Researchers at Queen Mary, University of London and Montanuniversität Leoben in Austria have shown that bilayer graphene was noticeably softer than both two-dimensional (2D) graphene and three-dimensional (3D) graphite along the stacking direction.

This surprising result differs from previous research which showed that 2D graphene, a flat single layer of carbon atoms, had many of the same mechanical properties as 3D graphite, which is a naturally occurring form of carbon made up from a very weak stack of many layers of graphene.

The team explained that while graphene is a 2D material, it is known to have 3D properties such as its stiffness in the 'out-of-plane' direction, perpendicular to the plane of the graphene sheets.

The behavior of π electrons within multilayer graphene determine its out-of-plane stiffness. In this study, the researchers found that when bilayer graphene is compressed out-of-plane, some π electrons are 'squeezed' through the graphene planes, which are impenetrable to small molecules such as water. This response makes the material softer and much easier to compress.

Dr. Yiwei Sun, lead author of the study from Queen Mary University of London, said: "Our previous study showed that 2D graphene and 3D graphite have many of the same mechanical properties, so we were surprised to see that bilayer graphene is much softer than both of these materials. We think that the softness of bilayer graphene results from the 'squeezing' of pi-electronic orbitals through the graphene layers. For example, if the bread on a burger is replaced by a bagel it is even easier to compress because the contents can be squeezed out of the bagel hole."

This study provides insight into the complex interactions between graphene bilayers and enables quantification of its properties, which is critical for exploring future applications of the material in devices such as vertical transistors and pressure sensors.