An easy method for 2D nanomaterials

Materials World magazine
4 Jan 2017

2D nanomaterials could have extensive applications in everyday technology if used on a large scale. Ellis Davies reports on a method to achieve this. 

2D nanomaterials possess excellent physical properties that have the potential to advance technology significantly. However, they are difficult to translate to real world applications, such as biosensors and imaging, because of the challenge of making and manipulating them on a large scale. 

Researchers at University College London (UCL), UK, have developed an approach to overcome these issues that allows the nanomaterials to be applied over large areas at a low cost, using standard industry equipment. The method dissolves layered materials in a liquid to form a solution for easy application. 

Dr Chris How ard of UCL’s Physics and Astronomy Department explained the difficulties of working with 2D nanomaterials. ‘Many methods for the manufacture and manipulation of 2D materials [that are] cheap and scalable processes tend to make materials that are smaller, contain a range of layers and are defective, which is detrimental to the materials’ properties. Larger, higher quality sheets can be produced, but the processes required are expensive and often non-scalable or unreproducible.’

The method developed by Howard’s team can be used for a variety of materials, including semiconductors and thermoelectric materials. ‘By inserting positively charged lithium and potassium ions in-between the layers of a bulk material, such as molybdenum disulphide, bismuth telluride and titanium disulphide, with a process driven by charge transfer from these atoms to the sheets, we created a material composed of negatively charged layered sheets alternating with arrays of positively charged intercalated metal cations,’ he said. This compound becomes a layered material salt, which can be added to certain polar solvents to dissolve. This thermodynamically driven process is similar to an ionic salt dissolving in water, and produces 2D nanomaterial sheets with a negative charge. 

Howard claims that the charge is key to the process as it is what makes the layers spontaneously dissolve. The charged layers allow the polar solvent to order themselves around the solutes – the charged sheets and cations. ‘Furthermore, the fact that the nanosheets are charged is technologically useful. This charge enables us to controllably electroplate a range of nanomaterials onto electrodes over large areas. Surface charge can also enable the tethering of specific functional groups onto nanosheets,’ Howard said, adding that the team believes the charge is also responsible for the phenomenon of 2D hexagonal nanosheets “self-tiling” or self-assembling upon drying.

The method’s biggest advantage is its scalability. Most processes are difficult to scale because they require an ultracentrifugation step. The solutions, not suspensions, produced by the team are thermodynamically stable. The method uses ‘highly prized monolayers’ as opposed to a mix of layer thicknesses, meaning they do not flocculate over time and can last up to two years. Howard elaborated on the benefits of using solutions, saying, ‘The dissolution process is very gentle. The solutions form spontaneously upon contact of the salt with the solvent. This means, unlike other liquid-exfoliation methods, we don’t have to input energy to break apart the layers and the nanosheets do not suffer damage associated with these more violent or chemically aggressive techniques. This is clearly evident for some of our examples, where we can show the 2D materials beautifully maintain their original in-plane crystallographic form.’ 

When the solution is painted onto a surface, it dries and arranges itself into different tiled shapes, which the team says has not been seen before. ‘The 2D materials we create can either be used in applications as they stand, or be engineered into advanced composites that can combine the beneficial properties of a series of them,’ Howard said, meaning that the solutions have the potential to be used for a range of technological applications, such as battery materials, LEDs and solar cells. 

The process has been patented and Howard says that it will be made commercially available as soon as possible. ‘It’s likely the first products will be nanosheet inks, sold for R&D. However, in the years to come we hope that our solutions will be integral to the incorporation of 2D materials into a range of everyday technologies.’