Local Structure of Metal Halide Perovskite Glassy-Crystalline Phase Switching

Amorphous metal halide perovskites (MHP) are a novel materials class with promising potential to broaden the MHP application space to include neuromorphic computing and switchable spintronics. We present a fundamental structural study of the first stable glassy MHP, [(S-(-)-1-(1- naphthyl) ethylammonium]₂PbBr₄or “S-NPB”, which demonstrates facile amorphous-crystalline phase switching with moderate cooling rates under ambient conditions. [1] Over the past several years, chiral 2D MHPs have garnered substantial research interest due to their chirality-induced Rashba-Dresselhaus spin splitting. Research interest is continually growing, but existing work is heavily focused on crystalline chiral MHPs for spintronic and optoelectronic applications. [2] Though crystallinity offers numerous advantages, we demonstrate further exploitation of the structural chirality transfer to induce glass formation and extend the associated MHP structure-property paradigm. The inherent tunability of MHPs is further enhanced through the inclusion of amorphous phases that offer unique physical properties. We present a comprehensive multimodal characterization strategy to interrogate the complex structure of the amorphous MHP phase and the inorganic-organic interfacial dynamics, addressing significant knowledge gaps within the community. This fundamental understanding of chiral MHP crystallization dynamics pioneers a new class of MHPs with promising potential for phase-change random access memory (PCRAM) neuromorphic computing architectures, and to replace high thermal-budget chalcogenides for optical storage applications.<br/><br/>Upon crystallization, the chirality originating in the chiral cation bilayer of the organic sublattice is transferred to the inorganic sublattice as in-plane angular octahedral distortions, inducing the high melt viscosity and melting depression that enable glass stabilization along with unprecedented thermal stability. [3] Due to S-NPB's inherent structural complexity, insightful structural analysis requires the development of a multimodal analysis strategy probing the local atomic order, thin film confinement effects, and inorganic-organic lattice coupling and interfacial dynamics. In our analysis, we employed several complex structural characterization techniques to probe the intrinsic phenomena over various length and time scales, namely Pair Distribution Function (PDF), polarization-dependent Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, Raman Spectroscopy, Nuclear Magnetic Resonance (NMR), Quasi-elastic Neutron Scattering (QENS), and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS). We conducted <i>in-situ</i> total scattering PDF studies supported with Reverse Monte-Carlo modeling, determining that the corner-sharing 6-fold octahedral coordination is largely maintained in the melt and glass, but disordered beyond the first coordination shell due to octahedral lattice distortions. The structure of the organic sublattice was informed with NEXAFS to selectively probe the chiral cation local coordination and orientation. To study the interfacial organic-inorganic dynamics, in-situ NMR and QENS were employed to identify the molecular motions facilitating the structural chirality transfer, attributed to H-bonding interactions with the terminal octahedral halide atoms. We conducted <i>in-situ </i>GIWAXS studies under various annealing conditions and temperature ramping rates to investigate and control S-NPB’s morphological evolution and determine the rate-dependance of crystallization dynamics and enable the critical manipulation of crystal nucleation and growth to obtain preferred orientations. This multimodal approach establishes a comprehensive fundamental understanding of amorphous MHP structural dynamics, and presents the processing-structure-property relationships required to propose S-NPB as a promising candidate for PCRAM neuromorphic computing. [4]<br/><b>Ref</b>: A. Singh (2020),Q. Xiang (2018),M. Jana (2020),L. Wang (2020)