Elisha Mercado 

For the past six weeks, I’ve been on an industrial placement with Oxford Instruments Plasma Technology working on developing a chemical vapour deposition (CVD) process for the 2-dimensional transition metal dichalcogenide (TMD) MoS₂. But what are TMDs? And why is it important to develop new techniques to make them?

Beyond Graphene

The world of 2D materials is a rapidly expanding research area. The most well-known of all is graphene, a single layer of carbon atoms in a honeycomb lattice that was first successfully isolated in 2004 using the ‘scotch tape method’. Graphene has attracted such interest due to the wide range of desirable properties it possesses; such as a high carrier mobility, incredible strength and very high thermal and electrical conductivity to name a few. On the face of it, graphene seems to be an ideal candidate for all sorts of applications. But if this is the case, why aren’t graphene-based devices commonplace?

Graphene has one fundamental problem; it lacks a bandgap. This makes it unsuitable for use in important applications such as transistors or optoelectronic devices. While there are ways to engineer a bandgap in graphene, these typically degrade the electronic quality of the material or are impractical for device applications. This limitation has led scientists to seek out new 2D materials.

Naturally, researchers directed their search towards materials with a similar layered, honeycomb structure to that of graphite. In graphite, the layers are held together by weak Van der Waals forces and so are easily pulled apart – this is why the ‘scotch tape’ method works so well. It turns out that a whole host of other materials possess a similar structure and can be exfoliated in much the same way. One of the most widely researched classes of these materials is the layered transition metal dichalcogenides (TMDs).

What are TMDs?

TMDs are materials of the form MX₂, where M represents a transition metal atom such as Mo, W, Ta etc., sandwiched between two chalcogen atoms (S, Se or Te), represented by X.  Monolayer TMDs have been found to possess qualities somewhat similar to graphene. Crucially though, all of the materials in this family are semiconducting – they have bandgaps of around 1-2eV for a single layer, making them well suited for use in electronic devices. These bandgaps are direct in monolayer form (they are indirect for the bulk materials) due to changes in electronic structure induced by being thinned down to the ultrathin limit (quantum confinement). This means that they are also prime candidates for use in optoelectronics. Indeed a wide range of different TMD-based devices have already been successfully demonstrated, such as high performance field-effect and thin film transistors, photodetectors and photovoltaics.

a) A transition metal dichalcogenide (MX2) monolayer with transition metal (M) atoms represented in blue and chalcogen atoms (X) in yellow. b) A top-down view of a TMD layer showing the hexagonal, ‘honeycomb’ structure and lattice vectors a1 and a2.

TMDs also have a number of interesting properties that make them suitable for less conventional, emergent applications. Strong spin-orbit interactions and a lack of inversion symmetry in their monolayer form have identified TMDs as particularly promising candidates for spintronics and valleytronics. Spintronics involves the manipulation of the spin degree of freedom to store and transfer information, while valleytronics instead exploits the valley degree of freedom. In TMDs the conduction/valence band has multiple extrema, or ‘valleys’, at different positions in momentum space. The inequivalent valleys can serve as multiple indices to be used to encode information. So far though, research in these areas has been hampered somewhat by problems in producing sufficiently large, pure monolayers.

Schematic showing the valley property of monolayer TMDs. a) Valley dependent spin split bands, where the hexagonal plane indicates the first Brillouin zone. Spin polarization arising from the spin-orbit interaction is represented by the red and blue arrows. (b) Band diagram of monolayer TMDs. The K (K’) valley is excited selectively by σ− (σ+) circularly polarized light. Copyright IOP Publishing Ltd 2015.

Scaling up

Although high quality monolayer TMD flakes can be mechanically exfoliated using the scotch tape method, this technique is not scalable. We need to develop a controlled way of producing 2D TMDs on a large scale in order to make use of them in real, commercial devices. Chemical vapour deposition (CVD) has emerged as one possible way of doing this due to its potential for high scalability and degree of morphological control. It requires the use of metal and chalcogen precursor materials, which are introduced into a vacuum chamber containing a substrate. The volatile precursors then react on the substrate surface, leaving behind deposited TMD material.

Recent developments have led to marked improvements in the quality of TMD layers produced via this method. For example, in the past CVD methods for TMDs have mostly relied on the use of solid precursors, which were evaporated simultaneously, limiting the control over growth parameters for the two precursors. A shift towards using gas-phase rather than solid-phase chalcogen reactants has significantly improved the uniformity and scalability of TMD monolayers produced. The use of metal-organic precursors (MOCVD) has also been shown to potentially allow for the production of wafer-scale high quality films.

The future

The field of 2D materials is an ever expanding research area, and the search for 2D materials beyond graphene is not limited to just TMDs. New monoelemental 2D materials such as silicene and phosphorene also present some promising characteristics for device applications if stability issues can be overcome. Additionally, it is possible that the full potential of TMDs may be realised with their incorporation into 2D heterostructures – layers of 2D materials stacked on top of one another.

I thoroughly enjoyed my time on placement and the experience I gained in working with these fascinating materials will likely prove extremely valuable in my future work. Their wide range of interesting properties and potential for use in emergent technologies suggest TMDs are likely to remain an important research area for years to come.

You can read more on Transition Metal Dichalcogenides in a dedicated special issue from the Journal of Physics: Condensed Matter.

This work is licensed under a Creative Commons Attribution 3.0 Unported License.

Figure 1 – Andor Kormányos et al 2015 2D Mater. 2 022001. Figure 2 – Y J Zhang et al 2015 2D Mater. 2 044004. Copyright IOP Publishing Ltd 2015.

Categories: Journal of Physics: Condensed Matter, JPhys+

Tags: 2D materials, Chemical vapour deposition, Condensed matter physics, Guest blog, Transition metal dichalcogenides