Preparing samples for scanning electron microscopy

Scanning electron microscopes usually image conductive or semi-conductive materials best. Non-conductive hydrated materials can be imaged by an environmental scanning electron microscope. Most biological and materials specimens are insulators. This means that when they are bombarded by the electron beam in a SEM, the electrons have nowhere to go and cause serious imaging problems. Further, the electron beam may damage the sample.

These problems are reduced or eliminated by coating the specimen with a thin layer (5-20 nanometers) of electrically conductive material, such as gold-palladium, from a sputtering machine (Denton Desk II). This is done to prevent the accumulation of static electric fields at the specimen due to the electron irradiation required during imaging. The layer deposited nearly evenly coats the surface of the specimen, faithfully reflecting the surface morphology.

If the elemental composition of a sample surface is also required, then a graphite deposition by high vacuum evaporation (Edwards Carbon Evaporator) would be applied. Graphite is a better choice of coating since the numerous energy lines from gold & palladium may interfere with the EDS analysis.

Stable conductive specimens such as carbon nanotubes and metal surfaces require no special treatment before being examined in the scanning electron microscope.