Molybdenum diselenide

Like many TMDCs, is a layered material with strong in-plane bonding and weak out-of-plane interactions. These interactions lead to exfoliation into two-dimensional layers of single unit cell thickness. The most common form of these TMDCs have trilayers of molybdenum sandwiched between selenium ions causing a trigonal prismatic metal bonding coordination, but it is octahedral when the compound is exfoliated. The metal ion in these compounds is surrounded by six ions. The coordination geometry of the Mo is sometimes found as octahedral and trigonal prismatic. == Synthesis ==

Synthesis of involves direct reaction of molybdenum and selenium in a sealed tube at high temperature. Chemical vapor transport with a halogen (usually bromine or iodine) is used to purify the compound at very low pressure (less than 10-6 torr) and very high temperature (600–700 °C). It has to be heated very gradually to prevent explosion due to its strong exothermic reaction. Stoichiometric layers crystallize in a hexagonal structure as the sample cools. The synthesis reaction of is: : Mo + 2 Se → Single-crystal-thick layers of are produced by scotch tape exfoliation from bulk crystals, by chemical vapor deposition (CVD) or molecular-beam epitaxy (MBE). == Properties ==

The electron mobility of 2D- is significantly higher than that of 2D-. 2D adopts structures reminiscent of graphene, although the latter's electron mobility is thousands of times greater still. In contrast to graphene, monolayer has a direct band gap, suggesting applications in transistors and photodetectors. However, the band gap of multilayer is indirect. Molybdenum diselenide exhibits anisotropy of the refractive index. The in-plane refractive index has exceptionally high values, extending 4 for wavelengths above 430 nm and reaching a maximum value of 5.117 for 825 nm, while the out-of-plane refractive index is near 3 in the visible and the infrared range of the spectrum. highly absorb the visible light but is transparent for the infrared. == Natural occurrence ==

Molybdenum(IV) selenide occurs in the nature as the extremely rare mineral drysdallite. == References ==