Probing Ultrafast Structural Dynamics in Nanomaterials with XFEL Diffraction Imaging

X-ray free-electron lasers (XFELs) provide unique opportunities to investigate nonequilibrium structural dynamics in nanoscale materials with unprecedented spatial and temporal resolution. By recording diffraction patterns from individual nanoparticles and nanocrystals, XFEL-based methods can reveal structural heterogeneity and transient lattice distortions that are inaccessible to conventional ensemble-averaged techniques. This talk will discuss how XFEL single-particle imaging and serial femtosecond crystallography can be used to probe photoinduced structural dynamics in semiconductor quantum dots and metallic nanoparticles. Using time resolved serial femtosecond crystallography (TR-SFX), we investigate ultrafast lattice responses in colloidal semiconductor quantum dots. In recent experiments on CsPbBr₃ quantum dots, resonant optical excitation produced measurable changes in diffraction intensities and Bragg peak profiles, demonstrating sensitivity to subtle photoinduced lattice distortions at the single-particle level [1].In parallel, we apply XFEL single-particle imaging to large ensembles of gold nanoparticles undergoing nonequilibrium processes such as growth and plasmon-induced melting. In these experiments, each diffraction pattern originates from an individual particle, and a single dataset can contain millions of snapshots. Unlike conventional methods that primarily provide ensemble-averaged information, single-particle imaging offers access to the full distribution of structures within the ensemble. To extract structural information from these highly heterogeneous datasets, we developed a high-throughput reconstruction framework based on Monte Carlo sampling of particle shape and orientation parameters. By combining diffraction patterns from millions of particles and exploiting geometric correlations across the dataset, the method reconstructs statistically significant distributions of transient particle morphologies and reveals structural evolution pathways during nonequilibrium transformations [2]. Together, these examples illustrate how XFEL diffraction imaging, combined with advanced analysis methodologies, enables direct observation of structural heterogeneity and ultrafast dynamics in nanomaterials. The talk will highlight both the experimental capabilities and computational challenges of extracting transient structural information from large-scale XFEL datasets. [1] Shen, Zhou, et al. "Direct Observation of the Exciton-Polaron in Single CsPbBr3 Quantum Dots." ACS nano 19.31 (2025): 28372-28382. [2] Shen, Zhou, et al. "Resolving nonequilibrium shape variations among millions of gold nanoparticles." ACS nano 18.24 (2024): 15576-15589.