Synthesis of Honeycomb‐Structured Beryllium Oxide via Graphene Liquid Cells

With the advent of layered two-dimensional crystals, surface dangling bonds-free planar sheets provide one emergent type of growth substrates to explore the new possibilities of crystal synthesis. While graphene has been widely exploited as the buffered substrate in van der Waals epitaxy, the encapsulation by two sheets can lock liquid droplets in between and more importantly, experience a van der Waals pressure up to 1 GPa, producing a miniaturized high-pressure container for the crystallization in solution. Here, using high-resolution transmission electron microscopy and electron energy loss spectroscopy, we show that beryllium oxide crystallizes in the hexagonal structure in the graphene liquid cell by the wet chemistry approach. The thickness of as-received crystals is beyond the thermodynamic ultra-thin limit above which the wurtzite phase is energetically more favorable according to the theoretical prediction. The crystallization of the layered phase is ascribed to the near-free-standing condition afforded by the graphene surface. And our calculation shows that the energy barrier of the phase transition is responsible for the observed hexagonal layers beyond the previously predicted limit. Our findings open a new door for exploring the aqueous solution approaches of more metal oxide layered semiconductors with the exotic phase structures and properties in the graphene-encapsulated confined cell.