Molybdenum diselenide
Chemical compound
From Wikipedia, the free encyclopedia
Molybdenum diselenide (MoSe
2) is an inorganic compound of molybdenum and selenium. Its structure is similar to that of MoS
2.[7] Compounds of this category are known as transition metal dichalcogenides, abbreviated TMDCs. These compounds, as the name suggests, are made up of a transition metals and elements of group 16 on the periodic table of the elements. Compared to MoS
2, MoSe
2 exhibits higher electrical conductivity.[8]
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 Top-view atomic images of MoSe2 before and after (right) ion irradiation[1] | |
| Names | |
|---|---|
| IUPAC name
bis(selanylidene)molybdenum | |
| Other names
molybdenum diselenide, molybdenumdiselenide, molybdenum selenide, diselanylidenemolybdenum, molybdenum(IV) selenide | |
| Identifiers | |
3D model (JSmol) |
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| ChemSpider | |
| ECHA InfoCard | 100.031.831 |
| EC Number |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |
| MoSe 2 | |
| Molar mass | 253.86 g/mol[2] |
| Appearance | crystalline solid |
| Density | 6.90 g/cm3[2] |
| Melting point | >1200 °C[2] |
| Band gap | ~0.85 eV (indirect, bulk) ~1.5 eV (direct, monolayer)[3][4] |
Refractive index (nD) |
1.4--3.4 (ultraviolet) 3.4--5.1 (visible) 4.2--4.9 (near infrared)[5] |
| Structure | |
| hP6, space group P6 3/mmc, No 194[6] | |
a = 0.3283 nm, c = 1.2918 nm | |
| Trigonal prismatic (MoIV) Pyramidal (Se2−) | |
| Related compounds | |
Other anions |
Molybdenum dioxide Molybdenum disulfide Molybdenum ditelluride Tantalum diselenide |
Other cations |
Tungsten diselenide |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Structure
Like many TMDCs, MoSe
2 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.[9]
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 Se2−
ions. The coordination geometry of the Mo is sometimes found as octahedral and trigonal prismatic.[10]
Synthesis
Synthesis of MoSe
2 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.[10] Excess selenium can be removed by sublimation under vacuum.[11] The synthesis reaction of MoSe
2 is:
- Mo + 2 Se → MoSe
2
Single-crystal-thick layers of MoSe
2 are produced by scotch tape exfoliation from bulk crystals,[12] by chemical vapor deposition (CVD) [13] or molecular-beam epitaxy (MBE).[14]
Properties
The electron mobility of 2D-MoSe
2 is significantly higher than that of 2D-MoS
2. 2D MoSe
2 adopts structures reminiscent of graphene, although the latter's electron mobility is thousands of times greater still.
In contrast to graphene, monolayer MoSe
2 has a direct band gap, suggesting applications in transistors and photodetectors. However, the band gap of multilayer MoSe
2 is indirect. [13]
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. MoSe
2 highly absorb the visible light but is transparent for the infrared.[5]
Natural occurrence
Molybdenum(IV) selenide occurs in the nature as the extremely rare mineral drysdallite.[15]
