New material offers potential for designing 2D nanomaterials

“In addition to thorough characterization in terms of stability, mechanical and electronic properties of the material, we have identified the origin of this half-auxetic character and believe that this mechanism can be used as a design concept for new half-auxetic materials”Thomas Heine

An international team led by Thomas Heine of Technische Universität Dresden have demonstrated a material that, regardless of whether it is being compressed or strained, will always expands in the same way. The breakthrough is based on auxetic materials, which have a negative Poisson's ratio, and when stretched become thicker perpendicular to the applied force. This unprecedented feature, where a material that expands normal to the direction of stress regardless of whether it is strained or compressed, is termed half-auxeticity.

The physical behavior in most materials is akin to stretching an elastic band. Upon strain, it will shrink normal to the direction of the applied strain, while if compressed will expand in the direction orthogonal to the pressure. However, there is an opposite behavior in some auxetic materials, which expand in the direction orthogonal to the strain, and then shrink when compressed. A good example of a material characterized by a negative Poisson's ratio is the bottle cork, which has a Poisson's ratio of zero, which allows the cork to be put into the thinner neck of the bottle.

In a paper entitled ‘Half-Auxeticity and Anisotropic Transport in Pd Decorated Two-Dimensional Boron Sheets’, published in the journal ACS Nano Letters [Ma et al. ACS Nano Lett. (2021) DOI: 10.1021/acs.nanolett.0c04154], a previously unknown phenomenon was discovered based on borophene, which is an atomically thin configuration of the element boron, for which a stable form was shown through the addition of palladium, yielding the chemical composition PdB4. With computational modelling, the team demonstrated that a borophene sheet stabilized by decorating Pd atoms behaves like an auxetic material under strain, but also expands similar to an ordinary material under compression.

From such properties, novel auxetic materials offer new functionalities for a range of products involving adjustable functional properties, such as in nanotechnology for sensing or magneto-optics, medical technology or the development of protective equipment. As Thomas Heine said, “In addition to thorough characterization in terms of stability, mechanical and electronic properties of the material, we have identified the origin of this half-auxetic character and believe that this mechanism can be used as a design concept for new half-auxetic materials”.

In investigating Pd-decorated borophene they identified three stable phases, of which one offers this property of half-auxeticity, with analysis of its stability and mechanical and electronic properties demonstrating this was a structural feature that could be used to design new 2D nanomaterials.