Perspectives on ultra-high vacuum transmission electron microscopy of dynamic crystal growth phenomena

Crystal nucleation and growth is a fundamental pillar of materials design. To advance our under-standing of the underlying mechanisms, in situ visual observation plays an important role by providing dynamic information unavailable through conventional post-growth analysis. Such in-formation includes nucleation and growth rates, diffusion phenomena, phase transformation kinetics, strain relaxation mechanisms, and defect formation. Here, we review the contributions of ultra-high vacuum transmission electron microscopy (UHV-TEM) to our understanding of dynamic crystal growth phenomena. We describe the vacuum, sample handling, and deposition capabilities essential for quantitative studies of reactive metals and semiconductors, and discuss how these capabilities are achieved while preserving the imaging performance of the microscope. We then show examples of growth processes explored using UHV-TEM, where the high spatial and temporal resolution provides unique insights into nanocrystal nucleation, thin film microstructure evolution, and oxidation in controlled environments. We assess these past accomplishments in the context of recent advances in transmission electron microscopy, discussing how aberration correction, modified sample environments, fast and sensitive detectors, and data science are unlocking powerful opportunities for atomic and temporal resolution measurements using UHV-TEM. We conclude by dis-cussing the challenges and future perspectives for scientific advances using this technique.