Molybdenum disulfide

Molybdenum disulfide is an inorganic compound made of molybdenum and sulfur. Its chemical formula is MoS2. This material is classified as a transition metal dichalcogenide. It is a silvery black solid that appears naturally as the mineral molybdenite.

Molybdenite is the primary ore used to extract molybdenum. The compound is relatively unreactive and stable. It remains unaffected by oxygen or dilute acids. This stability makes it a reliable choice for many industrial uses.

The way this material works depends on its unique atomic structure. Every layer is a sandwich of atoms. A single plane of molybdenum atoms is placed between two planes of sulfide ions. These three layers form a single monolayer. In bulk form, many of these monolayers are stacked on top of each other. They are held together by weak van der Waals interactions. These weak bonds allow the layers to slide past one another easily. This sliding motion is why the material is such an excellent dry lubricant.

Scientists have identified several different crystalline phases of MoS2. The two most common are the 2H-phase and the 3R-phase. The names refer to their hexagonal and rhombohedral symmetry. In these phases, each molybdenum atom is at the center of a trigonal prismatic coordination sphere. Each molybdenum atom is covalently bonded to six sulfide ions. Both of these phases are semiconductors. A third phase called the 1T-phase is also known. This phase is metastable and has trigonal symmetry. Unlike the others, the 1T-phase is metallic. It can be created by intercalating the 2H-phase with alkali metals.

Researchers can manipulate these different phases using various methods. For example, the 1T-phase can be stabilized by doping with electron donors like rhenium. It can also be converted back to the 2H-phase using microwave radiation. The transition between these phases can be controlled by creating sulfur vacancies. These vacancies are missing sulfur atoms in the crystal lattice.

Scientists also study different shapes of the material. They have discovered nanotube-like and buckyball-like molecules made of MoS2.

The physical properties of MoS2 change significantly depending on its thickness. In bulk form, it is an indirect bandgap semiconductor with a bandgap of 1.23 eV. However, when it is reduced to a single monolayer, it becomes a direct bandgap semiconductor. This monolayer has a larger bandgap of 1.8 eV. This shift in optoelectronic properties is a major focus of modern research. To create these thin flakes, scientists use micromechanical exfoliation. This is often called "Scotch-tape exfoliation." It involves using adhesive to peel the layers apart.

This method was famously used by Konstantin Novoselov and Andre Geim to isolate graphene.

Molybdenum disulfide is widely valued for its mechanical strength and lubricity. At ambient conditions, its coefficient of friction is 0.150. Its estimated shear strength is approximately 56.0 MPa, though direct measurements suggest it may be closer to 25.3 MPa. The material can be strengthened through doping. Adding chromium (Cr) increases its wear resistance. For instance, pure MoS2 nanopillars have a yield strength of 821 MPa. When doped with 50% chromium, the yield strength increases to 1017 MPa. However, this change also makes the material more brittle.

Beyond lubrication, MoS2 has many important applications in chemistry and electronics. It acts as a catalyst in petrochemistry, specifically for desulfurization. It is also used as a hydrogenation catalyst for organic synthesis. In the world of microelectronics, its layered structure is very useful. It can be used to build flexible electronic circuits and transistors. Researchers have even used it to fabricate a 115-transistor, 1-bit microprocessor. Because of its unique electronic properties, it is also being studied for use in valleytronics. This field explores how the valley degree of freedom can be used in electronic devices.