Area MoS2 Research Articles

Direct liquid injection pulsed-pressure MOCVD of large area MoS2 on Si/SiO2.

Large-scale, high-quality growth of transition metal dichalcogenides (TMD) of controlled thickness is paramount for many applications in opto- and microelectronics. This paper describes the direct growth of well-controlled large area molybdenum disulfide (MoS2) on Si/SiO2 substrates by direct liquid injection pulsed-pressure metal-organic chemical vapor deposition (DLI-PP-MOCVD) using low-toxicity precursors. It is shown that control of the deposited thickness can be achieved by carefully tuning the amount of molybdenum precursor evaporated and that continuous layers are routinely obtained. Homogeneity and reproducibility have also been examined, as well as the average size of the grains. When targeting monolayer thickness, the MoS2 showed near stoichiometry (S/Mo = 1.93-1.95), low roughness and high photoluminescence (PL) quantum yield, equivalent to exfoliated monolayers and CVD MoS2 grown on the same substrates.

Large area van der Waals MoS2–WS2 heterostructures for visible-light energy conversion

Large area MoS2–WS2 van der Waals heterostructures have been fabricated using a physical deposition process. The type-II band alignment promotes electro-optical photoconversion and enhances organic pollutant photodissociation.

Solubility of MoS2 and Graphite in Molten Salt: Flowers, Faceted Crystals, or Exfoliation?

2D materials are of interest in various applications such as energy conversion, storage, and sensing. These materials are prepared by bottom‐up or top‐down methods that are difficult to control and suffer from low yield. Synthesis in molten salt is suggested as an alternative in which the balance between exfoliation and solubility is explored. It is demonstrated that when a pellet of 2D material is insoluble in molten salt (graphite in any salt), it is exfoliated. For low solubility (MoS2 in NaCl/KCl), the 2D material nucleates and grows into a small flowerlike structure composed of thin MoS2 sheets through Ostwald ripening. For high solubility in the molten salt (MoS2 in CsCl), it forms larger flowers. Herein, the molten salt treatment of high and low surface area MoS2 (micron‐size particles and a single large pellet, respectively) is compared. The particles yielded facet MoS2 crystals through dissolution–nucleation–recrystallization process and the pellet yielded flowers. Herein, methods for synthesizing 2D materials with controllable size and shape are promoted by simple molten salt treatment that opens an avenue to the development of soluble (MoS2) and non‐soluble (graphite) materials with different morphologies in a simple and affordable way.

Large area MoS2 thin film growth by direct sulfurization

In this study, we present the growth of monolayer MoS2 (molybdenum disulfide) film. Mo (molybdenum) film was formed on a sapphire substrate through e-beam evaporation, and triangular MoS2 film was grown by direct sulfurization. First, the growth of MoS2 was observed under an optical microscope. The number of MoS2 layers was analyzed by Raman spectrum, atomic force microscope (AFM), and photoluminescence spectroscopy (PL) measurement. Different sapphire substrate regions have different growth conditions of MoS2. The growth of MoS2 is optimized by controlling the amount and location of precursors, adjusting the appropriate growing temperature and time, and establishing proper ventilation. Experimental results show the successful growth of a large-area single-layer MoS2 on a sapphire substrate through direct sulfurization under a suitable environment. The thickness of the MoS2 film determined by AFM measurement is about 0.73 nm. The peak difference between the Raman measurement shift of 386 and 405 cm−1 is 19.1 cm−1, and the peak of PL measurement is about 677 nm, which is converted into energy of 1.83 eV, which is the size of the direct energy gap of the MoS2 thin film. The results verify the distribution of the number of grown layers. Based on the observation of the optical microscope (OM) images, MoS2 continuously grows from a single layer of discretely distributed triangular single-crystal grains into a single-layer large-area MoS2 film. This work provides a reference for growing MoS2 in a large area. We expect to apply this structure to various heterojunctions, sensors, solar cells, and thin-film transistors.

Large area MoS2 films fabricated via sol-gel used for photodetectors

Two-dimensional molybdenum disulfide (2D-MoS2) has been extensively exploited to fabricate devices for applications ranging from optoelectronics to gas sensing and energy harvesting. The major limitations in the production of 2D-MoS2 regard the scalability and reproducibility of the fabrication limiting the possibility to produce large area devices. In order to bridge the scalability needs with facile fabrication techniques, solution processed MoS2 has recently been explored as a possible pathway. In this work, we optimized a recipe for the making of MoS2 films through sol-gel synthesis. The new recipe allows for the deposition of large area (∼cm2) MoS2 films on various dielectric and metallic substrates. The obtained MoS2 films are investigated through a wide range of characterization techniques that demonstrate that the films display chemical and physical properties expected for 2D-MoS2. Moreover, the properties are found to be homogeneous all over the film area. In order to test the material in practical applications, we fabricated and characterized photodetector devices. The photodetectors display responsivity values of few mA/W in the NUV-VIS-NIR spectrum, widening the spectral response of previously reported MoS2 based photodetectors. Although the device response time and light-to-dark current ratio are affected by residual electronic defects in the material, this work demonstrates a scalable and versatile process to fabricate 2D-MoS2 and photodetectors based thereof.

Fabrication of 3D ordered mesoporous MoS2/C composite with few-layered MoS2 for electrochemical hydrogen evolution

MoS2 is a highly active catalyst for hydrogen evolution reaction (HER), but its active site for HER is limited to the edges. Thus structures exposing plenty of edges is desired for MoS2 to apply in HER. Porous three-dimensional (3D) structure full of few-layered MoS2 satisfies the requirement. In this work, a 3D ordered mesoporous structure MoS2/C composite with few-layered MoS2 was synthesized by liquid phase nanocasting method, using SBA-15 as hard template, sucrose as carbon source and ammonium tetrathiomolybdate (ATTM) as MoS2 precursor. The limiting effect of amorphous carbon makes thin MoS2 slices evenly dispersed, and avoids occurrence of MoS2 agglomeration, resulting in the exposure of a large number of MoS2 edges as active sites. The 3D ordered mesoporous structure of the catalyst provides high specific surface areas and ensures transport channels for material and electron for electrochemical HER. As a result, the composite demonstrates efficient HER activity with an overpotential of 165 mV at current density of 10 mA/cm2, and a Tafel slope of 91 mV under acidic conditions. This study provides a basis for constructing 3D HER catalyst with high specific surface area and few-layered MoS2 uniformly dispersed.

Enhancing Raman spectra by coupling plasmons and excitons for large area MoS2 monolayers

Surface-enhanced Raman spectroscopy (SERS) has been applied to analyze 2D materials with plasmonic enhancement at hotspots, localized at gaps or junctions. However, the SERS technique strongly demands hotspots at excitation laser wavelengths to achieve energy confinement at the excitation wavelength. In this study, we report the enhancement of Raman signals by the plasmon-exciton coupling between large-area monolayer MoS2 and plasmonic nanogrooves (NGs). The plasmon-exciton coupling enhances Raman signal instead of SERS enhancement at an excitation wavelength. In addition, spectrally tunable plasmonic NGs are utilized to investigate coupled and de-coupled enhancements. The enhancement performance is further examined through electromagnetic field enhancement. We demonstrate that the proposed nanostructures integrated with atomically thin 2D materials, showing highly uniform Raman signals, are applicable for broad applications in materials science and nanophotonic.

Reliable and broad-range layer identification of Au-assisted exfoliated large area MoS2 and WS2 using reflection spectroscopic fingerprints

The emerging Au-assisted exfoliation technique provides a wealth of large-area and high-quality ultrathin two-dimensional (2D) materials compared with traditional tape-based exfoliation. Fast, damage-free, and reliable determination of the layer number of such 2D films is essential to study layer-dependent physics and promote device applications. Here, an optical method has been developed for simple, high throughput, and accurate determination of the layer number for Au-assisted exfoliated MoS$_2$ and WS$_2$ films in a broad thickness range. The method is based on quantitative analysis of layer-dependent white light reflection spectra, revealing that the reflection peak intensity can be used as a clear indicator for determining the layer number. The simple yet robust method will facilitate the fundamental study on layer-dependent optical, electrical, and thermal properties and device applications of 2D materials. The technique can also be readily combined with photoluminescence and Raman spectroscopies to study other layer-dependent physical properties of 2D materials.

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.

Surface-Enhanced Resonance Raman Spectroscopy by Plasmon-Exciton Coupling for Large Area Mos2 Monolayers Characterization

Surface-Enhanced Resonance Raman Spectroscopy by Plasmon-Exciton Coupling for Large Area Mos2 Monolayers Characterization

Substrate impact on the thickness dependence of vibrational and optical properties of large area MoS2 produced by gold-assisted exfoliation

The gold-assisted exfoliation is a very effective method to produce large-area (cm2-scale) membranes of molybdenum disulfide (MoS2) for electronics. However, the strong MoS2/Au interaction, beneficial for the exfoliation process, has a strong impact on the vibrational and light emission properties of MoS2. Here, we report an atomic force microscopy, micro-Raman, and micro-photoluminescence (μ-PL) investigation of 2H-MoS2 with variable thickness exfoliated on Au and subsequently transferred on an Al2O3/Si substrate. The E2g–A1g vibrational mode separation Δω (typically used to estimate MoS2 thickness) exhibits an anomalous large value (Δω ≈ 21.2 cm−1) for monolayer (1L) MoS2 on Au as compared to the typical one (Δω ≈ 18.5 cm−1) measured on 1L MoS2 on Al2O3. Such substrate-related differences, explained in terms of tensile strain and p-type doping arising from the MoS2/Au interaction, were found to gradually decrease while increasing the number of MoS2 layers. Furthermore, μ-PL spectra for 1L MoS2 on Au exhibit a strong quenching and an overall redshift of the main emission peak at 1.79 eV, compared to the 1.84 eV peak for 1L MoS2 on Al2O3. After PL spectra deconvolution, such redshift was explained in terms of a higher trion/exciton intensity ratio, probably due to the higher polarizability of the metal substrate, as well as to the smaller equilibrium distance at the MoS2/Au interface.

Mechanical transfer of electrochemically grown molybdenum sulfide layers to silicon wafer

Large area MoS2 ultra-thin film deposition is one of the big challenges in the recent years. Electrodeposition provides an opportunity to grow such ultra-thin films on large scale. However, the transfer of the electrochemically grown film is challenging. Standard transfer of those thin films is done by wet etching in which the underlying substrate is etched. In this work, the polymer coated electrodeposited MoS2 films on Au are separated mechanically from the underlying substrate by using ultra-sonication. Collapse of micron-sized bubbles produced by ultra-sonication at the interface of Au and silicon substrate provides enough energy for separation due to their weak adhesion. The Au layer is then removed by standard Au-etchant (K/KI) and the polymer coated film is transferred to a desired substrate. Ammonium tetrathiomolybdate (ATTM) has been used as precursor material for the electrodeposition of the films. Initial electrochemically grown films consist of MoS3 which is reduced to MoS2 by a post-annealing step at 450–900 °C. Obtained films are investigated by AFM, Raman, UV–Vis and XPS. Crystal quality improves by increasing the post-annealing temperature. The thickness of the thinnest film was found to be equivalent to 2 monolayers of MoS2, which is desirable for future electronics.Graphic abstract

Effect of precursor ratio on the morphological and optical properties of CVD-grown monolayer MoS2 nanosheets

Atmosphere pressure chemical vapor deposition (CVD) is one of the most powerful methods of synthesizing high quality and large area MoS2 films with a reasonable cost. In our work, the large-scale and high crystalline quality monolayer MoS2 nanosheets were synthesized on Silicon substrate with a 300 nm oxide layer using MoO3 and S powders as precursors by an atmosphere pressure CVD. The results suggest that the surface morphology, crystalline quality and luminescence of CVD-grown MoS2 nanosheets can be tunable by controlling the precursor ratio (the effective Mo: S ratio). Excessive S-rich atmosphere is favor to synthesize large-size and high crystalline quality monolayer MoS2 nanosheets with sharp corners and straight edges. This study may provide insight into the synthesis of large-scale and high crystalline quality MoS2 films.

Enhancement in optically induced ultrafast THz response of MoSe2MoS2 heterobilayer.

THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed.

Scalable lateral heterojunction by chemical doping of 2D TMD thin films

Scalable heterojunctions based on two-dimensional transitional metal dichalcogenides are of great importance for their applications in the next generation of electronic and optoelectronic devices. However, reliable techniques for the fabrication of such heterojunctions are still at its infancy. Here we demonstrate a simple technique for the scalable fabrication of lateral heterojunctions via selective chemical doping of TMD thin films. We demonstrate that the resistance of large area MoS2 and MoSe2 thin film, prepared via low pressure chalcogenation of molybdenum film, decreases by up to two orders of magnitude upon doping using benzyl viologen (BV) molecule. X-ray photoelectron spectroscopy (XPS) measurements confirms n-doping of the films by BV molecules. Since thin films of MoS2 and MoSe2 are typically more resistive than their exfoliated and co-evaporation based CVD counterparts, the decrease in resistance by BV doping represents a significant step in the utilization of these samples in electronic devices. Using selective BV doping, we simultaneously fabricated many lateral heterojunctions in 1 cm2 MoS2 and 1 cm2 MoSe2 films. The electrical transport measurements performed across the heterojunctions exhibit current rectification behavior due to a band offset created between the doped and undoped regions of the material. Almost 84% of the fabricated devices showed rectification behavior demonstrating the scalability of this technique.

Electrical properties tunability of large area MoS2 thin films by oxygen plasma treatment

MoS2 thin films prepared via sulfurization of molybdenum films have attracted great attention due to their advantage for scalable synthesis with a large area coverage. However, the MoS2 thin films are typically more resistive than their exfoliated and co-evaporation chemical vapor deposition based counterparts. The ability to modulate the electrical property of MoS2 thin films will have a significant impact on scalable device applications in electronics, sensors, and catalysis. Here, we report the tuning of electrical transport properties of large area MoS2 thin films with different oxygen plasma exposure times. The electrical transport measurements of the pristine and plasma treated samples reveal that with increasing oxygen plasma treatment, the resistance of the MoS2 thin films first decreases by almost an order of magnitude and then increases again. The x-ray photoelectron spectroscopy measurements show that the S:Mo ratio continuously decreases with increasing plasma exposure time. For a short plasma exposure time, the resistance decrease can be explained due to the creation of sulfur vacancies leaving unsaturated electrons with molybdenum (Mo) atoms which act as electron donors. With increasing plasma exposure, more sulfur vacancies and hence more Mo atoms are created, many of which get converted to insulating MoO3 resulting in an increase in the resistance of the MoS2 thin film. The results presented here are a major step forward in realizing the overreaching goals of MoS2 thin films for practical device applications.

Low pressure sulfurization and characterization of multilayer MoS2 for potential applications in supercapacitors

The emergence of two-dimensional molybdenum disulfide (MoS2) has prompted enormous research attention as supercapacitor electrode materials due to their exciting electrochemical, electrical and mechanical properties. In this work, we present a low-pressure technique to grow multilayer MoS2 by sulfurization of molybdenum thin films at three different temperatures (700, 750 and 800 °C) to study the effect of growth temperatures on surface morphology, film thickness and film quality. Large area growth of MoS2 throughout the Si/SiO2 substrate was achieved at all the sulfurization temperature. The as-prepared MoS2 films were characterized by several techniques including X-ray diffraction, atomic force microscopy, scanning electron microscopy, Raman spectroscopy and photoluminescence spectroscopy. Although MoS2 is successfully grown at different temperatures, the surface morphology, thickness and crystallite size have varied significantly with temperature. Our results presented here will contribute towards the reproducible growth of large area MoS2 at the desired temperature for energy storage applications.

High performance UV photodetector based on MoS2 layers grown by pulsed laser deposition technique

Highly efficient ultraviolet (UV) photodetector based on Molybdenum Disulfide (MoS 2 ) layers has been fabricated using pulsed laser deposition (PLD) technique. Systematic layer dependent photoresponse studies have been performed from single layer to 10 layers of MoS 2 by varying the laser pulses to see the effect of the number of layers on the photoelectrical measurements. Raman and Photoluminescence studies have been carried out to ensure the growth of high-quality MoS 2 layers. Layers of MoS 2 grown at 100 pulses were found to exhibit the characteristic Raman phonon modes i.e. E 1 2g and A 1g at 383.8 cm −1 and 405.1 cm −1 respectively and Photoluminescence (PL) spectra show B exciton peak for MoS 2 at around 625 nm suggesting the growth of high-quality MoS 2 layers. Atomic force microscopy (AFM) thickness profiling and cross sectional-high resolution transmission electron microscopy (HRTEM) analysis gives the thickness of grown MoS 2 to be 2.074 nm and 1.94 nm, respectively, confirming the growth of trilayers of MoS 2 . X-ray photoelectron spectroscopy (XPS) spectra of the grown trilayer sample show characteristic peaks corresponding to Molybdenum and Sulphur doublet (Mo 4+ 3d 5/2,3/2 and S 2p 3/2 , 1/2 ) confirming the chemical state of pure MoS 2 phase without the presence of any Molybdenum oxide state. Dynamic photoelectrical studies with Indium Tin Oxide (ITO) as contact electrode upon UV laser illumination show superior responsivity of 3 × 10 4 A/W at 24 μW optical power of the incident laser (λ = 365 nm) and very high detectivity of 1.81 × 10 14 Jones at a low applied bias of 2 V. The obtained results are highly encouraging for the realization of low power consumption and highly efficient UV photodetectors based on MoS 2 layers. • Optimization of high-quality large area MoS2 layers utilizing the PLD technique. • Demonstration of low power operated UV photodetector on optimized MoS2 layers. • Explained the Physics of the UV photodetector using energy band diagrams. • Work justifies the development of efficient photodetectors using PLD technique.

Annealing effects on sulfur vacancies and electronic transport of MoS2 films grown by pulsed-laser deposition

We have synthesized high quality and large area MoS2 films on flexible fluorophlogopite substrates using the pulsed-laser deposition (PLD) technique. Annealing in a sufficient sulfur atmosphere was adopted to eliminate oxide molybdenum and sulfur vacancies introduced during the growth in the vacuum chamber. X-ray photoelectron spectroscopy results demonstrate the advantages benefitted from the annealing process. The S/Mo ratio of the annealed MoS2 film was 1.98:1, which was much closer to the theoretical value. Raman spectroscopy, Photoluminescence spectroscopy, and X-ray diffraction spectroscopy provided direct evidence for the crystallinity improvement. Due to the elimination of molybdenum oxide, the Fermi level was shifted by 0.175 eV, and the conductive type changes from the Ohmic contact to the Schottky contact. The optimized method in this paper makes the PLD-derived MoS2 films promising candidates for microelectronic device application.

Synthesis and Application of Molybdenum Disulphide Thin Layers Grown By Chemical Vapour Deposition

Recently, two-dimensional (2D) molybdenum disulfide (MoS2) thin layers have been considered for the next generation optoelectronic devices, field effect transistors (FETs) and flexible electronics due to their flexibility, ultra-thin dimensions and excellent semiconducting properties. In this study, large area MoS2 mono-layers/ bi-layers/ tri-layers were grown on different types of substrates (SiO2/Si, Si) by chemical vapour deposition (CVD) technique. The MoS2 thin layers were synthesized at high temperatures (800 oC, 900 oC and 1000 oC) by using solid source precursors of molybdenum trioxide and sulphur in an argon atmosphere. The as-grown MoS2 thin layers were systematically characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) for phase analysis and to determine the surface morphology, respectively. Further, the thickness and surface roughness of the grown MoS2 thin layers were analysed by atomic force microscopy (AFM). Micro-Raman spectroscopy was used to identify the number of grown MoS2 layers and their crystalline quality. Finally, the electrical properties of the grown MoS2 thin layers were evaluated by using DC I-V measurements at room temperature in order to know the resistance of the grown layers.