Synthetic 2D materials without bulk analogues

Materials World magazine
27 Sep 2019

Two-dimensional materials can be synthesised without bulk analogues using a new method developed to make 2D glass. Idha Valeur reports.

A technique to synthesise two-dimensional (2D) materials, previously considered impossible using present technologies, has been created by researchers from the University of Manchester, UK.

In their paper, Two-dimensional covalent crystals by chemical conversion of thin van der Waals materials, published in Nano Letters, the team described how they created a 2D material without using a 3D counterpart, known as bulk analogue, by converting layers of van der Waals crystals using chemical conversion.

Breaking it down

2D materials are normally created by chemical exfoliation, where a 3D bulk material is stripped of layers until a single layer is left. However, the team wanted to find a way to make a 2D material outright.

‘Crystals with covalent bonding between atomic planes do not allow mechanical exfoliation, and to make such crystals down to atomic-scale thickness poses a clear challenge. This can be overcome by chemical conversion of atomically thin van der Waals crystals. The conversion leads to new 2D covalent crystals that cannot be obtained via mechanical cleaving.

‘Their thickness can be precisely controlled by choosing the number of atomic layers in the parent van der Waals material,’ National Graphene Institute and Department of Chemical Engineering and Analytical Science Professor, Rahul Nair, told Materials World.

Nair explained that to create these materials, they used 2D indium selenide (InSe) obtained by exfoliation of bulk InSe and converted it by direct fluorination into indium fluoride (InF3). ‘Fluorination was carried out by direct exposure to xenon difluoride (XeF2) at elevated temperatures,’ he said.

This new synthetic 2D material does not oxidise in contact with air, unlike most others. Nair explained that interest in advanced nanomaterials is increasing for several applications, including water filters and space technology. ‘The concept of chemical conversion of cleavable, thin van der Waals crystals into covalently bonded non-cleavable ones opens prospects for synthesising a wide variety of novel atomically thin covalent crystals with unknown properties and potentials,’ he said.

‘Stacking different 2D materials, similar to stacking Lego bricks, is an active area of research in preparing materials with on-demand properties. By demonstrating the possibility of 2D covalent solids, researchers now have more legos in their playground to create novel materials with custom-made properties.’

Applications and commercialisation

There are several applications for the synthetic 2D material. ‘2D indium fluoride is a semiconductor, exhibiting high optical transparency across the visible and infrared spectral ranges and could potentially be used as a 2D glass. Because of its optical properties, this could also be used in the optoelectronic application e.g. LED and photodiode.

‘However, to use this material in optoelectronic applications, we need further improvement in the crystalline quality, to get crystals without defects,’ Nair said. ‘We already demonstrated the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation into InF3 films. The next step will be improving the quality of the crystals and also exploring some unique applications for this material.’

For more information, access the paper here: