Tools and devices

Electron microscopy at the nanoscale

Light cannot resolve anything much smaller than half its wavelength, which puts individual nanoparticles beyond the reach of an ordinary microscope. Electrons, with far shorter wavelengths, can go much further. Electron microscopes are the instruments that let nanoscience see what it makes.

Why electrons

An optical microscope is limited by the wavelength of visible light, several hundred nanometres. Electrons accelerated through a high voltage behave as waves with wavelengths far shorter than that, so a microscope that focuses electrons instead of light can in principle resolve much finer detail. The practical difficulty is focusing them. Glass lenses are useless for electrons; instead, electron microscopes use magnetic coils. The press release for the 1986 Nobel Prize in Physics describes how Ernst Ruska, as a young student in Berlin at the end of the 1920s, found that a magnetic coil could act as a lens for electrons, combined two such lenses into a primitive microscope, and by 1933 had built an electron microscope whose performance clearly exceeded that of the light microscope. He later helped develop commercial instruments, and shared the 1986 prize for that work.

Transmission electron microscopy

In a transmission electron microscope, the beam passes through the sample. The Wikipedia article on transmission electron microscopy explains that the specimen is usually a section less than 100 nm thick or a suspension dried on a grid, and that the image forms from the interaction of the electrons with the sample as they pass through it. Because electrons have a much smaller wavelength than light, the instrument can image fine detail down to a single column of atoms. The article describes TEM as a major analytical method across the physical, chemical and biological sciences and names nanotechnology and semiconductor research among its uses.

The same article records the early history: a first instrument demonstrated by Max Knoll and Ernst Ruska in 1931, a version with better resolution than a light microscope in 1933, and the first commercial instrument in 1939. It also lists the modes a modern instrument offers beyond conventional imaging, including scanning transmission imaging, diffraction and spectroscopy. For a nanoparticle chemist, those modes answer different questions: imaging shows size and shape, diffraction shows crystal structure, and spectroscopy shows which elements are present.

Scanning electron microscopy

A scanning electron microscope works on the surface instead. According to the Wikipedia article on the scanning electron microscope, a focused beam is scanned across the sample in a raster, and the signals produced where it strikes, most commonly secondary electrons, are detected and assembled into an image. The number of secondary electrons that escape depends on the local topography, which is why SEM images have their characteristic three-dimensional look and large depth of field. The article notes that some instruments achieve resolution better than 1 nanometre, and that besides conventional high-vacuum instruments there are variable-pressure and environmental versions for wetter samples.

In the nanoscience literature, SEM is the usual way to show the shape of things larger than a few tens of nanometres: arrays of nanowires, patterned surfaces, fibre networks and porous films. TEM is usually needed for anything smaller or for internal structure. A paper on, say, a self-assembled peptide fibre network may show SEM images of the tangle and TEM images of individual fibres; the self-assembly page describes one early example in which scanning electron microscopy revealed a network of filaments.

Reading an electron micrograph

Electron micrographs are among the most common figures in nanoscience papers, and among the easiest to over-read. A few habits help.

• Check the scale bar first. Magnifications vary enormously from panel to panel, and a particle that looks large in one panel may be tiny in the next.
• Ask how representative the image is. A single field of view is a sample of a sample. Look for size histograms measured from many particles and for low-magnification overviews.
• Remember sample preparation. Drying a suspension onto a grid can make particles cluster, and a vacuum can change soft or hydrated materials. What appears in the microscope is not always what existed in solution.
• Remember the beam. High-energy electrons can damage delicate structures during imaging, particularly organic and biological ones.
• Look for complementary evidence. Diffraction patterns, spectroscopy, or an independent technique such as light scattering or probe microscopy make an image-based claim far stronger.

Electron beams as tools

The same physics that lets electrons image small things lets them write small things. Focused electron beams are used to draw patterns in electron-sensitive films with very high resolution, a technique described on the nanofabrication page. The line between measuring the nanoscale and making it is often a single instrument used in two different ways.

For the other great family of nanoscale imaging tools, which feel a surface with a sharp tip instead of illuminating it with a beam, see seeing and touching atoms.