Hybrid organic–inorganic perovskites exhibit extraordinary photovoltaic performance. This is believed to arise from almost liquid-like low-energy interactions among lattice ions and charge carriers. While spatial variations have recently been identified over multiple length scales in the optoelectronic response of perovskites, the relationship between the heterogeneity and the soft cation–lattice interactions has remained elusive. Here, we apply multivariate infrared vibrational nanoimaging to a formamidinium (FA)–methylammonium (MA)–cesium triple-cation perovskite by using the FA vibrational resonance as a sensitive probe of its local chemical environment. The derived correlation among nanoscale composition, cation–lattice coupling, and associated few-picosecond vibrational dynamics implies a heterogeneous reaction field and lattice contraction that we attribute to a spatially nonuniform distribution of cesium cations. The associated spatial variation in elasticity of the lattice leads to disorder in charge–phonon coupling and related polaron formationthe control of which is central to improving perovskite photovoltaics.

Much of the electronic transport, photophysical, or biological functions of molecular materials emerge from intermolecular interactions and associated nanoscale structure and morphology. However, competing phases, defects, and disorder give rise to confinement and many-body localization of the associated wavefunction, disturbing the performance of the material. Here, we employ vibrational excitons as a sensitive local probe of intermolecular coupling in hyperspectral infrared scattering scanning near-field optical microscopy (IR s-SNOM) with complementary small-angle X-ray scattering to map multiscale structure from molecular coupling to long-range order. In the model organic electronic material octaethyl porphyrin ruthenium(II) carbonyl (RuOEP), we observe the evolution of competing ordered and disordered phases, in nucleation, growth, and ripening of porphyrin nanocrystals. From measurement of vibrational exciton delocalization, we identify coexistence of ordered and disordered phases in RuOEP that extend down to the molecular scale. Even when reaching a high degree of macroscopic crystallinity, identify significant local disorder with correlation lengths of only a few nanometers. This minimally invasive approach of vibrational exciton nanospectroscopy and -imaging is generally applicable to provide the molecular-level insight into photoresponse and energy transport in organic photovoltaics, electronics, or proteins. infrared spectroscopy | molecular vibrations | scattering-scanning near-field optical microscopy (s-SNOM) | vibrational exciton | molecular energy transport

Order, disorder, and domains affect many of the functional properties in self-assembled monolayers (SAMs). However, carrier transport, wettability, and chemical reactivity are often associated with collective effects, where conventional imaging techniques have limited sensitivity to the underlying intermolecular coupling. Here we demonstrate vibrational excitons as a molecular ruler of intermolecular wave function delocalization and nanodomain size in SAMs. In the model system of a 4-nitrothiophenol (4-NTP) SAM on gold, we resolve coupling-induced peak shifts of the nitro symmetric stretch mode with full spatio-spectral infrared scattering scanning near-field optical microscopy. From modeling of the underlying 2D Hamiltonian, we infer domain sizes and their distribution ranging from 3 to 12 nm across a field of view on the micrometer scale. This approach of vibrational exciton nanoimaging is generally applicable to study structural phases and domains in SAMs and other molecular interfaces.

We develop precision infrared nano-imaging and -spectroscopy to probe vibrational excitons and associated vibrational wavefunction delocalization as a molecular ruler to study order and domains in molecular materials and self-assembled monolayer on the molecular scale.