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Electron Microscopy

Introduction

Electron microscopy is an imaging technique that uses an electron beam to probe a material. Since the wavelength of an electron is much smaller than the wavelength of visible light, diffraction effects occur at much smaller physical dimensions. The imaging resolution in electron microscopy is on the order of 1 nm, which is much better than the µm resolution of light microscopy. When electrons penetrate a sample, they are diffracted to form a diffraction pattern. This diffraction pattern can be transformed with a lens to obtain the sample image. Electron microscopy finds its widest use in materials science and microbiology.


Instrumentation

The use of electron beams requires that the sample be placed in a vacuum chamber for analysis. An electron beam is produced by applying a high voltage to a hot tungsten filament, and accelerating the emitted electrons through a high electric field, typically 10-100 keV. The electron beam is then focused with magnetic field lenses to a typical spot diameter of 1-100 nm on the sample.


Transmission electron microscopy (TEM)

Transmission electron microscopy (TEM) images the electrons that pass through a sample. Since electrons interact strongly with matter, electrons are attenuated as they pass through a solid and require the samples to be prepared in very thin sections, less than 100 nm. Specialized electron microscopes can use electron energies on the order of a MeV to study samples as thick as 1 µm or more. Materials samples are or for particles dispersed as a monolayer on a support grid. Biological samples are embedded in a solid matrix and microtomed. In all cases, the sample is placed on some type of support grid. Either the diffraction pattern or the image of the sample is observed on a phosphor screen below the sample, and can be recorded with film.


High resolution electron microscopy (HREM)

High resolution electron microscopy (HREM) is a variation of TEM that uses phase-contrast microscopy to achieve a resolution of approximately 0.2 nm, which is sufficient to provide atomic-scale resolution. The phase-contrast method uses the phase difference of multiple diffracted beams in addition to the intensity of the diffracted beams. The lattice image that is reconstructed is valid only when the electron beam has a spatial resolution of approximately 0.2 nm and the sample is thin enough that electrons are not scattered multiple times (less than approximately 10 nm). Under these conditions the reconstructed lattice image represents the true structure.


Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) uses the secondary electrons that are ejected from a sample to image a surface. These images are useful for studying surface morphology or measuring particle sizes. The electron beam is rastered across the sample by ramping the voltages on x- and y-deflection plates through which the electron beam passes (the z axis is the electron-beam direction). A detector above the sample detects the secondary electrons to produce an intensity map as a function of electron-beam position, which is displayed on a video or computer screen. Nonconductive samples require an evaporated gold or graphite coating over the sample to prevent charging effects that would distort the electric fields in the electron microscope.

A recent innovation in scanning electron microscopy is to use several stages of differential pumping between the electron gun and the sample, which is placed in a vacuum of a few torr. This method is called environmental scanning electron microscopy (E-SEM) and allows imaging of samples that would quickly vaporize in a high-vacuum environment.


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