Structural Dynamics and Symmetry-Dictated Properties in Inorganic Halide Perovskites

Structural transformation and structure-property relationship are fundamentally important topics in both chemistry and material science research. Perovskite is a classical crystal structure or structural unit as the basis of many functional materials including the conventional oxide perovskites and the emerging halide perovskites. As a class of new-generation semiconductors, halide perovskites hold great promise for various energy conversion applications such as photovoltaics, light-emitting diodes, and even electromechanical devices. Their advantages including the ease of processing, bandgap tunability, phase modification and defect tolerance are largely due to the soft and dynamical ionic lattice of halide perovskite structure. On the one hand, the rich chemical/phase transformations in halide perovskites have been utilized for tailoring their electronic structures and functionalities, though the fundamental understanding of these transformation dynamics are always elusive or indirect. On the other hand, the structural phase transitions in some cases also induce changes of the crystal symmetry in materials which would certainly dictates different physical properties as stated in Neumann’s principle, but such structure-property relation in halide perovskites has not been fully unraveled. This dissertation is to present the study of structural dynamics and symmetry-dictated properties in halide perovskites. In particular, it focuses on inorganic halide perovskites because their better thermal stability and rich diagram of chemical/phase transformations making them ideal platforms for the fundamental understanding of their structural dynamics as well as studying the emerging properties. Chapter 1 provides an overview of the structural features and physical properties in inorganic perovskites including both halides and oxides. It introduces the crystal structure, electronic structure, optoelectronic performance, soft ionic lattice behaviors of halide perovskites and is followed by an illustration of typical symmetry-dictated physical properties. Chapter 2 demonstrates the synthesis of halide perovskite nano- to micro-scale single crystals, via the chemical vapor transport method, with controllable compositions, morphologies, and sizes. These single crystals are ideal platforms for all the lattice dynamics and structure-property studies presented in the following chapters. A facile anion exchange chemistry is one of the examples of their dynamical lattice behaviors, which has been utilized in tuning the electronic bandgaps while the host perovskite structure remains. Chapter 3 presents a quantitative study of the anion exchange kinetics by directly visualizing the transformation process on single-particle level with spatial, temporal, and spectroscopic information, enabled by the confocal photoluminescence imaging microscopy and a design of vapor-phase chemical treatment. The two-step transformation for CsPbBr3 to CsPbI3 was identified as: 1) initial surface reaction; 2) subsequent lattice anion diffusion. The quantitative imaging of anion exchange also revealed a diffusion-limited reaction mechanism. Structural phase transition is also an example of the dynamical lattice in halide perovskites. Chapter 4 focuses on discussing a type of structural phase transition in CsGeX3 (X=Cl, Br, I) where Ge (II) cation displacement leads to a rhombohedral distortion that introduces an inversion symmetry breaking and the emergence of ferroelectricity. Through a combination of ab initio theory and experiments from structural characterizations to multiscale material behaviors, we identified the role of lone pair stereochemical activity of Ge (II) in determining the ferroelectric origin and found out their spontaneous polarizations. Characteristic ferroelectric domain patterns on CsGeBr3 nanoplates are imaged with piezo-response force microscopy. Chapter 5 details the study of another symmetry-dictated property in CsGeX3, i.e., nonlinear optical behavior and specifically second harmonic generation (SHG). A polarization-resolved SHG microscopic imaging technique was utilized as a noninvasive all-optical tool for probing the ferroelectric domains in CsGeBr3 nanoplates. As a closing remark for both Chapter 4 and Chapter 5, this new semiconducting ferroelectric system CsGeX3 is expected to expand the understanding of physical phenomena related to ferroelectrics and introduce new possibilities to halide perovskites as well. Chapter 6 closes this dissertation by summarizing the overall research and provides an outlook for future studies.

Advisor: Yang, Peidong