An electron microscope is an instrument that allows the imaging of objects through the use of electrons instead of light. It was developed in 1926 by Hans Busch, who discovered that magnetic fields could serve as lenses by making electron beams converge to create a point of focus. The first prototype was created by Max Knoll and Ernst Ruska.
Thanks to the advancement of technology, the electron microscope in use today is far improved compared to that first prototype. One of the main things that were improved over the years is the resolution of the microscope. The resolution refers to the finest detail that can be captured and distinguished in an image. The standard resolution differs depending on which type of electron microscope is involved.
There are two main types of an electron microscope, and their resolutions differ greatly from each other.
1. Transmission Electron Microscope
In a transmission electron microscope or TEM, a monochromatic beam of electrons accelerates through 40 to 100 kV and passes through a strong magnetic field that serves as the lens. Modern transmission electron microscopes have a resolution of about 0.2 nm. This is 1000 times greater than the resolution of a light microscope and around 500,000 times greater than the resolution of the human eye.
In a TEM, resolution tends to be limited by certain factors, such as spherical and chromatic aberration, which can be overcome by aberration correctors and software correction. Through corrective processes, images with enough resolution to expose carbon atoms can be achieved.
At the same time, resolution can be improved by reducing the wavelength. To shorten the wavelength, electrons must travel at a faster rate. Thus, if the accelerating voltage of the microscope’s electron beam is increased, the wavelength shortens and its resolution is improved.
2. Scanning Tunneling Microscope
In a scanning tunneling (STM) microscope, an electron beam is used to scan quickly over the surface of the sample to create an image of the surface topography. An STM has a resolution of around 2 to 10 nm; it is a bit limited compared to the TEM because of the width of the electron beam and the interaction of the electrons in a solid. Since the STM relies on electron interactions on the surface, it can provide bulk samples of images and can capture a greater depth of view. They are most ideal for capturing a 3D structure of samples, but until now, STMs can only produce black and white images.
Choosing the correct microscope to use depends on the necessary resolution as dictated by the needs of the user.