Charging of carbon thin films in scanning and phase-plate transmission electron microscopy

•Study on charging of C thin films under intense electron-beam illumination.•Contamination and charging effects are separated by suitable measures.•Observation of weak negative charging of the C thin films at moderate temperatures.•Negative charging is explained by electron-stimulated desorption of adsorbates.•The model is a possible explanation for the functionality of hole-free phase plates. A systematic study on charging of carbon thin films under intense electron-beam irradiation was performed in a transmission electron microscope to identify the underlying physics for the functionality of hole-free phase plates. Thin amorphous carbon films fabricated by different deposition techniques and single-layer graphene were studied. Clean thin films at moderate temperatures show small negative charging while thin films kept at an elevated temperature are stable and not prone to beam-generated charging. The charging is attributed to electron-stimulated desorption (ESD) of chemisorbed water molecules from the thin-film surfaces and an accompanying change of work function. The ESD interpretation is supported by experimental results obtained by electron-energy loss spectroscopy, hole-free phase plate imaging, secondary electron detection and x-ray photoelectron spectroscopy as well as simulations of the electrostatic potential distribution. The described ESD-based model explains previous experimental findings and is of general interest to any phase-related technique in a transmission electron microscope.