Inducing ferroelastic domains in single-crystal ${\mathrm{CsPbBr}}_3$ perovskite nanowires using atomic force microscopy

Ferroelectric and ferroelastic domains have been predicted to enhance metal halide perovskite (MHP) solar cell performance. While the formation of such domains can be modified by temperature, pressure, or strain, established methods lack spatial control at the level of single domains. Here, we induce the formation of ferroelastic domains in ${\mathrm{CsPbBr}}_3$ nanowires at room temperature using an atomic force microscope (AFM) tip and visualize the domains using nanofocused x-ray diffraction with a 60 nm beam. Regions scanned with a low AFM tip force show orthorhombic 004 reflections along the nanowire axis, while regions exposed to higher forces exhibit 220 reflections. The applied stress locally changes the crystal structure, leading to lattice tilts that define ferroelastic domains, which spread spatially and terminate at {112}-type domain walls. The ability to induce individual ferroelastic domains within MHPs using AFM gives new possibilities for device design and fundamental experimental studies.

Figure 1

Schematic representation of the nanofocused scanning x-ray diffraction (XRD) experimental setup. The nanowire was rotated to the Bragg angle θ, and the x-ray detector was positioned at 2θ. The scattering vector components $q$_x, $q_y$, and $q_z$, as well as lattice tilts α (rotation around $q_y$), β (rotation around $q_x$), and γ (rotation around $q_z$) are defined in the inset. The nanowire axis was set along $z$ and the sample normal along $y$, so that $q_z$ is the main scattering component of the 004 Bragg reflection. The coordinate system rotates as the nanowire is rotated around the sample normal ($q_y$ axis).

Figure 2

(a) Atomic force microscopy (AFM) image of nanowire A after manipulation. (b) Sum of the diffraction frames acquired after AFM manipulation, evidencing the presence of multiple Bragg peaks. The white stripe is a detector module gap. (c) Map of the $d$ spacing in the lattice. Insets show higher resolution images near the manipulated regions.

Figure 3

(a) Atomic force microscopy (AFM) image of nanowire B before manipulation. Another nanowire can be seen crossing its left part. The image is stitched together from three AFM images. The inset shows a zoom of the right part of nanowire B, acquired after manipulation and evidencing the damaged region, represented by the black square. The arrow indicates the AFM scan direction, which is angled 30 ° relative to the nanowire long axis. (b) Sum of the x-ray diffraction after manipulation, evidencing the presence of multiple Bragg peaks. (c) Map plotting the real space distribution of domains associated with the different peaks, numbered from 1 to 5, as marked in (b). The inset shows a more detailed view near the peak force scan area, same as in (a), marked with the black box. (d) Schematic model of domains orientation near a {112}-type domain wall. Straight lines represent the atomic planes. Note slight differences in line spacing and rotation representing the variations in $d$ spacing and lattice tilt for neighboring domains.

Figure 4

(a) and (b) Sum of the diffraction frames acquired with the scattering vector Q parallel and orthogonal to the nanowire axis, respectively. Peaks are indexed from 1 to 5, followed by letters P and O, indicating parallel and orthogonal geometries, respectively. (c) Maps plotting the $d$ spacing and the α and β tilts, with Q parallel to the nanowire axis. (d) Maps plotting the $d$ spacing and the α and γ tilts, with Q orthogonal to the nanowire axis. Insets show magnified plots from the marked areas in (c) and (d). Note that the map in (d) is shown rotated to the same orientation as (c) to make comparisons easier, although the acquisition was done with the nanowire rotated orthogonal to the Q vector. The indicated coordinate vectors show the actual nanowire orientation in each geometry, always with the axis along $z$.

Figure 5

(a) Sum of the diffraction frames acquired from nanowire B when the Q vector was oriented diagonal to the nanowire axis. (b)–(d) Maps plotting the real space distribution of domains associated to different peaks, retrieved with the nanowire aligned (b) parallel, (c) orthogonal, and (d) diagonal to the Q vector. Maps in (b)–(d) were set in the same orientation for better comparison, while the coordinate vectors evidence the actual nanowire orientation in each geometry, always with the axis along $z$.