Quality MoS2 Research Articles

Foams-to-Films: A Facile Approach Towards Space-Confined CVD Growth of MoS2.

2D materials have rapidly become the building blocks for the next generation of semiconducting materials and devices, with Chemical vapor deposition (CVD) emerging as a prefered method for their synthesis. However, the predictable and reproducible growth of high quality, large 2D monolayers remains challenging. An important facet is controlling the local environment at the surface of the substrate - here, space-confinement techniques have emerged as promising candidates. We demonstrate that space-confined CVD growth using microstructured MoOx grown on Ni foam is an appealing approach for rapid growth of high quality MoS2 monolayers; a very important subset of 2D materials. This method eschews the use of powders which can be more difficult to control. By incorporation of a porous barrier in the Ni foam support, the rate of delivery of both the Mo and S source to the substrate is dampened, leading to coverage of large, high quality, mono-to-few layer triangular domains as confirmed by Raman and photoluminescence (PL) spectroscopies together with atomic force microscopy (AFM) height measurements.

MoS2/porous carbon nanofiber heterostructures for efficient evaporation-driven generators.

Evaporation power generators (EPGs) based on natural water evaporation can directly convert heat energy from the surrounding environment into electrical energy. However, EPGs are difficult to commercialize due to the low charge generation and transport efficiency of single material systems, resulting in unsatisfactory open-circuit voltages and short-circuit currents. Here, we step by step prepared MoS2/porous carbon nanofiber (PCNF) heterogeneous systems by electrospinning and hydrothermal methods. Electron microscope measurements proven the uniform coating of high-crystalline quality MoS2 nanosheets on PCNF fabrics, and the uneven concave-convex surface increased the specific surface area. MoS2 covered PCNF fabrics retained good hydrophilicity, which was suitable for absorbing water and keeping the surface wet during long-term evaporation. Moreover, layered MoS2 with rich surface charge improved the charge transfer of the MoS2/PCNF fabrics. As a result, the open-circuit voltage and short-circuit current of the EPGs prepared with MoS2/PCNF fabrics were improved to 0.25 V and 75 μA, respectively, compared with those based on PCNF fabrics, which demonstrated that the MoS2 coatings improved the interaction area with water and the charge transfer effect of the EPGs. This heterogeneous combination strategy provides ideas for the preparation of high-performance EPG materials.&#xD.

Development and optimization of large-scale integration of 2D material in memristors

Two-dimensional (2D) materials like transition metal dichalcogenides (TMD) have proved to be serious candidates to replace silicon in several technologies with enhanced performances. In this respect, the two remaining challenges are the wafer scale growth of TMDs and their integration into operational devices using clean room compatible processes. In this work, two different CMOS-compatible protocols are developed for the fabrication of MoS2-based memristors, and the resulting performances are compared. The quality of MoS2 at each stage of the process is characterized by Raman spectroscopy and x-ray photoemission spectroscopy. In the first protocol, the structure of MoS2 is preserved during transfer and patterning processes. However, a polymer layer with a minimum thickness of 3 nm remains at the surface of MoS2 limiting the electrical switching performances. In the second protocol, the contamination layer is completely removed resulting in improved electrical switching performances and reproducibility. Based on physico-chemical and electrical results, the switching mechanism is discussed in terms of conduction through grain boundaries.

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.

Visualized and Nondestructive Quality Identification of Two-Dimensional MoS2 Based on Principal Component Analysis.

To date, the common quality characterizations for MoS2 are inefficient or cause irreversible damage to the samples, which have limited scalability and low throughput. Here, we propose a visualized and nondestructive approach to evaluate the quality of MoS2 based on the PCA machine learning method. Through PCA processing of PL mapping, the CVD grown MoS2 with different edge defect densities can be well distinguished. Furthermore, six twin GBs along the sulfur zigzag direction of the six pointed MoS2 stars are also successfully identified. To verify the correctness of the identification results, we measured the lifetime mapping and thermal expansion coefficient of the synthesized MoS2 samples. It is found that the high quality MoS2 samples have a shorter carrier lifetime (∼0.291 ns) and lower thermal expansion coefficient (∼2.03 × 10-5K-1). Therefore, our work offers a new approach to evaluate the quality of MoS2 to drive their practical application.

Comparative Study between Sulfurized MoS2 from Molybdenum and Molybdenum Trioxide Precursors for Thin-Film Device Applications.

Two-dimensional (2D) materials have been studied as an emerging class of nanomaterials owing to their attractive properties in nearly every field of science and technology. Molybdenum disulfide (MoS2) is one of the more promising candidates of these atomically thin 2D materials for its technological potential. The facile synthesis of MoS2 remains a matter of broad interest. In this study, MoS2 was synthesized by chemical vapor deposition sulfurization at various temperatures (550 °C, 650 °C, and 750 °C) of either precursor molybdenum metal (Mo) or molybdenum trioxide (MoO3) deposited on silicon/silicon dioxide (Si/SiO2) via e-beam evaporation. Monolayer, bilayer, and few layers sulfurized samples have been grown and verified by Raman, photoluminescence spectroscopy, XRD, XPS, and AFM. MoO3 sulfurization provided monolayer growth in comparison to Mo sulfurization under the same conditions and precursor thicknesses. Optical microscopy showed the homogeneous nature of grown samples. A main finding of this work is that MoO3 sulfurization produced higher quality MoS2 as compared to those grown by an Mo precursor. Device characteristics based on monolayer MoO3 sulfurized MoS2-x include nonvolatile resistive switching with Ion/Ioff ≈ 104 at a relatively low operating bias of ±1 V. In addition, field-effect transistor characteristics revealed p-type material growth with a carrier mobility ∼ 41 cm2 V-1 s-1, which is in contrast to typically observed n-type characteristics.

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.

Magnetic NiFe thin films composing MoS2 nanostructures for spintronic application

We demonstrate a nanostructure layer made of Ni80Fe20 (permalloy:Py) thin film conjugated MoS2 nano-flakes. Layers are made based on a single-step co-deposition of Py and MoS2 from a single solution where ionic Ni and Fe and MoS2 flakes co-exist. Synthesized thin films with MoS2 flakes show increasing coercivity and enhancement in magneto-optical Kerr effect. Ferromagnetic resonance linewidth as well as the damping parameter increaseed significantly compared to that of the Py layer due to the presence of MoS2. Raman spectroscopy and elemental mapping is used to show the quality of MoS2 within the Py thin film. Our synthesis method promises new opportunities for electrochemical production of functional spintronic-based 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.

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.

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.

Importance of crystallinity improvement in MoS2 film by compound sputtering even followed by post sulfurization

The MoS2 film for chip-size area was synthesized by two step processes consisting of MoS2-compound sputtering and post sulfurization. We intentionally revealed that the crystallinity of sulfurized MoS2 film depends on that of just-after-sputtered film. Therefore, a crystallinity improvement just-after sputtering is mandatory to achieve an excellent quality MoS2 film after sulfur-vapor annealing for thin film transistor, sensor and human interface device applications.

Charge transfer accelerated by internal electric field of MoS2 QDs-BiOI p-n heterojunction for high performance cathodic PEC aptasensing

Integrating the advantages of excellent anti-interference and high stability superior to photoanode, cathodic photoelectrochemical (PEC) bioanalysis is promising and competitive in precise monitoring targets in complex matrices. However, serious consideration of the photocathode is far behind the anodic one. Herein, we report a high quality MoS2 QDs-BiOI p-n heterojunctioned material and exemplify the feasibility of constructing cathodic PEC aptasensing platform. Intense visible light-harvesting, high photoelectric conversion efficiency and magnified photocurrent is observed, which origin is detailed explored by various spectroscopic and electrochemical study. Taking the tumor necrosis factor-alpha (TNF-α) as a model analyte and by coupling the simplest target-induced conformational change mechanism, high selectivity and ultrasensitive self-power cathodic PEC detection is exemplified. Stable and reproducible PEC responses are achieved in a wide linear range of 10 fg/mL to 0.1 ug/mL with an ultralow detection limit (5.2 fg/mL) surpassing most sensors reported so far. Monitoring TNF-α in biological system is realized with desired accuracy and satisfactory recovery.

Influence of precursor thin-film quality on the structural properties of large-area MoS2 films grown by sulfurization of MoO3 on c-sapphire

In recent years, molybdenum disulfide (MoS2) has been investigated due to its unique electronic, optical, and mechanical properties with a variety of applications. Sulfurization of pre-deposited MoO3 layers is one of the methods of the preparation of large-area MoS2 thin films. The MoO3 layers have been grown on c-sapphire substrates, using two different techniques (rf sputtering, pulsed laser deposition). The films were subsequently annealed in vapors of sulfur at high temperatures what converted them to MoS2 films. The quality of MoS2 is strongly influenced by the properties of the precursor MoO3 layers. The pre-deposited MoO3, as well as the sulfurized MoS2, have been characterized by several techniques including Raman, Rutherford backscattering spectroscopy, atomic force microscopy, scanning electron microscopy, and X-ray diffraction. Here we compare two types of MoS2 films prepared from different MoO3 layers to determine the most suitable MoO3 layer properties providing good quality MoS2 films for future applications.

Dependence of laser parameters on structural properties of pulsed laser-deposited MoS2 thin films applicable for field effect transistors

The pulsed laser deposition (PLD) technique can be efficient and cost-effective in the fabrication of high-quality MoS2 thin films. The laser pulse parameters, such as the number of pulses, the pulse energy density, and the frequency, that influence the MoS2 thin films quality have been investigated. The optimum parameters of laser pulse for the best crystalline quality MoS2 films were determined by experiments. Back-gated field effect transistors (FETs) were fabricated based on the MoS2 thin film. The carrier mobility of the MoS2 back-gated FETs has reached 4.63 cm2 V−1 S−1. The responsivity of the MoS2 back-gated FETs is approximately 0.06 AW−1 at drain voltage of − 2 V. These results show that the back-gated FETs based on MoS2 thin films prepared by PLD can be applied to photodetectors.

Synthesis of Ultrahigh-Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules.

Monolayer transition metal dichalcogenides are 2D materials with many potential applications. Chemical vapor deposition (CVD) is a promising method to synthesize these materials. However, CVD-grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, thiol is proposed to be used as a liquid precursor for CVD growth of high quality and uniform 2D MoS2 . Atomic-resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS2 grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS2 . Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS2 and repair these defects during the growth of MoS2 , resulting in high-quality MoS2 . This work provides a facile and controllable method for the growth of high-quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications.

Enlargement of grain size for MoS2 film fabricated by RF magnetron sputtering with additional DC bias by optimization of deposition parameters and its evaluation with Raman spectroscopy

Raman spectroscopy was used to evaluate crystal quality of MoS2 films fabricated by RF magnetron sputtering with additional DC bias. Raman spectroscopy is a powerful tool to evaluate different physical properties of a film since phonons are sensitive to those properties. From the results of Raman spectroscopy measurements, improvement of crystal quality was shown corresponding to increase of the DC bias voltage, increase in the substrate temperature, and decrease of the deposition rate. Furthermore, the grain size was quantitatively evaluated by using the frequency deference between E12g and A1g Raman peaks. Through these evaluation it was revealed that the sputter parameter adjustment listed above (increase in DC bias voltage, etc.) increased grain size showing the film quality improvement quantitatively. Therefore, optimization of these sputtering parameters contributes to the fabrication of high quality MoS2 film at low-temperature.

Photoluminescence of CVD-grown MoS2 modified by pH under aqueous solutions toward potential biological sensing

Transition metal dichalcogenide (TMD) represented by molybdenum disulfide (MoS2) is a promising platform for versatile applications in biosensing. The semiconducting properties of MoS2 coupled with its large surface area allows for highly sensitive detection of external environments. Although chemical vapor deposition (CVD) is a promising method for synthesizing large and high quality MoS2, optical properties of CVD-grown MoS2 under physiological solution is not clear. Furthermore, understanding of the pH effect on its optical properties is still limited.Here, we report their photoluminescence (PL) responses to solution pH under aqueous condition by means of PL spectroscopy and imaging. MoS2 with different Mo sources and additive such as MoS2, MoO2 and NaCl were synthesized. It was found that the PL response was highly affected by the Mo sources. While MoS2 made from MoO2 showed the highest modulation of PL by solution pH, MoS2 made from MoS2 did not show significant response. Furthermore, we demonstrated enhanced pH-sensitivity by UV/O3 treatments. Spectral analysis on PL and x-ray photoelectron spectroscopy revealed that the PL responses to solution pH were not directly correlated to the defect density (S/Mo ratio) of MoS2, but likely correlated to its state of defects and initial carrier density. These findings will be a practical guideline to optimize synthesis condition toward developing MoS2-based optoelectronic biosensing devices.

Molybdenum disulfide thin films fabrication from multi-phase molybdenum oxide using magnetron sputtering and CVD systems together

Abstract Molybdenum disulfide (MoS2) is a layered 2D semiconducting material with a tunable bandgap and a promising materials for next generation optoelectronics applications. In this study, the characterization of large-scale MoS2 films obtained by sulfurization of Mo–O films grown in different thicknesses with reactive magnetron sputtering method at 400 °C are reported. At a critical deposition temperature of 400 °C, different phases of Mo–O structure with high photoluminescent and bandgap were observed. Although there are no triangular domains, bandgaps and PL properties are close to few-layered MoS2. The enhanced PL intensities attributed to the increasing amount of MoO2 that may cause MoS2's folding and the large number of electrons from MoO2 in the MoO2–MoS2 hetero-structure. The UV-VIS spectroscopy analysis shows that two bandgaps are presented with a low and high values which may extend a wide absorption range. One of these bandgaps is compatible with few-layer MoS2s and the other increases by the thickness of the Mo–O film grown by magnetron sputtering. This explains the absorption at low wavelengths, but also shows that the MoO2 structure can be used to adjust the band gaps of MoS″s. The combined growth technique of magnetron sputtering and CVD is provided a high quality and homogeneous MoS2 thin films for next-generation optoelectronics and nanoelectronic devices, as well as for other potential applications.