“If you have this kind of cut-and-paste growth and removal, combined with the different functionality of putting single-crystal oxide materials together, you have a tremendous possibility for making devices and doing science”Chang-Beom Eom
Researchers from the University of Wisconsin-Madison and MIT have created a new method for stacking ultrathin and complex oxide single-crystal layers that can produce stacked-crystal materials in nearly infinite combinations. The breakthrough could improve high-tech electronic devices due to the diverse functional properties of complex oxides, seen as crucial to the development of new components for applications such as data storage, sensing, energy technologies and biomedical devices.
As the geometrically arranged atoms of complex oxide single-crystal layers have useful magnetic, conductive and optical properties, the innovative platform and crystal-stacking process described in the journal Nature [Kum et al. Nature (2020) DOI: 10.1038/s41586-020-1939-z], could be used to develop structures with hybrid properties and a range of functions, as producing perfect interfaces while coupling different classes of complex materials allows new behaviors and tunable properties.
The researchers had previously added an ultrathin intermediate layer of graphene, before using epitaxy – where a material is deposited on top of another material in an orderly way – to grow a thin semiconducting material layer on top, with the grapheme acting as a peel-away backing since it is only a single molecule thick and therefore has weak bonding. This left a freestanding ultrathin sheet of semiconducting material.
Here, their layering method managed to overcome a key issue with conventional epitaxy in that every new oxide layer must be very compatible with the atomic structure of the underlying layer. If they align as a mismatch, the layers won’t stack properly. In the conventional method, a perfect single crystal can be grown on top of a substrate, but there is a problem in that growing the next material the structure must be the same and the atomic spacing similar, a constraint to growth.
For instance, while magnetic materials and piezoelectric materials cannot be grown on top of each other as they have different crystal structures, but with this method the layers can be grown separately, and then peeled off and integrated. The team showed the effectiveness of their approach with materials including perovskite, spinel and garnet, and they also can stack single complex oxide materials and semiconductors.
Such complex oxide materials can have a broad range of tunable properties that most other materials do not have but are significantly more difficult to grow and integrate, so the peel-away approach was effective. As team leader Chang-Beom Eom said, “If you have this kind of cut-and-paste growth and removal, combined with the different functionality of putting single-crystal oxide materials together, you have a tremendous possibility for making devices and doing science”.