Exfoliating layered materials

Layered materials can be separated into individual sheets with enhanced electronic properties using a new quick and simple method, says an international team of researchers.

Very thin sheets of a material can exhibit enhanced properties such as increased electrical conductivity, compared with the bulk three dimensional (3D) layered material. Graphene, a sheet of carbon one atom thick, is a good example of this, and is the subject of much research.

A team led by Jonathan Coleman at Trinity College Dublin in Ireland and Valeria Nicolosi at the University of Oxford, UK, used common solvents such as n-methyl pyrrolidone to exfoliate (separate) layered materials such as boron nitride (BN), molybdenum disulfide (MoS?₂), tungsten disulfide (WS₂), and bismuth telluride (Bi₂Te₃) into individual sheets.

Traditional methods to separate layered materials such as ion intercalation (in which metal ions are inserted between layers and a charge is transferred to the layers causing them to repel each other) can change the chemical structure of the layers, are time consuming and need to be carried out under an inert atmosphere.

Coleman’s process of liquid exfoliation can take as little as 10 minutes, can be carried out on in air and is straightforward. Individual sheets of layered materials are held together using van der Waals forces. By submerging the material in a solvent and shaking it gently in a sonicator, the layers separate easily.  

Coleman explains that the key is to find a solvent that has an interaction energy with the layers that is greater than the interaction energy between the layers in the bulk material themselves. ’It is therefore thermodynamically more favourable for the layers to separate and interact with the solvent than stay together,’ he says. ’Here you have an exfoliation method for layered compounds just by clever selection of solvents,’ he adds. 

’The work shows a beautifully uncomplicated yet effective methodology,’ says Adrian Wright, an expert in layered materials at the University of Birmingham in the UK. ’With the ability to now isolate such nano-dimensional inorganic layers, there are exciting possibilities to use these as building blocks to engineer nano-composite materials in a layer by layer approach,’ he adds. 

After separating a wide variety of inorganic materials into sheets, the team were able to add other materials into the structure to generate hybrid composites. ’When we dispersed some of the layered materials in liquids, we could add other compounds such as carbon or graphene that gives a new material with the same properties as the layered material, but also has electrical conductivity of a billion times more,’ says Coleman. 

’In terms of the chemistry behind it, it is the simplicity of the method that the authors have proposed that is the key attraction,’ says Richard Walton, an expert in inorganic materials from the University of Warwick, in the UK. ’The challenge now is to put it into practice,’ he adds. 

In the future, Coleman and his team hope to make more hybrid composite materials to develop a new generation of thermoelectric materials - materials that can be used in devices to generate energy from waste heat. 

Mike Brown