Molybdenum disulfide (MoS2) – the new frontier of innovation


An international team of researchers have found a way to refine and reliably produce an unpredictable and hard-to-control material that could impact environmental conservation, energy and consumer electronics.

The material, Molybdenum Disulfide (MoS2), holds tremendous potential for numerous applications in energy storage, water treatment, gas, chemical and light sensing. But high costs and fabrications challenges have held back wider use.

“There are many different ways to fabricate this material, but no one has yet been able to make it in a controlled and tunable dimension in large quantities, in a low-cost, reproduceable fashion,” said Donglei (Emma) Fan, an associate professor in the Cockrell School of Engineering’s Walker Department of Mechanical Engineering and Texas Materials Institute.

As reported in Advanced Materials, Fan and the research team have created a method to fabricate thin nanoribbons of MoS2 at a large scale.

Previously, researchers have only been able to make the material in small amounts, attached at random to silicon substrates. This limited the material’s use, and when attached to the substrate it became very challenging to manipulate.

The research team created a free-standing version of MoS2 in a powder form that can be dispersed into solutions for several different applications, most notably water treatment. Yun Huang, graduate student and first author of the work, said their process has cut the cost of fabricating a gram of the material by 3,000 times, compared to previous published research focused on producing MoS2 nanoribbons.

Removing the dangerous element mercury from water represents one of the most impactful uses of MoS2, Fan said. A 2016 study led by the U.S. Geological Survey found mercury contamination is widespread at various levels across the Western United States, in air, soil, sediment, plants, fish and wildlife. High levels of mercury can lead to brain and kidney damage, especially in younger people. Aside from contaminated water, humans are most exposed to mercury issues by eating fish, which can pile up high concentrations of the element in their bodies as they consume other organisms that have been exposed.

When introduced to water in powder form, the team’s version of MoS2 can be dispersed with the ability to suck up mercury and remove it from water. There are several methods for removing mercury from water already, but with these new low-cost and large-scale manufacturing capabilities, MoS2 makes for a strong alternative solution.

“This is an attractive material because it has unique properties for various applications with potential to change people’s lives. Being able to make the material with controlled dimensions and in a large quantity, assemble it and integrate it with pre-made devices brings MoS2 a step closer to practical applications, not just staying in the lab,” Fan said.

MoS2 also has potential as a component in light-based microprocessors, which offer the promise of much faster computing speeds over today’s devices.

And it could serve as a low-cost catalyst for generating hydrogen fuel from water.

Creating MoS2 is a challenge. It comes from adding sulfur to a morphed “pre-cursor” material. Splitting this process into two steps—first performing the sulfurization at a lower temperature and then upping the heat—represented one of the key innovations in making MoS2 more controllable.

Previous experiments making MoS2 nanoribbons have only been able to create a microscopic amount of the material. However, the researchers are able to obtain a “spoon full” of MoS2 nanoribbons with a single synthesis, and the researchers’ say there no barrier to hold back scaling up the procedure to create larger quantities of the material.

MoS2 is part of a class of 2-D materials that have received a lot of attention from researchers lately. They are thin, flexible and capable semiconductors, traits that make them valuable as part of sensors for everything from heart monitors to pollutant emission detectors.

Two-dimensional (2D) materials such as molybdenum disulfide (MoS2) have been studied for use in potential applications(1−3) such as transistors,(4) light emitters,(5) photodetectors,(6) modulators,(7) pressure sensors,(8) resonators,(9) biosensors,(10) gas sensing,(11) photocatalysis,(12) and electrochemical applications.(13)

For using these 2D materials in device applications, it is critically important to realize the industrial-scale and reliable synthesis of high-quality monolayers of the 2D materials at low cost.

Chemical vapor deposition (CVD) is a potential large-scale 2D material synthesis approach suitable for industrial applications and one of the most developed approaches for the preparation of large-area MoS2 of good quality.(14−17)

For the development and optimization of CVD-grown single-layer MoS2, it is important to characterize the properties of the grown MoS2 layers that typically consist of irregularly shaped individual grains that connect the adjacent grains(18−21) through grain boundaries.

As a result, the grain sizes and grain boundaries of the domains or continuous films of MoS2 have an important impact on its electrical,(18,22−24) optical,(18) optoelectronic,(25,26) mechanical,(27,28) and chemical properties(29) as well as on the characteristics of devices made of MoS2.

Furthermore, the properties of grain boundaries in MoS2 might be beneficially used in specific applications by controlled defects engineering.(30−32) For the above reasons, fast and simple methods to directly observe the large-area distribution of grains and grain boundaries in MoS2 are of increasing importance.

Grains and grain boundaries in MoS2 films can be characterized by atomic resolution using transmittance electron microscopy (TEM)(18−20) and scanning tunneling microscopy (STM),(32,33) providing detailed information about the crystal structure of the grains and the grain boundaries. The MoS2 grain boundaries can also be identified using atomic force microscopy (AFM) by decorating a self-assembled octadecylphosphonic acid monolayer on the MoS2 surface.(34)

However, these techniques are time-consuming, require complex sample preparation procedures, and/or are limited to the characterization of very small areas. Another approach to observe grain boundaries in MoS2 is the use of nonlinear optics.(35−38) For instance, the grain boundaries between the adjacent MoS2 grains can be distinguished by stacking MoS2 bilayers and using photoluminescence imaging based on second harmonic generation.(37)

However, photoluminescence imaging is generally limited to MoS2 domains that feature a large rotation of the crystal axis as compared to the neighboring grains, requires sophisticated optical systems, and is typically slow.(39) Compared to photoluminescence imaging, a faster approach for visualizing grain boundaries in CVD-grown MoS2 is multiphoton microscopy based on third-harmonic generation, which is also independent of the degree of crystal axis rotation.(39)

Yet, this approach still requires a relatively sophisticated optical system and therefore is not easily accessible. Grain boundaries in CVD-grown MoS2 layers can also be visualized by oxidizing MoS2 using UV irradiation in a moisture-rich environment and subsequently imaging the layer with scanning electron microscopy (SEM) or AFM.(40)

However, with this approach, additional visible oxidized line defects within the MoS2 grains were produced alongside the grain boundaries because of easy oxidation of MoS2.(40) The grains and grain boundaries of large MoS2 layers were also visualized by depositing nematic liquid crystals on the MoS2 layers in combination with polarized optical microscopy(41) or by visualizing the differential diffusion of gold on the surface of MoS2 between the grain boundaries and the inner grain areas after depositing gold on the MoS2 surface using optical microscopy.(42) However, these techniques require elaborate and controlled preparation of the MoS2 samples.

Here, we present a simple and rapid method for visualizing grain boundaries in large areas of CVD-grown single-layer MoS2 on a SiO2 surface using SEM, optical microscopy, or Raman spectroscopy. Similar to the previously reported method for observing grain boundaries in CVD-grown graphene placed on a SiO2 surface using vapor hydrofluoric acid (VHF) exposure,(43) in the proposed method, we first expose the MoS2 layer on the SiO2 surface to VHF, which causes VHF molecules to diffuse through the defects in the lattice structure of the MoS2 grain boundaries.

The diffusion of the VHF through these defects results in etching of SiO2 underneath MoS2, with a difference between the etching speed of SiO2 directly at the grain boundaries and the etching speed of SiO2 in the areas below the grains away from the grain boundaries.

The resulting etch pattern in the SiO2 layer along the MoS2 grain boundaries is then visible and can be imaged using optical microscopy, SEM, or Raman spectroscopy. Because the MoS2 and the underlying SiO2 layers are exposed to VHF and etched to some extent in our method, this is an invasive approach.

SiO2 is one of the most commonly used growth substrates for large-area CVD-grown MoS2, and thus, our method will be useful in the development, characterization, and optimization of large-area MoS2 synthesis processes.

Figure 1. SEM and optical microscopy observation of the top view of a single layer of CVD-grown MoS2 on a SiO2 surface before and after exposure to VHF for 120 s. (a) SEM image of a continuous CVD-grown MoS2 film on a SiO2 substrate without visible grain boundaries or line patterns. (b) SEM image, (c) optical image, and (d) optical dark-field image of MoS2 of the same area as (a) after exposure to VHF for 120 s. After exposure to VHF for 120 s, line patterns are visible within the continuous MoS2 film. (e) SEM image of CVD-grown MoS2 on a SiO2 substrate, with visible MoS2 grain boundaries between the adjacent grains that were not completely stitched to each other during CVD growth. (f) SEM image, (g) optical image, and (h) optical dark-field image of the same position as (e) after exposure to VHF for 120 s. After exposure to VHF for 120 s, the previously visible grain boundaries in the MoS2 film get more pronounced, indicating that the VHF interaction and resulting line patterns occur at the MoS2 grain boundaries.
o explore the impact of the VHF exposure time on the resulting line patterns in both the continuous CVD-grown MoS2 films and individual domains placed on a SiO2 substrate, we exposed such samples to VHF at 40 °C for different times, that is, 30, 60, and 120 s. We observed that the line patterns appear increasingly pronounced with the increasing exposure time to VHF (Figures 2 and 3). We also observed that the patterns did not change in structure for the different evaluated VHF exposure times (Figures 2 and 3). Generally, when the VHF exposure time was increased from 30 to 60 and to 120 s, thin line patterns appeared inside the large MoS2 domains and became increasingly distinct when the VHF exposure time was increased to 120 s (Figures 1–3 and S4). This indicates that a longer time (i.e., 120 s) of exposure to VHF reveals more information about the grain boundaries in MoS2 than shorter VHF exposure time (i.e., 30 and 60 s). This is in agreement with the observation of grain boundaries in CVD-grown graphene on SiO2/Si substrates after exposure to VHF.(43)

reference link

More information: Advanced Materials (2020). DOI: 10.1002/adma.202003439


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