High-quality Monolayer MoS2 Research Articles

Growth of Very Large MoS2 Single Crystals Using Out-Diffusion Transport and Their Use in Field Effect Transistors

Monolayer molybdenum disulfide (MoS2) is an attractive 2D material with a wide range of potential applications in the field of electronics and optoelectronics. To obtain the best performance, it is very necessary to grow large area single crystals of MoS2 (single domain) to avoid the effects of grain boundaries, but is exceptionally challenging to do this. Here, we report a novel method which we call out-diffusion vapor transport to grow large area single crystal monolayer MoS2 using an otherwise conventional chemical vapor deposition system. In this method, microchannels were created on the boat to significantly limit the region where MoOx vapor can react with S vapor to form crystals. This growth method resulted in triangular monolayer MoS2 single crystals up to ∼640 μm on a side grown on an oxidized silicon substrate, the largest crystals reported to date. Most of these crystals were multilayer at the center. This common feature has been identified in the literature as partially reduced transition metal oxide nucleates a second layer. We also achieved fully monolayer MoS2 single crystals up to ∼450 μm on a side, the largest demonstrated without the MoOx. Fabricated field effect transistors (FET) using MoS2 monolayer crystal as the active layer demonstrate a conventional n-type behavior, room-temperature mobility up to 45.5 cm2 V−1 s−1 and a maximum ON-Current (ION)/OFF-current (IOFF) ratio of 1.8 × 107. Raman and Photoluminescence results indicate that the as-grown large area monolayer crystals have high crystalline quality and uniformity with minimal defects, a finding that is consistent with the high electron mobility. This research work provides a superior technique to grow large-area high-quality single-crystal monolayer MoS2 without resorting to exotic equipment or techniques.

Excitons, trions and Rydberg states in monolayer MoS2 revealed by low-temperature photocurrent spectroscopy

Exciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temperature photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS2 phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons ({mathrm{X}}_{1{mathrm{s}}}^{mathrm{A}} and {mathrm{X}}_{1{mathrm{s}}}^{mathrm{B}}) and charged trions (TA and TB) as well as up to eight additional spectral lines at energies above the {mathrm{X}}_{1{mathrm{s}}}^{mathrm{B}} transition, which we attribute to the Rydberg series of excited states of XA and XB. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states.

High-Quality CVD-MoS2 Synthesized on Surface-Modified Al2O3 for High-Performance MoS2 Field-Effect Transistors

In this article, an effective way of chemical modification on the dielectric surface (Al2O3) is investigated to chemical vapor deposition (CVD)-grown high-quality monolayer MoS2 and the relevant back-gated FETs are fabricated without MoS2 transfer. As a result, the size of the triangle MoS2 is increased and its quality is improved as the surface of Al2O3 is treated by H2SO4. Furthermore, as compared with the gate dielectrics of the SiO2 and as-deposited Al2O3, the fabricated transistor with the H2SO4-treated Al2O3 as gate dielectric achieves better electrical properties: high carrier mobility of 12.9 cm2/Vs (~10 times higher than the untreated sample, ~5.2 times higher than the SiO2 gate-dielectric sample), small subthreshold swing of 110 mV/dec, and high ON/OFF ratio of $3\times 10^{{6}}$ . The involved mechanisms are attributed to the fact that the H2SO4-treated Al2O3 not only can increase its surface roughness to promote the nucleation and high-quality growth of MoS2 but also can improve the quality of the MoS2/Al2O3 interface. This simple chemical-modification treatment will open up an effective approach of combining the high-crystallinity CVD-MoS2 with the high- ${k}$ dielectric without MoS2 transfer required.

Tunable friction of monolayer MoS2 by control of interfacial chemistry

In this report, we utilize the heterostructures of monolayer MoS2 and self-assembled monolayers (SAMs) of organic molecules as a platform to understand how carrier density of nanomaterials affects their friction behaviors. Previous studies on the friction of two-dimensional materials have explored the effect of lattice structures and morphologies. Given the same normal force and scanning speed of the AFM tip, we observe the sliding friction of high-quality n-type monolayer MoS2 could be reduced by SAMs. The friction tunability is attributed to the charge transfer from MoS2to SAMs, which modulated the carrier density and hence the carrier-mediated friction behaviors in 2D MoS2. ab initio simulations and nanoscale mapping of work functions support this hypothesis. These SAMs-based heterostructures provide a potential tool to control friction in low-dimensional materials, and also an original perspective on the effect of electron–phonon coupling on friction at nanoscale.

Wafer-Scale Highly Oriented Monolayer MoS2 with Large Domain Sizes.

Two-dimensional molybdenum disulfide (MoS2) is an emergent semiconductor with great potential in next-generation scaled-up electronics, but the production of high-quality monolayer MoS2 wafers still remains a challenge. Here, we report an epitaxy route toward 4 in. monolayer MoS2 wafers with highly oriented and large domains on sapphire. Benefiting from a multisource design for our chemical vapor deposition setup and the optimization of the growth process, we successfully realized material uniformity across the entire 4 in. wafer and greater than 100 μm domain size on average. These monolayers exhibit the best electronic quality ever reported, as evidenced from our spectroscopic and transport characterizations. Our work moves a step closer to practical applications of monolayer MoS2.

Defect Passivation and Photoluminescence Enhancement of Monolayer MoS2 Crystals through Sodium Halide-Assisted Chemical Vapor Deposition Growth.

Recent success in the chemical vapor deposition (CVD) growth of atomically thin transition metal dichalcogenide (TMD) crystals opens up prospects for exploiting these materials in nanoelectronic and optoelectronic devices. However, CVD-grown TMDs often suffer from weak crystal quality because of the formation of defects during the growth, which makes a large impact on their electrical and optical properties. Here, we report a facile synthesis of high-quality MoS2 monolayers through a sodium halide-assisted CVD method. Our results show that the addition of sodium halides into MoO3 precursors leads to the rapid growth of highly crystalline MoS2 monolayers. Moreover, the overall photoluminescence (PL) intensity of MoS2 monolayers can be greatly enhanced by up to 2 orders of magnitude. The PL enhancement originates from that the deep trap states induced by sulfur vacancies are passivated by the substitution doping of halogen atoms, which promotes the emission of excitons and trions. Density functional theory calculations indicate that the band gaps of halogen-doped MoS2 monolayers are slightly smaller than those of pristine MoS2 monolayer, which is responsible for the small red shift of PL peaks (∼30 meV). These findings provide a new route toward engineering electronic and optical properties of MoS2 and other TMD monolayers.

Synthesis of High-Quality Monolayer MoS2 by Direct Liquid Injection

We report the synthesis of high-quality single monolayer MoS2 samples using a novel technique that utilizes direct liquid injection (DLI) for delivery of precursors. The DLI system vaporizes a liquid consisting of a selected precursor dissolved in a solvent into small, micron-sized droplets in an expansion chamber maintained at a selected temperature and pressure, before delivery to the deposition chamber. We demonstrate synthesis of monolayer MoS2 on SiO2/Si substrates using the DLI technique with film quality superior to exfoliated samples or those grown by traditional tube furnace chemical vapor deposition (CVD) methods. Photoluminescence measurements of DLI monolayers exhibit consistently brighter emission, narrower line width and higher emission energy than their exfoliated and CVD counterparts. Conductive atomic force microscopy identifies a defect density of 8.3 x 10^11/cm2 in DLI MoS2, lower than the measured density in CVD material and nearly an order of magnitude improvement over the exfoliated MoS2 investigated under the same conditions. The DLI method is directly applicable to many other van der Waals materials which require the use of challenging low vapor pressure precursors, to growth of alloys, and sequential growths of dissimilar materials leading to van der Waals heterostructures.

Remarkable quality improvement of as-grown monolayer MoS2 by sulfur vapor pretreatment of SiO2/Si substrates.

Monolayer MoS2 is a direct bandgap semiconductor which is believed to be one of the most promising candidates for optoelectronic devices. Chemical vapor deposition (CVD) is the most popular method to synthesize monolayer MoS2 with a large area. However, many defects are always found in monolayer MoS2 grown by CVD, such as sulfur vacancies, which severely degrade the performance of devices. This work demonstrates a concise and effective method for direct growth of high quality monolayer MoS2 by using SiO2/Si substrates pretreated with sulfur vapor. The MoS2 monolayer obtained using this method shows about 20 times PL intensity enhancement and a much narrower PL peak width than that grown on untreated substrates. Detailed characterization studies reveal that MoS2 grown on sulfur vapor pretreated SiO2/Si substrates has a much lower density of sulfur vacancies. The synthesis of monolayer MoS2 with high optical quality and low defect concentration is critical for both fundamental physics studies and potential practical device applications in the atomically thin limit.

Effect of Growth Temperature on Physical Properties of MoS2 Thin Films Synthesized by CVD

Due to the application of two-dimensional crystals in different fields, high-quality growth of these materials has attracted more attention from researchers. High-quality monolayer MoS2 with single crystals up to 20 microns in size has been formed on Si substrate by the chemical vapor deposition method. A comprehensive study was carried out on the prepared MoS2 thin films using optical microscopy, atomic force microscopy, x-ray diffraction (XRD) analysis, and Raman spectroscopy. It was concluded that the growth temperature affected the morphology and structure of the synthesized MoS2 sheets. The XRD spectra confirmed that the peak intensity and resolution were highly dependent on the growth temperature. Raman spectroscopy showed that monolayer MoS2 was grown on the silicon substrate at higher temperature, as proved by the Raman frequency difference (∼ 19 cm−1) between two characteristic modes ($$ {\hbox{E}}^{1}_{{2{\rm{g}}}} $$ and A1g). Atomic force micrographs of the films showed the evolution of the surface morphology as a function of the growth temperature.

Revealing the Grain Boundary Formation Mechanism and Kinetics during Polycrystalline MoS2 Growth.

Controllable synthesis of MoS2 with desired grain morphology via chemical vapor deposition (CVD) or physical vapor deposition (PVD) remains a challenge. Hence, it is important to understand polycrystalline growth of MoS2 and further provide guidelines for its CVD/PVD growth. Here, we formulate a kinetic Monte Carlo (kMC) model aiming at predicting the grain boundary (GB) formation in the CVD/PVD growth of polycrystalline MoS2. In the kMC model, the grain growth is via kink nucleation and propagation, whose energetic parameters and initial nucleus details are either from first-principles calculations or from experiments. Using the kMC model, we perform extensive simulations to predict the GB formation by using two, three, four, and five initial nuclei and compare the simulation results with previous experimental results. The obtained GB morphologies are in an excellent agreement with those experimental results. These agreements suggest that the proposed kMC model can correctly capture the mechanism and kinetics of GB formation. In particular, we reveal that the formation of smooth/rough GB is dictated by the two growth vectors for the kink propagation at the two associated grain edges, which is validated by our high-resolution scanning transmission electron microscopy images for PVD growth of MoS2 grains. Besides, we have made predictions beyond reproducing experimental observations, including the growth with artificially designed nuclei, the morphology transformation by tuning the Mo and S sources, and the formation of high-quality single-crystalline monolayer MoS2 by using single-crystalline substrates with vicinal steps. Our kMC model may serve as a powerful predictive tool for the CVD/PVD growth of monolayer MoS2 with desired GB configurations.

Controlled growth of MoS2 via surface-energy alterations

Monolayer MoS2 in triangular configurations with rich edges or high-quality uniform films are either catalytically active for the hydrogen evolution reaction or flexible for functional electronic and optoelectronic devices. Here, we have experimentally discovered that these two types of MoS2 products can be selectively synthesized on graphene or sapphire substrates, which are associated with both different adsorption energy and diffusion-energy barrier for vapor precursors during growth. Our study not only provides insights into the on-surface synthesis of high-quality MoS2 monolayers, but also can be applied to the growth of vertically-stacked and large-scale in-plane lateral MoS2–graphene heterostructures.

Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors

With the increasing demand for high integration and multi-color photodetection for both military and civilian applications, the research of multi-wavelength detectors has become a new research hotspot. However, current research has been mainly in visible dual- or multi-wavelength detectors, while integration of both visible light and ultraviolet (UV) dual-wavelength detectors has rarely been studied. In this work, large-scale and high-quality monolayer MoS2 was grown by the chemical vapor deposition method on transparent free-standing GaN substrate. Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors was demonstrated using common semiconductor fabrication technologies such as photolithography, argon plasma etching, and metal deposition. High performance of a 280 nm and 405 nm dual-wavelength photodetector was realized. The responsivity of the UV detector reached 172.12 A/W, while that of the visible detector reached 17.5 A/W. Meanwhile, both photodetectors achieved high photocurrent gain, high external quantum efficiency, high normalized detection rate, and low noise equivalent power. Our study extends the future application of dual-wavelength detectors for image sensing and optical communication.

Wafer-scale MOCVD growth of monolayer MoS2 on sapphire and SiO2

High-quality and large-scale growth of monolayer molybdenum disulfide (MoS2) has caught intensive attention because of its potential in many applications due to unique electronic properties. Here, we report the wafer-scale growth of high-quality monolayer MoS2 on singlecrystalline sapphire and also on SiO2 substrates by a facile metal-organic chemical vapor deposition (MOCVD) method. Prior to growth, an aqueous solution of sodium molybdate (Na2MoO4) is spun onto the substrates as the molybdenum precursor and diethyl sulfide ((C2H5)2S) is used as the sulfur precursor during the growth. The grown MoS2 films exhibit crystallinity, good electrical performance (electron mobility of 22 cm2·V-1·s-1) and structural continuity maintained over the entire wafer. The sapphire substrates are reusable for subsequent growth. The same method is applied for the synthesis of tungsten disulfide (WS2). Our work provides a facile, reproducible and cost-efficient method for the scalable fabrication of high-quality monolayer MoS2 for versatile applications, such as electronic and optoelectronic devices as well as the membranes for desalination and power generation.

Probing the Growth Improvement of Large-Size High Quality Monolayer MoS₂ by APCVD.

Two-dimensional transition metal dichalcogenides (TMDs) have attracted attention from researchers in recent years. Monolayer molybdenum disulfide (MoS2) is the direct band gap two-dimensional crystal with excellent physical and electrical properties. Monolayer MoS2 can effectively compensate for the lack of band gap of graphene in the field of nano-electronic devices, which is widely used in catalysis, transistors, optoelectronic devices, and integrated circuits. Therefore, it is critical to obtain high-quality, large size monolayer MoS2. The large-area uniform high-quality monolayer MoS2 is successfully grown on an SiO2/Si substrate with oxygen plasma treatment and graphene quantum dot solution by atmospheric pressure chemical vapor deposition (APCVD) in this paper. In addition, the effects of substrate processing conditions, such as oxygen plasma treatment time, power, and dosage of graphene quantum dot solution on growth quality and the area of the monolayer of MoS2, are studied systematically, which would contribute to the preparation of large-area high-quality monolayer MoS2. Analysis and characterization of monolayer MoS2 are carried out by Optical Microscopy, AFM, XPS, Raman, and Photoluminescence Spectroscopy. The results show that monolayer MoS2 is a large-area, uniform, and triangular with a side length of 200 μm, and it is very effective to treat the SiO2/Si substrate by oxygen plasma and graphene quantum dot solution, which would help the fabrication of optoelectronic devices.

MoS2-OH Bilayer-Mediated Growth of Inch-Sized Monolayer MoS2 on Arbitrary Substrates.

Due to remarkable electronic property, optical transparency, and mechanical flexibility, monolayer molybdenum disulfide (MoS2) has been demonstrated to be promising for electronic and optoelectronic devices. To date, the growth of high-quality and large-scale monolayer MoS2 has been one of the main challenges for practical applications. Here we present a MoS2-OH bilayer-mediated method that can fabricate inch-sized monolayer MoS2 on arbitrary substrates. This approach relies on a layer of hydroxide groups (-OH) that are preferentially attached to the (001) surface of MoS2 to form a MoS2-OH bilayer structure for growth of large-area monolayer MoS2 during the growth process. Specifically, the hydroxide layer impedes vertical growth of MoS2 layers along the [001] zone axis, promoting the monolayer growth of MoS2, constrains growth of the MoS2 monolayer only in the lateral direction into larger area, and effectively reduces sulfur vacancies and defects according to density functional theory calculations. Finally, the hydroxide groups advantageously prevent the MoS2 from interface oxidation in air, rendering high-quality MoS2 monolayers with carrier mobility up to ∼30 cm2 V-1 s-1. Using this approach, inch-sized uniform monolayer MoS2 has been fabricated on the sapphire and mica and high-quality monolayer MoS2 of single-crystalline domains exceeding 200 μm has been grown on various substrates including amorphous SiO2 and quartz and crystalline Si, SiC, Si3N4, and graphene This method provides a new opportunity for the monolayer growth of other two-dimensional transition metal dichalcogenides such as WS2 and MoSe2.

Self-Limiting Growth of High-Quality 2D Monolayer MoS2 by Direct Sulfurization Using Precursor-Soluble Substrates for Advanced Field-Effect Transistors and Photodetectors

Monolayer transition metal dichalcogenides (TMDs) have demonstrated great potential in next-generation electronics due to their unique optical and electronic properties. However, it remains challenging to produce uniform high-quality TMDs over a large scale. The direct sulfurization method holds great promise in achieving large-scale synthesis, but the obtained materials suffer from small grain size and multilayer regions. Herein, low-cost glass substrate is used to achieve facile growth of large-area uniform and large-size monolayer MoS2 crystals via a modified direct sulfurization method. We find that the ability of glass to incorporate predeposited precursors into its molten state is the key to the production of high-quality monolayer MoS2 crystals. The monolayer MoS2 crystals possess the largest average crystal size (∼100 μm) for MoS2 grown by the direct sulfurization method, with large-area uniformity, which is only limited by the size of the substrate. A combination of low-cost, uniformity, scalability, simplicity, and high quality is achieved in our method which showed great promise for the development of wafer scale electronic devices based on 2D materials. Our work also provides a new look at the role substrates could play in the synthesis and the possibilities of using other glass or liquid substrates for the growth of high-quality 2D materials.

Effects of Strategically Placed Water Droplets on Monolayer Growth of Molybdenum Disulfide

Two-dimensional (2D) molybdenum disulfide (MoS2) films with a tunable bandgap hold great promise for next-generation electronic and optoelectronic devices. Synthesis of large areas of high-quality MoS2 monolayers lacks experimental reproducibility. Moreover, the outcome of MoS2 growth by chemical vapor deposition is dependent on several interconnected growth parameters. In this study, we present results of MoS2 monolayer growth by strategically placing water droplets on the growth substrate and/or in the source prior to its loading in the growth chamber. The volume and distribution of water on the growth substrate and in the source had a direct impact on the morphology of the as-grown MoS2. Characterized by scanning electron microscopy (SEM), Raman microscopy, and atomic force microscopy (AFM), the number and size of MoS2 layers as well as its distribution on the growth substrate were found to have a strong dependence on the positioning of the water droplet. This study on MoS2 monolayer growth using water droplets as a promoter provides a simple and reproducible experimental technique enabling growth with high reliability.

High-Temperature Continuous-Wave Pumped Lasing from Large-Area Monolayer Semiconductors Grown by Chemical Vapor Deposition.

The realization of low-energy-consumption lasers based on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for the development of optical communications, flexible displays, and lasers on the chip level. However, among the as-demonstrated TMDC-based lasers so far, the gain materials are mainly achieved by a mechanical exfoliation approach accompanied by poor reproducibility and controllability. In this work, we report a controllable design for generating large-scale lasing from chemical vapor deposition (CVD)-derived high-quality monolayer MoS2 film. Strong continuous-wave optically driven whispering-gallery-mode lasing is achieved in a wide temperature range from 77 to 400 K. The eminent lasing performances result from the strong spatial confinement of carriers and the enhanced efficiency of spontaneous emission owing to the lensing and screening effects of silica microsphere cavities. These findings not only advance the fundamental understanding of 2D lasing effects but also provide solutions to fabricate low-cost, scalable, and integratable TMDC-based lasers.

High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS2.

Two-dimensional semiconductors such as monolayer MoS2 are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, vsat. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS2. We find that in typical device geometries (e.g. on SiO2 substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron vsat = (3.4 ± 0.4) × 106 cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS2 could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal management.

Atomic-scale defects and electronic properties of a transferred synthesized MoS2 monolayer

MoS2 monolayer samples were synthesized on a SiO2/Si wafer and transferred to Ir(111) for nano-scale characterization. The samples were extensively characterized during every step of the transfer process, and MoS2 on the final substrate was examined down to the atomic level by scanning tunneling microscopy (STM). The procedures conducted yielded high-quality monolayer MoS2 of milimeter-scale size with an average defect density of 2 × 1013 cm–2. The lift-off from the growth substrate was followed by a release of the tensile strain, visible in a widening of the optical band gap measured by photoluminescence. Subsequent transfer to the Ir(111) surface led to a strong drop of this optical signal but without further shifts of characteristic peaks. The electronic band gap was measured by scanning tunneling spectroscopy (STS), revealing n-doping and lateral nano-scale variations. The combined use of STM imaging and density functional theory (DFT) calculations allows us to identify the most recurring point-like defects as S vacancies.