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

Abstract

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.

Investigation of hole-free phase plate performance in transmission electron microscopy under different operation conditions by experiments and simulations

“…In fact, additional measurements at HFPP film temperatures of 100 °C (not shown here), where contamination builds up more slowly, showed both positive and negative phase shifts. This is in agreement with the findings from [4,20] and indicates that contamination and negative charging can occur simultaneously.…”

“…e Experimental and simulated image intensities obtained with the phase shift profile in (d) are in a very good agreement. Including drift in ϕ PP (q) explains different halo intensities on the left and right sides of the CNT bundles frequencies, which qualitatively confirms the previous suggestions [20].…”

“…The operation temperature was limited to 200 °C in this work, which prevented further optimization of ϕ PP (0) towards -0.5π. The sign change of ϕ PP (0) indicates that a completely different mechanism is responsible for the phase shift compared to case A. Hettler et al [20] suggested that the negative phase shift can be explained by a change of the local work function after desorption of adsorbed (water) molecules on the HFPP surface by the intense ZOB.…”

“…This can be explained by two effects. It was suggested that the negative ϕ PP (0) increases with the diameter of the charged patch [20]. In addition, the concentration of adsorbed molecules should be higher at low temperature, leading to a higher work function.…”

“…With adsorbed (water) molecules acting as a dipole layer, the work function is modified if some adsorbed molecules are desorbed by the intense ZOB [20]. The induced phase shift strongly depends on ambient conditions such as the temperature of the HFPP, the electron dose, the size of the ZOB on the film, and residual gases in the vacuum [9,10,20,21]. However, a clear understanding of the influence of these parameters and their control has not been achieved yet and requires further investigation.…”