Production of Extremely Thin Metal Films by Evaporation on to Liquid Surfaces

Abstract

IN two articles in Nature of March 15, 1947, a method is given for making very thin metal films, 10–20 A. or more in material thickness. On the surface of a film of cellulose nitrate, etc., there is evaporated a small amount of, for example, beryllium or aluminium. Then the basic film is dissolved away by amylacetate or other appropriate solvent, the metal being left free. Such films of beryllium or aluminium are especially useful as supporting membranes for electron microscopical studies. As a rule these metal films have no visible structure. It may, however, happen that a faint structure appears even if the film is very transparent in the electron microscope. This structure effect may depend upon the fact that all the cellulose molecules are not nitrated to the same degree, some of them being insoluble in amyl acetate. Other explanations may be that a molecular layer of the cellulose nitrate adheres very closely to the metal surface and does not go into solution, or that some of the very active metal atoms have penetrated into the cellulose molecules. I have now found that the structure effect mentioned above does not exist when using acrylates instead of cellulose nitrate, collodion, etc., as the basic film. The solvent then used is ethyl acetate. The metal films made are almost invisible in the electron microscope when made 20 A. in thickness, and the photographic contrast effect of the object particles is increased compared with that obtained with ordinary supporting membranes, which is of the greatest importance when studying molecules or other minute particles.