Basics of Electron Microscopy

Electron microscopy is a powerful technique to study matter at the nano and atomic scale. Optical microscopes, which have traditionally been used for imaging micro and nanostructures, typically have a spatial resolution of a few hundreds of nanometers, limited by the wavelength of visible light. In contrast, electron microscopes make use of very energetic electrons (up to hundreds of keV, corresponding to a speed in the order of 100,000,000 km/h) to excite the sample. The wavelength of these electrons can be down to a few picometers, which is much smaller than that of visible light (100s of nanometers), allowing electron microscopes to achieve very high spatial resolutions.

A typical electron microscope is composed of different parts. In brief, electrons are initially emitted from an electron source, usually a micro or nanotip of LaB6 or tungsten, and subsequently accelerated in the electron column to voltages up to 200 kV, depending on the type of electron microscope. The electron column also contains electrostatic and electromagnetic lenses to stir, collimate and/or focus the electron beam. The electron gun, column and sample area are in vacuum.

The resulting electron beam impacts on the sample, prompting a plethora of processes inside the material. The figure below shows a schematic of different types of signals that can be collected after excitation of matter with an electron beam. The impact of the initial electron beam on the sample can result in the collection of other electrons, such as secondary electrons, which are low-energy electrons that have been knocked out from atoms in the sample, and backscattered electrons, which are electrons deflected by the nuclei of the atoms. In the case of thin samples, the energy, angle and amount of transmitted electrons can also be analyzed. Moreover, electromagnetic radiation, such as X-rays and visible light (cathodoluminescence) can also result from the interaction of the primary electron beam with the sample. The type and amount of signal given by a sample depends mainly on the type of sample, its thickness and the electron acceleration voltages. Aside from imaging, all of these signals offer complementary information about the sample, such as its composition, its optical properties, and its atomic lattice structure, among others.

Types of electron microscopes

There are different types of electron microscopes, which have different applications, depending on the type of sample, signal that is being analyzed and spatial resolution needed.

In a transmission electron microscope (TEM), the electrons that are transmitted from the sample are collected. For this, very thin samples are used, typically well below 100 nm-thick, and the electrons are accelerated to high voltages, between 50 and 200 kV, thus allowing to achieve the very spatial resolutions, down to the atomic scale. TEMs have different operating modes, such as imaging, in which the electron beam is either focused or collimated onto the sample, and the intensity of the transmitted beam is recorded; diffraction, based on the analysis of the diffraction pattern after excitation with a parallel electron beam; and spectroscopy, in which the energy lost by the primary electron beam is analyzed.

In contrast to a TEM, a scanning electron microscope (SEM) uses lower electron voltages (1 to 30 kV) and is based on the collection of the secondary electrons generated in the sample, as well as some of the other signals shown above. SEMs enable the study of bulk samples, given that there is no need for the electron to pass through, but their spatial resolution is typically limited to a few nanometers. In this app we provide estimations of the depth and lateral extent that electrons can reach inside the material, taking into account the range of acceleration voltages used in SEMs. In TEMs, the samples are typically very thin, thus knowing the interaction volume is less critical.

More information

There are many great sources of information about electron microscopes and electron-matter interaction. Here you can find a few of them:

[1] R. F. Egerton, Physical principles of Electron Microscopy (Springer US, Boston, MA, 2005)

[2] L. Reimer, Scanning Electron Microscopy: Physics of Image Formation and Microanalysis, Measurement Science and Technology 11, 1826 (2000)

[3] F. J. García de Abajo, Optical excitations in electron microscopy, Reviews of Modern Physics 82, 209 (2010)