University of Minnesota Twin Cities Professor Bharat Jalan is co-leading a team that has developed a new method for making nano-membranes of “smart” materials, which will allow scientists to harness their unique properties for use in devices such as sensors and flexible electronics. Credit: Olivia Hultgren
Perovskite oxide semiconductors are a family of “smart” materials with a range of applications due to their unique properties and responsiveness to stimuli such as light, magnetic fields and electric fields. Creating thin films of perovskite oxide could enable the development of new technologies such as sensors, smart textiles and flexible electronics, but researchers face challenges producing freestanding membranes of oxide materials, limiting their functionality. A team led by University of Minnesota researchers has now developed a new method that enables the production of freestanding thin films of perovskite oxides, which relies on a technique known as hybrid molecular beam epitaxy.
Traditional epitaxy involves building a thin film of material on a substrate, but in many cases, the film cannot be removed from the substrate, limiting its use. Another technique, called remote epitaxy, uses a layer of graphene between the substrate and thin film; this allows for the creation of freestanding membranes but risks oxidation of the graphene when building oxide materials. To produce the perovskite oxide material strontium titanate (SrTiO3) as a freestanding membrane, the researchers turned to hybrid molecular beam epitaxy, which uses titanium tetraisopropoxide (TTIP), rather than elemental titanium and oxygen sources, to build SrTiO3. The oxygen that is already bonded to titanium in TTIP is sufficient for the production of SrTiO3 and is less likely to oxidize graphene compared with a separate oxygen source.
The team first used the hybrid technique using TTIP on a substrate without graphene, and using atomic force microscopy (AFM) and high-resolution X-ray diffraction, showed that the method produced a high quality thin film of SrTiO3. The researchers then grew the film on a bilayer of graphene and showed the film could be exfoliated from the graphene, forming a freestanding membrane that can then be transferred to another substrate. Scanning transmission electron microscopy high angle annular dark-field (STEM-HAADF) imaging and STEM energy-dispersive X-ray spectroscopy (STEM-EDS) elemental mapping were performed on the films before and after exfoliation and transfer, confirming the successful production and transfer of the SrTiO3 film. Additionally, the hybrid molecular beam epitaxy technique allows for self-regulated stoichiometric control during film growth. This study was published in Science Advances.
“We showed for the first time, and conclusively by doing several experiments, that we have a new method which allows us to make a complex oxide while ensuring that graphene is not oxidized. That’s a major milestone in synthesis science,” said co-senior author Bharat Jalan. “And, we now have a way to make these complex oxide membranes with an automatic stoichiometric control. No one has been able to do that.”
The method could be further applied to produce freestanding membranes of other complex oxides, which will be useful to produce a range of technologies such as extremely small transistors and flexible sensors, said co-senior author Steven Koester.