Tungsten diselenide is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula WSe₂. The compound adopts a hexagonal crystalline structure similar to

molybdenum disulfide Molybdenum disulfide (or moly) is an inorganic chemistry, inorganic compound composed of molybdenum and sulfur. Its chemical formula is . The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as ...

. The

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

atoms are covalently bonded to six

selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...

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 A van is a type of road vehicle used for transporting goods or people. There is some variation in the scope of the word across the different English-speaking countries. The smallest vans, microvans, are used for transporting either goods or p ...

. WSe₂ is a very stable

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

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 Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...

, 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 phase, 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 A fullerene is an allotropes of carbon, allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may ...

-like structure. The

Young's modulus Young's modulus (or the Young modulus) is a mechanical property of solid materials that measures the tensile or compressive stiffness when the force is applied lengthwise. It is the modulus of elasticity for tension or axial compression. Youn ...

varies 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 Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is ...

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 dichalcogenide : 220px, Cadmium sulfide, a prototypical metal chalcogenide, is used as a yellow pigment. A chalcogenide is a chemical compound consisting of at least one chalcogen anion and at least one more electropositive element. Although all group 16 elements ...

s in

solar cell A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

s and photonics are often discussed. Bulk has an

optical band gap In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. In graphs of the electronic band structure of solids, the band gap refers to the ...

of ~1.35 eV with a temperature dependence of −4.6 eV/K. photoelectrodes are stable in both acidic and basic conditions, making them potentially useful in electrochemical solar cells. The properties of monolayers differ from those of the bulk state, as is typical for semiconductors. Mechanically exfoliated monolayers of 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.

Superconductivity Superconductivity is a set of physical properties observed in superconductors: materials where Electrical resistance and conductance, electrical resistance vanishes and Magnetic field, magnetic fields are expelled from the material. Unlike an ord ...

has been reported in twisted bilayer , with a transition temperature of 200 mK.

References

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

Structure and properties

Synthesis

Potential applications

See also

References