Electron microscopy for particle size analysis

Electron microscopy is a particle size analysis technique that is either implemented as transmission electron microscopy or TEM, and scanning electron microscopy or SEM. The two techniques image individual particles dried from solution, and allow precise measurements of individual particles. These are described below.


  Electron microscopy (TEM and SEM)

Electron microscopy, specifically transmission electron and scanning electron microscopy (TEM and SEM) are the gold standard for particle size analysis. To prepare a sample for electron microscopy, the sample is dried from a few milliliters of the fluid containing the particles. The sample is in some cases then coated with a nm-thick layer of conductive carbon or gold, to prevent spurious charging while viewing the sample. The dry coated sample is then placed in the sample analysis chamber of the microscope; most commonly this is in vacuum, although more expensive modern microscopes allow sample examination in air. The sample is then illuminated with a high energy beam of electrons generated by a cathode; the electrons can range in energy from a few kilovolts to a few hundred kilovolts, and are focused by coaxial magnets to a focus point 0.1 to 1 nm in diameter. This focus point is scanned over the sample in an SEM, while in a TEM the beam passes through the sample and the scattering occurs during the passage through the interior, after which scattered electrons are imaged on a CCD. Either the scattered electrons, or in some cases the secondary electrons generated by the primary electrons in the beam, or both, are collected and projected into an imaging system. Very high resolution images of particles can be formed in this way, and analyzed with precision measurement tools calibrated on the microscope using calibration standards. In a TEM, it is possible with crystalline materials to image the atoms that make up the sample.

Particle diameters can be measured with great precision, and the geometric details of each particle are revealed in the images. By forming many images and using automated software, the size distribution of particles can be calculated and a concentration versus particle diameter can be calculated.

The sample preparation and sample imaging as well as image analysis are very time-consuming. TEMs and to a lesser extent SEMs are large, room-filling pieces of equipment that need special spaces with low vibration, low stray magnetic fields, and good temperature control. Using this method as a regular sample measurement technique is clearly prohibitive in terms of both cost and time. The data are typically used as an orthogonal verification of some of the other techniques discussed here.

SEM image of nanoparticles

A typical image of nanoparticles taken with a scanning electron microscope (SEM).






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