Electron microscopy beam damage

Chemical fixation for transmission electron microscopy prepares cells for the preservation of damage due to subsequent washing with aqueous solvents, dehydration with organic solvents such as ethanol or acetone, embedding in plastic resins, polymerization of the resins by heat, exothermic catalysts, or ultraviolet radiation, and imaging with high-energy electron beams in an electron microscope. 

In electron microscopy, generally intense electron beams are used which can severely damage the surface (however, at high energies the electron-atom cross sections become smaller). Often, large magnetic fields are also present that could affect the surface structure. 

Better spatial definition can be attained using transmission microscopy but the opaqueness of the electrode support negates its effectiveness. The spatial distribution of elements on the surface can be resolved with a scanning Auger microprobe. The problem of electron beam damage to the modified surface has prevented widespread usage. 

The low degree of hard-segment crystallinity in the solvent-cast samples with the added problem of loss of crystallinity from electron-beam damage prevented visualization of either the hard- or soft-segment domains by dark-field microscopy. Therefore bright-field defocus electron microscopy was used to enhance contrast between the microphases (27, 28). Only for the 1/5/4 and 1/6/5 samples was a distinct domain... 

Transmission electron microscopy is valuable because it allows imaging of even the smallest supported clusters of a heavy metal such as platinum. Dark field microscopy has been used to advantage for platinum clusters consisting of < 20 atoms each in zeolite KLTL [30]. Even single platinum atoms can be detected on zeolite supports thinner than about 20 nm, but the precision with which clusters can be pinpointed in the structure is limited by beam damage-induced distortion of the zeolite framework [30]. 

The X-ray images of extended lattice imperfections within the crystal arise fi om variations in beam intensity caused by local distortions of the lattice. Dislocation densities of up to 10 mm can be resolved in transmission studies or ten times this by the reflection technique. This resolution is lower than that obtainable by transmission electron microscopy, but the sample used may be thicker and does not have to be examined under high vacuum. X-ray beams also produce less radiation damage in the sample. When decomposition proceeds beyond a > 0.01, distortion of the lattice is such that the X-ray image loses resolution and the exposures required become even longer. Reflection data obtained at several different diffraction angles may be required to characterize the imperfections present. 

The atomic structure of the films was studied by transmission electron microscopy (TEM) using a JEM lOOC electron microscope in the microdiffraction mode. The diffraction patterns were obtained at a low electron beam intensity using the CCD high-sensitive registration system to prevent the films from radiation damage by the electron beam. 

It is worth while noting that in the electron microscopy of biological samples there is considerable interest in reducing radiation damage using liquid helium temperatures. Improvement factors between 87x (Knapek and Dubochet 1980 Dietrich et al 1980) and 4x (Lamvik, Kopf and Robertson 1983) have been reported. Henderson (1990) has compared the behaviour of frozen crystals in X-ray and electron beams. 

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