Tungsten diselenide is aninorganic compound with the formula WSe2.[6] The compound adopts a hexagonal crystalline structure similar tomolybdenum disulfide. Thetungsten atoms are covalently bonded to sixselenium 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.[7] It is a well studied example of alayered material. The layers stack together viavan der Waals interactions. WSe2 is a very stablesemiconductor in thegroup-VI transition metal dichalcogenides.
The hexagonal (P63/mmc) polymorph 2H-WSe2 is isotypic with hexagonalMoS2. The two-dimensional lattice structure has W and Se arranged periodically in layers with hexagonal symmetry. Similar tographite, 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.[8]
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.[8][9] WSe2 can form afullerene-like structure.
TheYoung's modulus 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.[10]
Heating thin films of tungsten under pressure from gaseous selenium and high temperatures (>800 K) using thesputter deposition technique leads to the films crystallizing in hexagonal structures with the correct stoichiometric ratio.[11]
The properties ofWSe 2 monolayers differ from those of the bulk state, as is typical for semiconductors.Mechanically exfoliated monolayers ofWSe 2 are transparentphotovoltaic materials withLED properties.[18] The resulting solar cells pass 95 percent of the incident light, with one tenth of the remaining five percent converted into electrical power.[19][20] 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.[21]
Superconductivity has been reported in twisted bilayerWSe 2, with a transition temperature of 200 mK.[22]
^abAgarwal, M. K.; Wani, P. A. (1979). "Growth conditions and crystal structure parameters of layer compounds in the series Mo1−xWxSe2".Materials Research Bulletin.14 (6):825–830.doi:10.1016/0025-5408(79)90144-2.
^Prakash, Abhijith; Appenzeller, Joerg (2017-02-28). "Bandgap Extraction and Device Analysis of Ionic Liquid Gated WSe2 Schottky Barrier Transistors".ACS Nano.11 (2):1626–1632.doi:10.1021/acsnano.6b07360.ISSN1936-0851.PMID28191930.
^Yun, Won Seok; Han, S. W.; Hong, Soon Cheol; Kim, In Gee; Lee, J. D. (2012). "Thickness and strain effects on electronic structures of transition metal dichalcogenides: 2H-MX2 semiconductors (M = Mo, W;X = S, Se, Te)".Physical Review B.85 (3) 033305.Bibcode:2012PhRvB..85c3305Y.doi:10.1103/PhysRevB.85.033305.
^O'Hare, P.A.G.; Lewis, Brett M.; parkinson, B.A. (June 1988). "Standard molar enthalpy of formation by fluorine-combustion calorimetry of tungsten diselenide (WSe2). Thermodynamics of the high-temperature vaporization of WSe2. Revised value of the standard molar enthalpy of formation of molybdenite (MoS2)".The Journal of Chemical Thermodynamics.20 (6):681–691.doi:10.1016/0021-9614(88)90019-5.
^Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.),Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter,ISBN0-12-352651-5
^Upadhyayula, L.C.; Loferski, J.J.; Wold, A.; Giriat, W.; Kershaw, R. (1968). "Semiconducting Properties of Single Crystals of n- and p-Type Tungsten Diselenide (WSe2)".Journal of Applied Physics.39 (10):353–358.Bibcode:1968JAP....39.4736U.doi:10.1063/1.1655829.
^Gobrecht, J.; Gerischer, H.; Tributsch, H. (1978). "Electrochemical Solar Cell Based on the d-Band Semiconductor Tungsten-Diselenide".Berichte der Bunsengesellschaft für physikalische Chemie.82 (12):1331–1335.doi:10.1002/bbpc.19780821212.
^Li, Hai; Wu, Jumiati; Yin, Zongyou; Zhang, Hua (2014). "Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets".Accounts of Chemical Research.47 (4):1067–1075.doi:10.1021/ar4002312.PMID24697842.
