Abstract

X-rays are an important tool for nanoparticle characterization due to their element specificity and chemical sensitivity, and due to their short wavelength, which allows them to be used for imaging nanostructures with atomic resolution. However, when applying common X-ray-based imaging and spectroscopy techniques to characterize nanoparticles, signal-level and sample-handling requirements typically make it necessary to perform these measurements on an ensemble of particles, which are often deposited on a substrate. This limits the amount of specific information, such as particle size and morphology, which can be extracted for individual, nonidentical, or non-reproducible nanoparticles. The development of accelerator-based, intense, and short-pulse X-ray sources, called X-ray free-electron lasers, has now enabled X-ray imaging techniques that can capture images of single, unsupported nanoparticles and even snapshots of ultrafast nanoparticle reactions with sub-nanometer spatial and femtosecond temporal resolution. This opens up new avenues, e.g., for studying formation and reactivity of nanoparticles in their native aerosol environment. This chapter presents a brief description of single-particle X-ray imaging techniques along with recent examples that highlight the power and versatility of these novel techniques.