Graphene Bubbles: Layers, Pressure, and Changing Shapes

Nanobubbles made of graphene can lock into several different shapes, each with a distinct number of stacked layers inside. When atoms such as helium or xenon are trapped under a flat graphene sheet, the membrane stretches only directly over the cluster. This creates concentric circular layers that look like a stepped pyramid with a flat top, while the rest of the sheet stays level on the substrate.

The maximum layers that can form grow steadily as more atoms are added; for a cluster of 4000 atoms the peak number is six. The graphene’s pull on the surface squeezes the atoms together, producing internal pressures around one gigapascal.

Simulations of how the bubble vibrates at different temperatures show that there is always one lowest‑energy (ground) state. When the bubble is heated, this ground state gradually becomes a liquid‑like layerless shape. Every other possible configuration will collapse into the ground state once the temperature reaches a certain threshold that depends on the layer count. For 4000 atoms, the ground state has four layers.

Because multiple stable shapes can coexist at low temperatures, the bubble’s height compared to its radius is not fixed. The ratio can vary from zero up to about 0.28, depending on how many layers are present. The often‑quoted ratio of around 0.2 only appears for the ground state configuration.

Applying external pressure does not erase this multistability, but it can change the bubble’s shape if the trapped atoms crystallize. Crystallization lowers the height‑to‑radius ratio, while a noncrystalline bubble simply compresses uniformly without altering that ratio.