2D Layered Transition Metal Dichalcogenides Research Articles

In situ decorated 2D vanadium disulphide on optimized reduced graphene oxide-based interface for highly sensitive nonenzymatic glutamate detection

2D layered transition metal dichalcogenides (TMDs) possess eccentric nanostructures, large surface areas, and unique semiconducting properties, making them exceptional for non-enzymatic applications compared to metal oxides. However, TMDs like vanadium disulfide (VS2), with peroxidase-like nanozymatic characteristics, suffer from limited electronic conductivity, restricting their use as electrochemical (EC) sensors. To address this, reduced graphene oxide (rGO) is introduced to enhance electronic conductivity and leverage the enzyme-like property of VS2. This study introduces a novel enzyme-free EC sensor, rGO/VS2 nanocomposite-based, for accurate and efficient glutamate (Glut) detection. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrated that rGO/VS2 electrodes with varying rGO to VS2 composition ratios exhibit superior EC properties compared to pure VS2 electrodes. The EC response of the rGO/VS2 modified electrode towards Glut was investigated using CV and differential pulse voltammetry (DPV). The fabricated electrode displayed a linear relationship with Glut concentration (4–300 μM) (normal levels 30–80 μM), boasting a low limit of detection (LOD) (0.056 μM) and higher sensitivity (0.797 μA/(mM·cm2)) compared to existing non-enzymatic Glut sensors. Structural and morphological characterizations employed Raman spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDX). Furthermore, the proposed electrode's electroanalytical activity was assessed in spiked real samples, establishing its reliability and precision in Glut detection. The developed rGO/VS2-based EC sensor holds practical utility in biomedical research, clinical diagnostics, and environmental monitoring, where Glut detection is essential.

Trends and prospects of 2-D tungsten disulphide (WS2) hybrid nanosystems for environmental and biomedical applications

Recently, 2D layered transition metal dichalcogenides (TMDCs) with their ultrathin sheet nanostructure and diversified electronic structure have drawn attention for various advanced applications to achieve high-performance parameters. Unique 2D TMDCs mainly comprise transition metal and chalcogen element where chalcogen element layers sandwich the transition metal element layer. In such a case, various properties can be enhanced and controlled depending on the targeted application. Among manipulative 2D TMDCs, tungsten disulphide (WS2) is one of the emerging nano-system due to its fascinating properties in terms of direct band gap, higher mobility, strong photoluminescence, good thermal stability, and strong magnetic field interaction. The advancement in characterization techniques, especially scattering techniques, can help in study of opto-electronic properties of 2D TMDCs along with determination of layer variations and investigation of defect. In this review, the fabrication and applications are well summarized to optimize an appropriate WS2-TMDCs assembly according to focused field of research. Here, the scientific investigations on 2D WS2 are studied in terms of its structure, role of scattering techniques to study its properties, and synthesis routes followed by its potential applications for environmental remediation (e.g., photocatalytic degradation of pollutants, gas sensing, and wastewater treatment) and biomedical domain (e.g., drug delivery, photothermal therapy, biomedical imaging, and biosensing). Further, a special emphasis is given to the significance of 2D WS2 as a substrate for surface-enhanced Raman scattering (SERS). The discussion is further extended to commercial and industrial aspects, keeping in view major research gaps in existing research studies.

Composition-Controllable Syntheses and Property Modulations from 2D Ferromagnetic Fe5 Se8 to Metallic Fe3 Se4 Nanosheets.

Exploring new-type 2D magnetic materials with high magnetic transition temperature and robust air stability has attracted wide attention for developing innovative spintronic devices. Recently, intercalation of native metal atoms into the van der Waals gaps of 2D layered transition metal dichalcogenides (TMDs) has been developed to form 2D non-layered magnetic TMDs, while only succeeded in limited systems (e.g., Cr2 S3 , Cr5 Te8 ). Herein, composition-controllable syntheses of 2D non-layered iron selenide nanosheets (25% Fe-intercalated triclinic Fe5 Se8 and 50% Fe-intercalated monoclinic Fe3 Se4 ) are firstly reported, via a robust chemical vapor deposition strategy. Specifically, the 2D Fe5 Se8 exhibits intrinsic room-temperature ferromagnetic property, which is explained by the change of electron spin states from layered 1T'-FeSe2 to non-layered Fe-intercalated Fe5 Se8 based on density functional theory calculations. In contrast, the ultrathin Fe3 Se4 presents novel metallic features comparable with that of metallic TMDs. This work hereby sheds light on the composition-controllable synthesis and fundamental property exploration of 2D self-intercalation induced novel TMDs compounds, by propelling their application explorations in nanoelectronics and spintronics-related fields.

Performance enhancement of CZTS-based solar cells with tungsten disulfide as a new buffer layer

2D layered Transition Metal Dichalcogenide materials (TMDCs) have shown promising potential for ultra-thin photovoltaic and solar cells applications owing to their outstanding photon absorption and electrical and optoelectronics features. This paper intended to discuss a numerical exploration of the CZTS based solar cells employing the solar cell capacitance simulator (SCAPS-1D), using a novel non-toxic n-type WS2 TMDCs as a buffer layer. The cell parameters, such as the thickness and defect density of the CZTS absorber, are optimized. Then, the impact of the energy bandgap (Eg) and the back contact work function of the WS2 buffer layer on cell performance is investigated. An optimized Eg of 2.2 eV is declared. The results refer to the promoting conduction band alignments at the interface of the buffer absorber (i.e., WS2/CZTS). Further, we have studied the photovoltaic cell performance versus the defect level of the WS2 buffer layer. It was resolved that deep defect levels exceeding 1 × 1018 cm−3 degrade cell efficiency. The results show an optimized power conversion efficiency of about 26.81% with Voc = 1.17 V, Jsc = 27.7 mA/cm2, and FF = 83.66%. The simulation was further analyzed and discussed at various operating temperatures. The novel device architecture using WS2 as a buffer layer might encourage the fabrication of non-toxic CZTS solar cells.

MoSe2 nanoflowers grown on 3D carbon network as an advanced anode for lithium ion batteries

2D layered transition metal dichalcogenides deliver much higher specific capacity than the commercial graphite. However, they suffer from severe problems in practical applications such as low conductivity and poor cycling stability. Herein, we report a nanoflower-like MoSe2 grown on a 3D carbon network synthesized by efficient hydrothermal method. It shows a high specific capacity of 717.9 mAh/g in the initial cycle, with no capacity loss from the second to the fiftieth cycle. With the help of the 3D carbon network, MoSe2 nanoflowers present a stable structure with less agglomeration and then provide more active sites for the lithium storage, leading to better electrochemical performance. The present work suggests fabricating 3D hierarchical architectures applying simple synthetic methods could be an efficient strategy for designing advanced anode materials.

High-speed and broadband spectral photodetectors based on β-In2Se3/Si heterojunction

Two-dimensional Indium selenide (2D In2Se3), a direct band gap semiconductor in group III-VI, shows a potential application in constructing novel photodetectors with broadband spectral response. The device performances of these In2Se3-based photodetectors are limited by synthesis of the large-scale In2Se3 with high crystal quality and optimization of device structure and constructing process. Herein, a vertical β-In2Se3/Si p-n heterojunction is reported by in situ depositing a β-In2Se3 film with high quality on Si substrate by pulsed laser deposition technique.The resultant β-In2Se3/Si p-n heterojunction showed a broadband spectral response range from 265 nm to 1300 nm with a high responsivity up to 6.4 AW-1 at bias of −2 V. More importantly, a high response speed approaching to 2.2 μs at zero bias is achieved, which are much better than most of previous III-VI/Si based photodetectors and some of 2D layered transition metal dichalcogenides (TMDCs)/Si heterojunction. Moreover, the responsivity for 1300 nm NIR light is as large as 0.12 AW-1, which is superior than most of previous Si compatible 2D-3D heterojunction. The results suggest that our work paves the way toward the construction of novel III-VI/Si 2D-3D heterojunctions for high-performance and broadband photodetectors.

Hybridization of 2D MoS2 Nanoplatelets and PtAu Hybrid Nanoparticles for the SERS Enhancement of Methylene Blue

AbstractSERS is a powerful technique that can offer an extraordinary capacity in the enhancement of weak Raman vibration and identification of molecules at a low concentration. In this work, a versatile hybrid SERS platform is demonstrated by the hybridization of 2D MoS2 nanoplatelets and core–shell PtAu hybrid nanoparticles (HNPs) for the enhancement of Raman vibration of methylene blue (MB). The MB is mixed with the MoS2 nanoplatelets and applied on the uniquely developed PtAu core–shell HNPs and significantly improved SERS is achieved as compared to a general SERS. The PtAu core–shell HNPs can offer the core–shelled PtAu NP configuration along with the highly dense background Au NPs, which exhibit much enhanced distribution of localized surface plasmon resonance (LSPR) and hot spots. The MoS2 nanoplatelets are 2D layered transition metal dichalcogenide (TMD) semiconductors, which can offer an ample charge transfer to the MB. The hybrid SERS platform takes the advantages from both the electromagnetic mechanism (EM) and chemical mechanism (CM). The finite‐different time‐domain (FDTD) simulation demonstrates the enhanced local e‐field distributions and increased maximum local e‐field intensity of the core–shell PtAu HNPs and combination of 2D MoS2 nanoplatelets/core–shell PtAu HNPs.

Cover Feature: A Review on the Current Progress and Challenges of 2D Layered Transition Metal Dichalcogenides as Li/Na‐ion Battery Anodes (ChemElectroChem 13/2021)

The Cover Feature illustrates the advancement of the metal-ion battery (Li/Na-ion) using different transition metal dichalcogenides (TMDs) as anode material. Multi-electron transfer reaction and unique physico-chemical properties will be the highlight for TMDs as alternative anode material for future rechargeable battery. More information can be found in the Review by R. Sahoo et al.

A Review on the Current Progress and Challenges of 2D Layered Transition Metal Dichalcogenides as Li/Na‐ion Battery Anodes

AbstractEnergy storage systems play a crucial role to effectively utilize the recurrent renewable energy sources. In the last few decades, lithium‐ion battery (LIB) has emerged as the most convenient and viable energy storage systems and consequently, sodium‐ion battery (SIB) has drawn most of the attention of researchers as a potential alternative to LIB, especially in grid storage. Further development on LIB and SIB requires detection of novel electrode materials, precisely the anode. Captivating physico‐chemical properties and unique structure of layered transition metal dichalcogenides (TMDs) nanostructured materials have been rendered supremely appealing to the electrochemical energy conversion and storage devices researchers. In this review, synopsis of current state‐of‐art LIB and SIB anode materials, brief discussion on the different crystal structures and the common synthesis process of the TMDs have been articulated. Apart from these, detailed discussion on the electrochemical reaction and battery performance of different TMDs have been summarized and discussed in this review. The individuality of this review paper compared to the other reported review papers has also been highlighted in details. Finally, the challenges and limitations of the current TMDs materials, and outlook for the future improvement of TMDs in the energy sector have been proposed.

Electronic properties of hybrid WS2/MoS2 multilayer on flexible PET

Two-dimensional (2D) layered materials transition metal dichalcogenide compound (TMDC), which stack-together and form van der Waals heterostructures, have created interesting phenomena due to their interlayer interactions and their great potential for atomic-scale devices. Various electrical properties have been investigated. The presence of vacancies and their related charge trappings have been reported to affect the electrical properties. In this study, we investigate the electrical properties of hybrid WS2/MoS2 multilayer film deposited on polyethylene terephthalate (PET). The hybrid morphology and signatures are confirmed by the scanning electron microscope image and Raman shift spectra, respectively. We observed a semiconductor like behaviour as well as the large hysteresis which indicates the vacancies inducing charge trappings. This characteristics is different with the electronic characteristics of WS2 and MoS2 multilayer which tend to exhibit insulating behaviours and small hysteresis. This study shows how hybrid dichalcogenide WS2/MoS2 multilayer might create new features for future electronic devices.

Nonlinear Optical Interactions and Relaxation in 2D Layered Transition Metal Dichalcogenides Probed by Optical and Photoacoustic Z-Scan Methods

Atomically thin 2D materials, currently being at the forefront of scientific and technological interest, can be categorized as metallic, semimetallic, semiconducting, insulating, or superconducting...

Temperature-dependent Raman modes of MoS2/MoSe2 van der Waals heterostructures

Two-dimensional (2D) monolayer transition metal dichalcogenides (TMDs) show extra-ordinary properties compared to their bulk forms, which has inspired a large number of researchers to investigate these materials recently. Design and fabrication of different combinations of 2D TMDs layers can allow for high-performance and novel heterostructure-based devices, of which the performance will depend also on their thermal properties. On the other hand, the temperature-dependent behavior of such heterolayers and their interaction at different temperatures is still not comprehensively studied in a wide temperature range. In this work, we have performed a systematical temperature-dependent (83 K—483 K) Raman spectroscopic analysis of the MoS2/MoSe2 van der Waals (vdW) heterostructures and discussed their stability. After the transfer process of the MoS2 monolayers onto the MoSe2/SiO2/Si, we annealed the samples, which is a commonly used process to increase the crystallinity. Associatively, the thermal annealing process leads to a decrease in the thermal coefficients of the E1 2g and A1g modes of MoS2 and MoSe2 monolayers. Our study shows that the peak positions of the Raman modes in the heterostructures redshift with an increase in temperature. Furthermore, the full width at half maximum (FWHM) of the E1 2g and A1g modes of the layers broaden at higher temperatures. This phenomenon is attributed to increasing phonon–phonon interactions and thermal expansion effects with the ascending temperature. To the best of our knowledge, for the first time, temperature-dependent Raman analysis of MoS2/MoSe2 vdW heterostructures before and after annealing are carried out; and peak positions, FWHMs, and thermal coefficients of the layers are extracted. We do not observe any deformation in the heterobilayer structure even at very low (83 K) or very high temperatures (483 K). This is the first step to confirm the durability of the MoS2/MoSe2 heterolayered devices under extreme temperatures by studying their thermal properties.

(Invited) Wafer-Scale Integration of 2D TMD Heterostructures of Controlled Layer Orientation on Arbitrary Substrates Towards Mechanically-Reconfigurable Electronic Devices

Advancements of modern electronics have demanded to incorporate a diverse set of additional functionalities into device platforms such as high mechanical deformability and improved material/process sustainability. Traditional silicon (Si) wafers-based device manufacturing is intrinsically limited in realizing such novel aspects owing to their rigid/bulky nature as well as complex and unsustainable process schemes. Two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors are highly promising owing to their extremely large mechanical flexibility and near atom thickness coupled with van der Waals (vdW) attraction-enabled relaxed assembly requirement. Major challenges for realizing such opportunities for emerging electronics have been associated with a lack of reliable manufacturing methods to precisely separate 2D TMD layers from original growth wafers and integrate them on desired functional substrates in a controllable, scalable, and sustainable manner. In this talk, I will discuss recent efforts in my group on exploring viable manufacturing strategies to assemble wafer-scale 2D TMD layers of heterogeneously tailored components on arbitrary substrates. We grew various 2D TMD layers of controlled layer orientation on specially-treated growth substrates with high hydrophilicity or water solubility via a chemical vapor deposition (CVD) process. By taking advantage of the large surface energy contrast between growth substrates vs. grown 2D TMDs, we precisely peel off wafer-scale 2D TMD layers from their original substrates using water preserving their intrinsic structural/chemical integrity. We then integrate them on substrates of virtually unrestricted kinds and shapes in a layer-by-layer fashion, realizing heterogeneously-assembled wafer-scale 2D TMDs layers on a variety of exotic substrates impossible with any conventional approaches. The achieved material quality has been characterized via extensive microscopy/spectroscopy techniques, and the original substrates have been sustainably recycled for sequential growth and integration. Several demonstrations of 2D TMDs-enabled mechanically reconfigurable electronic devices will be presented, which will be impossible with any other traditional materials. This novel manufacturing strategy is believed to greatly broaden the applicability of 2D TMDs in emerging areas of electronics such as three-dimensionally conformal electronic devices of unconventional forms factors.

Automated Assembly of Wafer-Scale 2D TMD Heterostructures of Arbitrary Layer Orientation and Stacking Sequence Using Water Dissoluble Salt Substrates.

We report a novel strategy to assemble wafer-scale two-dimensional (2D) transition metal dichalcogenide (TMD) layers of well-defined components and orientations. We directly grew a variety of 2D TMD layers on "water-dissoluble" single-crystalline salt wafers and precisely delaminated them inside water in a chemically benign manner. This manufacturing strategy enables the automated integration of vertically aligned 2D TMD layers as well as 2D/2D heterolayers of arbitrary stacking orders on exotic substrates insensitive to their kind and shape. Furthermore, the original salt wafers can be recycled for additional growths, confirming high process sustainability and scalability. The generality and versatility of this approach have been demonstrated by developing proof-of-concept "all 2D" devices for diverse yet unconventional applications. This study is believed to shed a light on leveraging opportunities of 2D TMD layers toward achieving large-area mechanically reconfigurable devices of various form factors at the industrially demanded scale.

Thickness-Independent Semiconducting-to-Metallic Conversion in Wafer-Scale Two-Dimensional PtSe2 Layers by Plasma-Driven Chalcogen Defect Engineering.

Platinum diselenide (PtSe2) is an emerging class of two-dimensional (2D) transition-metal dichalcogenide (TMD) crystals recently gaining substantial interest, owing to its extraordinary properties absent in conventional 2D TMD layers. Most interestingly, it exhibits a thickness-dependent semiconducting-to-metallic transition, i.e., thick 2D PtSe2 layers, which are intrinsically metallic, become semiconducting with their thickness reduced below a certain point. Realizing both semiconducting and metallic phases within identical 2D PtSe2 layers in a spatially well-controlled manner offers unprecedented opportunities toward atomically thin tailored electronic junctions, unattainable with conventional materials. In this study, beyond this thickness-dependent intrinsic semiconducting-to-metallic transition of 2D PtSe2 layers, we demonstrate that controlled plasma irradiation can "externally" achieve such tunable carrier transports. We grew wafer-scale very thin (a few nm) 2D PtSe2 layers by a chemical vapor deposition (CVD) method and confirmed their intrinsic semiconducting properties. We then irradiated the material with argon (Ar) plasma, which was intended to make it more semiconducting by thickness reduction. Surprisingly, we discovered a reversed transition of semiconducting to metallic, which is opposite to the prediction concerning their intrinsic thickness-dependent carrier transports. Through extensive structural and chemical characterization, we identified that the plasma irradiation introduces a large concentration of near-atomic defects and selenium (Se) vacancies in initially stoichiometric 2D PtSe2 layers. Furthermore, we performed density functional theory (DFT) calculations and clarified that the band-gap energy of such defective 2D PtSe2 layers gradually decreases with increasing defect concentration and dimensions, accompanying a large number of midgap energy states. This corroborative experimental and theoretical study decisively verifies the fundamental mechanism for this externally controlled semiconducting-to-metallic transition in large-area CVD-grown 2D PtSe2 layers, greatly broadening their versatility for futuristic electronics.

Ultra-broadband photon harvesting in large-area few-layer MoS2 nanostripe gratings.

Flat optics nanoarrays based on few-layer MoS2 are homogeneously fabricated over large-area (cm2) transparent templates, demonstrating effective tailoring of the photon absorption in two-dimensional (2D) transition-metal dichalcogenide (TMD) layers. The subwavelength subtractive re-shaping of the few-layer MoS2 film into a one-dimensional (1D) nanostripe array results in a pronounced photonic anomaly, tunable in a broadband spectral range by simply changing the illumination conditions (or the lattice periodicity). This scheme promotes efficient coupling of light to the 2D TMD layers via resonant interaction between the MoS2 excitons and the photonic lattice, with subsequent enhancement of absorption exceeding 400% relative to the flat layer. In parallel, an ultra-broadband absorption amplification in the whole visible spectrum is achieved, thanks to the non-resonant excitation of substrate guided modes promoted by MoS2 nanoarrays. These results highlight the potential of nanoscale re-shaped 2D TMD layers for large-area photon harvesting in layered nanophotonics, quantum technologies and new-generation photovoltaics.

Wafer-scale 2D PtTe2 layers for high-efficiency mechanically flexible electro-thermal smart window applications.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers have gained increasing attention for a variety of emerging electrical, thermal, and optical applications. Recently developed metallic 2D TMD layers have been projected to exhibit unique attributes unattainable in their semiconducting counterparts; e.g., much higher electrical and thermal conductivities coupled with mechanical flexibility. In this work, we explored 2D platinum ditelluride (2D PtTe2) layers - a relatively new class of metallic 2D TMDs - by studying their previously unexplored electro-thermal properties for unconventional window applications. We prepared wafer-scale 2D PtTe2 layer-coated optically transparent and mechanically flexible willow glasses via a thermally-assisted tellurization of Pt films at a low temperature of 400 °C. The 2D PtTe2 layer-coated windows exhibited a thickness-dependent optical transparency and electrical conductivity of >106 S m-1 - higher than most of the previously explored 2D TMDs. Upon the application of electrical bias, these windows displayed a significant increase in temperature driven by Joule heating as confirmed by the infrared (IR) imaging characterization. Such superior electro-thermal conversion efficiencies inherent to 2D PtTe2 layers were utilized to demonstrate various applications, including thermochromic displays and electrically-driven defogging windows accompanying mechanical flexibility. Comparisons of these performances confirm the superiority of the wafer-scale 2D PtTe2 layers over other nanomaterials explored for such applications.

Probing the Origin and Suppression of Vertically Oriented Nanostructures of 2D WS2 Layers.

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) such as WS2 are promising materials for nanoelectronic applications. However, growth of the desired horizontal basal-plane oriented 2D TMD layers is often accompanied by the growth of vertical nanostructures that can hinder charge transport and, consequently, hamper device application. In this work, we discuss both the formation and suppression of vertical nanostructures during plasma-enhanced atomic layer deposition (PEALD) of WS2. Using scanning transmission electron microscopy studies, formation pathways of vertical nanostructures are established for a two-step (AB-type) PEALD process. Grain boundaries are identified as the principal formation centers of vertical nanostructures. Based on the obtained insights, we introduce an approach to suppress the growth of vertical nanostructures, wherein an additional step (C)—a chemically inert Ar plasma or a reactive H2 plasma—is added to the original two-step (AB-type) PEALD process. This approach reduces the vertical nanostructure density by 80%. It was confirmed that suppression of vertical nanostructures goes hand in hand with grain size enhancement. The vertical nanostructure density reduction consequently lowers film resistivity by an order of magnitude. Insights obtained in this work can contribute toward devising additional pathways, besides plasma treatments, for suppressing the growth of vertical nanostructures and improving the material properties of 2D TMDs that are relevant for nanoelectronic device applications.

(Invited) ALD and ALEt of Transition Metal Dichalcogenide Materials

Ultra-thin layer of two dimensional (2D) materials mainly transition metal dichalcogenide (TMD) semiconductor possess exceptional properties such as electrical, optical, magnetic, mechanical and chemical. This allows the exploration of internal quantum degrees of freedom of electrons and their potential for use in optoelectronic, energy, and sensor applications. At present, research on 2D-layered TMDs mostly rely on (i) flakes produced by exfoliation from bulk, (ii) synthesis by high temperature (800-1000oC) mostly by chemical vapor deposition or (iii) solution based methods. Although exciting results are being obtained, but all these methods have limitations such as poor quality of the material, lack of precise control of thickness, interface or ability to integrate them into complex 3D devices. Nevertheless, the biggest challenge in realizing their full potential has been the lack of doable material synthesis methods of such films with high uniformity, conformality and interfacing with other materials such as oxides and metals. Among the various thin film growth methods, atomic layer deposition (ALD) offers the best combination of a controlled layer-by-layer growth of the material with high conformality and suitable integration with other oxides such as dielectrics and metals. At the same time, precisely control thermal-based etching method for 2D-TMD materials are needed. Secondly at present, there are tremendous attention and efforts on development of the thermal vapor based well control etching methods especially on atomic layer etching (ALEt). The successful combination of both ALD and ALEt of 2D-TMD can pave the way to control the synthesis of 2D-TMD layers over large areas. This will be important for 2D-TMD material integration for future successful large-scale complex device structures fabrication. Here we will first discuss the ALD growth of 2D-TMDs (e.g. MoS2, HfS2 and MgS2) on metals (e.g. Mo, W and TiN), dielectric oxides (e.g. Al2O3, MgO, HfO2, and TiO2) and vice versa. Then precise control etching processes based on molybdenum hexafluoride (MoF6) and H2O vapor will be discussed. The MoF6-H2O precursors based etching process offer several advantages including low cost, low processing temperature, rapid and effective etching spanning the range from continuous etching to atomic layer etching (ALEt). We have used in-situ quartz crystal microbalance (QCM) and Fourier-transform infrared spectroscopy (FTIR) measurements to monitor the deposition and etching 2D-TMDs layers. Next, the deposited and etched 2D-TMDs thin films were analyzed by spectroscopic ellipsometry to determine the thickness and refractive index, and the composition was determined by X-ray photoelectron spectroscopy (XPS). These ex-situ measurements confirmed the 2D-TMDs deposition and etching behavior findings from our in-situ studies.

Recent Progress in CVD Growth of 2D Transition Metal Dichalcogenides and Related Heterostructures.

In recent years, 2D layered materials have received considerable research interest on account of their substantial material systems and unique physicochemical properties. Among them, 2D layered transition metal dichalcogenides (TMDs), a star family member, have already been explored over the last few years and have exhibited excellent performance in electronics, catalysis, and other related fields. However, to fulfill the requirement for practical application, the batch production of 2D TMDs is essential. Recently, the chemical vapor deposition (CVD) technique was considered as an elegant alternative for successfully growing 2D TMDs and their heterostructures. The latest research advances in the controllable synthesis of 2D TMDs and related heterostructures/superlattices via the CVD approach are illustrated here. The controlled growth behavior, preparation strategies, and breakthroughs on the synthesis of new 2D TMDs and their heterostructures, as well as their unique physical phenomena, are also discussed. Recent progress on the application of CVD-grown 2D materials is revealed with particular attention to electronics/optoelectronic devices and catalysts. Finally, the challenges and future prospects are considered regarding the current development of 2D TMDs and related heterostructures.