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From a structural mechanical point of view, multilayers consisting of alternating soft and hard layers offer enhanced toughness compared to all-hard structures. Thus, engineering such hard-soft multilayer system is highly sought after in the realm of mechanical engineering for its high plastic deformation resistance and increased fracture hardness. Thus, the fabrication process is complex for inorganic hard-soft multilayers that alternates at nanometer scale periodicity. Here, we present a completely new perspective that two-dimensional Ruddlesden-Popper perovskites (RPPs) consisting of alternatively stacked layers of soft RNH₃⁺ organic molecular layers and rigid PbX₄^2- inorganic layers from a ‘natural’ undulating hard-soft system that work synergistically to resist deformation. Although there have been numerous work on hydrostatic pressure induced changes in the photoluminescence or absorption properties of hybrid perovskites, none of these work considers the mechanism of how these crystals resist deformation. Little is known so far about the behaviors of the organic cations (tilting, bending and twisting, etc.), and the plastic deformation or strain hardening process when the organic molecules re-orientate under compression. In this work, we describe the molecular scale mechanism of how compression and tensile strain develops in (BA)₂MA_n-1Pb_nI_3n+1 (n = 1, 2, 3, 4, ...) RPPs during compression, and elucidate the mechanism of why n = 1 RPP shows the greatest elastic recovery following decompression, compared to the higher homologues.

Raman spectroscopic evidence of the pressure-driven formation of high energy rotational isomers.

Our measured in situ high-pressure Raman spectra of n = 2 RPP under compression, as shown in Figure 1. The Raman bands at 0 GPa in the frequency region of 450 - 890 cm^-1 are the skeletal vibrational modes of BA molecules in the tt conformation, as represented by grey solid line. The Raman bands appearing at ~ 475.3 and 484.7 cm^-1 belong to the scissoring modes of tt-BA, i.e., δ(CH₂-CH₂-CH₃) (δ(CCC)) and δ(CH₂-CH₂-NH₃⁺) (δ(CCN)), which are associated CH³ and HN³⁺ stretch in opposite directions (Figure 1a). The observed characteristic band at ~ 864.7 cm^-1 belongs to the rocking mode of tt-BA, i.e., ρ_tt (Figure 1b). Upon compression at 0.2 GPa, the Raman spectrum shows distinct changes. First, the δ(CCC) and δ(CCN) bands almost merge into one band at ~ 487.2 cm^-1, which is the spectrally overlapped scissoring modes of BA in the trans-gauche and gauche-gauche form, i.e., δ_tg and δ_g+g-, respectively (Figure 1c). Besides, the intensity of ρ_tt weakens (Raman inactive-like) under compression, while the intensity of the Raman band at ~ 840 cm^-1 is enhanced due to the formation of Raman-active tg and g⁺g^- BA conformers (Figure 1c). Above 3.0 GPa, the distortion of Pb-I sublattice lowers the lattice symmetry, which splits the degenerate scissoring and rocking bands of BA in tg and g⁺g^- conformations, as demonstrated by the Raman spectrum measured at 3.7 GPa. However, all Raman peaks becomes weak and disappears beyond 6.0 GPa due to the structural amorphization. Interestingly, the fingerprint of Raman bands of tg and g⁺g^- conformers is still clear after decompression from high-pressure amorphous state (9.8 GPa), thus the BA conformers are strain-hardened and stabilized under high-pressure treatment (Figure 1d, e). At ambient conditions, the high-energy isomers could partially transform back to the tt isomer.

Figure 1 Raman characterization of the BA tg^- and g⁺g^- conformers. a, b Calculated Raman spectra for the BA scissoring and rocking modes in the tt, tg and g⁺g^- structures. c Typical Raman spectral evolution of n = 2 RPP with increasing pressure. d, e Raman spectral comparison of RPP sample before and after compression. Δ and ρ refers to scissoring and rocking vibrations. The grey, dark (light) blue and red lines in (a, b) represent BA vibrations in the tt, tg and g⁺g^- conformations, respectively. Insets represent the corresponding vibration patterns of BA isomers, green arrows on the atoms indicate the vibrational directions and relative amplitude of displacements in the given Raman modes.

Photoluminescence evidence of the elastic and plastic deformation of RPPs.

The role of the organic cations in buffering stress of 2D RPPs is unambiguously proven by tracking the photoluminescence (PL) spectroscopy under compression-decompression cyclings, as shown in Figure 2. The PL spectra of exfoliated n = 2 RPP flakes with thickness of ~ 17 nm that are collected under compression in Figure 2a, while Figure 2b shows the one-to-one corresponding spectra after decompression. Before compression, the sample displays a clear intrinsic excitonic emission at ~ 2.14 eV arising from the bulk-like state (BS), this corresponds to the state, where the majority of its BA molecules adopt the trans configuration. A low-energy PL emission centered at ~ 2.06 eV is related to surface state (SS) emission introduced by a small population of disordered tt-conformer BA molecules. Upon compression to 2.5 GPa, the BS and SS emissions red shift continuously and merge into a single peak at 3.9 GPa. This single peak blue shifts with further increase in pressures until a drastic drop in PL intensity occurs due to amorphization. Importantly, below the pressure of 2.5 GPa, the PL spectra can recover to the initial state after decompression, as judged from the suppression of broadband emission at the low-energy side in Figure 2b.The ability of the PL to recover is due to the elastic recovery of the Pb-I sublattice. In other words, the soft organic cation buffers the compressive strain and allows the inorganic lattice to remain in the elastic regime without crossing the yield point. At pressure higher than 2.5 GPa, the pressure yield point is exceeded in the stress-strain curves and the PL peak broadened irreversibly. The corresponding optical photographs and fluorescence micrographs before and after compression are shown in Figure 2c, it can be observed that emission is changed from original red color to orange.

Figure 2 PL characterization of the reversible and irreversible deformation in RPP. In-situ PL spectra of n = 2 RPP; individual spectrum in (a) shows compression at specific “X” GPa, and the corresponding spectrum in (b) after decompression of “X” GPa indicated as Re “X” GPa. The blue arrows in gradient color represent the proportion of the broadband emission. c Optical photographs and fluorescence images of n = 2 RPP exfoliated flakes before and after pressure treatments. Scale bar is 50 µm. d Plot summarizing the threshold pressure to cross from elastic to plastic regime for n = 1 to 4 RPPs, which is concomitant with the transition from a state where PL is reversible after decompression, to one where it is not. Dashed line guides to the eye, and the blue-shaded area represents the plastic deformation region.

The role of the organic cations in buffering stress becomes clear when we examine the deformation resistance of the inorganic lattice for n = 1 and n = 2 RPP. As n increases, the ratio of the number of layers of inorganic slab (hard) to organic slab (soft) increases, the ability to buffer compressive stress in the hybrid system decreases. As a results, there is increasing strain in the Pb-I inorganic lattice, leading to an increasingly lower pressure threshold from n = 1 to 4 RPPs for permanent deformation, as demonstrated by the pressure threshold as shown in Figure 2d, where the occurrence of non-recoverable PL after decompression for n =1 to 4 RPPs. The pressure threshold for irreversible deformation from n = 1 to n = 4 RPPs demonstrates that the flexibility of organic cation tilting in n = 1 acting synergistically with the octahedral tilt of the inorganic cages, resulting in the best elastic recovery ability for n = 1 RPP among the homologous series.

Our results provide the essential experimental and theoretical basis of understanding the synergistic action of organic-inorganic system in nanometer scale multilayer hard-soft superlattices under compression, which is useful for engineering new types of deformation-resistant multilayer coatings and the isothermal change related barocaloric effect in perovskites.

This work titled “Pressure driven rotational isomerism in 2D hybrid perovskites” was published in the Nature Communications journal (https://www.nature.com/articles/s41467-023-36032-y).