Unusual Pressure‐Driven Phase Transformation and Band Renormalization in 2D vdW Hybrid Lead Halide Perovskites

Abstract

The application of high pressure allows control over the unit cell and interatomic spacing of materials without any need for new growth methods or processing while accessing their materials properties in situ. Under these extreme pressures, materials may assume new structural phases and reveal novel properties. Here, unusual phase transition and band renormalization effects in 2D van der Waals Ruddlesden−Popper hybrid lead halide perovskites, which have shown extraordinary optical properties and immense potential in light emission and conversion technologies, are reported. The results show that (CH3(CH2)3NH3)2(CH3NH3)Pb2Br7 (n = 2) layers undergo two distinct phase transitions related to PbBr6 octahedra, butylammonium (BA), and methylammonium (MA) molecule tilting motion that leads to rather unique/anomalous bandgap variation with pressure. In contrast, (CH3(CH2)3NH3)PbBr4 (n = 1) lacks MA molecules and possesses only one pressure‐induced phase transition related to PbBr6 octahedra and BA tilting. In this range, the bandgap reduces monotonically, much similar to other inorganic semiconductors and display surprisingly large redshift from 3 to 2.4 eV. Together with theoretical calculations, this study offers unique insights into these pressure‐induced changes and extends the understanding of these highly anisotropic layered soft organic perovskite materials under extreme conditions.

Raman spectroscopy in layered hybrid organic-inorganic metal halide perovskites

“…These include diamond anvil cells (DACs) and cryostats (figures 1(c) and (d)). Anvil cells exploit the transparency of diamond to visible light for spectroscopic access, and can apply uniaxial stress as well as isotropic stress with a pressure transmitting medium, such as silicone oil [68,69] or NaCl [70], and calibrate the pressure using ruby [68,70]. Cryostats with optical access can be He-or liquid-N 2 cooled allowing to reach temperatures in the range 4-350 K.…”

“…In 3D HOIPs PL tunability has been demonstrated for MAPbX 3 <80 meV and for FAPbX 3 ∼120 meV before PL quenching due to lattice distortion and bond bending. Instead, this tunability can be further increased using 2D HOIPs, reaching up to 520 meV, due to their layered nature and large A-site organic cations [69,70]. Considering the sensibility of the vibrational properties to changes in the crystal structure and environment, Raman spectroscopy is a powerful tool to monitor these materials under pressure.…”

Anomalous Behavior of 2D Janus Excitonic Layers under Extreme Pressures

“…The pressure-induced band reduction effect is generally well explained within the tight-binding approximation as well as more advanced theory simulations. [27][28][29][30] In contrast to these traditional semiconductor materials, 2D Janus layers exhibit rather different pressure dependence as evidenced by the collected PL spectra at different pressures (Figure 5a). The overall PL peak position shows very small changes with pressure but has a noticeable amount of blue-shift as shown in Figure 5b.…”

Pressure Driven Rotational Isomerism in 2D Hybrid Perovskites

“…In the ground state of C4 alkyl chain, each CH2 moiety adopts the trans-trans (tt) conformation to avoid steric repulsion. Under compression, it can be expected that the lamellar layers will contract and the strain in the metal halide bonds will affect optical and electronic properties [12][13][14][15][16][17][18][19][20][21] . However, little is known about the conformational change of the organic cations and the plastic deformation or strain hardening process when the organic molecules re-orientate under pressure, this is because X-ray diffraction is mostly sensitive to the heavier inorganic components.…”