In Situ Imaging of Ferroelastic Domain Dynamics in CsPbBr3 Perovskite Nanowires by Nanofocused Scanning X-ray Diffraction

The interest in metal halide perovskites has grown as impressive results have been shown in solar cells, light emitting devices, and scintillators, but this class of materials have a complex crystal structure that is only partially understood. In particular, the dynamics of the nanoscale ferroelastic domains in metal halide perovskites remains difficult to study. An ideal in situ imaging method for ferroelastic domains requires a challenging combination of high spatial resolution and long penetration depth. Here, we demonstrate in situ temperature-dependent imaging of ferroelastic domains in a single nanowire of metal halide perovskite, CsPbBr3. Scanning X-ray diffraction with a 60 nm beam was used to retrieve local structural properties for temperatures up to 140 °C. We observed a single Bragg peak at room temperature, but at 80 °C, four new Bragg peaks appeared, originating in different real-space domains. The domains were arranged in periodic stripes in the center and with a hatched pattern close to the edges. Reciprocal space mapping at 80 °C was used to quantify the local strain and lattice tilts, revealing the ferroelastic nature of the domains. The domains display a partial stability to further temperature changes. Our results show the dynamics of nanoscale ferroelastic domain formation within a single-crystal perovskite nanostructure, which is important both for the fundamental understanding of these materials and for the development of perovskite-based devices.


Nano-XRD on reference nanowire
We have performed measurements in a reference nanowire, named ref-nanowire, 23 µm long, 1.1 µm wide and 60 nm height, at RT. We initially aligned the (004) orthorhombic (Pbnm) Bragg reflection, an intense fundamental reflection expected to be accessible with a rotation around the refnanowire short axis that is parallel to the substrate. With this choice, we can collect information regarding the structural properties along the ref-nanowire long axis. Making use of the piezoelectric stage we were able to map the ref-nanowire, collecting the diffraction frame for different real space positions with high real space resolution.
A normalized real space map projection of this nanowire is shown in Figure S1(a) (upper panel).
In this map, bright regions are associated with more diffracted intensity, which indicates a well Bragg alignment, while fluctuations might be mainly caused by misalignments on the Bragg condition due to local lattice tilts. A set of projection maps collected around the Bragg condition was used to a full reciprocal space map reconstruction. Note that the RT measurements of the nanowire in the main text was only done for a single angle. Summing the total diffraction frames for each real space position, we could retrieve the integrated intensity map. In this map, each pixel represents the sum of the total diffracted intensity for that specific real space position. Such procedure also leads to the disentanglement and unambiguous determination of the strain field and both α and β lattice tilt maps along the refnanowire. All the maps are shown in Figure S1(a). These maps indicate a crystal with very small strain variations, except near both ends. The lattice tilts show an undulating profile perpendicular to the substrate along the ref-nanowire long axis, with rotations around q x ranging from -0.46° to +0.34° in approximately 500 nm. A more detailed view of the selected area can be seen in Figure S1(b), for each corresponding map. These measurements show that the domains seen in the single projection map (top pannel) are due to variations in crystal tilt, not strain. We expect that the same is true for the nanowire in the main text, for which we don't have the full rocking curve dataset at RT.

Crystalline structure change
We have tracked the nanowire along the heating process. Real space diffraction mappings, performed using the piezo stage below the sample, allowed the investigation of the crystalline structure at different temperatures. The Figure S2(a) shows the sum of the diffraction frames acquired along the nanowire for all the steps between 30 °C and 80 °C, when the appearance of new diffraction peaks at different reciprocal space positions was observed. For each temperature, an associated real space projection of the nanowire can be seen in Figure S2(b). Each pixel corresponds to a real space position, and the total intensity, represented by the colormap, which is the sum of the diffraction intensity, is normalized. Figure S2(c) shows a more detailed view of the selected areas marked with the black box in Figure S2

Low angle measurements
We performed a real space mapping of the nanowire with the Bragg angle set at at θ = 4.28°, at 80 °C. This angle corresponds to half of the value used on the other real space mappings. A sum of the detector frames collected along the nanowire can be seen in Figure S3. The peak appearing in the right part of the Figure, at higher q z , must correspond to the tetragonal (110) Bragg peak, while the left one can be associated to the tetragonal (001) or to the orthorhombic (002). A theoretical ΔQ = 0.0077 Å -1 would be expected for the case where these peaks correspond to tetragonal (001) and (110) Bragg 5 reflections, while the scenario where the lower angle peak is associated to the orthorhombic (002) Bragg reflection leads to ΔQ = 0.0094 Å -1 . The measured average difference in the momentum transfer vector is ΔQ = 0.0098 Å -1 , which is 27% higher than the theoretical value obtained for the first scenario and 4% higher when compared with the second case.

Cooling down cycle
Real space diffraction mappings allowed us to identify local changes on the nanowire structure for every 10 °C step, as well as at 25°C, corresponding to the RT. The sum of the diffraction frames acquired along the nanowire for each temperature are shown in Figure S5 (left panels). Part of the real 7 space map projections, corresponding to each respective temperature, can be seen in Figure S5 (right panels). Figure S5. Sum of the diffraction frames collected along the nanowire, as well as part of the real space map projections for temperatures below 80 °C acquired in the cooling down process, pointing out that the structure, instead of returning to its original state, preserves a memory of the high temperature shape. The nanowire was mapped for every 10 °C step, as well as at 25 °C (RT).

Temperatures above 80 °C
The temperature was then increased directly to 80 °C and, again, the nanowire was mapped for each 10 °C step to 140 °C. The sum of the diffraction frames acquired along the nanowire can be seen 8 in Figure S6 (left panels). Part of the real space projection maps are shown in Figure S6 (right panels), revealing a progressive extinction of both the stripes-like pattern, as well as the tilted features along the nanowire edges. Figure S6. Sum of the diffraction frames collected along the nanowire, as well as part of the real space map projections acquired at high temperatures, evidencing the extinction of the stripes-like pattern. The nanowire was mapped for every 10 °C step. It is notable that peaks are moving in the q z direction (horizontal direction in this Figure), merging at high temperatures, indicating that all domains are gradually changing to a similar crystalline structure. 9

Final cooling down cycle
Finally, we cooled the nanowire down a second time and performed new real space mappings at 75 °C and RT. The sum of the diffraction frames acquired at 75 °C and 25 °C are depicted in Figure S7 (left panels). Part of the corresponding real space projection maps are also shown on Figure S7 (right panels).