Vortex flow induced self-assembly in CsPbI₃ rods leads to an improved electrical response towards external analytes

We present a facile and versatile strategy for enabling CsPbI₃ rods to self-assemble at an air-water interface. The CsPbI₃ rods, which float at the air-water interface, align under the influence of the rotational flow field due to the vortex motion of a water subphase. The aligned CsPbI₃ rods could be transferred onto various substrates without involving any sophisticated instrumentation. The temperature of the subphase, the concentration of the CsPbI₃ aliquot, the rotational speed inducing vortex motion, and the lift-off position and angle of the substrate were optimized to achieve high coverage of the self-assembled rods of CsPbI₃ on glass. The Rietveld refinement of the XRD profile confirms that the aligned CsPbI₃ is in the pure orthorhombic phase ascribed to the Pnma space group. The hydrophilic carboxylic group of the oleic acid attaches to the Pb atoms of the halide perovskite rods, while their hydrophobic tails encapsulate the rods within their shell, creating a shielding barrier between the water and the perovskite surface like a reverse micelle. The aligned CsPbI₃ rods exhibit a nearly 47-fold increment in current upon exposure to ammonia gas (amounting to 5.6 times higher sensitivity in ammonia sensing) compared to the non-aligned CsPbI₃ rods.