Negative Pressure Engineering with Large Cage Cations in 2D Halide Perovskites Causes Lattice Softening

Organic–inorganic hybrid halide
perovskites are promising
semiconductors with tailorable optical and electronic properties.
The choice of A-site cation to support a three-dimensional (3D) perovskite
structure AMX3 (where M is a metal and X is a halide) is
limited by the geometric Goldschmidt tolerance factor. However, this
geometric constraint can be relaxed in two-dimensional (2D) perovskites,
providing us an opportunity to understand how various A-site cations
modulate the structural properties and thereby the optoelectronic
properties. Here, we report the synthesis and structures of single-crystal
(BA)2(A)­Pb2I7 where BA = butylammonium
and A = methylammonium (MA), formamidinium (FA), dimethylammonium
(DMA), or guanidinium (GA), with a series of A-site cations varying
in size. Single-crystal X-ray diffraction reveals that the MA, FA,
and GA structures crystallize in the same Cmcm space
group, while the DMA imposes the Ccmb space group.
We observe that as the A-site cation becomes larger, the Pb–I
bond continuously elongates, expanding the volume of the perovskite
cage, equivalent to exerting “negative pressure” on
the perovskite structures. Optical studies and DFT calculations show
that the Pb–I bond length elongation reduces the overlap of
the Pb s- and I p-orbitals and increases the optical bandgap, while
Pb–I–Pb tilting angles play a secondary role. Raman
spectra show lattice softening with increasing size of the A-site
cation. These structural changes with enlarged A cations result in
significant decreases in photoluminescence intensity and lifetime,
consistent with a more pronounced nonradiative decay. Transient absorption
microscopy results suggest that the PL drop may derive from a higher
concentration of traps or phonon-assisted nonradiative recombination.
The results highlight that extending the range of Goldschmidt tolerance
factors for 2D perovskites is achievable, enabling further tuning
of the structure–property relationships in 2D perovskites.