IMS theory shows thin-film optics alone can mimic molecular signals in perovskite infrared spectra
A theoretical study from Japan's Institute for Molecular Science reproduces most of a puzzling infrared signal in a perovskite film without any change in molecular vibrations.
Jun Nishida, Assistant Professor at the Institute for Molecular Science (IMS) in Japan, has shown in a theoretical study published in The Journal of Chemical Physics that light-generated charges can change how infrared light is reflected and transmitted within the layers of a thin film. This produces sharp spectral features that resemble changes in molecular vibrations even when the vibrations themselves are unaltered aip.org
The Journal of Chemical Physicsaip.orgOpen source ↗. The paper was selected as an Editor's Pick. The author names the mechanism carrier-induced photonic resonance (CIPR).
Where the infrared signal comes from
In time-resolved infrared spectroscopy, a visible light pulse excites the material and an infrared pulse probes its molecular vibrations. In lead halide perovskites, several studies have reported vibrational absorption that appears to grow dramatically upon excitation, along with Fano-like line shapes. This is hard to explain because the organic cations are not directly bonded to the inorganic framework that carries the charges.
Nishida analyzed earlier measurements on a perovskite film using multilayer transmission theory, which accounts for multiple reflections of infrared light at each interface of the layered sample. The model included only the broad optical response of the charges and assumed no change in the vibrations of the organic cations. It nevertheless reproduced most of the narrow, blue-shifted signal observed in the experiment, including its position and amplitude.
According to the analysis, the same mechanism can mimic increased absorption by excited molecules, a shift of a vibrational peak, or bleaching of an absorption band, depending on the spectral character of the charge response. Calculations also predict that an adjacent silica layer can produce additional resonances although it is not itself excited. Such features could be mistaken for charge trapping or charge transfer into that layer aip.org
The Journal of Chemical Physicsaip.orgOpen source ↗.
A control experiment and nanoscale data
The study proposes a way to separate the optical effect from genuine molecular dynamics: measuring perovskite nanocrystals dispersed in a dilute solution instead of a densely packed film. In the calculations, CIPR weakened as the concentration was lowered, while the longer optical path through the sample kept the molecular vibrational signal strong. This approach has not yet been tested experimentally.
Nishida also revisited earlier ultrafast infrared nanospectroscopy measurements, which probe the same perovskite with a sharp metallic tip at a spatial resolution below 100 nanometers. The predicted CIPR contribution depended strongly on how the tip-sample interaction was modeled. In the models considered most realistic, the optical effect accounted for only a minor part of the observed resonance.
This suggests that a genuine shift in molecular vibration, possibly linked to light-induced expansion of the perovskite lattice, may also contribute. Resolving the discrepancy between conventional and nanoscale measurements will require further work aip.org
The Journal of Chemical Physicsaip.orgOpen source ↗.
