Tungsten diselenide is an

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

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

molybdenum disulfide Molybdenum disulfide (or moly) is an 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 mineral molybdeni ...

. Every

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

atom is covalently bonded to six

selenium Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal (more rarely considered a metalloid) with properties that are intermediate between the elements above and below in the periodic table, sulfur and telluriu ...

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 In material science, layered materials are solids with highly anisotropic bonding, in which two-dimensional sheets are internally strongly bonded, but only weakly bonded to adjacent layers. Owing to their distinctive structures, layered materials ...

. The layers stack together via

van der Waals interactions A van is a type of road vehicle used for transporting goods or people. Depending on the type of van, it can be bigger or smaller than a pickup truck and SUV, and bigger than a common car. There is some varying in the scope of the word across th ...

. WSe₂ is a very stable

semiconductor A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...

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 crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...

, 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, WSe₂ exists in another polymorph, a metallic octahedral coordination phase 1T-WSe₂ 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 allotrope of carbon whose molecule consists 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 seven atoms. The molecule may be a hollow sphere, ...

-like structure.

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

Transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that c ...

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 are semiconductors with potential applications in

solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.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.

References

{{Selenides Tungsten compounds Selenides Transition metal dichalcogenides Monolayers