CONTRAST IN LOW-VOLTAGE SEM AND CRYO-SEM

Modern scanning electron microscopes (SEM) play a very important role in the characterization of nanomaterials and related applications. 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. While specimen charging is a very well-known unwanted phenomena for SEM, the new cutting-edge technology of high resolution SEM, designed to operate at low acceleration voltage, opens an opportunity to investigate conductive, or insulating systems, without conductive coating. Moreover, slight charging can be exploited to enhance contrast between different materials and phases that interact distinctively with the electron beam, without leading to 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 liquids, especially, in biological research. The operating SEM parameters can be adjusted to a specific system based on a prior knowledge of the interaction of similar systems with the electron beam, and the type of information one needs 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 specimens, focusing on materials built mainly of carbon and oxygen, thus naturally providing low contrast in SEM. We used cryogenic SEM (cryo-SEM) to study emulsions in their native state. Cryo-SEM employs thermal fixation of the system to provide a reliable method for the study of structured liquid and biological systems at their native state down to the nanometric level. We also studied carbon nanotubes (CNTs) dispersed in water and dissolved in superacid by cryo-SEM. HR-SEM at room temperature was performed on CNT films, deposited on glass. While studied emulsions represent insulators, the CNT-based samples represent combined conductor/insulator systems. We show that the data acquired with different detectors, at different acceleration voltages, has an impact on the final micrographs and their contrast. Judicious selection of the SEM operation parameters leads to optimal picture contrast between domains of different composition.