Synthesis Of Transition Metal Dichalcogenides Research Articles

Unusual Defect-Related Room-Temperature Emission from WS2 Monolayers Synthesized through a Potassium-Based Precursor.

Alkali-metal-based synthesis of transition metal dichalcogenide (TMD) monolayers is an established strategy for both ultralarge lateral growth and promoting the metastable 1T phase. However, whether this can also lead to modified optical properties is underexplored, with reported photoluminescence (PL) spectra from semiconducting systems showing little difference from more traditional syntheses. Here, we show that the growth of WS2 monolayers from a potassium-salt precursor can lead to a pronounced low-energy emission in the PL spectrum. This is seen 200-300 meV below the A exciton and can dominate the signal at room temperature. The emission is spatially heterogeneous, and its presence is attributed to defects in the layer due to sublinear intensity power dependence, a noticeable aging effect, and insensitivity to washing in water and acetone. Interestingly, statistical analysis links the band to an increase in the width of the A1g Raman band. The emission can be controlled by altering when hydrogen is introduced into the growth process. This work demonstrates intrinsic and intense defect-related emission at room temperature and establishes further opportunities for tuning TMD properties through alkali-metal precursors.

Boosting the gas sensing performances of 2H–MoSe2 bilayer upon alkali metal intercalation: Insights from DFT calculations

Realizing the multilayer or bulk MoSe2 cross from indirect bandgap to direct bandgap has drawn increasing attention due to the more practically useable and novel optoelectronic properties. In this work, the feasibility to realize indirect-to-direct band gap transition by intercalating alkali metals into 2H–MoSe2 bilayer was explored and its effect on the gas adsorption performance was evaluated utilizing first-principles calculation. The calculated results show that only Li and Na can be stably intercalated into the interlayer of 2H–MoSe2 bilayer. And the intercalated alkali metals as n-type dopant can realize the band gap transition from indirect to direct of 2H–MoSe2 bilayer, which is mainly ascribed to the decoupled interlayer interaction and built-in electric field in 2H–MoSe2 bilayer. Additionally, the adsorption performance of reducing gases is basically unchanged, while that of oxidizing gases (NO, NO2 and O2) is gradually enhanced with the intercalation concentrations of alkali metals increasing. The enhancement in adsorption performance for oxidizing gases is mainly ascribed to an indirect-to-direct bandgap transition along with the built-in electric field along the z-axis of 2H–MoSe2 bilayer. Through alkali metal intercalation, this research can provide a new method for the design and synthesis of transition metal dichalcogenides with enhanced gas-sensing performances.

Improved Photo‐Excited Carriers Transportation of WS2‐O‐Doped‐Graphene Heterostructures for Solar Steam Generation (Small 19/2023)

Solar Steam Generation In article number 2204898, Dan Liu, Weiwei Lei, and co-workers report a viable and general method that allows an effective synthesis of transition metal dichalcogenides (TMDs)-O-doped graphene nanosheet heterostructures by utilizing commercial TMD powder and sucrose as precursors. The solar evaporators of WS2-O-graphene exhibit high-performance water evaporation due to the improved photo-excited carrier transportation. This study provides a general approach for the design of heterostructure materials for solar-driven steam evaporation.

Effect of Solution pH on the Synthesis of Two-Dimensional Molybdenum–Tungsten Sulfide Nanostructures

Compared with binary transition-metal dichalcogenides (TMDCs), ternary alloys or heterojunctions have more abundant properties. The spin-coating method has been widely used in the synthesis of TMDCs. However, there is still a lack of research on the effect of acidity and alkalinity of precursor solutions on the epitaxial growth of TMDCs. It is found that the MoxW1–xS2 ternary alloy is formed when an acidic solution of (NH4)2MoO4 and (NH4)2WO4 is used for spin-coating, while the MoS2/WS2 lateral heterojunction with a sharp boundary is formed when an alkaline solution of Na2MoO4, Na2WO4, and NaOH is used. According to the calculation, under alkaline conditions, sulfur atoms preferentially bind to MoO42– due to lower energy and finally form a lateral heterojunction. However, under acidic and neutral conditions, MoO42– and WO42– in the precursor solution will undergo a complex reaction, respectively. Because there is no significant difference in the binding energy of the reaction between the sulfur atom and two complexes, a ternary alloy will be formed. The controllable synthesis of different types of TMDCs can be achieved by regulating acidity and alkalinity.

Migration-Enhanced Metal–Organic Chemical Vapor Deposition of Wafer-Scale Fully Coalesced WS2 and WSe2 Monolayers

Metal–organic chemical vapor deposition (MOCVD) is widely employed for the wafer-scale synthesis of transition metal dichalcogenide (TMDC) monolayers (MLs). Despite large efforts devoted to understanding the intricate nucleation and lateral growth mechanisms of TMDCs, little attention has been paid to the migration of adatoms on the top of an ML and its influence on parasitic/premature bilayer (BL) nucleation. In this work, using a commercial multi-wafer MOCVD platform, a novel two-stage migration-enhanced MOCVD process is introduced to realize the deposition of wafer-scale fully coalesced tungsten disulfide (WS2) and tungsten diselenide (WSe2) MLs with only sparse BL nucleation in a reasonable deposition time. With the WS2 ML coverage exceeding 99% on 2 in. sapphire substrates within 3 h, BL coverage is suppressed to ∼15%. Following the same migration enhancement approach, WSe2 MLs are synthesized in 90 min with <20% BL coverage. The migration of W adatoms on the already formed stable WS2 (or WSe2) ML domains is promoted by ramping down the delivery of the tungsten precursor. From the qualitative analysis of the nanomorphology, the migration length of W adatoms is estimated to be ≤100 nm. This approach can be seen as a reliable and solid basis for the development of future large-scale TMDC ML deposition techniques.

Solvothermal exfoliation assisted synthesis of transition metal dichalcogenide based tungsten disulphide quantum dots (WS2 QDs) and cellular QD-bio interaction in LN-229 human glioblastoma cells

WS2 is a typical TMDC candidate which marked a great sensation due to its similar characteristics with that of graphene. In the present study, highly cyan green emissive and excellent water dispersive WS2 QDs were prepared via top down strategy by solvothermal liquid exfoliation method using NMP solvent. The optical and physico-chemical characteristics of WS2 QDs was investigated systematically. The resultant WS2 QDs exhibit stable fluorescence (λmax = 500 nm), spherical morphology with a uniform thickness of ∼4nm. Moreover, the synthesised QDs exhibit a size-dependant photoluminescence in the wide visible region. Under 365 nm longer wavelength UV illumination, bright cyan green fluorescence was visualised due to excellent quantum confinement effect. XRD analysis revealed the efficient exfoliation potential of the material via this synthesis route. The as synthesised WS2 QDs exhibits excellent properties such as stable aqueous dispersion, extreme low cytotoxicity as well as photoluminescent properties, which makes them suitable candidates for optoelectronic and biological applications. Cellular interaction mediated cytotoxic response was evaluated by MTT assay in LN-229 human glioblastoma cells with a wide ranging concentrations and confirmed the non-destructive status of these QDs in biological subjects. The results from the study also confirm the potential of WS2 QDs to be used as probes for real-time optical cellular imaging and further in vivo trajectory imaging studies.

A facile strategy for the growth of high-quality tungsten disulfide crystals mediated by oxygen-deficient oxide precursors.

Chemical vapor deposition (CVD) has been established as a versatile route for the large-scale synthesis of transition metal dichalcogenides, such as tungsten disulfide (WS2). Yet, the precursor composition's role on the CVD process remains largely unknown and remains to be explored. Here, we employ Pulsed Laser Deposition (PLD) in a two-stage approach to tune the oxygen content in the tungsten oxide (WO3-x) precursors and demonstrate the presence of oxygen vacancies in the oxide films leads to a more facile conversion from WO3-x to WS2. Using a joint study based on ab initio density functional theory (DFT) calculations and experimental observations, we unravel that the oxygen vacancies in WO3-x can serve as niches through which sulfur atoms enter the lattice and facilitate an efficient conversion into WS2 crystals. By solely modulating the precursor stoichiometry, the photoluminescence emission of WS2 crystals can be significantly enhanced. Atomic resolution scanning transmission electron microscopy imaging (STEM) reveals that tungsten vacancies are the dominant intrinsic defects in mono- and bilayers WS2. Moreover, bi- and multilayer WS2 crystals derived from oxides with a high V0 content exhibit dominant AA'/AB or AA(A…) stacking orientations. The atomic resolution images reveal local strain buildup in bilayer WS2 due to competing effects of complex grain boundaries. Our study provides means to tune the precursor composition to control the lateral growth of TMDs while revealing insights into the different pathways for forming grain boundaries in bilayer WS2.

Wafer‐Scale Two‐Dimensional Molybdenum Diselenide Phototransistor Array via Liquid‐Precursor‐Assisted Chemical Vapor Deposition

AbstractMonolayer transition metal dichalcogenides (TMDs) have received considerable interest as a candidate material for ultrathin photodetectors, but their practical applications are hindered by difficulties in synthesizing high‐quality films over centimeter‐scale areas. Although chemical vapor deposition (CVD) based on liquid‐phase precursors has shown promise for large‐area synthesis of TMDs, the resulting films typically exhibit much lower optoelectronic performance due to the lack of wafer‐scale uniformity with monolayer thickness and inferior electrical properties induced by defects. In this article, the authors present a wafer‐scale, gate‐tunable photodetector array from high‐quality monolayer molybdenum diselenide (MoSe2) films synthesized with the liquid‐phase CVD process assisted by a growth promoter. Continuous monolayer MoSe2 can form by introducing potassium iodide as a growth promoter for selenizing the metal‐precursor film. Side‐by‐side comparison between MoSe2 formed with and without the potassium iodide reveal that the promoter‐assisted growth strategy significantly improves the crystallinity, which results in enhanced optoelectronic properties. The resulting photodetector array is highly photosensitive over the visible wavelengths with photoresponsivities higher than those of the previously reported devices based on CVD‐synthesized monolayer TMDs and even comparable to the devices fabricated from mechanical exfoliation approach.

Low-temperature synthesis of molybdenum sulfides, tungsten sulfides, and composites thereof as efficient electrocatalysts for hydrogen evolution reaction

In this work, we report a one-step approach for synthesizing molybdenum sulfides and tungsten sulfides, using (N2H5)2MS4 (M = Mo, W) as highly energetic self-catalytic redox precursors and inducing a thermolysis process at various temperatures. These materials’ thermodynamic advantages facilitate the formation of molybdenum/tungsten sulfides at low temperatures. As expected, MoSx-100 °C (which features the [Mo3S13]2− active site model) exhibits the highest catalytic activity of the reported molybdenum sulfides. In addition, an optimized sample of crystalline WS2 was reported to produce hydrogen at 400 °C. Furthermore, the combination of carbon materials significantly enhanced the hydrogen production performance. The optimal sample of reduced graphite oxide (rGO)/MoSx-100 °C required an overpotential of only 125 mV to achieve a current density of 10 mA cm−2 and a shallow Tafel slope of 48.8 mV dec−1; this was attributed to the increased charge transfer from rGO. Furthermore, the catalyst exhibited good stability after 2000 cycles and 12 h of testing. This work may provide an alternative approach for the large-scale synthesis of transition metal dichalcogenides in high-performance catalyst applications.

Synthesis of Transition Metal Dichalcogenides based hybrid nanostructures by thermal decomposition method

In this work, we report synthesis of hybrid nanostructures of Transition Metal Dichalcogenides via thermal decomposition method. Ammonium tetrathiomolybdate was used as not only growth templates but also as starting precursor for synthesis of hybrid nanostructures. The conditions for the synthesizing method were optimized using electron microscopy and x-ray diffraction. In this hybrid nanostructure synthesis, it was found that MoO3 nanorods are interspersed on exfoliated MoS2 nanosheets. The structural and optical properties of the hybrid nanostructure were investigated using transmission electron microscopy (TEM), Scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy and Ultraviolet Visible spectrophotometry (UV-VIS). The hybrid nanostructure of MoO3 on MoS2 shows a band gap of 2.2 eV. It was also found that by tuning the preparation parameters viz temperature of heating and time of heating, the composition of the hybrid nanostructure can be varied.

Design of High-Performance Rram through Interfacial Engineering Toward Neuromorphic Application by Low-Temperature Plasma Selenization Process

The probabilistic formation of conducting filament (CF) in two-terminal Resistive switches affects the device functionality. The spontaneity of formation of CF is the key factor in determining the device reliability and can be controlled by functional layer dynamics near the interface. This functional layer can act as a series resistance as well as a barrier layer against the random migration of metallic ion and oxygen vacancies. Here, we propose the interfacial engineering by inserting WO3/WSe2 layer between the electrolyte and the active electrode. The proposed WO3/WSe2 layer provides fine control over diffusion of oxygen vacancy across AL2O3/WO3 interface, thus provide reliable and high performance for RRAM devices. In addition, inductively coupled plasma (ICP) process is used to change the WO3/WSe2 ratio to obtain the high-performance Resistive switch. The use of ICP process facilitates the synthesis of transition metal dichalcogenide (TMDs) through plasma selenization process at a temperature as low as 3000C. Therefore, the temperature required for TMDs synthesis is drastically reduced in comparison to conventional CVD process. The plasma synthesized WO3/ WSe2 is coupled with AL2O3 to form a hybrid structure for Resistive switching device. Compared with Pt/AL2O3/W device, the optimized Pt/Al2O3/WO3/WSe2/W hybrid structure show improvement in on/off ratio (104-105), multilevel characteristics (uniform LRS/HRS distribution) and retention (10 years). The results indicate that plasma engineered interface engineering can be a promising approach for improved and high-performance RRAMs.

Role of Nanodiamond Grains in the Exfoliation of WS2 Nanosheets and Their Enhanced Hydrogen-Sensing Properties.

Herein, for the first time, a combination of detonation nanodiamond (DND)-tungsten disulfide (WS2) was devised and studied for its selective H2-sensing properties at room temperature. DND-WS2 samples were prepared by a sonication-assisted (van der Waals interaction) liquid-phase exfoliation process in low-boiling solvents with DND as a surfactant. The samples were further hydrothermally treated in an autoclave under high pressure and temperature. The as-prepared samples were separated as two parts named DND-WS2 BH (before hydrothermal) and DND-WS2 AH (after hydrothermal). The exfoliated bilayer to few-layer DND-doped WS2 nanosheets were confirmed by ultraviolet-visible spectra, atomic force microscopy, and transmission electron microscopy studies. It was observed that the DND powder not only acted as a surfactant but also doped and expanded on WS2 nanosheets. The difference between samples BH and AH treatment was further investigated using Raman spectroscopy. The WS2 and DND-WS2 samples on SiO2/Si were fabricated using a sputtered Pd/Ag interdigitated electrode and utilized for H2 gas-sensing measurements. Surprisingly, the DND-WS2 exhibits an ultrahigh sensor response of 72.8% to H2 at 500 ppm when compared to only 9.9% for WS2 alone. Also, the DND-WS2 shows a fast response/recovery time, high selectivity, and stability toward H2 gas. It can be attributed to the correlation of the intergrain phase of DND nanoparticles and WS2 nanosheets, which contributes to the easy transportation of charge carriers when exposed to the air and H2 gas atmosphere. Moreover, it is believed that DND-induced WS2 exfoliation might inspire future synthesis of transition metal dichalcogenides induced by DND in green solvents.

Large-area synthesis of transition metal dichalcogenides via CVD and solution-based approaches and their device applications.

For the last decade, two-dimensional transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their unique physical and chemical properties. Novel devices based on these materials are commonly fabricated using the exfoliated samples, which lacks control of the thickness and cannot be scaled. Therefore, the synthesis of large-area TMDC thin films with a high uniformity to advance the field is required. This article reviews the latest advances in the synthesis of wafer-scale thin films using chemical vapor deposition methods. The key factors that determine the electrical performance of TMDCs are introduced, including the interfacial properties and defects. The latest solution-based techniques which suggest the opportunity to obtain large-area TMDC thin films with a low-cost process and the potential applications in electronics and optoelectronics are also discussed. The outlook for future research directions, challenges, and possible development of 2D materials are further discussed.

Synthesis of MoS2 Thin Film by Ionized Jet Deposition: Role of Substrate and Working Parameters

The lack of scalable synthesis of transition metal dichalcogenides, such as molybdenum disulfide (MoS2), has proved to be a significant bottleneck in realization of fundamental devices and has hindered the commercialization of these materials in technologically relevant applications. In this study, a cost-efficient and versatile thin-film fabrication technique based on ionized jet deposition (IJD), i.e., a technique potentially providing high processing efficiency and scalability, is used to grow MoS2 thin films on silicon substrates. The operating conditions of IJD were found to influence mainly the ablation efficiency of the target and only slightly the quality of the deposited MoS2 thin film. All as-deposited films show chemical properties typical of MoS2 with an excess of free, elemental sulfur that can be removed by post-deposition annealing at 300–400 °C, which also promotes MoS2 crystallization. The formation of an interface comprised of several silicon oxide species was observed between MoS2 and the silicon substrate, which is suggested to originate from etching and oxidizing processes of dissociated water molecules in the vacuum chamber during growth. The present study paves the way to further design and improve the IJD approach for TMDC-based devices and other relevant technological applications.

Titanium Disulphide (TiS2) Dichalcogenide Thin Films as Inorganic Hole Transport Layer for Perovskite Solar Cells Synthesized from Ionic Liquid Electrodeposition

AbstractIn high efficiency organic-inorganic perovskite solar cells formed as a multilayer structure, the hole transporting layer (HTL) at the perovskite absorber layer interface has a critical role. Organic HTLs based on Spiro-OMeTAD and PTAA have led to high efficiencies but displayed poor long-term stability and involves expensive purification processes that hinders universal low-cost commercialization goals for perovskite solar cells. Though as an inorganic alternative, transition metal chalcogenides have been investigated for HTL recently, the hot-injection method often used in synthesis has shown poor reproducibility and difficulty in scaling-up. In this work we demonstrate an ab initio facile inexpensive scalable synthesis of transition metal dichalcogenide (TiS2) by electrodeposition from ionic liquids as a low-cost inorganic HTL for perovskite solar cells. The TiS2 thin films were electrodeposited from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C with Na2S2O3 as sulphur and TiCl4 as titanium source. From cyclic voltammetry studies the deposition potential of TiS2 was optimized at -0.8V vs Pt. The as-deposited TiS2 HTL exhibited polycrystalline structure with preferential growth along (001), (100), (002), (102), (110), (111) planes. The Raman spectroscopy of the films showed peaks around 225 cm−1 and 332 cm−1 attributed to the Eg and A1 g Raman modes respectively. The synthesized thin films demonstrated sharp optical bandgap edge along with bandgap tunability as the bandgap (direct) decreased from 1.53 eV to 1.49 eV, 1.40 eV, and 1.34 eV with gradual change in deposition potential from −0.8 V to −0.9 V, −1.0 V, and −1.1 V vs Pt, respectively. This aspect has potential for alignment of valance band edge in facilitating the hole transport at the perovskite-TiS2 interface. The absorption coefficient in visible-light range of the as-deposited TiS2 thin films likewise has shown a dependence on the synthesis potential which is highly conducive for application as an HTL in multilayer solar cell structure. The TiS2 thin films were observed to be p-type as shown from the Hall effect studies with a carrier mobility up to 14.4 cm2V−1s−1. A detailed study of the effect of the synthesis parameters on the structural, optical, band-edge, and electronic properties of TiS2 thin films suitable for application as HTL in perovskite solar cells is presented.

Low-temperature synthesis and electrocatalytic application of large-area PtTe2 thin films

The synthesis of transition metal dichalcogenides (TMDs) has been a primary focus for 2D nanomaterial research over the last 10 years, however, only a small fraction of this research has been concentrated on transition metal ditellurides. In particular, nanoscale platinum ditelluride (PtTe2) has rarely been investigated, despite its potential applications in catalysis, photonics and spintronics. Of the reports published, the majority examine mechanically-exfoliated flakes from chemical vapor transport (CVT) grown crystals. This method produces high quality-crystals, ideal for fundamental studies. However, it is very resource intensive and difficult to scale up meaning there are significant obstacles to implementation in large-scale applications.In this report, the synthesis of thin films of PtTe2 through the reaction of solid-phase precursor films is described. This offers a production method for large-area, thickness-controlled PtTe2, potentially suitable for a number of applications. These polycrystalline PtTe2 films were grown at temperatures as low as 450 °C, significantly below the typical temperatures used in the CVT synthesis methods. Adjusting the growth parameters allowed the surface coverage and morphology of the films to be controlled. Analysis with scanning electron- and scanning tunneling microscopy indicated grain sizes of above 1 µm could be achieved, comparing favorably with typical values of ∼50 nm for polycrystalline films. To investigate their potential applicability, these films were examined as electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). The films showed promising catalytic behavior, however, the PtTe2 was found to undergo chemical transformation to a substoichiometric chalcogenide compound under ORR conditions. This study shows while PtTe2 is stable and highly useful for in HER, this property does not apply to ORR, which undergoes a fundamentally different mechanism. This study broadens our knowledge on the electrocatalysis of TMDs.

Synthesis of MoX2 (X = Se or S) monolayers with high-concentration 1T′ phase on 4H/fcc-Au nanorods for hydrogen evolution

Controlled synthesis of transition metal dichalcogenide (TMD) monolayers with unusual crystal phases has attracted increasing attention due to their promising applications in electrocatalysis. However, the facile and large-scale preparation of TMD monolayers with high-concentration unusual crystal phase still remains a challenge. Herein, we report the synthesis of MoX2 (X = Se or S) monolayers with high-concentration semimetallic 1T′ phase by using the 4H/face-centered cubic (fcc)-Au nanorod as template to form the 4H/fcc-Au@MoX2 nanocomposite. The concentrations of 1T′ phase in the prepared MoSe2 and MoS2 monolayers are up to 86% and 81%, respectively. As a proof-of-concept application, the obtained Au@MoS2 nanocomposite is used for the electrocatalytic hydrogen evolution reaction (HER) in acid medium, exhibiting excellent performance with a low overpotential of 178 mV at the current density of 10 mA/cm2, a small Tafel slope of 43.3 mV/dec, and excellent HER stability. This work paves a way for direct synthesis of TMD monolayers with high-concentration of unusual crystal phase for the electrocatalytic application.

Esaki Diodes Based on 2-D/3-D Heterojunctions

Esaki diodes based on interband tunneling have the characteristics of negative differential resistance (NDR) and ultrafast transient time, which lead to broad applications including oscillators, multivalue memories, and terahertz detectors. In this paper, we present the first demonstration of Esaki diodes based on 2-D/3-D heterojunctions-more specifically, chemical vapor deposition MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> on degenerately doped silicon. As compared to traditional 3-D heterostructures, these 2-D/3-D heterostructures have the following advantages: dislocation-free 2-D crystals even when the lattices are mismatched, dangling bond-free surface, and capability for large-scale synthesis at low cost. In this paper, monolayer, bilayer, and trilayer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> are synthesized directly on degenerated Si substrate, forming the ultraclean heterostructures without surface contamination from tape and resist. Based on these pristine heterostructures, we are able to observe prominent NDR effect at room temperature. This NDR effect is attributed to the degenerately p-type doping in silicon and the natural n-type doping in MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . We also found that the peak voltage corresponding to the local maximum tunneling current depends on the MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thickness. While MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is changing from the monolayer (0.7 nm) to bulk (9.5 nm), the peak voltage increases from 0.8 to 1.6 V. This phenomenon can be explained by the energy-level differences between the monolayer and bulk MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . This paper provides the experimental groundwork for the synthesis of transition metal dichalcogenides on degenerately doped Si substrates and opens up new and exciting opportunities for electronic applications of 2-D/3-D heterostructures.

Synthesis of transition metal dichalcogenides and their heterostructures

As compared to the heterostructures based on bulk materials, the heterostructures based on transition metal dichalcogenides (TMDCs) have the advantages of atomically sharp hetero-interface and freedom from lattice-match restriction, which make them promising for nanoscale electronic and photonic devices. In this paper, we systematically investigate the synthesis of TMDC heterostructures using two-step processes via chemical vapor deposition (CVD). We found that if a MoS2 growth at 730 °C is followed by a WSe2 growth at 875 °C, the selenium atoms in the precursor will replace the sulfur atoms in MoS2 during the second step and form MoSe2/WSe2 lateral heterostructures. On the other hand, if WSe2 growth is followed by MoS2 growth, WSe2/MoS2 lateral heterostructures were formed and no ion-exchange was observed. This result indicates that in the two-step processes, low-temperature growth followed by high-temperature growth may result in ion-exchange in the first material, while high-temperature growth followed by low-temperature growth results in epitaxy growth along the edge without ion-exchange. In addition, we have investigated the synthesis of MoS2 and WSe2 on sapphire substrates. In contrast to MoS2 grown on SiO2/Si substrates where the domains are randomly oriented, the domain edges of MoS2 grown on sapphire substrates are mostly aligned to the primary directions, which can significantly reduce the amount of grain boundaries when the domains are merged together to form continuous films. This finding will be critical to enable the fabrication of large-scale electronic devices with high-performance and uniform distributions across the wafers based on these materials.

Effect of Carrier Localization on Electrical Transport and Noise at Individual Grain Boundaries in Monolayer MoS2.

Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries (GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS2 layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is ∼30-70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases exponentially when the temperature is reduced. The microscopic framework of electrical transport and noise developed in this paper may be readily extended to other strongly localized two-dimensional systems, including other members of the TMDC family.