Multistability of graphene nanobubbles

TL;DR

This study demonstrates that graphene nanobubbles on flat substrates are multistable, capable of adopting multiple stable layered states with distinct shapes, influenced by atom encapsulation and thermal effects, revealing complex stability and shape behaviors.

Contribution

The paper introduces the concept of multistability in graphene nanobubbles, showing multiple stable layered configurations and their dependence on atom number and temperature, which was not previously understood.

Findings

Nanobubbles can have multiple stable layered states.

Maximum layers increase with encapsulated atoms, reaching 6 for 4000 atoms.

Thermal vibrations lead to transitions to a liquid state at characteristic temperatures.

Abstract

Using He, Ne, Ar, Kr, and Xe atoms as a model system, it is demonstrated that graphene nanobubbles on flat substrates are multistable systems. A nanobubble can adopt multiple stable stationary states, each characterized by the number of layers $l$ within the cluster of encapsulated atoms. The layers are circular, concentrically stacked, and form an $l$-stepped pyramid with a flat top. Encapsulation of this pyramid by the graphene sheet is achieved through local stretching of the membrane: the valence bonds elongate only directly above the confined atoms. Outside this coverage zone, the sheet remains undeformed and lies flush against the substrate. The maximum number of possible layers, $l_m$, increases monotonically with the number of encapsulated atoms $N$, reaching $l_m=6$ for $N=4000$. The graphene membrane, through van der Waals interaction with the substrate, compresses the internal atomic cluster, generating pressures on the order of $P\sim 1$~GPa. Numerical simulations of thermal vibrations reveal that among all $l$-layer configurations, one ground state always exist. Upon heating, this state smoothly transitions into a layerless liquid configuration. All other stationary states transform into this ground state once a characteristic temperature $T_l$ is reached. For $N=4000$, the ground state corresponds to the four-layer packing ($l=4$). The coexistence of multiple stable states with distinct layer numbers at low temperatures leads to the absence of a universal shape for the nanobubbles. In this scenario, the height-to-radius ratio, $H/R$ is not constant and can vary from 0 to 0.28, depending on the number of layers.