Traditionally, it has always been thought that the interactions between a 2-dimensional (2D) material and another solid substrate adhered through conventional solid-solid interactions. However, as it transpires, a team of researchers from China and the US have found that there is actually an energy conversion between the heat and mechanical work during attachment and detachment of the two materials. The discovery of these interactions has prompted the researchers to stipulate that the interactions during adhesion act more in the way of a gas-like adsorption, and not a solid-solid interaction.

The adhesion between two molecules is one of the most common and important phenomenon’s and interactions in nature. Many reactions, chemical, and biological processes rely on surface interactions to perform specific functions and reactions. This has never been more true than at the nanoscale, where the high surface-volume ratios can become the most dominant driving force for any nanoscale process. However, it has always been poorly understood how nanomaterials, particularly those that operate in 2-dimensions (2D), interact with other solid surfaces.

2D materials are growing ever popular, and not only in academic research as people are now starting to realise the commercial implications that this class of materials can bring. The common 2D materials that people recognise are graphene, silicene, borophene and phosphorene. However, there are many more than this to date. To truly get the most out of these materials, a better understanding of their interactions is a key research area, as it can define how these materials interact when fabricated into various commercially-viable devices.

There has been a lot of interest in 2D material-to-substrate interactions. Recent research has treated 2D materials as mechanical sheets and their adhesion properties have been considered to utilise mechanical contact. However, 2D materials can conform to the surface of another material much more closely than other materials, and as a result, the interactions are very different than those of bulk solid-solid interactions. Many 2D materials have found to exhibit temperature-dependent adhesion phenomena, thus, the interactions cannot be viewed in a purely mechanical way.

The researchers have used a series of computational models based on phonon analysis to determine that the interactions between 2D materials and other solid substrates is born out of gas-like adsorption rather than bulk solid-solid interactions. As they are two of the most common, the research team focused on both graphene and boron nitride when performing their calculations. The researchers found that the two main contributions to the interactions between a 2D material and a solid substrate are a low bending stiffness and a large concentration of contacting atoms.

Both the adhesion energy and forces were found to be constants that do not solely rely on physical interactions; but also by the temperature of the system. During the attachment and detachment phases, there lies a near reversible energy conversion between the thermal and mechanical work forces.

In their research, the 2D materials exhibited temperature changes, with variations showing a tendency towards thermodynamic heat release. Physical interactions can cause friction between the molecules and can lead to heat release during adhesion. However, in gas-like adhesion, the adsorption of a molecule onto a surface releases heat, but under desorption, the surface will adsorb heat. The 2D materials showed heat-exchange characteristics of gas-like adhesion interactions during the absorption/desorption process with the solid substrate.

In addition to the heat transfer, gas molecules come closer to a surface than solid materials and form a film. 2D materials have the ability to get close and conform to the surface of the substrate, which in essence mimics the film conformity shown by gas molecules and is a completely different mechanism shown by bulk solid-solid interactions.

The mechanism of gas-like adhesion for 2D materials is dependent on the entropy of the freestanding sheet being higher than the adhered one. The adhesion process causes a drop in the entropic contributions through confinement of the out-of-plane fluctuations in the adhered 2D material and reduces its degrees of freedom.

There is also no difference in heat adsorption when multi-layered 2D materials are used, which suggests that only the layer in contact with the substrate is responsible for the change in entropy.

The results now provide a fundamental understanding of the interactions of 2D materials, but are bitter-sweet. On one hand, the result provides a new approach to developing nanoscale actuators- something which has been a challenge due to the lack of an efficient energy conversion mechanism. But on the other hand, it may raise issues in other 2D material applications, especially in oscillators where it has been proposed that a high sensitivity can be generated using shear forces- a temperature dependent force.

Source:

Guo Z., Chang T., Guo X., Gao H., Gas-like adhesion of two-dimensional materials onto solid surfaces, Scientific Report, 2017 7, 159