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The facile synthesis and electronic properties of two-dimensional hybridorganic-inorganic perovskites (2D HOIPs) make these self-assembled systems animportant class of energy materials. The basic building blocks of thesematerials include inorganic lattice frameworks that often consist oflead-halide octahedra and organic molecules possessing ammonium functionalgroups. Understanding the coupling between the inorganic and organic layers iskey to unraveling how the electronic properties of 2D HOIPs relate to theirstructures. In this work, we leverage Raman spectroscopy measurements andfirst-principles calculations to characterize the Raman-active modes in four 2DHOIPs: hexylammonium lead iodide [(HA)$_2$PbI$_4$, HA = C$_6$H$_{13}$NH$_3^+$],hexylammonium lead bromide [(HA)$_2$PbBr$_4$], butylammonium lead iodide[(BA)$_2$PbI$_4$, BA = C$_4$H$_9$NH$_3^+$], and benzylammonium lead iodide[(BNA)$_2$PbI$_4$, BNA = C$_6$H$_5$CH$_2$NH$_3^+$]. We focus on the 1400-1600cm$^{-1}$ range where the Raman intensity of the molecular constituents is thestrongest, and assign the major peaks observed in experiments as ammoniumbending vibrations. We employ a combination of density functional perturbationtheory based on the local density approximation and the frozen-phonon approachbased on the vdw-DF-cx functional to find quantitative agreement betweenexperimental and calculated Raman spectra. Furthermore, by comparing thevibrational spectra of isolated molecular cations with those near lead-halideclusters, we show how the inorganic lattice framework modulates the vibrationalproperties of the organic cations. We conclude that the properties of theRaman-active ammonium bending modes could effectively probe the localmicroscopic structure of the inorganic lattice framework in 2D HOIPs.

journal of physical chemistry. c./journal of physical chemistry. c

Characterization of the Ammonium Bending Vibrations in Two-Dimensional  Hybrid Lead-Halide Perovskites from Raman Spectroscopy and First-Principles  Calculations