The study of atomic ordering on bismuth nanocrystals during photo-excited ultrafast phase transition

Abstract

Solid-to-liquid phase transition in materials is a fundamental phenomenon, which has been relatively well understood at the macroscopic scale and longer timescale. With the advent of ultrafast optics, ultrafast phenomena in photo-excited materials have been extensively studied with interest in microscopic understanding on melting transitions at a fundamental level in solids. In particular, nonequilibrium processes provide fresh views on the mechanisms behind the melting transition with new observations previously unseen with stimuli close to equilibrium. However, the investigations have remained challenging without clear experimental verifications owing to the difficulty in imaging materials undergoing ultrafast and irreversible transitions with high resolution. In this thesis, we introduce a novel ultrafast imaging method, called “single-pulse time-resolved imaging”, which is a combination of coherent diffraction imaging (CDI) and X-ray free electron laser (XFEL). High-resolution imaging from CDI is combined with ultrafast measurement from XFEL pulse to explore the transient dynamics in an ultrafast irreversible phase transition driven at nonequilibrium condition. We applied single-pulse time-resolved imaging on the investigation of ultrafast melting of bismuth nanocrystals. During ultrafast melting, hidden phase transitions were revealed that contradicted the conventional understanding of melting. This new discovery stimulates new studies of phase transitions that are important in materials sciences and physics. In addition, technical developments for single-particle imaging (SPI) at XFEL will be introduced. We proposed general routes and new efficient method for signal-to-noise ratio enhancements in SPI experiments. Moreover, we developed an aerosol injector as an efficient sample delivery method in SPI experiments. With further improvements, it aims public use at PAL-XFEL. This thesis will assist to experience the advanced imaging techniques at XFEL and photo-induced ultrafast science in solids.