Verification of water presence in graphene liquid cells

• Published in: Micron (Oxford, England : 1993) Vol. 149; p. 103109
• Main Authors: Keskin, Sercan, Pawell, Carly, de Jonge, Niels
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-10-2021
• Subjects: Bubble; Electron energy loss spectroscopy; STEM; TEM; Transmission electron microscopy; Bubble; Transmission electron microscopy; TEM; Electron energy loss spectroscopy; STEM
• ISSN: 0968-4328; 1878-4291
• DOI: 10.1016/j.micron.2021.103109

•Graphene liquid cells (GLCs) provide sample enclosures for liquid phase electron microscopy (LPEM) with the thinnest possible liquid-enclosing membranes.•However, the preparation technique of GLCs is still challenging, and often, a GLC contains no liquid at all because it is expelled from the device during assembly or evaporated through cracks in the grapheme sheets when placed in the vacuum of the electron microscope.•Several methods are available for testing the presence of liquid, including observation of higher contrast in the image, or bubble formation, electron energy loss spectroscopy (EELS) to measure elemental oxygen, and thickness measurement.•We show here that applying a single criterion is not always sufficient as test, and artifacts in the sample may hamper interpretation.•The most reliable method is direct observation of the water exciton peak using EELS but if this experiment is impossible, it is recommended to use the combination of higher contrast in the image, bubble formation, and EELS analysis of the oxygen signal.
Graphene liquid cells (GLCs) present the thinnest possible sample enclosures for liquid phase electron microscopy. However, the actual presence of liquid within a GLC is not always guaranteed. Of key importance is to reliably test the presence of the liquid, which is most frequently water or saline. Here, the commonly used methods for verifying the presence of water were evaluated. It is shown that depending on the type of sample, applying a single criterion does not always conclusively verify the presence of water. Testing liquid filling for a specific GLC sample preparation protocol should thus be considered critically. The most reliable method is direct observation of the water exciton peak using electron energy loss spectroscopy (EELS). But if this method cannot be carried out, water filling of the GLC can be verified from a combination of higher contrast in the image, the presence of bubbles, and an oxygen signal in the EEL spectrum, which can be accomplished at a high electron dose in spot mode. Nanoparticle movement does not always occur in a GLC.