Speaker
Description
When a crystalline solid melts, long-range order collapses into a disordered liquid — a 1st-order transition characteristic of 3D materials. In 2D, however, melting can proceed differently: through defect-mediated unbinding of dislocations and disclinations as described by Kosterlitz–Thouless–Halperin–Nelson–Young theory, via a 1st-order grain-boundary process, or through a combination of both. These scenarios predict an intermediate hexatic phase, which retains short-range positional but quasi-long-range orientational order, akin to a liquid crystal.
Here, we study melting of covalent hexagonal monolayers of AgI encapsulated in graphene. Encapsulation stabilizes the 2D phase and prevents transformation into bulk polymorphs, while lattice incommensurability minimizes substrate constraints. Time- and temperature-resolved in situ scanning transmission electron microscopy combined with convolutional neural network analysis reveals a hexatic phase — the first such observation in a covalent material — and supports a mixed defect- and grain-boundary-mediated melting scenario [1].
- Bui, T. A., Lamprecht, D., et al. (accepted). Hexatic phase in covalent 2D silver iodide. Science.
