High-quality MoS2 Research Articles

A modified CVD method for the synthesis of monolayer MoS2 and photoelectric improvement by HfO2 passivation

Molybdenum disulfide (MoS2) is an emerging class of new materials with a wide range of potential practical applications. However, the uncontrollability of monolayer MoS2 synthesized by traditional chemical vapor deposition method and the low responsivity of MoS2 photodetectors limit its further development in the field of photoelectric detection. To achieve controlled growth of monolayer MoS2 and construct MoS2 photodetectors with a high responsivity, we propose a novel single crystal growth strategy of high-quality MoS2 by controlling the Mo to S vapor ratio near the substrate, and deposit a layer of hafnium oxide (HfO2) on the surface of MoS2 to enhance the performance of the pristine metal-semiconductor-metal structure photodetector. At a reverse bias of 8 V, the HfO2 passivated MoS2 photodetector features an extremely high responsivity of a response time of around 0.5 s, and a detectivity of Meanwhile, we deeply investigate the effect of the HfO2 layer on the performance of the fabricated MoS2 photodetector and propose a physical mechanism to interpret the obtained experiment results. These results might facilitate a better understanding on the performance modulation of the MoS2 photodetectors and accelerate the development of MoS2-based optoelectronic devices.

Transferring 2D TMDs through water-soluble sodium salt catalytic layer

This study reports a clean and damage-free transfer method that enables the ultrafast transfer of two-dimensional (2D) transition metal dichalcogenides (TMDs) onto desired substrates with a remarkably high yield. We employ a water-soluble sodium salt as both a transfer sacrificial layer for facile transfer and a catalytic layer for the growth of high-quality large-area MoS2 using liquid-phase chemical vapor deposition via a catalyzed kinetic growth. We show that the pristine structural and electrical properties of the grown MoS2 can be reliably preserved by avoiding detrimental effects during the prolonged harsh-environment transfer process. We demonstrate the technological versatility of the proposed transfer method by fabricating as-transferred MoS2-based back-gated field-effect transistors (FETs). The MoS2 FETs exhibit excellent charge mobility as high as 28.7 cm2 V−1 s−1 and an on–off ratio up to ∼107 at room temperature, indicating no performance degradation after the transfer process. The proposed transfer method offers universal applicability for various 2D TMDs, mechanical supporting polymers, and target substrates, thus facilitating the facile fabrication of 2D TMD-based electronics and optoelectronics.

Modulation Effect of Substrate Interactions on Nucleation and Growth of MoS2 on Silica

Chemical vapor deposition is a well-established bottom-up technique to produce a high-quality single-crystal MoS2 film. In this technique, the substrate (e.g., silica) plays a crucial role in the segregation and chemical interaction of precursors to form grain-sized MoS2. However, the mechanisms of the surface interactions that influence the properties of MoS2 during growth are still poorly understood. Here, we combine ReaxFF reactive molecular dynamics simulations, density functional theory (DFT) calculations, and experimental growth of MoS2 to provide an atomic insight into the coupling between the surface chemistry and the MoS2 nucleation and growth on a silica surface. Our experimental results show that the dehydroxylated surface stays mainly inert during the MoO3 flow, while the presence of hydroxyl groups leads to MoO3 nucleation on the substrate. ReaxFF and DFT simulations further confirm that the reaction between MoO3 and the hydroxylated surface is both thermodynamically and kinetically driven, indicating that hydroxyl groups formed on silica enhance the chemical reactivity of the surface toward MoO3 molecules and promote the growth. Additionally, the MoS2 growth on the hydroxylated silica support initiates with MoO3 nucleation followed by the chalcogen–surface interactions. Moreover, the existence of the substrate catalyzes the growth by lowering the growth temperature, providing an effective way for energy saving and cost reduction. These results demonstrate the intricate role of surface engineering in controlling and promoting large-area MoS2 growth.

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.

Inkjet-Printed MoS2 Nanoplates on Flexible Substrates for High-Performance Field Effect Transistors and Gas Sensing Applications

Owing to the simplicity, scalability, and cost-efficiency, solution-processable two-dimensional (2D) semiconductors have attracted great interest in electronic applications, especially as the channel material for field-effect transistors (FETs). Inkjet printing is a lithography-free technique to achieve drop-on-demand patterning of solution-processable 2D ink. However, thus far, inkjet-printed 2D FETs exhibit limited performance due to the coffee-ring effect and consequent discontinuity of the printed 2D material films. Here, we report high-performance and flexible inkjet-printed MoS2 FETs with high mobilities and high on/off ratios and their gas sensing applications. By preparing high-quality MoS2 ink comprised of MoS2 nanoplates using electrochemical exfoliation and then applying a binary solvent comprised of 2-butanol and isopropanol, the obtained ink was printed to form a continuous and relatively uniform MoS2 film, and high-performance printed MoS2 FETs were demonstrated, with mobilities of 11 cm2 V–1 s–1 and on/off ratios of 106. Furthermore, low-voltage gate modulation was achieved by applying an ion gel gate, and robust electrical performance under tensile strain was observed for the ion gel-gated MoS2 FETs printed on flexible substrates. As the printed MoS2 film is abundant in edge sites and sulfur vacancies, we further demonstrated our MoS2 FETs as high-performance gas sensors with a limit of detection of 10 ppb for NO2 and 0.5 ppm for NH3, together with a fast recovery rate.

Controllable p-type doping of 2D MoS2via Sodium intercalation for optoelectronics

We synthesized high-quality MoS2 flakes via the NaOH assisted CVD method. The morphological evolution of MoS2 flakes was investigated at different growth temperatures, and high-quality MoS2 were achieved with a photoresponsivity of 4.2 A W−1 and a specific detectivity of 8.1 × 108 Jones.

Hydrothermal synthesis of MoS2-decorated silicon nanowires heterostructure with enhanced performance of photocatalytic activity under visible light

Molybdenum disulfide (MoS2) is weakly stacked by vander Waals force, and the interaction force between the layers of MoS2 and Silicon is weak. Therefore, direct growth of homogeneous high quality MoS2 films on silicon substrates stays challenging. Herein, for the first time, few layered MoS2nanosheetswere successfully deposited onto silicon nanowires (SiNWs) through hydrothermal process. First, as prepared MoS2/SiNWs heterostructure was characterized by using Raman spectroscopy, Scanning Electron Microscopy (SEM) and by means of the Energy-Dispersive X-ray spectroscopy (EDX). Second, the photocatalytic activity ofsynthesizedMoS2/SiNWs photocatalyst was systematically evaluated for the decoloration of methylene blue (MB) under visible light irradiation. It was found that MoS2/SiNWs heterostructure exhibited excellent photocatalytic activity against this dye compared with synthesized SiNWs films. Also, it was found that after irradiation during 180 min, the MoS2/SiNWs film exhibited the highest photocatalytic efficiency (90%), in comparison to SiNWs (62%). Finally, it is assumed that the photocatalytic activity improvement was attributed to visible light absorption improvement, the enhancement of the photogenerated electron-hole pair’s separation efficiency. Moreover, thanks to the nanoflowers structure assembled by nanosheets with few layers, the specific surface area was increased offering more sites for MB pollutant adsorption.

Synthesis and ellipsometric characterizations of large-scale MoS2 sub-monolayer

Two-dimensional (2D) transitional metal dichalcogenides (TMDs) have drawn significantly interest motivated by their prominent properties for electronics beyond silicon. Despite extensive efforts, continuous TMDs monolayer films are still difficult to grow with high yield, and universal characterization methods potential for industrial mass production are lacking, which significantly hinder their applications. Here, we designed a controllable chemical vapor deposition method to directly synthesis high-quality and large-scale MoS2 monolayer with various surface coverage ratios (CRs), representing growth stages from nucleation to connected film. A series of optical characterization methods and dielectric analysis were conducted to systemically study the monolayer feature of MoS2. Then, we demonstrated an alternative characterization strategy by disclosing the relations of measured Spectroscopic Ellipsometry (SE) spectra ∆ and Ψ to the CRs of MoS2 sub-monolayer, achieving a direct model-free assessment. The SE spectra show extraordinary sensitivity to the sub-monolayer CRs. This work presents a paradigm to generally evaluate large-scale 2D materials and offers basic SE characterization toward industrial production.

Atomistic reaction mechanism of CVD grown MoS2 through MoO3 and H2S precursors

Chemical vapor deposition (CVD) through sulfidation of MoO3 is one of the most important synthesis techniques to obtain large-scale and high-quality two-dimensional (2D) MoS2. Recently, H2S precursor is being used in the CVD technique to synthesize 2D MoS2. Although several studies have been carried out to examine the mechanism of MoS2 growth in the presence of sulfur and MoO3 precursors, the growth of MoS2 in the presence of H2S precursor has largely remained unknown. In this study, we present a Reactive molecular dynamics (RMD) simulation to investigate the reaction mechanism of MoS2 from MoO3 and H2S precursors. The intermediate molecules formation, the reason behind those formations, and the surface compositions of MoOxSyHz during the initial steps of CVD have all been quantified. Surprisingly, a sudden separation of sulfur atoms from the surface was observed in the H2S precursor system due to the substantial oxygen evolution after 1660 K. The sulfur detachments and oxygen evolution from the surface were found to have a linear relationship. In addition, the intermediate molecules and surface bonds of MoS2 synthesized by MoO3 and H2S precursors were compared to those of a system using S2 and MoO3 precursors. The most stable subsidiary formation from the H2S precursor was found to be H2O, whereas in case of S2 precursor it was SO. These results provide a valuable insight in the formation of large-scale and high-quality 2D MoS2 by the CVD technique.

NaCl-Assisted Chemical Vapor Deposition of Large-Domain Bilayer MoS2 on Soda-Lime Glass.

In recent years, two-dimensional molybdenum disulfide (MoS2) has attracted extensive attention in the application field of next-generation electronics. Compared with single-layer MoS2, bilayer MoS2 has higher carrier mobility and has more promising applications for future novel electronic devices. Nevertheless, the large-scale low-cost synthesis of high-quality bilayer MoS2 still has much room for exploration, requiring further research. In this study, bilayer MoS2 crystals grown on soda-lime glass substrate by sodium chloride (NaCl)-assisted chemical vapor deposition (CVD) were reported, the growth mechanism of NaCl in CVD of bilayer MoS2 was analyzed, and the effects of molybdenum trioxide (Mo) mass and growth pressure on the growth of bilayer MoS2 under the assistance of NaCl were further explored. Through characterization with an optical microscope, atomic force microscopy and Raman analyzer, the domain size of bilayer MoS2 prepared by NaCl-assisted CVD was shown to reach 214 μm, which is a 4.2X improvement of the domain size of bilayer MoS2 prepared without NaCl-assisted CVD. Moreover, the bilayer structure accounted for about 85%, which is a 2.1X improvement of bilayer MoS2 prepared without NaCl-assisted CVD. This study provides a meaningful method for the growth of high-quality bilayer MoS2, and promotes the large-scale and low-cost applications of CVD MoS2.

Chemical Vapor Deposition of Uniform and Large-Domain Molybdenum Disulfide Crystals on Glass/Al2O3 Substrates.

Two-dimensional molybdenum disulfide (MoS2) has attracted significant attention for next-generation electronics, flexible devices, and optical applications. Chemical vapor deposition is the most promising route for the production of large-scale, high-quality MoS2 films. Recently, the chemical vapor deposition of MoS2 films on soda-lime glass has attracted great attention due to its low cost, fast growth, and large domain size. Typically, a piece of Mo foil or graphite needs to be used as a buffer layer between the glass substrates and the CVD system to prevent the glass substrates from being fragmented. In this study, a novel method was developed for synthesizing MoS2 on glass substrates. Inert Al2O3 was used as the buffer layer and high-quality, uniform, triangular monolayer MoS2 crystals with domain sizes larger than 400 μm were obtained. To demonstrate the advantages of glass/Al2O3 substrates, a direct comparison of CVD MoS2 on glass/Mo and glass/Al2O3 substrates was performed. When Mo foil was used as the buffer layer, serried small bilayer islands and bright core centers could be observed on the MoS2 domains at the center and edges of glass substrates. As a control, uniform MoS2 crystals were obtained when Al2O3 was used as the buffer layer, both at the center and the edge of glass substrates. Raman and PL spectra were further characterized to show the merit of glass/Al2O3 substrates. In addition, the thickness of MoS2 domains was confirmed by an atomic force microscope and the uniformity of MoS2 domains was verified by Raman mapping. This work provides a novel method for CVD MoS2 growth on soda-lime glass and is helpful in realizing commercial applications of MoS2.

Electrochemical Construction of Edge-Contacted Metal-Semiconductor Junctions with Low Contact Barrier.

2D semiconductors, such as MoS2 have emerged as promising ultrathin channel materials for the further scaling of field-effect transistors (FETs). However, the contact barrier at the metal-2D semiconductor junctions still significantly limits the device's performance. By extending the application of electrochemical deposition in 2D electronics, a distinct approach is developed for constructing metal-2D semiconductor junctions in an edge-contacted configuration through the edge-guided electrodeposition of varied metals. Both high-resolution microscopic imaging and electrical transport measurements confirm the successful creation of high-quality Pd-2D MoS2 junctions in desired geometry by combining electrodeposition with lithographic patterning. FETs are fabricated on the obtained Pd-2D MoS2 junctions and it is confirmed that these junctions exhibit a reduced contact barrier of ≈20meV and extremely low contact resistance of 290Ωµm and thus increase the averaged mobility of MoS2 FETs to ≈108cm2 V -1 s-1 . This approach paves a new way for the construction of metal-semiconductor junctions and also demonstrates the great potential of the electrochemical deposition technique in 2D electronics.

Modulated Photoluminescence of Single-Layer MoS2 on Various Rutile TiO2 Surfaces: Implications for Photocatalytic Applications

Hybridization with oxide semiconductors provides a versatile strategy for tailoring physicochemical properties of two-dimensional materials (2DMs). However, the direct impacts of specific interface interactions have not yet been very well categorized, in particular at an atomic level. In the present work, through a chemical vapor deposition (CVD) method, we successfully grew monolayer MoS2 flakes on an atomically smooth rutile TiO2 single crystal with (100), (110), and (001) terminations. We found that the fabrication of comparable high-quality MoS2 on all of the TiO2 substrates can only be achieved via finely varying the growth parameters. Moreover, the photoluminescence of MoS2 also changes against the substrate terminations, showing a gradually reduced A0/A– exciton ratio following the sequence of (100) > (110) > (001). Detailed X-ray photoelectron spectroscopy measurements showed the same tendency for the binding energy shifts of both Ti and O in the MoS2/TiO2 samples, which were attributed to the varied dipole fields established at the MoS2/TiO2 interfaces. Our work not only reinforces the important role of interface charge redistribution in tailoring the properties of hybridized systems but also emphasizes that the facet effect may be applied as an efficient strategy for optimizing the photocatalytic activities of compositional systems.

Boosting the photovoltaic performance of MoS2/Si heterojunction solar cells with thiourea-doped MoS2 films

Two-dimensional molybdenum disulfide film has unique physical and chemical properties, which make it a prominent candidate for optoelectronic device applications. In this work, the high-quality few-layer MoS2 films were successfully deposited by spin-coating the ammonium tetrathiomolybdate aqueous solution doped with an appropriate amount of thiourea on p-Si substrates. The effects of thiourea doping ratio on the microstructural properties of the prepared MoS2 films were analyzed by Raman spectrum, X-ray diffraction, and X-ray photoelectron spectroscopy. It was found that the few-layer MoS2 films with high crystallinity and low defect density can be formed at the thiourea doping ratio of 0.5. The electrical and photovoltaic properties of the MoS2/Si heterojunction solar cells fabricated by using the MoS2 film with different thiourea doping ratios were investigated by current density-voltage, external quantum efficiency, and capacitance-voltage measurements. The MoS2/Si heterojunction solar cell based on the optimized thiourea doping ratio achieves a power conversion efficiency of 9.81%. The results display a simple and convenient method for preparing the high-quality MoS2 films and boosting the performance of MoS2/Si heterojunction solar cells, providing a possibility to realize the low-cost and large-area MoS2-based optoelectronic devices.

Optimization of Growth Parameters of Thermal Chemical Vapor Deposition Method for 2D MoS2 Synthesis

Even after a decade of research on MoS2, it is still quite challenging to obtain high quality MoS2 films using a controlled synthesis technique. Out of all the available methods chemical vapor deposition (CVD) has proven to be a reliable technique to produce MoS2 thin films. The CVD growth process is sensitive to variety of parameters involved in the process. In the present work, we report the effect of pressure, temperature, gas flow, and position of substrate on the growth of MoS2 films. After several failures, the technique is optimized to synthesize high quality MoS2 films on quartz substrate. Further, the films are characterized using XRD, UV-Vis Spectroscopy, Photoluminescence Spectroscopy and Raman Spectroscopy. The main objective of this work is to formulate a method that produces MoS2 films using thermal CVD process by optimizing various parameters.

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.

Monolayer MoS2 of high mobility grown on SiO2 substrate by two-step chemical vapor deposition

We report a novel two-step ambient pressure chemical vapor deposition (CVD) pathway to grow high-quality MoS2 monolayer on the SiO2 substrate with large crystal size up to 110 μm. The large specific surface area of the pre-synthesized MoO3 flakes on the mica substrate compared to MoO3 powder could dramatically reduce the consumption of the Mo source. The electronic information inferred from the four-probe scanning tunneling microscope (4P-STM) image explains the threshold voltage variations and the n-type behavior observed in the two-terminal transport measurements. Furthermore, the direct van der Pauw transport also confirms its relatively high carrier mobility. Our study provides a reliable method to synthesize high-quality MoS2 monolayer, which is confirmed by the direct 4P-STM measurement results. Such methodology is a key step toward the large-scale growth of transition metal dichalcogenides (TMDs) on the SiO2 substrate and is essential to further development of the TMDs-related integrated devices.

Ambient Pressure Chemical Vapor Deposition of Flat and Vertically Aligned MoS2 Nanosheets

Molybdenum disulfide (MoS2) got tremendous attention due to its atomically thin body, rich physics, and high carrier mobility. The controlled synthesis of large area and high crystalline monolayer MoS2 nanosheets on diverse substrates remains a challenge for potential practical applications. Synthesizing different structured MoS2 nanosheets with horizontal and vertical orientations with respect to the substrate surface would bring a configurational versatility with benefit for numerous applications, including nanoelectronics, optoelectronics, and energy technologies. Among the proposed methods, ambient pressure chemical vapor deposition (AP-CVD) is a promising way for developing large-scale MoS2 nanosheets because of its high flexibility and facile approach. Here, we show an effective way for synthesizing large-scale horizontally and vertically aligned MoS2 on different substrates such as flat SiO2/Si, pre-patterned SiO2 and conductive substrates (TaN) benefit various direct TMDs production. In particular, we show precise control of CVD optimization for yielding high-quality MoS2 layers by changing growth zone configuration and the process steps. We demonstrated that the influence of configuration variability by local changes of the S to MoO3 precursor positions in the growth zones inside the CVD reactor is a key factor that results in differently oriented MoS2 formation. Finally, we show the layer quality and physical properties of as-grown MoS2 by means of different characterizations: Raman spectroscopy, scanning electron microscopy (SEM), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). These experimental findings provide a strong pathway for conformally recasting AP-CVD grown MoS2 in many different configurations (i.e., substrate variability) or motifs (i.e., vertical or planar alignment) with potential for flexible electronics, optoelectronics, memories to energy storage devices.

Synthesis and characterization of large-sized monolayer MoS2 nanoflakes by sulfurization of exfoliated MoO3 powder

We demonstrate a novel approach based on sulfurization of MoO3 powder pre-exfoliated on the SiO2/Si substrate to synthesize high crystalline quality and large-sized monolayer MoS2 films uniformly covering the whole substrate. The surface morphology, thickness, crystalline quality, and luminescence mechanism of the MoS2 films were characterized by the optical microscopy, Raman spectroscopy, and photoluminescence spectroscopy. The results reveal that MoS2 films directly synthesized on the bare SiO2/Si substrate by our method have high surface coverage superior over those of one-step chemical vapor deposition (CVD) method and exhibit uniform monolayer with larger lateral length up to ∼120 µm compared to those of the traditional two-step CVD method. Our proposed growth strategy opens up new pathways for CVD synthesis of high crystalline quality and large-sized uniform monolayer MoS2 films and can also be utilized for synthesizing other two-dimensional materials.

Strain-Assisted Single Pt Sites on High-Curvature MoS 2 Surface for Ultrasensitive H 2 S Sensing

Strain-Assisted Single Pt Sites on High-Curvature MoS ₂ Surface for Ultrasensitive H ₂ S Sensing