Transition metal dichalcogenides (TMDs) are atomically thin semiconductors with the formula MX2, where M is a transition metal like molybdenum or tungsten and X is a chalcogen like sulfur or selenium, forming a three-layer sandwich structure only about 6.5 Å thick. They are part of the 2D materials family and can be synthesized using methods such as chemical vapor deposition or molecular-beam epitaxy.