An international team of researchers have used nano focused X-rays at the NanoMAX beamline to image the complex structure of metal halide perovskite nanowires. The high-resolution imaging made it possible to see domains inside the nanowire, as the temperature was increased across a structural phase transition. The structure of perovskite materials plays an important role in their properties for solar cells and light-emitting device applications.
For this study, the researchers produced the metal halide perovskite CsPbBr3 in the form of nanowires. It is a type of nanomaterial that is often considered in electronics and energy applications. The research team studied the effect of temperature on the structure of the material.
“We started measuring at room temperature, and we could observe structural inhomogeneities within the nanowire. This had not been observed with such high resolution using nano focused X-rays before, so that was already exciting. However, the domains did not have any special pattern,” says Dr Lucas Marçal from Lund University. “We gradually increased the temperature, and suddenly, at 80°C, the pattern changed to a highly ordered state.”
The crystal structure in the nanowire is divided into smaller, more local structural units, so-called domains. Each of these domains is a perfect crystal that has a specific orientation, and the domains generally have a characteristic size.
In the recent study, the nano focused beam was smaller than the domains, making it possible for the researchers to image them directly. The ordered domains in the nanowires is a signature of ferroelasticity, which is a property of a material where strain arises internally in the structure even without external load. It is the mechanical and structural equivalent of the more well-known ferromagnetism.
“The highly ordered pattern that we could observe at elevated temperatures is fascinating. We were also surprised that we could get such a high spatial resolution at high temperature at NanoMAX since heating often leads to thermal drift and instabilities.” continues Marçal. “We could see much of these results immediately, thanks to the analysis tools at the beamline.”
The research team have already conducted more studies. They also see the importance of this type of experiments, using nano focused X-rays, for other materials.
“We have already made new investigations in which we try to improve the spatial resolution even further,” concludes Marçal. “The method we have demonstrated, nano diffraction of ferroelastic domains, should be useful for a much wider range of materials. Ferroelasticity is closely related to ferroelectricity, which is used for computer memories.”
Publication (open access)
Lucas A. B. Marçal, Eitan Oksenberg, Dmitry Dzhigaev, Susanna Hammarberg, Amnon Rothman, Alexander Björling, Eva Unger, Anders Mikkelsen, Ernesto Joselevich, and Jesper Wallentin, In Situ Imaging of Ferroelastic Domain Dynamics in CsPbBr3 Perovskite Nanowires by Nanofocused Scanning X-ray Diffraction, ACS Nano 2020, published online October 19, DOI: https://dx.doi.org/10.1021/acsnano.0c07426
Top image: The sample holder, holding the perovskite material ready to be measured at NanoMAX. The wires sticking out from the sample holder are for heating the sample. This photo shows an experiment conducted after the one reported in the paper.