Laser-induced particle synthesis and spectroscopy: a microsynthetic study of caesium lead bromide perovskite

Inorganic caesium lead halide perovskite nanocrystals are rapidly emerging in many fields, such as photovoltaics and display technology. However, the mechanism by which these nanocrystals grow is poorly understood, with synthesis reactions occurring within seconds. This project used a Multifunctional Fluorescence Microscope to conduct laser-induced reactions from solid lead bromide and dissolved caesium oleate in a single drop of solution. A 405 nm continuous-wave laser prompted fluorescence with the nanocrystals changing from blue to green over 100 s. These techniques calculated a reaction activation energy of 25.6 ± 8.2 kJ mol-1 over the laser intensity range of 0.7-6.6 kW cm-2. After approximately 20 s, the nanocrystals seemingly detached from the lead bromide surface, changing the dimensionality of their growth. The detachment was caused by lead bromide photodecomposition from the isolation of electrons and holes under the photo-Dember effect, with a gradual release of bromine gas and a more rapid release of strain energy bursting the crystals apart. Density functional theory calculations demonstrated that the isolation of negative and positive charges would morph the unit cell resulting in regions of expansion (15%) and contraction (16%), respectively. A further consequence of this decomposition was the formation of lead nanoparticles. These nanoparticles produced localised surface plasmon resonances at increased laser intensities and triggered Marangoni flows. Marangoni effects are of interest in microfluidic studies, such as the aforementioned laser-induced reaction conducted within a drop of solution. Therefore, these effects were investigated using experimental methods and computer simulations. These techniques were used to calculate the central stokeslet force (approximately 0.05-0.35 nN for Pb0) and revealed a threshold laser intensity for Marangoni flows to occur within this system. For example, in this system, Pb0 had a threshold of 140 kW cm-2