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Researchers grew freestanding crystalline films of two perovskite oxides—strontium titanate (SrTiO3) and bismuth ferrite (BiFeO3)—almost down to a single unit cell. Their 2019 study used a water-soluble layer to release the films from the growth substrate. It showed that the films studied retained crystalline order near the monolayer limit; it did not establish that every perovskite can do so at any thickness.
How the researchers made the films
Perovskite oxides cannot be peeled into thin sheets in the same straightforward way as graphite: the bonds between their layers are too strong. Instead, Dianxiang Ji, Songhua Cai and colleagues grew SrTiO3 and BiFeO3 films using reactive molecular beam epitaxy, with a water-soluble Sr3Al2O6 layer between the film and its growth support.
- The researchers grew the oxide film on the sacrificial Sr3Al2O6 buffer.
- They dissolved the buffer in water, freeing the oxide film from the original support.
- They transferred the released film to other substrates, including crystalline silicon wafers and holey carbon films.
The buffer is key to the approach: it makes it possible to grow the oxide in an ordered form and then release it for transfer. The Nature paper reports the materials and process; Chemistry World’s 2019 account describes immersion and transfer of square-millimetre-scale sheets.
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Ji and colleagues reported high crystalline quality almost down to a single unit cell. Chemistry World described the samples as roughly two to four unit cells thick, with some regions only one unit cell thick. Those figures describe the reported samples and regions; they do not mean every transferred sheet was uniformly one unit cell thick.
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The authors summarized the result as demonstrating “the absence of a critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films.” That conclusion applies to the freestanding films they studied. It is not evidence that crystallinity has no thickness limit in every oxide or under all growth and transfer conditions.
What changed in BiFeO3 and SrTiO3?
BiFeO3: structural and electrical changes
As freestanding BiFeO3 approached the two-dimensional limit, the researchers reported unexpected giant tetragonality and polarization. These are findings about the material’s structure and electrical behavior, not evidence of a finished device or a practical application.
SrTiO3: flexibility in an ultrathin sheet
Chemistry World reported that the ultrathin SrTiO3 sheet was flexible, unlike bulk strontium titanate, which behaves as a brittle ceramic. This is a material observation; the study does not establish a flexible consumer product.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsCould 2D perovskite oxides become the next graphene?
That remains a possibility, not a demonstrated outcome. The researchers, as quoted by Chemistry World, said they anticipated that “2D perovskite oxides could become as useful as graphene.” The experiments establish a way to make and characterize freestanding crystalline oxide films near the monolayer limit; they do not demonstrate graphene-like devices, performance or commercial uses.
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The paper points to future exploration of two-dimensional correlated phases and interfacial phenomena. Those are research prospects that the fabrication result may enable, rather than applications already verified by the study.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What the 2019 study establishes
- Reactive molecular beam epitaxy and a water-soluble Sr3Al2O6 buffer enabled researchers to release and transfer SrTiO3 and BiFeO3 films.
- The studied freestanding films retained crystalline order almost down to a single unit cell.
- Freestanding ultrathin BiFeO3 showed reported structural and polarization changes, while ultrathin SrTiO3 was reported as flexible.
- The result does not prove unlimited crystallinity across all materials, nor does it establish a device or commercial application.
The primary paper, “Freestanding crystalline oxide perovskites down to the monolayer limit,” by Dianxiang Ji and colleagues, appeared in Nature on 5 June 2019 (issue date 6 June 2019). Its data are available from the corresponding authors upon reasonable request, according to the paper.
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