Micrograph contrast in low-voltage SEM and cryo-SEM

Abstract

Modern high-resolution scanning electron microscopes (SEM), equipped with field emission guns (FEGs), designed to operate at low acceleration voltage, have opened new opportunities to study conductive or insulating systems, without conductive coating. Better electron sources, optics, vacuum, and detectors allow high-resolution SEM to serve as a powerful characterization and analytical tool, and provide invaluable information about structure-property relations of nanomaterials and related applications. Slight specimen charging can be exploited to enhance contrast between different materials and phases, with minimum imaging artifacts. Optimization of charging effects and improved micrograph contrast are essential for the study of different-scale features in ceramics, polymers, organic materials, and thermally fixed liquids, including in biological research. The operating SEM parameters can be adjusted to a specific specimen based on prior knowledge of interaction of the electron beam with similar specimens, and the type of information one wishes to acquire. In this work we examined the effect of the acceleration voltage and the use of different detectors on the contrast formation in several types of specimens, focusing on materials formed mainly of carbon and oxygen, with low inherent contrast in the SEM. That includes cryogenic SEM (cryo-SEM) to study emulsions in their native state. We also studied by cryo-SEM carbon nanotubes (CNTs) dispersed in water and dissolved in superacid. HR-SEM at room temperature was performed on CNT films, deposited on glass. We show how micrograph contrast changes with different detectors, at different acceleration voltages. Judicious selection of the SEM operation parameters leads to optimal picture contrast between domains of different composition.