High-quality Monolayer MoS2 Research Articles

Experimental and theoretical investigation on pre-deposited precursor as growth sites for monolayer MoS2 growth by supercritical fluid deposition

The assistance of promoters, Molybdates or alkali metal salts can regulate the nucleation site of MoS2 in chemical vapor deposition. However, the introduction of impurities inevitably reduces the MoS2 quality. Here, we present the growth of monolayer MoS2 by supercritical fluid deposition to pre-deposit Mo(CO)6 precursors as nucleation sites, replacing difficult to clean conventional promoters. Domain-limited homogeneous and heterogeneous reactions occur at nucleation sites, and through optimization of sulfurization parameters, high-quality monolayer MoS2 with about 20 μm domain size can be obtained by growing at 760 °C and 40 sccm argon for 20 min. Monolayer MoS2 coverage ranged from 2.6 % to 39.1 % by regulating dissolution mass in range of 100–400 mg, utilizing the highly soluble Mo(CO)6 in supercritical CO2. Density functional theory calculations show that decarbonylated Mo(CO)6 possesses higher sulfide reactivity. This study provides new insights into the impurity-free controlled site growth of two-dimensional materials.

Molten glass-mediated conditional CVD growth of MoS2 monolayers and effect of surface treatment on their optical properties

In the rapidly developing field of optoelectronics, the utilization of transition-metal dichalcogenides with adjustable band gaps holds great promise. MoS2, in particular, has garnered considerable attention owing to its versatility. However, a persistent challenge is to establish a simple, reliable and scalable method for large-scale synthesis of continuous monolayer films. In this study, we report the growth of continuous large-area monolayer MoS2 films using a glass-assisted chemical vapor deposition (CVD) process. High-quality monolayer films were achieved by precisely controlling carrier gas flow and sulfur vaporization with a customized CVD system. Additionally, we explored the impact of chemical treatment using lithium bistrifluoromethylsulfonylamine (Li-TFSI) salt on the optical properties of monolayer MoS2 crystals. To investigate the evolution of excitonic characteristics, we conditionally grew monolayer MoS2 flakes by controlling sulfur evaporation. We reported two scenarios on MoS2 films and flakes based on substrate-related strain and defect density. Our findings revealed that high-quality monolayer MoS2 films exhibited lower treatment efficiency due to substrate-induced surface strain. whereas defective monolayer MoS2 flakes demonstrated a higher treatment sensitivity due to the p-doping effect. The Li-TFSI-induced changes in exciton density were elucidated through photoluminescence, Raman, and x-ray photoelectron spectroscopy results. Furthermore, we demonstrated treatment-related healing in flakes under variable laser excitation power. The advancements highlighted in our study carry significant implications for the scalable fabrication of diverse optoelectronic devices, potentially paving the way for widespread real-world applications.

Batch production of centimeter-scale monolayer MoS2 with low nucleation by face-to-face metal precursor supply

Transition-metal dichalcogenides (TMDs) show great potential for next-generation semiconductor due to its atomic level thickness, tunable band gap and high carrier mobility. However, high-quality monolayer MoS2 (ML-MoS2) films require stringent growth conditions during preparation. Here, we report a face-to-face metal precursor supply approach during chemical vapor deposition (CVD) to batch production of uniform ML-MoS2 films. Mo foils provide a stable concentration of metal precursors leading to uniform film generation with low nucleation. Comprehensive characterization techniques, including spectroscopic, surface topography and atomic scale analysis confirm the centimeter-scale homogeneity and high crystallographic quality of the monolayer films. Back-gate field effect transistors (FET) fabricated by ML-MoS2 exhibit excellent electrical performance. This methodology provides a pathway for the growth of TMDs, which should promote their applications in high-performance electronics or other fields.

High-quality MoS2 monolayers with largely enhanced electrical properties by plasma-treated SiO2/Si substrates based chemical vapor deposition

The atomic chalcogen vacancies are generally considered to be intrinsic defects of two-dimensional transition metal dichalcogenides prepared either by chemical vapor deposition or mechanical exfoliation, which could be detrimental to the intrinsic properties and device performance. Here we report a plasma-pretreated SiO2/Si substrate based chemical vapor deposition strategy for the synthesis of high quality MoS2 single crystal monolayers with largely enhanced electrical properties. First, the plasma-pretreated SiO2/Si substrate possesses much better hydrophilicity than the pristine one, and this promotes the adsorption of sodium molybdate solution (metal source) via spin coating, and thus facilitates the growth of high quality and large size MoS2 single crystal monolayers with fewer sulfur vacancies. As a result, the as-fabricated MoS2 FETs exhibit enhanced ON-state current density by nearly three order of magnitude, improved carrier mobility by around 70-fold up to 38.8 cm2 V−1 s−1. Second, the treated SiO2 surface have active oxygen bonds, which can bond with S atoms of the subsequently growing MoS2 monolayers, introducing n-type doping to the final sample, as verified by optical and electrical measurements. Our work provides a non-destructive but effective strategy towards reducing intrinsic defect density and tuning carrier density through engineering the growing substrate surface.

One-step method to control the temperature of S and Mo source and its effect on the size and morphology of MoS2

Molybdenum disulfide (MoS2) is a typical two-dimensional material, which is widely used in many fields because of its excellent electrical and optical properties. In recent years, the high-quality growth of MoS2 has attracted more and more attention from researchers. Many studies have reported a two-step method for the production of MoS2 by pre-deposition of molybdenum (Mo) source and then vulcanization, which is cumbersome and time-consuming, and the preparation temperature is mostly above 800 °C, increasing energy consumption and experimental risk. In this paper, S source and Mo source can be distinguished by double temperature zone tube furnace to prepare MoS2 material in one-step method. The temperatures of S and Mo sources were designed separately, and the effect of each temperature on the quality of MoS2 was investigated. And the monolayer MoS2 with high crystalline quality was obtained at a S source temperature of 165 °C and a Mo source temperature of 680 °C. We produce MoS2 in one-step method at low temperature, which provides a reference for the synthesis of high-quality monolayer MoS2.

MoS2-Thin Film Transistor Based Flexible 2T1C Driving Circuits for Active-Matrix Displays.

Two-dimensional (2D) semiconductors offer great potential as high-performance materials for thin film transistors (TFTs) in displays. Their thin, stable, and flexible nature, along with excellent electrical properties, makes them suitable for flexible displays. However, previous demonstrations lacked clear superiority in pixel resolution and TFT performance. Here we present the flexible 2T1C pixel driving circuit for active-matrix displays based on high-quality large-scale monolayer MoS2. A gate-first fabrication process was developed for flexible MoS2-TFTs, showing a remarkable carrier mobility (average at 52.8 cm2 V-1 s-1), high on/off ratio (average at 1.5 × 108), and negligible hysteresis. The driving current can be modulated by pulsed input voltages and demonstrates a stable and prompt response to both frequency and amplitude. We also demonstrated a 10 × 10 active-matrix with high resolution of 508 pixels per inch, exhibiting 100% yield and high uniformity. The driving circuit works well under bending up to ∼0.91% strain, highlighting its normal functions in flexible displays.

Scalable growth of atomically thin MoS2 layers in a conventional MOCVD system using molybdenum dichloride dioxide as the molybdenum source

To bring atomically thin transition metal dichalcogenides (TMDs) to practical application, a highly reproducible process to grow them over a large scale is indispensable. Here, we develop a carbon-free reproducible route for the scalable growth of high-quality monolayer MoS2 by selecting molybdenum dichloride dioxide (MoO2Cl2) as the Mo source and integrating the precursor with a standard low-pressure cold-wall metalorganic chemical vapor deposition (MOCVD) system. Specifically, combined with H2S as the sulfur source and using catalytic Dragontrail glass (DT-glass) as the main substrate, we investigate the effect of MoO2Cl2 flux, temperature, and deposition time on the growth, confirming MoO2Cl2 with reasonably high vapor pressure is well compatible with the MOCVD system that enables precise control of sources for uniform nucleation and growth of MoS2 over a large area. We successfully demonstrate the growth of carbon-free MoS2 monolayers on DT-glass with decent crystalline, optical, and electrical properties. Additionally, the initial attempts also manifest that the proposed strategy could be generalized for growing ultrathin MoS2 on other technologically important substrates, such as SiO2/Si and quartz, exhibiting superior optical quality and wafer-scale uniformity. This work provides a new avenue for the large-scale production of high-quality MoS2 monolayers and facilitates their use in practical applications.

Probing Sulfur Vacancies in CVD-Grown Monolayer MoS2 on SiO2/Si in the Temperature Range 750–900°C

This work reports morphologically alike, high-quality monolayer MoS2 flakes with a similar strain at various growth temperatures (750–900°C) achieved by adjusting sulfur temperature. The growth dynamics of MoS2 are correlated with changes in the photoluminescence (PL) and Raman peak positions. Monolayer MoS2 crystals are synthesized at different growth temperatures from 750°C to 900°C using chemical vapor deposition (CVD). We examined the structural quality and aimed to extract the recombination mechanisms in MoS2 using low-temperature, variable, and low-laser-intensity PL measurements. Our studies of the defect-associated bound exciton emission are well correlated with the blueshift in the A1g mode of Raman spectra, blueshift in PL spectra, and x-ray photoelectron spectroscopy results for crystal grown at 900°C. Our research findings not only shed light on a thorough, non-intrusive method for modifying growth parameters to enhance optical performance, but they also suggest a way to modify the optical characteristics of MoS2 while maintaining the morphology.

Nucleation and Growth of Monolayer MoS2 at Multisteps of MoO2 Crystals by Sulfurization

Two-dimensional (2D) materials and their heterostructures are promising for next-generation optoelectronics, spintronics, valleytronics, and electronics. Despite recent progress in various growth studies of 2D materials, mechanical exfoliation of flakes is still the most common method to obtain high-quality 2D materials because precisely controlling material growth and synthesizing a single domain during the growth process of 2D materials, for the desired shape and quality, is challenging. Here, we report the nucleation and growth behaviors of monolayer MoS2 by sulfurizing a faceted monoclinic MoO2 crystal. The MoS2 layers nucleated at the thickness steps of the MoO2 crystal and grew epitaxially with crystalline correlation to the MoO2 surface. The epitaxially grown MoS2 layer expands outwardly on the SiO2 substrate, resulting in a monolayer single-crystal film, despite multiple nucleations of MoS2 layers on the MoO2 surface owing to several thickness steps. Although the photoluminescence of MoS2 is quenched owing to efficient charge transfer between MoS2 and metallic MoO2, the MoS2 stretched out to the SiO2 substrate shows a high carrier mobility of (15 cm2 V-1 s-1), indicating that a high-quality monolayer MoS2 film can be grown using the MoO2 crystal as a seed and precursor. Our work shows a method to grow high-quality MoS2 using a faceted MoO2 crystal and provides a deeper understanding of the nucleation and growth of 2D materials on a step-like surface.

Highly Efficient Deposition of Centimeter-Scale MoS2 Monolayer Film on Dragontrail Glass with Large Single-Crystalline Domains.

Highly efficient growth of a centimeter-scale MoS2 monolayer film by oxide scale sublimation chemical vapor deposition (OSSCVD) in a time as short as 60 s is reported. Benefiting from the superior catalytic ability of Dragontrail glass (DT-glass) substrate and the controlled large vapor supersaturation of the molybdenum source, the ultrafast deposition of MoS2 is realized with maintaining large-sized single-crystalline domains over 20µm at maximum in the film. It is comparable to those reported for MoS2 grown in tens of minutes and even hours. Similar to the face-to-face precursor feed route, the gas-controlled OSSCVD with a showerhead configuration facilitates a homogeneous and controllable source supply. It enables high-quality monolayer MoS2 film deposition on 2 × 2 cm2 DT-glass with centimeter-scale uniformity confirmed by microscopic, spectroscopic, and electrical characterizations. Back-gate MoS2 field-effect transistors fabricated on polycrystalline continuous film exhibit the maximum field-effect mobility of 5.1 cm2 V-1 s-1 and a peak Ion /Ioff ratio of 5 × 108 . They reach 40 cm2 V-1 s-1 and 1.2 × 109 , respectively, on single-crystalline domains. These results are even greater than those for MoS2 grown using 1-2 orders of magnitude longer deposition time and higher temperatures. This study highlights the opportunities for low-cost high-throughput production of large-area high-quality monolayer MoS2 .

Robust room-temperature ferromagnetism induced by defect engineering in monolayer MoS2

Magnetism in two-dimensional materials is of great importance for exploring new physical phenomena and developing novel devices in nanoscale. It’s still a challenge to induce robust room temperature ferromagnetism in monolayer transition metal dichalcogen compound materials. In this paper, high quality monolayer MoS2 was prepared by chemical vapor deposition and low energy Cu ions were implanted into MoS2 via ion implantation. The surface morphology, chemical states, optical and magnetic properties were systematically investigated. Robust room-temperature ferromagnetism with saturation magnetization as high as 58 μB/Cu (∼2148 emu/cm3) was successfully induced in monolayer MoS2 after Cu implantation. Moreover, the magnetic properties are weakly temperature dependent, which is clearly the characteristic of defect-induced magnetism. First principles density functional theory calculations reveal that various structural defects, including nanohole, CuMo, Cui + VS and CuMo + VS can induce high magnetic moments. Meanwhile, CuMo and Cui + VS have lower formation energies and may contribute significantly to the extremely large magnetism in the implanted monolayer MoS2. Our work has demonstrated that through defects engineering by Cu implantation, robust room temperature ferromagnetism can be achieved in monolayer MoS2, which provides a potential way to induce large magnetization in other transition metal dichalcogen compound materials, and opens up new opportunities for their applications in nano-spintronics.

Pass-Transistor Logic Circuits Based on Wafer-Scale 2D Semiconductors.

2D semiconductors, such as molybdenum disulfide (MoS2 ), have attracted tremendous attention in constructing advanced monolithic integrated circuits (ICs) for future flexible and energy-efficient electronics. However, the development of large-scale ICs based on 2D materials is still in its early stage, mainly due to the non-uniformity of the individual devices and little investigation of device and circuit-level optimization. Herein, a 4-inch high-quality monolayer MoS2 film is successfully synthesized, which is then used to fabricate top-gated (TG) MoS2 field-effect transistors with wafer-scale uniformity. Some basic circuits such as static random access memory and ring oscillators are examined. A pass-transistor logic configuration based on pseudo-NMOS is then employed to design more complex MoS2 logic circuits, which are successfully fabricated with proper logic functions tested. These preliminary integration efforts show the promising potential of wafer-scale 2D semiconductors for application in complex ICs.

Layer-by-layer epitaxy of multi-layer MoS2 wafers.

The 2D semiconductor of MoS2 has great potential for advanced electronics technologies beyond silicon. So far, high-quality monolayer MoS2 wafers have been available and various demonstrations from individual transistors to integrated circuits have also been shown. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer. However, achieving high-quality multi-layer MoS2 wafers remains a challenge. Here we report the growth of high-quality multi-layer MoS2 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to six. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements in device performances were found in thicker-layer field-effect transistors (FETs), as expected. For example, the average field-effect mobility (μFE) at room temperature (RT) can increase from ∼80 cm2·V–1·s–1 for monolayers to ∼110/145 cm2·V–1·s–1 for bilayer/trilayer devices. The highest RT μFE of 234.7 cm2·V–1·s–1 and record-high on-current densities of 1.70 mA·μm–1 at Vds = 2 V were also achieved in trilayer MoS2 FETs with a high on/off ratio of >107. Our work hence moves a step closer to practical applications of 2D MoS2 in electronics.

Imaging and analysis of multidimensional atomically thin transition metal dichalcogenide layers on diatom frustules

Diatom frustules can exhibit multiple unique properties owing to their multidimensional and hierarchical pore structures. Herein, we report on the growth of transition metal dichalcogenides (TMDCs) on diatom frustules using metal–organic chemical vapor deposition. Structural analysis results revealed the conformal growth of primarily monolayer TMDCs not only on the outer surface but also inside the pore structures. Characterization and imaging using photoluminescence and micro-Raman analyses revealed that the resulting structures exhibit high-quality and uniform monolayer MoS2 films on the diatom frustules throughout the complex morphologies. Furthermore, these atomically thin layers provided a uniform conducting path for electrons from the sample surface to ground during scanning electron microscopy operations. This feature can minimize the reduction and/or deformation of the signals obtained with a conventional conducting layer coating, thereby enabling imaging and analysis of the diatom frustules during energy-dispersive X-ray spectroscopy measurements and focused ion-beam processing.

A Novel Carbon-Assisted Chemical Vapor Deposition Growth of Large-Area Uniform Monolayer MoS2 and WS2

Monolayer MoS2 can be used for various applications such as flexible optoelectronics and electronics due to its exceptional optical and electronic properties. For these applications, large-area synthesis of high-quality monolayer MoS2 is highly desirable. However, the conventional chemical vapor deposition (CVD) method using MoO3 and S powder has shown limitations in synthesizing high-quality monolayer MoS2 over a large area on a substrate. In this study, we present a novel carbon cloth-assisted CVD method for large-area uniform synthesis of high-quality monolayer MoS2. While the conventional CVD method produces thick MoS2 films in the center of the substrate and forms MoS2 monolayers at the edge of the thick MoS2 films, our carbon cloth-assisted CVD method uniformly grows high-quality monolayer MoS2 in the center of the substrate. The as-synthesized monolayer MoS2 was characterized in detail by Raman/photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. We reveal the growth process of monolayer MoS2 initiated from MoS2 seeds by synthesizing monolayer MoS2 with varying reaction times. In addition, we show that the CVD method employing carbon powder also produces uniform monolayer MoS2 without forming thick MoS2 films in the center of the substrate. This confirms that the large-area growth of monolayer MoS2 using the carbon cloth-assisted CVD method is mainly due to reducing properties of the carbon material, rather than the effect of covering the carbon cloth. Furthermore, we demonstrate that our carbon cloth-assisted CVD method is generally applicable to large-area uniform synthesis of other monolayer transition metal dichalcogenides, including monolayer WS2.

Gas-Phase "Prehistory" and Molecular Precursors in Monolayer Metal Dichalcogenides Synthesis: The Case of MoS2.

Two-dimensional MoS2 is one of the most promising materials for nanoelectronics due to its semiconducting nature and plethora of extraordinary properties. The main method for mass production of large-scale, high-quality MoS2 monolayers is chemical vapor deposition (CVD). Yet, the details of the chemistry occurring during the synthesis remain largely unknown, hindering process optimization. Combining ab initio molecular dynamics (AIMD) simulations and first-principles calculations allows us to explore the complete processes of MoS2 monolayer growth at the atomic level. We find that solid MoO3 precursor sublimates forming ringlike molecules, such as Mo3O9, which can later be regarded as gas-phase Mo-carrier reactants, undergoing sulfurization in three main stages: ring opening, chain breaking as the rate-limiting step, and further sulfurization. The fully sulfurized MoS6 molecule emerges as an immediate gas precursor to the crystal growth, as it reacts to join the MoS2-layer edge, with the release of a S4 molecule. Our comprehensive study provides detailed insights into the microscopic reaction mechanisms of MoS2 CVD growth and guidance for optimizing the synthesis parameters for transition metal dichalcogenides.

Synthesis of High-Performance Monolayer Molybdenum Disulfide at Low Temperature.

The large-area synthesis of high-quality MoS2 plays an important role in realizing industrial applications of optoelectronics, nanoelectronics, and flexible devices. However, current techniques for chemical vapor deposition (CVD)-grown MoS2 require a high synthetic temperature and a transfer process, which limits its utilization in device fabrications. Here, the direct synthesis of high-quality monolayer MoS2 with the domain size up to 120µm by metal-organic CVD (MOCVD) at a temperature of 320°C is reported. Owing to the low-substrate temperature, the MOCVD-grown MoS2 exhibits low impurity doping and nearly unstrained properties on the growth substrate, demonstrating enhanced electronic performance with high electron mobility of 68.3 cm2 V-1 s-1 at room temperature. In addition, by tuning the precursor ratio, a better understanding of the MoS2 growth process via a geometric model of the MoS2 flake shape, is developed, which can provide further guidance for the synthesis of 2D materials.

Wafer-Scale Oxygen-Doped MoS2 Monolayer.

Monolayer MoS2 is an emergent 2D semiconductor for next-generation miniaturized and flexible electronics. Although the high-quality monolayer MoS2 is already available at wafer scale, doping of it uniformly remains an unsolved problem. Such doping is of great importance in view of not only tailoring its properties but also facilitating many potential large-scale applications. In this work, the uniform oxygen doping of 2 in wafer-scale monolayer MoS2 (MoS2- x Ox ) with tunable doping levels is realized through an in situ chemical vapor deposition process. Interestingly, ultrafast infrared spectroscopy measurements and first-principles calculations reveal a reduction of bandgaps of monolayer MoS2- x Ox with increased oxygen-doping levels. Field-effect transistors and logic devices are also fabricated based on these wafer-scale MoS2- x Ox monolayers, and excellent electronic performances are achieved, exhibiting promise of such doped MoS2 monolayers.

Salt-assisted growth of monolayer MoS2 for high-performance hysteresis-free field-effect transistor

Atomically thin layered materials such as MoS2 have future versatile applications in low power electronics. Here, we demonstrate the growth of a salt-assisted large scale, high-quality monolayer MoS2 toward the realization of a high-performance hysteresis-free field-effect transistor (FET). Density functional theory calculations are implemented to monitor the effects of the Schottky barrier and metal-induced gap states between our metal electrodes and MoS2 for achieving high carrier transport. The role of absorbed molecules and oxide traps on the hysteresis are studied in detail. For the first time, a hysteresis-free intrinsic transistor behavior is obtained by an amplitude sweep pulse I–V measurement with varying pulse widths. Under this condition, a significant enhancement of the field-effect mobility up to 30 cm2 V−1 s−1 is achieved. Moreover, to correlate these results, a single-pulse time-domain drain current analysis is carried out to unleash the fast and slow transient charge trapping phenomena. Our findings on the hysteresis-free transfer characteristic and high intrinsic field-effect mobility in salt-assisted monolayer MoS2 FETs will be beneficial for future device applications in complex memory, logic, and sensor systems.

Growth of Monolayer and Multilayer MoS2 Films by Selection of Growth Mode: Two Pathways via Chemisorption and Physisorption of an Inorganic Molecular Precursor.

We report facile growth methods for high-quality monolayer and multilayer MoS2 films using MoOCl4 as the vapor-phase molecular Mo precursor. Compared to the conventional covalent solid-type Mo precursors, the growth pressure of MoOCl4 can be precisely controlled. This enables the selection of growth mode by adjusting growth pressure, which facilitates the control of the growth behavior as the growth termination at a monolayer or as the continuous growth to a multilayer. In addition, the use of carbon-free precursors eliminates concerns about carbon contamination in the produced MoS2 films. Systematic studies for unveiling the growth mechanism proved two growth modes, which are predominantly the physisorption and chemisorption of MoOCl4. Consequently, the thickness of MoS2 can be controlled by our method as the application demands.