Contamination-Free TEM for High-Resolution Imaging of Soft Materials

Element-selective imaging and analysis at atomic resolution have become possible by the recent advancements in TEM and STEM. However, the spatial resolution in images of soft materials can be limited by electron beam damage and/or contamination. This contamination is a carbonaceous layer deposited on the specimen surface as a result of electron bombardment. Beam-induced specimen contamination is caused by polymerization of hydrocarbons that are present in a TEM specimen chamber. The electron beam reacts with stray hydrocarbons in the beam's path to create hydrocarbon ions, which then condense and form carbon-rich polymerized film on the area being irradiated. Figure 1a shows contamination spots created on a carbon thin foil by illuminating a beam with an intensity of 5.6 × 104 el/nm·s at an accelerating voltage of 200 kV. The thickness of the contamination spots can be estimated by electron energy-loss spectroscopy (EELS). With increase in the irradiation period, the intensity of the zero-loss peak decreases, but the overall intensities in the energy-loss regions of the spectrum increase (Figure 1b). The thickness (D) can be estimated using the equation, D = Λ·ln(It/I0), where Λ is the total mean free path for inelastic scattering, and It and I0 are the integral intensities of the overall spectrum and the zero-loss peak, respectively. Using this equation, the thickness of the contamination was found to be about 600 nm with a 10-minute irradiation.