A process was developed for producing oxide perovskite crystals in flexible, free-standing layers. A two-dimensional rendition of this substance is intriguing because 2D materials have been shown to possess remarkable electronic properties, including high-temperature superconductivity. Such compounds are prized as potential building blocks in multifunctional high-tech devices for energy and quantum computing, among other applications.

Fabricating ultrathin perovskite oxides down to the monolayer limit enabled creation of a new class of two-dimensional materials. (Xiaoqing Pan/UCI)

By fabricating ultrathin perovskite oxides down to the monolayer limit, a new class of two-dimensional materials was created. Since the crystals have strongly correlated effects, they exhibit qualities similar to graphene.

For all of their promising physical and chemical properties, oxide perovskites are difficult to render in flat layers due to the clunky, strongly bonded structure of their crystals. Earlier efforts at making freestanding, monolayer films of the material through the pulsed laser deposition method failed.

A technique called molecular beam epitaxy was applied to grow the thin oxide films layer-by-layer on a template with a water-dissolvable buffer, followed by etching and transfer.

Most known two-dimensional materials can be synthesized by exfoliation or by chemical deposition, as their bulk crystals consist of unique layered structures in which many strong covalently bonded planes are held together by weak van der Waals interactions. Oxide perovskite is different; like most oxide materials, it has strong chemical bonds in three dimensions, making it especially challenging to fabricate into two dimensions.

Molecular beam epitaxy is a more precise method for growing oxide perovskite thin films with almost no defects, as seen at atomic resolution using aberration-corrected transmission electron microscopy (TEM). TEM provided important feedback for the optimization of film growth conditions and allowed the researchers to directly observe novel phenomena, including the crystal symmetry breaking and unexpected polarization enhancement under the reduced dimension.

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