General Guidelines for TEM Sample Preparation
TEM samples must be electron transparent. This means they must be thin – typically about 100 nm or less depending on the average atomic number of the material. Certain types of investigations demand more stringent requirements such as thickness on the order of 20 to 30 nm. It is impossible to anticipate every sample preparation problem. However, there are three common types of samples.
1. Particulate materials (powders, nanoparticles, nanowires)
Desirable particle size is about 500 nm or less. Powders that are more coarse than this should be ground with a mortar and pestle. The specimen preparation consists in transferring a suspension of the particles in a solvent such as isopropanol to a carbon coated grid and letting the solvent evaporate.
2. Preparation from bulk material
The preparation process can sometimes be time-consuming and often requires careful grinding and polishing techniques, skills for which may take time to develop. We recommend that the person studying the sample is the same one preparing the sample.
All of our TEM holders accommodate a 3-mm disc. Ultimately, the TEM sample you prepare will be in the form of such a disc. It may be monolithically cut from bulk material. Alternatively, a small (< 3 mm) fragment can be thinned and mounted on an appropriate support.
Simple metals or single-phase alloys can often be electropolished with an appropriate electrolytic solution. Even multiple phase alloys can sometimes be prepared in this fashion.
More commonly, samples from bulk material are thinned with an ion beam. Before the final ion-beam thinning, however, the sample should be first mechanically thinned by lapping and polishing so that the final thickness in the center of the sample is about 30 microns.
3. Cross section of thin films
Cross-section samples can usually be made using the same ion-thinning process as for bulk samples. However, one must first make a stack by gluing together two substrate fragments film-to-film. Then a cross-section can be cut or simply ground down with lapping film. Final preparation is with ion thinning as described above.
Site Specificity
Occasionally the need arises to prepare TEM samples from a specific location on or near the surface of a bulk sample or thin film sample. In order to achieve such site specificity, we recommend the use of the Focused Ion Beam instrument for sample preparation.