Transmission electron microscopy (TEM) involves a beam of high-energy electrons passing through a thin sample, typically about 100nm thick.
The image formed shows variations in the scattering of the electrons by the sample related to the mass of the atoms, the thickness of the sample or the orientation of the sample (if it is crystalline).
Information can also be obtained on the composition and chemistry of the sample using a range of optional microanalysis techniques.
The TEM has two standard imaging modes - bright field (BF) and dark field (DF). In BF images, areas of high scattering appear dark indicating regions of high mass, thickness or strong diffraction effects. In DF mode, the image is formed with electrons scattered in a specific direction, usually as a result of diffraction from a particular atomic plane or planes. Thus, regions of high intensity represent strong scattering.
HRTEM is a phase-contrast imaging technique, which makes it possible to obtain images with atomic resolution. It can be used to investigate the crystallinity of the sample, including identification of lattice planes and some defects. As a crystallographic tool it can be used to identify crystal phases and orientations. HRTEM images can be obtained on the JEOL 3000F and JEOL 2100. The best results will be obtained on the JEOL 3000F.
SAED patterns provide crystallographic information from selected regions of the sample from the micron to the ~100nm scale. The spacings and orientations of the diffraction spots can be interpreted in terms of the planar spacings and orientations in the sample. Similar to HRTEM, it can be used to identify crystal phases and their orientations, though not on such a small spatial scale. SAED patterns can be obtained on all of the Centre’s TEMs. They can be recorded digitally on the JEOL 2100.
EDS is a microanalysis technique allowing us to determine the composition of the sample. When the electron beam hits the sample X-rays are generated which have characteristic energies for each element. This provides a qualitative analysis of the elements present and, with further analysis, a quantitative composition can be determined. EDS can be conducted from the micron- to the nano-scale.
Electron energy-loss spectroscopy (EELS) EELS is another microanalysis technique that provides complementary information to EDS. With EELS we analyse the amount of energy lost by the electrons after they have passed through the sample. Characteristic energy loss peaks (similar to EDS) allow us to identify the elements present. The fine structure of these peaks provides information on valence states and bonding.
EFTEM involves selecting a range of energy-losses from an EEL spectrum and turning them back into an image. High intensity in this image corresponds to regions generating high signals in that energy range. By selecting energies corresponding to characteristic element signals, and removing the background, it is possible to obtain element distribution images or maps. Iron element map (left) from individual (arrowed) and accumulations of ferritin granules in biological tissue courtesy of the Centre's Biomineralisation Research Group.
STEM imaging involves scanning a small, focussed electron beam across the sample (similar to SEM). Detectors count the electrons transmitted through the sample, building up an image of the sample point by point as the beam is scanned. Bright field (BF) images are obtained by collecting electrons scattered through small angles. High-angle annular dark-field (HAADF) images are obtained by collecting the electrons scattered through large angles and provide information on mass variations in the sample (also called Z-contrast imaging).