In @PNASNews, we report direct time-resolved electron imaging of photoexcited hot carriers transfer across a semiconductor heterojunction using our scanning ultrafast electron microscope (SUEM). With SUEM, we took movies (with picosecond time resolution) of photocarriers migrating across a Si/Ge heterojunction and examined how the heterojunction potentials and band offsets interplay with the migration process. We found that, with the band alignment in the junction we studied, the junction potential leads to charge trapping and significantly slows down hot photocarrier diffusion. Our findings suggest that heterojunctions need to be carefully designed to facilitate hot carrier transport in hot-carrier-based photovoltaics, photosensing, and photocatalysis. We gratefully acknowledge our collaborator Prof. Mark Goorsky at UCLA, who provided the high-quality samples for this study. pnas.org/doi/10.1073/pn…