Conformational Disorder in a Hybrid 2D Perovskite with a Long Aliphatic Chain under Pressure

Two-dimensional hybrid lead halide perovskites are importantmaterialsfor solar energy and optoelectronic applications. Although the inorganiccomponent normally determines the electronic structure, there hasbeen an emerging interest in organic sublattice engineering. The organicspacer can influence the stability, the mechanical and optoelectronicproperties, and the device performance, at least through steric effects.Order-disorder transitions facilitated by the organic moleculesare some of the pathways that mediate this influence. Herein, we investigatedthe effect of hydrostatic pressure on the optical properties of hexadecylammonium(HDA) lead iodide ((HDA)(2)PbI4), where the pliableorganic part contains a long aliphatic chain with many conformationaldegrees of freedom, by means of optical, Raman, and solid-state NMRspectroscopy, as well as density functional theory simulations. Undermoderate pressures of 2 GPa and above, the perovskite crystal exhibitsdomains with differing photoluminescence (PL) energies. The PL peakdistribution exceeds 100 nm at higher pressure, and the changes arepartly irreversible after pressure release. We explain the observedphenomena by conformational transitions, which occurred around thedifferent C-C bonds proximate to the cationic NH3 end HDA and affected the lead iodide nanosheets and thus the optoelectronicproperties. The micrometer-scale bandgap junctions formed in one materialcan be applied in novel optoelectronic devices.