Pressure-Induced Detrapping from Self-Trapped Excitons to Free Excitons toward Enhanced Emission and Piezochromism in Ruddlesden–Popper (110)-Oriented Perovskites

Two-dimensional (2D) lead halide perovskites are emerging
as excellent
materials for optoelectronic applications including light-emitting
diodes, photovoltaics, and photodetectors, owing to their efficient
excitonic emission originating from both self-trapped excitons (STEs)
and free excitons (FEs). Recently, many efforts have been focused
on enhancing the emission intensity, modulating the dominant emission
mechanism, and establishing direct correlations between optical properties
and underlying structural motifs. Across the hybrid organic–inorganic
layers in 2D perovskites, the nanorange modulation of the density,
stiffness, strain, and ionicity can be efficiently tuned by external
stimuli, including the substrate–film strain. Here, we report
a pressure-induced narrowing of the band gap and an enhancement of
STE and FE emission in the topology of the (110)-oriented Ruddlesden–Popper
(RP) perovskite ACE2PbBr4 (ACE = acetamidinium),
accompanied by the rare phenomenon of the reversible detrapping process
from STEs to FEs under compression. Specifically, the STE-related
emission exhibits a 6.4-fold increase of intensity up to 2.56 GPa,
while the FE emission dominates under higher pressures, up to 11.11
GPa, causing a pronounced change in the emission color, from orange-yellow
to greenish-blue, across the compression range. In situ single-crystal
X-ray diffraction and Raman spectroscopy reveal that these emission
changes arise from the rarely observed pressure-induced reduction
of lead bromide octahedral distortion and confinement in the direction
of the corrugated structure and changes in amine-framework interactions,
as well as pressure-induced phase transitions (PTs) occurring near
2, 5, 5.9, and 6.7 GPa. These findings elucidate the structure–property
relationship in (110)-oriented RP perovskites and underscore the utility
of strain engineering for realizing light-emitting materials with
tailored and enhanced functionalities.