US researchers have used X-ray scattering while carrying out molecular beam epitaxy (MBE) to help them understand the formation of layered oxides and to build a computer model for predicting the characteristics of new materials and whether or not those materials would be a metastable material. [Freeland et al., Nature Mater. (2014) 13, 879-883; DOI:10.1038/nmat4039]
"MBE is the construction of new materials one layer at a time - and each layer is one-atom thick, explains John Freeland of Argonne National Laboratory in Illinois who worked with colleagues there and at the University of Wisconsin-Madison, Northwestern University and the University of Connecticut-Storrs on the study. "We used a new type of MBE system to observe what happens during the growth of oxide thin films. We found that the layers spontaneously rearrange to reach a lower energy, preferred configuration - but not necessarily the configuration we intended. It is important to know that this shuffling of layers takes place when designing new materials of this class.
The team worked with strontium titanates and found that when they applied a layer of titanium on to two layers of strontium, the titanium layer switched places with the second strontium layer, thus becoming the central layer. When titanium was layered on multiple layers of strontium, titanium always switched places with the strontium layer directly beneath it. The researchers have now developed a density functional theoretical (DFT) approach to explain the energetics that drive the rearrangements of the different layers.
Perhaps inevitably, the DFT studies revealed that the final sequence of layers formed during MBE corresponds to the lowest energy configuration. Of greater interest though is that the calculations suggest that the layer-shuffling phenomenon will not be limited to strontium and titanium oxide systems and will most likely occur in many different materials, suggesting a way to understand and even control the growth of oxide thin films at the monolayer level.
"What we have here is a new strategy for materials design and synthesis," suggests team member Dillon Fong. "Our combination of in situ X-ray scattering with computational theory can be extended to other layered materials and structures, even theoretical ones that haven't been made yet because they are challenging to manufacture." The strategy should point materials scientists to the optimal growth strategy for a given layered material in the Ruddlesden-Popper homologous series with their intriguing dielectric, ferroelectric, magnetic and catalytic properties, and hopefully reduce the time from design to production. Indeed, the team has already demonstrated proof of principle with a lanthanum nickel oxide.
David Bradley blogs at http://www.sciencebase.com and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".