33.13:
Transmission Electron Microscopy
A transmission electron microscope or TEM is used to study a sample's internal structure and composition.
The instrument consists of an electron gun which emits an electron beam. The beam is then focused on the sample using two or more electromagnetic condenser lenses.
The electrons transmitted through the sample are collected and focused by objective lenses to form the intermediate image. This image is further magnified by additional intermediate and projector lenses.
The final image can be captured using electron-sensitive imaging devices, such as a phosphorescent screen or CCD camera.
TEM forms a two-dimensional black and white image of a sample. Darker areas represent dense regions that transmit few or no electrons. In contrast, lighter areas indicate the regions through which more electrons are transmitted.
TEM is also used to analyze sample composition. The electron beam-sample interaction generates characteristic X-rays that can be used to identify and quantify different elements present in a sample.
33.13:
Transmission Electron Microscopy
In 1931, physicist Ernst Ruska—building on the idea that magnetic fields can direct an electron beam just as lenses can direct a beam of light in an optical microscope—developed the first prototype of the electron microscope. This development led to the development of the field of electron microscopy. In the transmission electron microscope (TEM), electrons are produced by a hot tungsten element and accelerated by a potential difference in an electron gun, which gives them up to 400 keV in kinetic energy. After leaving the electron gun, the electron beam is focused by electromagnetic lenses (a system of condensing lenses) and transmitted through a specimen sample to be viewed. The image of the sample is reconstructed from the transmitted electron beam. The magnified image may be viewed either directly on a fluorescent screen or indirectly by sending it, for example, to a digital camera or a computer monitor.
The entire setup consisting of the electron gun, the lenses, the specimen, and the fluorescent screen are enclosed in a vacuum chamber to prevent energy loss from the beam. Modern high-resolution models of a TEM can have resolving power greater than 0.5 Å and magnifications higher than 50 million times. For comparison, the best resolving power obtained with light microscopy is currently about 97 nm.
A limitation of the TEM is that the samples must be about 100 nm thick, and biological samples require a special preparation involving chemical “fixing” to stabilize them for ultrathin slicing. To overcome the limitations, several advancements in TEM techniques, such as cryo-TEM, have been made to get rid of artifacts and allow for direct sample imaging without the sample-damaging stages of sample fixation and dehydration.
This text is adapted from Openstax, University Physics Volume 3, Chapter 6 Photons and Matter Waves, Section 6.6: Wave-Particle Duality.