Atomic Imaging of Octahedral Tilting in Two-dimensional Ruddlesden-Popper Perovskites

In the first part of the talk, I will present our surface science investigation of excitons and electron tunneling in 2D Ruddlesden-Poppers Perovskites (RPP). These molecularly soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilting, which directly impacts charge carriers and excitons. We used scanning tunneling microscopy (STM and q-plus AFM) to directly visualize surface octahedral tilting in <i>freshly exfoliated </i>2D Ruddlesden-Popper (RP) perovskite across the homologous series, whereby the steric hindrance imposed by long organic cations is unlocked by exfoliation. The in-plane and out-of-plane surface octahedral tilting across <i>n = 1</i> to <i>n = 4</i> RPP had also been studied using density functional theory calculations, which agrees well with the surface structural relaxation observed by STM. The structure titling is correlated to excitonic redshift observed in photoluminescence (PL) of RPPs which is distinguished from previous reported broad band PL emission. In addition, I will report the first observation of pressure-driven rotation isomerism in n&gt;1 RPP. The formation of rotational isomers under hydrostatic compression acts in concert with the in-plane slip of octahedral anion cages to resist deformation. In the second part of the talk I will talk about novel 2D trivalent metal-based hybrid perovskites recently synthesized in our lab that exhibit ferroelectric properties. Interestingly, we found that the intercalation of these perovskites causes the ferroic order to change from ferroelectric to ferroelastic.