Low-voltage electron microscopy
Low-voltage electron microscope (LVEM) is an electron microscope which operates at accelerating voltages of a few kiloelectronvolts or less. Traditional electron microscopes use accelerating voltages in the range of 10-1000 keV.
Low voltage imaging in transmitted electrons is possible in many new scanning electron microscopes, equipped with transmitted electron detector.
Low cost alternative is dedicated table top low voltage transmission electron microscope. While its architecture is very similar to a conventional transmission electron microscope, it has a few key changes that enable it to take advantage of a 5 keV electron source, but trading off many advantages of higher voltage operations, including higher resolution, possibility of X-ray microanalysis and EELS, etc... Recently a new low voltage transmission electron microscope has been introduced that operates at variable voltage ranges between 6-25kV.
A substantial decrease of electron energy allows for a significant improvement of contrast of light elements. The comparison images below show that decreasing the acceleration voltage from 80 kV to 5 kV significantly enhances the contrast of test samples. The improved contrast is a direct result of increased electron scattering associated with a reduced accelerating voltage.
LVEM brings an enhancement of imaging contrast nearly twenty times higher than for 100 kV. This is very promising for biological specimens which are composed from light elements and don't exhibit sufficient contrast in classical TEMs.
Further, a relatively low mean free path (15 nm) for organic samples at 5 kV means that for samples with constant thickness, high contrast will be obtained from small variations in density. For example, for 5% contrast in the LVEM bright field image, we will only need to have a difference in density between the phases of 0.07 g/cm3. This means that the usual need to stain polymers for enhanced contrast in the TEM (typically done with osmium or ruthenium tetraoxide) may not be necessary with the low voltage electron microscopy technique.
The improved contrast allows for the significant reduction, or elimination, of the heavy metal negative staining step for TEM imaging of light elements (H, C, N, O, S, P). While staining is beneficial for experiments aimed at high resolution structure determination, it is highly undesirable in certain protein sample preparations, because it could destabilize the protein sample due to its acid pH and relatively high heavy metal concentration. The addition of stain to sectioned samples such as biological materials or polymers can also introduce imaging artifacts.
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