Advancing into new microdimensions
The wavelength of light limits the possibilities of optical magnification. If the objects are smaller than half a micrometer, they can no longer be imaged with a conventional light microscope. Although most bacteria can be identified in this manner, the much smaller viruses, for example, cannot. Other physical units are required to make them visible.
Ernst Ruska and Max Knoll relied on electrons. In 1931, they developed the first transmission electron microscope (TEM) at the Technische Hochschule (Technical University) in Berlin-Charlottenburg. This enabled them to open up previously unknown microdimensions for the human eye. They were the first to be able to image and observe the crystal structure of a thin metal foil.
Particles instead of light
In light microscopy, the light waves are passively captured and split up through lenses. The magnification results from this optical effect. It works completely differently with the TEM, which uses an electron source: The electrons are actively accelerated and directed as a focused beam onto the object to be observed. There is an interaction of the particles with the observed object. Microscope images are created by evaluating their paths after contact. Since the wavelength of the tiny particles is in the range of a few picometers, electron microscopy can show structures in the nanometer range in a differentiated way.
With the TEM, the beam electrons that have penetrated the object are evaluated. However, this only works with very thin layers; the samples often require complex preparation. This is superfluous with the scanning electron microscope (SEM). Its electron beam can also scan three-dimensional objects in a grid pattern. A part of the beam electrons is reflected, while further electrons are released from the object. The device "catches" these particles and creates the image from them. This is how, for example, images of tiny creatures are generated, which often remind us of fantasy monsters when enormously magnified.
Imaging works in the TEM like in the SEM but only if the electrons are not deflected on their path to and from the object. So there must not be any air molecules in the way. High vacuum is therefore always inside electron microscopes, which is produced by a suitable vacuum pump. The Busch Group offers various solutions for this.
The first electron microscope achieved 400 times magnification in 1931. For this breakthrough, its co-inventor Ernst Ruska received the Nobel Prize for Physics in 1986, 55 years later. He introduced the technology to the market at Siemens in 1938. The resolution became better and better in the course of its development. Today's devices can magnify millions of times, with the resolution of the transmission electron microscope reaching 0.08 nanometers. Molecular structures, among other things, can be mapped in detail.
With the help of an electron microscope, it was possible to examine the interior of a cell in detail for the first time. To this day, the device continues to play a crucial role in research into viruses. It can be used to decipher their morphological configuration, the spatial structures. From this, important findings on the risk of infection and dissemination mechanisms can be derived. In medicine and biology, the electron microscope is an indispensable tool, not least for this reason. Materials research is also an important field of application.