Electron microscope

An electron microscope operates on the same basic principles as a light microscope but uses a beam of electrons instead of photons. Electrons are emitted from a source (typically a tungsten filament or a field-emission gun) and accelerated by an electric field, then focused by electromagnetic lenses. The entire electron path is maintained under high vacuum to prevent scattering by air molecules. The short wavelength of electrons—about 100,000 times shorter than visible light—enables much higher resolution. The theoretical resolution limit is about 0.1 nm, but in practice, aberrations and sample limitations often reduce this to a few nanometers. Modern aberration-corrected instruments can achieve sub-50-picometer resolution, allowing imaging of individual atoms.¹

The two main types are the transmission electron microscope (TEM) and the scanning electron microscope (SEM). In a TEM, electrons pass through an ultrathin specimen, and the resulting image is magnified and focused onto a detector, revealing internal structure. TEMs can achieve atomic resolution and are used to study the internal morphology of cells, viruses, and materials. In an SEM, a focused electron beam scans the surface of a sample, and secondary or backscattered electrons are collected to produce a three-dimensional topographical image. SEMs have lower resolution than TEMs but offer greater depth of field and simpler sample preparation. Other variants include the scanning transmission electron microscope (STEM), which combines features of both, and the environmental SEM (ESEM), which allows imaging of wet or gaseous samples.

The first electron microscope was built by Ernst Ruska and Max Knoll in 1931, for which Ruska received the Nobel Prize in Physics in 1986. The instrument was based on the theoretical work of Louis de Broglie, who proposed the wave nature of electrons in 1924. Early electron microscopes were limited by aberrations, but by the 1940s, commercial instruments became available. The development of the field-emission gun in the 1970s improved brightness and coherence. Aberration correction, using multipole lenses, was introduced in the late 1990s, enabling atomic-resolution imaging. The history also includes contributions from Reinhold Rüdenberg, who patented an electron microscope in 1931, and Bodo von Borries, who helped commercialize the technology.²

Electron microscopes are indispensable in biology, materials science, and nanotechnology. In biology, they have revealed the ultrastructure of cells, viruses, and protein complexes; cryo-electron microscopy (cryo-EM) allows imaging of frozen-hydrated samples, earning the 2017 Nobel Prize in Chemistry for Jacques Dubochet, Joachim Frank, and Richard Henderson. In materials science, TEM and SEM are used to characterize crystal structures, defects, and surface morphology. In the semiconductor industry, electron microscopes are used for failure analysis and quality control. They also play a role in forensics, geology, and even art conservation, where they help identify pigments and degradation products.³

Beyond the standard TEM and SEM, there are specialized techniques: electron energy loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) provide elemental composition; electron holography maps electric and magnetic fields; and 4D-STEM captures diffraction patterns at every scan point. A niche application is the use of electron microscopes in the semiconductor industry for 'nanoprobing' to test individual transistors. Historically, the first commercial electron microscope was built by Siemens in 1939. A lesser-known fact is that the resolution of electron microscopes is limited not only by wavelength but also by the 'information limit' due to lens aberrations and instabilities. Also, some electron microscopes can be operated in 'low-dose' mode to minimize radiation damage to sensitive biological samples.⁴

Aberration

A defect in lens focusing that causes image blurring; corrected using multipole lenses.

Cryo-EM

Electron microscopy of frozen-hydrated samples, preserving native structure.

Secondary electron

Low-energy electrons emitted from a sample surface when struck by the primary beam; used in SEM.

Backscattered electron

High-energy electrons reflected from a sample; provide compositional contrast in SEM.