Access to Ultrafast Surface and Interface Carrier Dynamics Simultaneously in Space and Time

Charge carrier dynamics at material surfaces and interfaces play a fundamental role in controlling the performance of photocatalytic reactions and photovoltaic devices; however, precise characterization of the surface dynamical properties of a material with nanometer (nm) and femtosecond (fs) spatial and temporal resolutions, respectively, is a precondition for profound understanding and is thus urgently needed. Many techniques have been developed to meet this demand, but barely any of them have simultaneous excellent surface sensitivity (depth resolution) and sufficient spatiotemporal resolutions, except for a one-of-a-kind second-generation scanning ultrafast electron microscope (S-UEM), which has been established and developed at KAUST to provide direct and controllable dynamical information about the ultrafast charge carrier dynamics and the localization of electrons and holes on the photoactive material surface and interfaces. In this feature article, the instrumentation, working principles, new capabilities, and unique applications of S-UEM in the ultrafast characterization of material surfaces and interfaces, including charge carrier injection, surface carrier diffusion, surface carrier trapping, and recombination, are systematically summarized and inspected. Future developments from both theoretical and experimental perspectives are also discussed.