2D materials – realising the potential

Materials World magazine
1 Jun 2018

Andrew Pollard and Dr Ravi Sundaram* outline the growing potential of 2D materials, such as graphene and molybdenum disulphide, and how they can make their
way into everyday life.

There are an estimated 325 million smart watches and other wearables in use worldwide, according to a 2016 report from Statista. And, as consumers demand more from their devices, the need for smaller scale electronics and optoelectronics is also growing. 

Manufacturers are searching for new production techniques and materials, as traditional silicon-based technology limits the size and flexibility of devices. Novel, atomic-scale fabrication technologies, however, are allowing for the creation of devices using two-dimensional (2D) materials to make the seemingly impossible a reality.

Beyond graphene

2D materials have attracted significant interest in recent years, not only because of their atomically thin dimensions, but also because of their electrical and optical properties. 

The first such material to be studied in detail, graphene, revealed exciting potential, with its strength, conductivity, and transparent properties making it an ideal candidate for applications such as lightweight displays and solar cells. Now, the focus has moved to other 2D materials with a diverse range of complementary properties.

One, molybdenum disulphide (MoS2), is a semiconducting 2D material that is similar to graphene. 

A single layer of MoS2 has a direct band gap – a physical property of a material – that enables very high photoluminescence efficiency, which is the ability to emit light. Electronic devices made from these materials can be transparent and flexible, while exhibiting high on-off ratios and electrical mobility.  

Such characteristics are important when it comes to creating new devices that will be both smaller and flexible, such as ultra-thin and flexible light-emitting diodes (LEDs), thin film transistors (TFTs) and devices that can perform at a scale of less than three nanometres.

Bringing such material to market, however, requires researchers to go further than demonstrating the mere properties of MoS2 and look for ways to standardise its capabilities for commercialisation. Only with clear ways to measure when the material is of suitable quality can manufacturers produce these on an industrial scale and develop a supply chain. This, in turn, enables companies to introduce the material into shrinking electronic devices. 

The industry, therefore, faces a significant challenge in controlling the quality of this single molecule thick film over large areas, which can be integrated in a large scale. Furthermore, manufacturers risk destroying or damaging the material due to its nanoscale nature. Up until recently, assessing the quality of this 2D layer has only been possible using destructive techniques. 

This is not a sustainable method, as only off-line testing would be possible, therefore increasing the time and costs to develop viable products.

Also, the slightest defect can critically impact on the performance of MoS2-based electronic devices, by interrupting the flow of current through the material. This makes the ability to investigate and quantify the number of defects during production, without causing damage, crucial. 

Development of a quick and non-destructive quality control method, which is compatible with device fabrication processes, would enable the industry to monitor the electronic quality of the material until the final components are ready. 

Finding the fingerprints

To overcome this challenge, Oxford Instruments – the first technology business spun out of Oxford University that designs, supplies, and supports high-technology tools and systems with a focus on research and industrial applications – looked to develop process solutions for the fabrication of 2D materials and devices without damaging them.

Using advanced atomic scale processing technologies such as chemical vapour deposition (CVD), atomic layer deposition (ALD) and atomic layer etching (ALE), they developed a suite of deposition and etching processes to produce MoS2 devices in an industrially-scalable manner. These processes control the thickness and morphology of
2D materials. 

To ensure that these materials pass through advanced fabrication processes without sustaining damage, a quality control strategy that could be used as a standard approach was necessary. Oxford Instruments collaborated with the research team at the National Graphene Metrology Centre (NGMC), based at the National Physical Laboratory (NPL), the UK’s national measurement institute, in Teddington.

This research was particularly relevant for Oxford Instruments, as the NPL had investigated the use of Raman spectroscopy for characterising MoS2 – a method that can demonstrate a material’s fingerprint by showing the vibrational energy levels of the molecules present. 

By introducing known defects into MoS2 and then performing this spectroscopy, using examples from previous defects in graphene, the NPL team was able to demonstrate a method to help support non-destructive, quality control methods for MoS2 in manufacturing. 

By drawing on the results of this research, scientists at Oxford Instruments were then able to use the NPL study as a framework for developing their own quality control measures using Raman spectroscopy to quantify defects in MoS2 produced using CVD. Similar techniques have also been used for graphene.

The resulting methodology has since led to the development of quality control processes to characterise the 2D MoS2 layers without having a destructive impact on the material’s structure or functionality. 

Meeting the challenge

While this development remains at the technology readiness level of 4–5 – meaning that the component has been proven in appropriate environments, but that prototypes and actual systems are yet to be fully integrated. Once it has been commercialised, the production and characterisation of 2D materials like MoS2 will achieve their predicted success, with potential uses for biomedical devices, energy storage, flexible electronics, and hydrogen fuel catalysts for automobiles.  

Just as this technique has helped to pave the way for MoS2, other 2D materials with similar and complementary properties are also rising up in technology readiness, providing a fertile field of R&D to address long-term industrial challenges.

*Andrew Pollard is Senior Research Scientist at the National Physical Laboratory, UK. He leads the NPL’s Surface and Nanoanalysis Group’s research into the structural and chemical characterisation of graphene and related 2D materials. 

Dr Ravi Sundaram is a Senior Scientist at Oxford Instruments, UK, where he leads efforts on 2D materials R&D. He is now responsible for scoping out and developing a strategy for emerging technology markets.