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Tungsten diselenide

WSe2 monolayer on graphene (yellow) and its atomic image (inset)
Identifiers
CAS Number	12067-46-8
3D model (JSmol)	Interactive image
ECHA InfoCard	100.031.877
EC Number	235-078-7
PubChem CID	82910
CompTox Dashboard (EPA)	DTXSID8065240
InChI InChI=1S/2Se.WKey: ROUIDRHELGULJS-UHFFFAOYSA-N
SMILES ==
Properties
Chemical formula	WSe2
Molar mass	341.76 g/mol
Appearance	grey to black solid
Odor	odorless
Density	9.32 g/cm
Melting point	> 1200 °C
Solubility in water	insoluble
Band gap	~1 eV (indirect, bulk) ~1.7 eV (direct, monolayer)
Structure
Crystal structure	hP6, space group P6 3/mmc, No 194
Lattice constant	a = 0.3297 nm, c = 1.2982 nm
Coordination geometry	Trigonal prismatic (W) Pyramidal (Se)
Thermochemistry
Std enthalpy of formation (ΔfH298)	-185.3 kJ mol
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	External MSDS
Related compounds
Other anions	Tantalum diselenide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). Infobox references

Tungsten diselenide is an inorganic compound with the formula WSe₂. The compound adopts a hexagonal crystalline structure similar to molybdenum disulfide. The tungsten atoms are covalently bonded to six selenium ligands in a trigonal prismatic coordination sphere while each selenium is bonded to three tungsten atoms in a pyramidal geometry. The tungsten–selenium bond has a length of 0.2526 nm, and the distance between selenium atoms is 0.334 nm. It is a well studied example of a layered material. The layers stack together via van der Waals interactions. WSe₂ is a very stable semiconductor in the group-VI transition metal dichalcogenides.

Structure and properties

The hexagonal (P6₃/mmc) polymorph 2H-WSe₂ is isotypic with hexagonal MoS₂. The two-dimensional lattice structure has W and Se arranged periodically in layers with hexagonal symmetry. Similar to graphite, van der Waals interactions hold the layers together; however, the 2D-layers in WSe₂ are not atomically thin. The large size of the W cation renders the lattice structure of WSe₂ more sensitive to changes than MoS₂.

In addition to the typical semiconducting hexagonal structure, a second metallic polymorph of WSe₂ exists. This phasem 1T-WSe₂, is based on a tetragonal symmetry with one WSe₂ layer per repeating unit. The 1T-WSe₂ phase is less stable and transitions to the 2H-WSe₂ phase. WSe₂ can form a fullerene-like structure.

The Young’s modulus vary greatly as a function of the number of layers in a flake. For a single monolayer, the reported Young’s modulus is 258.6 ± 38.3 GPa.

Synthesis

Heating thin films of tungsten under pressure from gaseous selenium and high temperatures (>800 K) using the sputter deposition technique leads to the films crystallizing in hexagonal structures with the correct stoichiometric ratio.

W + 2 Se → WSe₂

Potential applications

The potential applications of transition metal dichalcogenides in solar cells and photonics are often discussed. Bulk WSe
₂ has an optical band gap of ~1.35 eV with a temperature dependence of −4.6×10 eV/K. WSe
₂ photoelectrodes are stable in both acidic and basic conditions, making them potentially useful in electrochemical solar cells.

The properties of WSe
₂ monolayers differ from those of the bulk state, as is typical for semiconductors. Mechanically exfoliated monolayers of WSe
₂ are transparent photovoltaic materials with LED properties. The resulting solar cells pass 95 percent of the incident light, with one tenth of the remaining five percent converted into electrical power. The material can be changed from p-type to n-type by changing the voltage of an adjacent metal electrode from positive to negative, allowing devices made from it to have tunable bandgaps.

See also

• Transition metal dichalcogenide monolayers

References

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• Tungsten(IV) compounds
• Selenides
• Transition metal dichalcogenides
• Monolayers