Low dose TEM operation

Assume now that the TEM is unavoidable, that it is really necessary to form an image from some transitory feature that is destroyed by 

When the low dose technique works well, the resolution in the image of a beam sensitive material approaches its theoretical limit (Section 3.4). Obtaining optimum imaging conditions beyond this point means improving this theoretical limit. The limit depends on the following four factors, which will be discussed further  

These problems mean that for most polymers it is not worthwhile using beam voltages above 200-300 kV to reduce radiation damage. Some types of specimens might be better at higher voltage [1071, but in practice little polymer work has been done above 200 kV. The penetration of thick specimens is the main advantage of the HVEM [1081. 

Low specimen temperatures reduce the damage rate, and crystalline material undergoes chemical change more slowly than amorphous material. Both effects can be explained by the reduced mobility of the molecules. Therefore it is to be expected that a physical restraint on the escape of material from the specimen should have a beneficial effect. Specimen coating is thus 

Reimer [1121 found a moderate reduction in mass loss rate for coated PMMA. Salih and Cosslett [113] coated an aromatic organic crystal and found that the dose needed to destroy the diffraction pattern increased by five times. Fryer and Holland [114] found factors of three to ten times improvement for a wide range of organic crystals. No reports have been published for diffraction lifetimes from coated polymer films, but with a very simple technique promising significant gains, it is worth a try. 

Cryomicroscopy is a second method for increasing the dose required to destroy image features. A low irradiation temperature increases the dose 

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TEM instruments and the improved operation procedures using devices such as a Minimum Dose System (MDS JEOL Ltd.) or a Low Dose Unit (LDU Philips) [2,3],... 

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