most efficient aqueous exfoliation method at present and is versatile for the choices of polyphenols and TMD nanomaterials. The as-exfoliated MoS2 nanosheets possess superior biomedical stability as nanocarriers to load antibiotic drugs. They show a high photothermal conversion effect and thus induce a synergetic effect of chemotherapy and photothermal therapy to harvest enhanced antibiofilm activity under near-infrared (NIR) light. All these results offer an appealing strategy toward the synthesis and application of ultrathin TMD nanosheets, with great implications for their development.

Nonvolatile field-effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating-gate organic-FET memory with an all-in-one floating-gate/tunneling layer of the solution-processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS2 ) is efficiently liquid-exfoliated by amine-terminated polystyrene with a controlled amount of MoS2 nanosheets in an all-in-one floating-gate/tunneling layer, allowing for systematic investigation of concentration-dependent charge-trapping and detrapping properties of MoS2 nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 104 , a program/erase endurance cycle over 400 times, and data retention longer than 7 × 103 s. All-in-one floating-gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically-flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer-bending events over 500 times at the bending radius of 4.0 mm.

The development of anodes for lithium‐ion batteries (LIBs) based on liquid‐phase‐exfoliated 2D transition metal dichalcogenide (TMD) nanosheets has been studied extensively because their intrinsic capacity is higher than graphite. Since most semiconducting TMDs possess low electrical conductivity and lithium‐ion diffusivity, expensive processes are necessary such as the addition of conductive fillers, chemically converted metallic phase transformation, and topological nano‐fabrication. Here, a novel conductor‐free TMD nanosheet anode with graft‐polymer ionic channels that ensures high stability and rate capability of the LIB is presented. The fluorinated polymer binder grafted with ionomers allows not only the efficient exfoliation of TMD nanosheets in the liquid phase to guarantee stable sheet‐to‐sheet separation but also provides self‐assembled ionic channels through which lithium ions in the electrolyte readily arrive close to the surface of the nanosheets. Efficient electrochemical reduction of lithium ions occurs on the surface of the binary anode of MoS2 nanosheets, self‐assembled with graft polymer ionic channels, resulting in a high‐performance LIB with stability (90% retention rate after 1,000 cycles), rate capability (50% at 5 A g−1), and high cell capacity (933.1 mAh g−1 at 0.1 A g−1). These TMD anodes that do not require additional processes, and offer a novel strategy for developing high performance large‐scale TMD‐based LIBs.

Two-dimensional alloyed transition metal dichalcogenide nanosheets: Synthesis and applications

Ecotoxicological assessment of water phase exfoliated two-dimensional Group-VI transition metal dichalcogenides using zebrafish embryo model

Size-dependent nonlinear optical properties of modification-free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS2 , WS2 , and NbSe2 . Firstly, a gradient centrifugation method is demonstrated to separate the TMD nanosheets into different sizes. The successful size separation allows the study of size-dependent nonlinear optical properties of nanoscale TMD materials for the first time. Z-scan measurements indicate that the dispersion of MoS2 and WS2 nanosheets that are 50-60 nm thick leads to reverse saturable absorption (RSA), which is in contrast to the saturable absorption (SA) seen in the thicker samples. Moreover, the NbSe2 nanosheets show no size-dependent effects because of their metallic nature. The mechanism behind the size-dependent nonlinear optical properties of the semiconductive TMD nanosheets is revealed by transient transmission spectra measurements.

Rapid and eco-friendly ultrasonic exfoliation of transition metal dichalcogenides supported on sonogel-nanocarbon black: A non-precious electrocatalyst for hydrogen evolution reaction

2D transition metal dichalcogenide (TMD) nanosheets, including MoS2, WS2, and TaS2, are used as hole injection layers (HILs) in organic light‐emitting diodes (OLEDs). MoS2, WS2, and TaS2 nanosheets are prepared using an exfoliation by ultrasonication method. The thicknesses and sizes of the TMD nanosheets are measured to be 3.1–4.3 nm and more than 100 nm, respectively. The work functions of the TMD nanosheets increase from 4.4–4.9 to 4.9–5.1 eV following ultraviolet/ozone (UVO) treatment. The turn‐on voltages at 10 cd m−2 for UVO‐treated TMD‐based devices decrease from 7.3–12.8 to 4.3–4.4 V and maximum luminance efficiencies increase from 5.74–9.04 to 12.01–12.66 cd A−1. In addition, this study confirms that the stabilities of the devices in air can be prolonged by using UVO‐treated TMDs as HILs in OLEDs. These results demonstrate the great potential of liquid‐exfoliated TMD nanosheets for use as HILs in OLEDs.

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Two-dimensional (2D) transition metal dichalcogenide (TMD) nanosheets exhibit remarkable electronic and optical properties. The 2D features, sizable bandgaps and recent advances in the synthesis, characterization and device fabrication of the representative MoS2, WS2, WSe2 and MoSe2 TMDs make TMDs very attractive in nanoelectronics and optoelectronics. Similar to graphite and graphene, the atoms within each layer in 2D TMDs are joined together by covalent bonds, while van der Waals interactions keep the layers together. This makes the physical and chemical properties of 2D TMDs layer-dependent. In this review, we discuss the basic lattice vibrations of 2D TMDs from monolayer, multilayer to bulk material, including high-frequency optical phonons, interlayer shear and layer breathing phonons, the Raman selection rule, layer-number evolution of phonons, multiple phonon replica and phonons at the edge of the Brillouin zone. The extensive capabilities of Raman spectroscopy in investigating the properties of TMDs are discussed, such as interlayer coupling, spin-orbit splitting and external perturbations. The interlayer vibrational modes are used in rapid and substrate-free characterization of the layer number of multilayer TMDs and in probing interface coupling in TMD heterostructures. The success of Raman spectroscopy in investigating TMD nanosheets paves the way for experiments on other 2D crystals and related van der Waals heterostructures.

Transition metal dichalcogenide (TMD) nanosheets exfoliated in the liquid phase are of significant interest owing to their potential for scalable and flexible photoelectronic applications. Although various dispersants such as surfactants, oligomers, and polymers are used to obtain highly exfoliated TMD nanosheets, most of them are electrically insulating and need to be removed; otherwise, the photoelectric properties of the TMD nanosheets degrade. Here, inorganic halide perovskite nanocrystals (NCs) of CsPbX3 (X = Cl, Br, or I) are presented as non-destructive dispersants capable of dispersing TMD nanosheets in the liquid phase and enhancing the photodetection properties of the nanosheets, thus eliminating the need to remove the dispersant. MoSe2 nanosheets dispersed in the liquid phase are adsorbed with CsPbCl3 NCs.The CsPbCl3 nanocrystals on MoSe2 efficiently withdraw electrons from the nanosheets, and suppress the dark current of the MoSe2 nanosheets, leading to flexiblenear-infrared MoSe2 photodetectors with a high ON/OFF photocurrent ratio and detectivity. Moreover, lanthanide ion-dopedCsPbCl3 NCs enhancethe ON/OFF current ratio to >106 . Meanwhile, the dispersion stability of the MoSe2 nanosheets exfoliated with the perovskite NCs is sufficiently high.

Transition metal oxides as the next generation of 2-D materials have attracted widespread attention because of their unique electrochemical properties. In this study, the preparation of metal oxides (MO) involves a two-step process of a hydrothermal reaction with subsequent air annealing. First, the transition metal dichalcogenides (TMD) nanosheets were prepared by a hydrothermal method. Subsequently, the TMD nanosheets were converted to MO nanosheets via the thermal annealing method. In addition, Au@NFs nanocomposites were also prepared using an efficiently combined approach of hydrothermal process and in situ chemical synthesis method. Both XRD and TEM results illustrated that MO nanosheets and Au@NFs nanocomposites were successfully prepared. For the first time, a novel electrochemical immunosensor modified with Au@NFs/MO nanocomposites was fabricated by the solution-casting method for detecting the calreticulin (CRT) biomarkers. This construction step provides a suitable and simple method for the covalent attachment of modified SPCE and anti-CRT molecules. Bio-affinity interactions between CRT biomarkers and anti-CRT molecules were investigated by CV and EIS techniques. The CV and EIS results indicated that L-Cysteine and EDC/NHS modified-Au@NFs/MO/SPCE could increase the electron transfer ability. When EN-LC/Au@NFs/MO/SPCE was sequentially modified with anti-CRT, BSA, and CRT, the Rct value gradually increased to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$4433.71\ \varOmega$</tex> , indicating that these non-conductive biomolecules were successfully immobilized. This result indicated that the CRT-immunosensors were successfully fabricated. In the future, the CRT-immunosensors are expected to detect real samples.