Scanning electron microscopy (SEM)

Information obtained

  • High resolution images showing sample morphology
  • Differences in sample composition and density on backscattered electron images
  • Focussed ion beam allows controlled erosion of material from a sample, for 3D image generation or for TEM sample preparation
  • Lateral resolution in the micrometre to submicrometre range
  • Information depth, about 2 μm

Principle

A focused electron beam interacts with a solid sample. Electrons are ejected close to the surface and are detected by a scintillator. The images produced are then displayed on a computer.

Special features

Vacuum modes from high vacuum to “environmental”: SEM typically uses high vacuum (10-6 to 10-8 bar), which gives the best resolution, but the environmental mode allows for observation of “wet samples”, materials that have water in their structure (e.g. clays, living organisms, biofilms). In environmental mode, vacuum is ~10-3 bar and water vapour is sometimes injected into the chamber. The presence of water vapour makes it possible to avoid charge buildup on surfaces, so good images can be made without the conductive coating, which can mask features and produce artefacts.

Focussed Ion Beam (FIB): Controlled erosion using a FIB can be used to prepare slices that are only a few Ångströms thick, such as for examination with our built in transmission electron microscope. The FIB allows controlled removal of layers of material for collecting a series of images which can then be converted into a 3D image where chemical composition from EDXS can be added.  The FIB is also be used for preparing samples for high resolution tomography scanning at a synchrotron.

combined_Kaolinite_final_copie2

SEM images from a wet paste of kaolinite. Using environmental mode (left), the water initially present is preserved, covering the clay. This water layer has evaporated when the same sample is observed under a high vacuum (right).

Energy dispersive X-ray spectroscopy (EDXS)

Information obtained

  • Quantitative element analysis
  • Chemical maps with submicrometre resolution

Principle

An electron beam energises a volume of the solid. X-rays are produced from a volume that is  usually about 1 µm in diameter, located 1 µm below the surface, with energy that is characteristic of their atom of origin. This technique is often called EDS or EDX and used in conjunction with SEM.

The back scattered electron image of a clay rich sample (upper left) shows a lighter grey area contrasting with the darker grey surrounding particles. This difference in contrast suggests a difference in composition, which is confirmed by EDXS: the light-grey area is NaCl whereas the rest of sample is clay particles. The detected titanium probably comes from a TiOparticle.