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Interpreting Electron Microscopy Data

Data generated by an electron microscope are presented as microimages showing an object along with a number of imaging parameters and a scale bar at the bottom of an image. As a rule, imaging parameters include accelerating voltage, working distance, magnification, operation mode, and a type of detector. The scale bar contains ten divisions, with the distance between two outermost divisions indicated directly under the bar. (um – micrometer, 10-6 m; nm – nanometer, 10-9 m). For example, the total length of the scale bar shown on Figure 1 is 10 micrometers, with the distance of 1 micrometer between scale points.

Figure 1. Example of a microimage obtained by a scanning electron microscope.

Images obtained by a scanning electron microscope (SEM) show the surface structure of a studied object and, unlike optical microscopy images, they are black-and-white (Figure 1). Brighter areas represent those sample fragments that emit more backscattered (BSE) and secondary (SE) electrons, while darker areas represent the fragments that emit fewer electrons. Surface topography is the main factor that effects the amount of registered secondary electrons (SE). Sample composition (effective atomic number, density, etc.) effects the amount of backscattered electrons (BSE) and allows studying particles of various chemical nature in the examined sample.

Microimages obtained by a transmission electron microscope include the following parameters along with the image of a studied object: magnification, operation mode, accelerating voltage, and imaging date and time (Figure 2).

Figure 2. Example of a microimage obtained by a transmission electron microscope.

Transmission electron microscope (TEM) registers electrons transmitted through a studied sample. In a standard bright-field mode (basic mode), brighter areas of an image represent less dense or less thick sample fragments with more electrons being transmitted through them. Darker areas of an image represent the sample fragments where electrons cannot be transmitted easily. These are either denser or thicker fragments. The darker an image area, the thicker or denser a corresponding sample fragment is. Thus, sample structures of different density and thickness can be distinguished by contrast.

See Equipment for more details on electron microscopy principles of operation.

Magnification of Microphotographs

For clarity, microphotographs can be grouped according to the magnification of an imaged object. Magnification is a convenient characteristic when it comes to image description. However, it should be noted that the meaning of magnification greatly depends on the size of a final image; magnification is changed with scaling. A more meaningful value is obtained by adjusting zoom to a standard frame size. It is this value that is shown at the bottom of an image together with a mandatory scale bar.

Magnification can be ranged as follows: 10 – 103 – low, 103 - 104 - medium, 104 - 105 - high, >105 - super high. Useful data can be obtained at different magnifications. Low magnification allows obtaining information about general structure of an object and its homogeneity; it also allows choosing areas that warrant taking more detailed microphotographs. Most electron microscopy studies are carried out at high magnifications. At high mode, it is possible to capture a large enough sample area, visualizing the morphology of micro-scale and sub-micro-scale objects. The smallest nano-scale details of a studied sample can be visualized at super high magnifications. However, it is not advisable to use this mode for studying higher levels of organization because the data is usually lost in such a case, especially with non-homogeneous objects.

Table. Selected images of studied objects at various magnifications.

10 - 103
Low magnification
103 - 104
Medium magnification
104 - 105
High magnification
Super high magnification
Micro Mechanics Flame Retardants Gold Particles Palladium on Carbon
Fashion Fabrics Integrated Circuits Copper Oxides Gold Particles
Metallic Spheres Copper Oxides Colloidal Palladium Colloidal Palladium
Synthetic Graphite Silver Chloride Silver Chloride Palladium on Graphite
Integrated Circuits Gold Particles Single-walled Carbon Nanotubes
Multi-walled Carbon Nanotubes
Synthetic Graphite

List of Terms

  • BSE – backscattered electrons
  • SE – secondary electrons
  • SEM – scanning electron microscopy
  • STEM – scanning transmission electron microscopy
  • TEM – transmission electron microscopy
  • HC – high contrast mode
  • HR – high resolution mode
  • (U) – upper detector
  • (T) – top detector
  • (L) – low detector
  • BF - bright-field imaging
  • ADF - annular dark-field imaging
  • EDX - energy-dispersive X-ray spectroscopy
  • PDBSE - photodiode-backscattered electron detector

  • Viewing and Downloading Microphotographs