In order to reveal the true nature of many specimens we have to study the third dimension of our sample. The majority of the TEM techniques record two-dimensional (2D) projections of a 3D structure. However the complexity of the new material and the biological structures highlights the need to develop tools and techniques to explore the morphologies and compositions of materials in three dimensions.An object viewed from many different angles will generate slightly different projections. These images can be recorded and analyzed to create a tomographic rendering of the specimen. The resolution obtained depends of the diameter of the object and the number of projections.
There is a limit in the angle that we can reach tilting the sample, missing wedge problem. This gap produces artifacts in the reconstructed object: the elongation factor
In biology field, bright-field TEM (BF-TEM) is the best imaging method for tomography tilt series acquisition because, for amorphous sample, the projected image intensities vary monotonically with material thickness. On the other hand, for material science this condition, the variation monotonically of the intensities is difficult to guarantee in BF/HRTEM, where image intensities in crystalline samples are dominated by phase-contrast. However, the technique of annular dark-field scanning transmission electron microscopy (ADF-STEM) and high angular ADF detector (HAADF) are more effectively suppressing phase and diffraction contrast, providing image intensities that vary with the projected mass-thickness of samples up to micrometres thick for materials with low atomic number. ADF-STEM also acts as a low-pass filter, eliminating the edge-enhancing artifacts common in BF/HRTEM.
Tomography can yield a reliable reconstruction of the underlying specimen which is extremely important for its application in nanoscience and nanotechnology.