Observation of subnanometre-high surface topography with X-ray reflection phase-contrast microscopy

The direct observation of molecular-scale features has been the exclusive realm of electron and probe microscopies(1-3), whereas X-ray imaging has been limited to the observation of objects greater than 10 nm in size(4-9) associated with the resolution of X-ray optics. Here, we describe a novel approach that extends hard X-ray microscopy to image the distribution of molecular-scale interfacial features directly and non-invasively with full-field imaging. Interfacial phase contrast from elementary defect structures allows direct observation of 0.6-nm-high monomolecular steps at a solid surface. This non-invasive technique opens up new opportunities to study interfacial processes in situ and in real time, particularly those taking place under aggressive chemical conditions, which currently can only be studied by ex situ approaches.