Towards Glasses from Two-Dimensional Hybrid Organic–Inorganic Perovskites

Abstract

Hybrid organic-inorganic perovskites (HOIPs) are an emerging class of functional materials with exceptional potential in optoelectronics. While research has long focused on their crystalline states, recent discoveries have revealed that certain HOIPs can melt, and be quenched into frozen liquids, i.e., glasses, opening up new avenues for disordered perovskite materials. Despite the growing number of glass-forming HOIPs, fundamental understanding of their vitrification processes remains limited. This thesis focuses on a prototypical two-dimensional system, (S-(−)-1-(1-naphthyl)ethylammonium)2PbBr4 ((S−NEA)2PbBr4) to provide insights into the preparation routes, vitrification mechanisms, stability, and functional properties of HOIP glasses, thereby advancing both the fundamental science and potential applications of these disordered hybrid perovskite systems. Chapters 1 and 2 introduce the basic concepts of glass science and HOIP chemistry and review the progress and challenges in HOIP glass formation, establishing the scientific context. Chapter 3 describes the synthesis protocols and characterisation techniques employed throughout the thesis. Chapter 4 introduces mechanochemistry as a rapid, green, and efficient alternative to conventional melt-quenching for HOIP glass formation. Both (S−NEA)2PbBr4 (meltable) and its racemic analogue (non-meltable) are shown to undergo direct crystal-to-glass transformation within minutes of ball milling. The microstructural evolution during milling is monitored using in situ time-resolved X-ray diffraction, offering insights into the mechanism of mechanochemically-induced amorphisation. Chapter 5 presents a comprehensive investigation into the structural dynamics of (S−NEA)2PbBr4 across its crystalline, liquid, and glassy phases. Through a combination of X-ray total scattering, spectroscopic techniques, and solid-state NMR, structural disorder is elucidated over various length scales, while dynamic molecular motion during melting and vitrification is probed. These insights are further correlated with the mechanical and optical responses of the glass, enabling the establishment of key structure-property relationships. Chapter 6 extends the scope to chemically modified systems by introducing a molecular doping strategy. Incorporation of fluorescent dyes such as coumarin-6 into (S−NEA)2PbBr4 melts suppresses recrystallisation and stabilises the doped glass. The influence of dopants on thermal behaviours, local structures, and optical properties is systematically investigated, demonstrating a route towards functional stable HOIP glass materials. Finally, Chapter 7 summarises the findings and outlines opportunities for future research, highlighting the broader implications of HOIP glasses as a platform for advancing fundamental glass science and for architecting hybrid glasses with tailored functionalities.