Preparation of Carbon Films by Use of a Glycerin Substrate

While carbon films are highly desirable for supporting specimens for electron microscopy, the difficulty in preparing them prevents their routine use in many laboratories. The usual method employed is to coat the grids with a plastic film which is then coated with carbon in an evaporator, followed by removal of the plastic with a suitable solvent (6). Satisfactorily removing the plastic film without damaging the carbon film, however, is arduous. Stripping carbon films from glass slides (2) or from cleaved mica surfaces (3) is tedious, and getting such films properly spread on grids and adequately attached to the grid bars presents other difficulties. It was reasoned that an ideal way to prepare grids coated with carbon would be to float them on the surface of a water-soluble liquid, the surface of which would serve as the substrate for the deposition of evaporated carbon. The liquid substrate could be removed by floating the grids on the surface of distilled water. Necessary properties of such a liquid would be a low vapor pressure, high solubility in water, and a surface tension strong enough to support the grids. Glycerin was found to serve the purpose adequately; it has a vapor pressure of about 0.2 # Hg at room temperature, it is very soluble in water, and its surface tension is sufficient to support grids. The idea of casting films onto liquid surfaces is not new. In 1948 Hast (4) recommended evaporation of metals onto glycerin as a method for producing very thin metal films. First attempts to use this principle were only partially successful. Grids were suspended on the surface of small individual droplets of glycerin on a microscope slide and placed in an evaporator for coating at a pressure of 0.3 # Hg. Due to traces of water in the glycerin, which is hygroscopic, bubbles formed in the glycerin and on bursting they often flipped the grids from the surfaces of the droplets. To avoid this problem, the procedure described below was adopted. A grid punch is used to make a series of holes spaced about { inch apart in a thin strip of celluloid. Thin celluloid rulers do very well for this. The celluloid strip is clamped in a holder which keeps it clear of the resting surface when the holder is set down. A suitable holder is a stack of micro-