Molybdenum Disulfide (MoS2) – Properties, Uses & Synthesis

Molybdenum Disulfide has outstanding properties therefore it is used in many applications. It is an inorganic compound made of Molybdenum and Sulphur and the chemical formula is MoS₂. It is a silvery black solid and occurs as a mineral Molybdenite which is a principal ore of Molybdenum.

MoS₂ is naturally found as Molybdenite or Jordisite (a low-temperature form of Molybdenite). The Molybdenite ore is processed to get pure MoS₂. The thermal treatment of all Molybdenum compounds can also obtain it.

Mos₂ is relatively unreactive and does not get affected by dilute acids and oxygen. In appearance and feel it is similar to Graphite. It is classified as a Transition Metal Dichalcogenide (TMD).

Its distinctive structure and band gap make it a preferred material to substitute Graphene and/or semiconductor devices. MoS₂ shows favourable electronic and quantum characteristics when going from bulk to 2D structure. The fabrication on MoS₂ is easier which means you can get a large production yield at low costs.

Properties – MoS₂

• All forms of MoS₂ have a layered structure and bulk MoS₂ consists of stacked monolayers. The bulk structure can be triagonal, hexagonal and rhombohedral. The hexagonal MoS₂ is semiconducting, the tri-agonal structure is metallic and hexagonal and rhombohedral structures are dry lubricants.
• The MoS₂ band gap changes with size and structure and it is used in optoelectronics. It has a large absorption coefficient for wavelengths between 400 nm to 500 nm. Different band gaps mean tunable photoresponsivity, specific selectivity and response time.
• The MoS₂ monolayer has high strength less than that of Graphene and good elasticity like Graphene oxide. A single layer of MoS₂ is more flexible than the bulk structures. Unlike other semiconductors, the flexibility of MoS₂ prevents deformation and bandgap shifts. Mechanical strain can be used to alter MoS₂ electronic characteristics and convert them to metals from semiconductors.
• As MoS₂ has a layered structure it is an excellent lubricating material and a low coefficient of friction. The shear strength of MoS₂ increases as the coefficient of friction increases. This property is called superlubricity. MoS₂ lubricant can be used in temperatures up to 350° C.

Applications – MoS₂

• It has less contact resistance and high performance and therefore it is used to make 1 nm gate transistors with excellent on/off switching characteristics.
• It is used in applications like biosensing, optical sensors and more. One of the most critical applications is DNA, cancer and Coronavirus detection. It may be effective in curing cancer and Alzheimer’s disease.
• The structure and the properties of MoS₂ make it promising for use in material for battery electrodes and electronic, microwave, sensing and terahertz applications.
• Due to its optical, electrical and chemical properties, MoS₂ is used in a wide range of applications. It is used in nanoelectronics and sensor applications. It is used in gas sensing, biosensing, refractive index sensing and photo-sensing.
• It has a high refractive index of 2 and therefore it can be used in coatings.
• It is used as a cocatalyst for desulphurisation in petrochemistry.

Synthesis – MoS₂

Different techniques are used to get MoS₂ in different shapes, sizes and quantities. The main techniques used in synthesising are the bottom-up techniques and the top-down techniques.

In the bottom-up technique, a substrate is used for stacking the crystals. The different methods are physical vapour deposition, chemical vapour deposition, atomic layer deposition and chemical solutions.

In the top-down technique, a substrate that has a crystal laid over it is used to etch the crystal planes. The different methods are mechanical exfoliation, liquid phase exfoliation and sputtering.

Bottom-up techniques

In the physical layer deposition method, an ion implantation technique such as ‘Molecular Beam Epitaxy’ is used. Chemical vapour deposition can be used with thick or thin MoS₂ layers. In this, Sulphur vapours are passed over a substrate that has Mo laid over it.

The atomic layer deposition method can be used to produce thin and thick layers of MoS₂. The layers produced have lesser impurities and the method is considered to be efficient. In the chemical solutions method, the elements Mo and S are added to a liquid solution where they react at high temperatures.

Top-down techniques

In the different exfoliation synthesising techniques, the weak forces that exist between the layers of MoS₂ called the ‘Van der Waal’ forces gave way. In mechanical exfoliation, an adhesive tape is taken and rubbed then it is put on a substrate that has the MoS₂ flakes. This method gives low yields but is suitable for use in the lab.

The liquid phase exfoliation method uses chemical compounds that are added and stirred, bubbled and ground. This process is economical and simple but output is of low quality.

Sputtering can be used to produce layers of MoS₂ lubricant.