Tungsten Disulfide (WS2) Properties

Tungsten disulfide (WS₂) exits in different forms, each with their own beneficial properties. Bulk WS₂ offers advantages such as exceptional lubricity and high thermal stability. Monolayer WS₂ has desirable optoelectronic properties, including strong light-matter interactions, as well as being strong and flexible. The 2D nature of WS₂ and its structure-property relationship make it a popular semiconducting material. WS₂ can be used in photonic circuits, photovoltaics, and high-resolution imaging.

On This Page

Chemistry of Tungsten Disulfide
Tungsten Disulfide Properties

Bulk Tungsten Disulfide
Optical and Electronic Properties

WS₂ Band Gap
WS₂ Second Harmonic Generation (SHG)
WS₂ Valleytronics

Chemistry of Tungsten Disulfide

Tungsten Disulfide Structure — Tungsten Disulfide 2H Structure

Tungsten disulfide (WS₂) exists as a 2D sheet-like material made from tungsten atoms covalently bonded to sulfur atoms. The sulfur atoms are arranged at the top and the bottom of the sheet with a layer of tungsten sandwiched in the middle. The atoms can be arranged in three different types of structure depending on the preparation method:

(2H) Hexagonal
(1T) Trigonal
(3R) Rhombohedral

The 2H hexagonal phase is the most stable and has the most desirable optical properties. The crystal structure of 2H tungsten disulfide (WS₂) forms a hexagonal lattice arrangement (see a top view). Six sulfur atoms coordinate to every tungsten atom in a trigonal prismatic arrangement to produce the lattice. For the 2H phase of WS₂, there are two layers in a unit cell. Bulk WS₂ exists as stacked layers which are bound by weak van der Waals forces. Monolayer WS₂ is ~ 0.7 nm thick and the spacing between the layers is ~ 0.6 nm.

WS₂ has the same structure as popular molybdenum disulfide (MoS₂). Both belong to a family of materials called transition metal dichalcogenides (TMDs). The chemical formula for these materials is MX₂ where the M is a transition metal, and the X is a chalcogen. The hexagonal lattice arrangement for all TMDs including tungsten disulfide means they have a three-fold rotational symmetry (D3h).

Optical and Electronic Properties

Bulk Tungsten Disulfide

Bulk WS₂ occurs naturally in mineral form and is referred to as tungstenite. It typically exhibits a hexagonal crystal structure where layers are held together by weak van der Waals forces. It is known for its high lubricity, thermal stability, and semiconducting properties, making it valuable for industrial lubricants and electronic applications.

Tungsten disulfide powder is an excellent lubricant, even under harsh conditions, as it has an extremely low coefficient of friction (CoF) of 0.03. WS₂ forms a robust, self-lubricating coating that can withstand high temperature and pressures. This is due to the weak van de Waals forces that can be easily overcome, allowing layers to slide over each other.

Bulk WS₂ has an indirect band gap of 1.2 eV. This changes when accessing monolayer WS₂ nanosheets and will be discussed later. While bulk WS₂ has a lower indirect band gap, it still has moderate carrier charge mobility (10-50 cm²/V.s). The combination of this with stability and semiconducting properties, means it is suitable for photovoltaics and transistors.

Tungsten Disulfide (WS2) — Bulk Tungsten Disulfide (WS₂)

WS₂ Band Gap

Monolayer tungsten disulfide has a direct band gap of 2.1 eV. This differs from bulk WS₂ as mentioned above that has an indirect band gap of 1.2 eV. The direct band gap allows for more efficient light absorption and emission, falling within the visible range of the electromagnetic spectrum. Due to the transition to direct band gap, monolayer WS₂ sees an increase in photoluminescence quantum yield (PLQY) by factor ∼10³ compared to the bulk material. The PLQY of WS₂ is also 20 times larger than for MoS₂, making WS₂ superior for application in emitting devices.

Monolayer WS₂: The valence band maximum (VBM) and the conduction band minimum (CBM) shift to the K-point in the Brillouin zone therefore the k-vectors become the same.

Bulk WS₂: The VBM and CBM of the bulk phase are located at the Γ-point and between the Γ- and the K-points, respectively.

The reduction in thickness from bulk to monolayer WS₂ causes a stronger out-of-plane quantum confinement. This increases the binding energy of excitons, making them more stable and enhancing optical interactions. Energy levels within monolayer WS₂ become quantized (discrete) which increases the band gap.

In monolayer WS₂, there is no Coulomb repulsion between the p_z orbitals of the chalcogenide elements as there are no adjacent layers to interact with. In bulk WS₂, interlayer coupling and hybridization lead to the stabilization of the Γ-state valence band.

WS₂ Second Harmonic Generation (SHG)

Tungsten disulfide monolayers have a large second harmonic generation susceptibility of d_eff = 0.77 nm/V. This nonlinear optical process is a result several key material properties:

2D atomic-scale thickness
Direct band gap
Non-centrosymmetric crystal structure (has no inversion symmetry)
Strong exitonic effects

These properties mean WS₂ has enhanced interaction with light. The lack of inversion symmetry allows WS₂ to efficiently generate second harmonic signals. When the electric field interacts with monolayer WS₂ it induces a nonlinear polarization. This means that two photons with the same frequency can interact with WS₂ and generate a new photon with twice the frequency of the original photons.

The ability to convert light to higher frequencies (SHG) means WS₂ can be integrated into photonic circuits for optical communications as well as into high-resolution imaging techniques.

Tungsten Disulfide Second Harmonic Generation

WS₂ Valleytronics

Monolayer tungsten disulfide has the same potential for use in valleytronics as other 2D transition metal chalcogenides like molybdenum disulfide (MoS₂). The D3h symmetry of WS₂ a result of its hexagonal lattice structure influences its electronic band structure. As the Brillouin zone is also hexagonal, it leads to the formation of energy-degenerate valleys at the K and K' points.

Symmetry operations of the D3h point group transform the electronic states at K into those at K' and vice versa without changing their energy. Strong spin-orbit coupling further splits the valence band at the K and K' points. This leads to spin-valley locking where each valley is associated with a particular spin orientation. This phenomenon is referred to as spin-locking as the spin and valley degrees of freedom are coupled.

K and K' valleys within WS₂ are sensitive to different circularly polarised light. Optical transitions are spin-selective, meaning that right-handed (𝜎+) and left-handed (𝜎−) circularly polarized light selectively couple to different valleys.

As optical transitions within the distinct valleys can be controlled by different types of light, WS₂ is suitable for valleytronic applications. Information can be coded, stored and processed, like 1s and 0s, depending on the spin state of the valley within the material.

More information about TMDs and valleytronics can be found:Valleytronics

Learn More

Introduction to 2D Materials

The foundation of technology is the understanding of material systems. Specific material properties are required depending on the application.

Resources and Support

Gultom, P. et al. (2023). Structural and Optical Properties of Tungsten Disulfide Nanoscale Films Grown by Sulfurization from W and WO₃.Nanomaterials, 13 . doi: 10.3390/nano13071276
Yan, P. et al.(2017). Large-area tungsten disulfide for ultrafast photonics. Nanoscale, 9, doi: 10.1039/C6NR09183K
Mohl, M. et al.(2020). 2D Tungsten Chalcogenides: Synthesis, Properties and Applications. Adv. Mater. Interfaces, 7, doi: 10.1002/admi.202000002

Han, C. et al.(2021). Strong anisotropic enhancement of photoluminescence in WS₂ integrated with plasmonic nanowire array. Sci Rep, 11, doi: 10.1038/s41598-021-89136-0
Frisenda, R. et al.(2018). Atomically thin p–n junctions based on two-dimensional materials. Chem. Soc. Rev., 47, doi: 10.1039/C7CS00880E
Ovchinnikov, D. et al.(2014). Electrical Transport Properties of Single-Layer WS₂. ACS Nano, 8, doi: 10.1021/nn502362b
Chen, Y-W. et al.(2020). Synthesis of high-quality monolayer tungsten disulfide with chlorophylls and its application for enhancing bone regeneration. npj 2D Mater Appl, 4, doi: 10.1038/s41699-020-00168-y
Oliva, R. et al.(2022). Strong Substrate Strain Effects in Multilayered WS₂ Revealed by High-Pressure Optical Measurements. ACS Appl. Mater. Interfaces, 17, doi: 10.1021/acsami.2c01726
Duan, X. et al.(2014). Lateral epitaxial growth of two-dimensional layered semiconductor heterojunctions. Nature Nanotech, 9, doi: 10.1038/nnano.2014.222
Janisch, C. et al.(2014). Extraordinary Second Harmonic Generation in Tungsten Disulfide Monolayers. Sci Rep, 4, doi: 10.1038/srep05530
Dasgupta, A. et al.(2019). Second-harmonic optical vortex conversion from WS₂ monolayer. Sci Rep, 9, doi: 10.1038/s41598-019-45424-4
Bianchi, M. et al.(2023). Engineering the Electrical and Optical Properties of WS₂ Monolayers via Defect Control. Adv Sci, doi: 10.1002/advs.202305162

Contributors

Written by

Dr. Amelia Wood

Application Scientist

Diagrams by

Sam Force

Graphic Designer