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MORE INFORMATION ABOUT TEM

Imaging

Contrast in the image arises either by absorption of electrons scattered through angles larger than the lens aperture (scattering contrast) or by interference between the scattered and incident wave at the image point (phase contrast). Information obtained from phase contrast is ultimately limited by lens aberrations and the limited coherence of the electron beam. The typical limit in a modern TEM is in the 0.2 to 0.4 nm range.

 

Transmission Electron Diffraction

In addition to imaging information, a TEM is also capable of providing transmission electron diffraction data. This is done quite simply by changing the post objective lens settings so that rather than projecting the objective-lens image they project the electron intensity in the objective back-focal plane.

 

Nanometer Scale Electron Probes and Scanning TEM (STEM)

Many modern TEMs are also capable of forming a small electron probe (5 nm or less) allowing diffraction information from nano-scale areas – so-called nano-beam diffraction. Production of characteristic x-rays (arising from excitation of inner atomic shells) is limited essentially to the illuminated area, since beam broadening is typically small for a thin sample. Detection of these x-rays allows for qualitative or semi-quantitative elemental analysis at the nano-scale.
Any modern TEM that can form a small electron probe may also be configured as a scanning TEM (STEM) simply by adding the capability of beam rastering together with appropriate detectors. This STEM configuration is often available on TEMs with field emission sources. STEM mode is convenient for analytical work (X-ray analysis or energy-loss spectroscopy) since the rastered image can be recorded almost simultaneously with the analytical signal without altering the illumination optics.