The Preparation of Carbon Films for Electron Microscopy

Carbon films prepared by the method of Bradley (1) are commonly used as specimen supports in preference to plastic films because they are more stable under electron bombardment. Their preparation involves, however, a number of operations, in each of which there is some risk of contamination. Techniques have been developed which reduce the number of operations, but they are useful only in the study of particular types of specimen. de Harven (2), forexample, has described a method of depositing carbon on sections which are to be studied, instead of on a plastic film which is later removed. He stated that certain precautions were necessary if artefacts were to be avoided. Davies and Wallace (3) have reported that a tissue culture may be grown on a carbon film deposited on glass, the film with its culture being floated off when ready for study and picked up on grids. They found it necessary to deposit films 250 to 300 A thick in order to float them off without damage. This Note describes a method of preparing carbon films with a measured thickness of about 50 A, which are suitable for all normal purposes, without the intermediate step of depositing on a plastic film. The principle of the method is that carbon is deposited on a freshly cleaved mica surface; the resulting film is then floated off and picked up on grids. The use of mica has already been described by Hall (4), who used it in preparing preshadowed replicas. When freshly cleaved it has a surface which is more hydrophilic than that of glass, and a film will usually float off it extremely readily. A carbon film is much more fragile than one of plastic, but by following closely the technique described below it has been found possible to prepare grids carrying practically undamaged films. The average thicknesses of typical films have been measured to within 20 A by multiple-beam interferometry (5) and found to lie in the range 30 to 70 A. Shadowed films have been found to show large areas with no structure other than that attributable to granular-ity in the shadowing metal deposit. The films have been found to be suitable for picking up sections from the surface of a liquid. Squares of best quality ruby mica 1 about 0.05 ram. thick and 2 cm. square were cleaved in air by bending down one corner and inserting a needle …