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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.

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).

Tungsten Disulfide 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)

Optical and Electronic Properties

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.

Tungsten Disulfide Bang Gap
Tungsten Disulfide Bang Gap

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 pz 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 deff = 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
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

Tungsten Disulfide

Tungsten Disulfide (WS<sub>2</sub>) Monolayer Film CAS 12138-09-9
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Molybdenum Disulfide (MoS2): Theory & Applications Molybdenum Disulfide (MoS2): Theory & Applications

Molybdenum disulfide belongs to a class of materials called 'transition metal dichalcogenides'.

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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

Contributors


Writen by

Dr. Amelia Wood

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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