Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites

The surface octahedral tilt in exfoliated 2D perovskites is directly visualized by STM and the degree of the tilt varies with the number of layers of inorganic slabs and result in different amounts of excitonic red shift in photoluminescence.
Published in Materials
Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites
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Octahedral tilt is a longstanding topic in perovskites because it is a long-range distortion of the inorganic metal halide cages that influences the structural symmetry, optical band gap, carrier effective masses and exciton-phonon coupling in these classes of materials. Although it has been widely studied in hybrid 3D perovskites that Goldschmidt tolerance factors can semi-quantitatively predict structural distortion on the basis of steric hindrance, the same cannot be applied directly for hybrid 2D Ruddlesden-Popper perovskites (RPPs) because the layered segregated structures in 2D perovskites allow the incorporation of larger organic cations into the inorganic cages, leading to their unique properties and novel applications.

In previous studies, the presence of octahedral tilt in perovskites is mostly inferred indirectly from the theoretical calculation and X-ray diffraction (XRD), but the actual tilt has not been directly visualized at an atomic scale. In addition, XRD provides bulk averaged information with no surface sensitivity, thus it cannot be used to probe the presence of surface reconstruction that occurs on exfoliated RPPs. Therefore, it can be said that the direct proof of octahedral tilt is lacking and correlations of physical effects to octahedral tilt remain highly contentious.

In this paper, we used scanning tunneling microscopy (STM) to directly visualize surface octahedral tilt in freshly exfoliated 2D RPPs (BA)2(MA)n-1PbnI3n+1 (n = 1-4; BA = CH3(CH2)3NH3, MA = CH3NH3+, n refers to the number of inorganic slabs per unit cell of the homologous series). Unlike traditional van der Waals stacked 2D materials (e.g., Graphene), the mechanical exfoliation of 2D RPPs induces large surface structural reorganization with respect to the bulk structure owing to their molecularly soft nature. As shown in Fig. 1, delaminating one of the two interlocked layers of BA chains removes the steric constraints for the remaining, thus causing it to tilt at a larger angle relative to its bulk position.

Fig. 1 Schematic illustration of exfoliation step liberating the steric constrains imposed by a bilayer of BA cations in bulk and unlocking surface octahedral tilt in exfoliated RPP by taking n = 4 as an example. Two layers of BA chains are interlocked in the bulk (a) and leaving only one layer of BA on top upon splicing (b).

The enhanced out-of-plane tilt of the two adjacent Pb-I cages towards each other will naturally bring their apical I atoms (large yellow spheres) closer, forming a “dimer-like” structure, which has been indeed verified by STM real-space atomic-scale imaging of the surface structure of the exfoliated n=4 RPP flakes (Fig. 2). Interestingly, the different orbitals of the 2D RPPs could be imaged by STM measurement by applying different bias voltages.

 

Fig. 2 STM study of the octahedral tilt in exfoliated n = 4 RPP. a, Schematic surface structure change by out-of-plane octahedral tilt in top view. The tilt leads to a “dimer-like” structure formed by two apical I atoms from the top of adjacent inorganic cages. b, Atomic-resolution STM image of the exfoliated n = 4 RPP at Vbias = +2.3 V. Scale bar, 2 nm. c, Simulated charge density plot. d-g, different orbitals of the 2D RPPs could be imaged by STM measurement by applying different bias voltages.

We have also imaged the atomic arrangements of the apical I atoms which determined octahedral tilts from n = 1 to n = 4 RPPs by STM. As shown in Fig.3, STM shows that n = 4 RPP has the shortest apical I-I distance of 4.55 Å, while n = 2 RPP has a shorter I-I distance of 4.85 Å than n = 3 RPP of 5.02 Å. Meanwhile, n = 1 RPP has the largest I-I distance of 5.69 Å. All the results agree well with the density functional theory (DFT) simulated models and validate that the out-of-plane surface octahedral tilt is enhanced compared to the bulk structure in exfoliated n ≥ 2 RPPs.

Fig. 3 Enlarged view of STM images scanned on freshly exfoliated RPP surfaces of n = 1-4. The degree of octahedral tilt as a function of dimensionality n in RPPs is determined by the apical I-I distance that agrees well with the DFT simulated models

Interestingly, the surface-enhanced out-of-plane octahedral tilt is correlated to the emergence of the excitonic redshift observed in photoluminescence (PL). A sharp red-shifted emission peak in addition to the main exciton peak occurs only in the exfoliated flakes rather than bulk structures in n ≥ 2 RPPs. We find the extent of the redshifts can be correlated to the relative octahedral tilt (surface versus bulk) in n ≥ 2 RPPs, which may arise from the exciton-phonon coupling. In addition, out-of-plane octahedral tilt at the surface enhances inversion asymmetry normal to the direction of quantum well and promotes Rashba spin splitting for n > 1.

Fig. 4 PL measurements of the exfoliated (up) and non-exfoliated bulk crystals (down) from n = 1 to n = 4 RPPs at 77 K. Red-shifted PL peak appears for n ≥ 2 RPPs, whereas n = 1 RPP shows blue-shifted PL peak caused by phase transition instead.

For more information, please visit our paper. https://www.nature.com/articles/s41467-021-27747-x

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