Functional engineering of metal halide perovskites: a nonlinear optical study

Crystalline materials are essential components of modern technology, so science is putting great efforts in the research of new and innovative materials. While size, shape and compositional effects on the functional properties of nanomaterials have been widely explored, the influence of the crystal phase has received less attention. The ability of a material to adopt multiple lattice structures, i.e. structural polymorphism, allows tuning of the functional properties (chemical, electrical, and optical) for specific applications.  This PhD project focuses on metal halide perovskites (MHPs) and how to gain control over their structure by tuning the experimental parameters such as synthesis method, composition, morphology and temperature. In fact, MHPs exhibit interesting electro-optic properties that strictly depend on the structure of the solid phase. In particular, the property towards which the interest of this project is aimed, is ferroelectricity. Because of what is aforementioned, a central component of this study is the characterization of the crystal phase. The primary technique used for the study of ferroelectricity is second harmonic generation (SHG) microscopy, which allows both to determine the presence of ferroelectricity and to perform structural imaging of MHPs. By combining SHG microscopy with other nonlinear optical techniques, such as third harmonic generation (THG) and multiphoton microscopy together with harmonic light scattering, it is possible to achieve the most complete structural characterization achievable by optical means. Besides this, a wide range of state-of-the-art techniques such as XRD, Raman micro-spectroscopy, SEM, AFM, absorption and emission spectroscopy are employed to characterize crystal structure and morphology. By establishing how the electro-optic properties (i.e. quantum yield, band gap, charge separation, etc.) relate to crystal structure, it will be possible to determine how we can take advantage of MHPs for different technological applications such as solar cells, LEDs or chemical sensors.