Scanning Electron Microscope (SEM)- Definition, Principle, Parts, Images


Electron microscopes are devices that use a beam of electrons as a source of illumination to magnify and examine the structure of samples with greater detail than the optical microscopes. They use electron optics that are analogous to the glass lenses of an optical light microscope. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light, electron microscopes have a higher resolution of about 0.1 nm, which compares to about 200 nm for light microscopes.

The history of electron microscopy dates back to the early twentieth century when the first electromagnetic lens was developed by Hans Busch in 1926. This opened the door of possibility to use the principles of the lens to invent a microscope that could examine the structure of samples with greater detail. This had the potential to exceed the theoretical limit for optical microscopy, which was about 0.2 micrometers at that time.

The first prototype electron microscope, capable of four-hundred-power magnification, was developed in 1931 by Ernst Ruska and Max Knoll, a physicist and an electrical engineer, respectively, at the Berlin Technical University . In 1933, Ruska built an electron microscope that was more powerful than the optical microscope and could resolve individual atoms. Ruska had a vast knowledge of electron wavelengths and invented the electron microscope while he was studying at the Technical University of Munich.

In 1937, Bodo von Borries and Helmut Ruska joined him to develop ways that the principles could be applied, such as to examine biological samples. In the same year, Manfred von Ardenne developed the first scanning electron microscope. Siemens-Schuckertwerke released the first commercial electron microscope to the public in 1938. From this point onwards, transmission electron microscopes became more readily available in other areas of the world, including North America.

In 1986, Ernst Ruska was awarded the Nobel Prize in Physics for the invention of the electron microscope, in conjunction with Heinrich Rohrer and Gerd Binnig for the development of the scanning tunneling microscope (STM).

Electron microscopes can be classified into two main types: transmission electron microscopy (TEM) and scanning electron microscopy (SEM). TEM uses a beam of electrons that passes through a thin sample and forms an image on a screen or a detector. SEM uses a beam of electrons that scans over a surface of a sample and collects signals such as secondary electrons, backscattered electrons, and X-rays that reflect the topography and composition of the sample. Both types of electron microscopes have various applications in different fields of science and technology.

In this article, we will focus on scanning electron microscopy (SEM), its definition, principle, parts, images, applications, advantages, and limitations. We will also introduce scanning-transmission electron microscopy (STEM), which combines the features of both TEM and SEM.