ELECTKON ^IICKOSCOPY 



smallest details (if the specimen surface 

 topo<i;raphy, down to the limit of resolution 

 of the electron microscope, and .yet it must 

 be \ery thin so that the electrons may pass 

 throuj^h it without excessive scattering. 



Replicas can be made from a number of 

 different materials each of which requires its 

 own particular specimen preparation tech- 

 niques. Plastics, certain metal-oxide films, 

 and ^•acuum-evaporated materials liaA^e been 

 used and the physical characteristics re- 

 quired of all three are basically the same. 

 The replica material must be transparent to 

 electrons so that the final replica can be 

 thick enough to mount on an electron micro- 

 scope specimen support grid without break- 

 ing. While the electron transparency of 

 plastics and some oxide films is high, many 

 evaporated materials are dense to electrons. 

 It is thus clear that while almost any plastic 

 is suitable for the preparation of a replica, 

 relatively few evaporated materials can be 

 used. 



Plastic Replicas 



The methods used in the preparation of 

 plastic replicas for the electron microscope 

 are essentially very simple (1). A thin plastic 



o 



replica, between 200 and 800 A in thickness, 

 can be formed by applying a solution of the 

 plastic onto the surface of the specimen. 

 After removal from the specimen, it is 

 mounted on an electron microscope support 

 grid for examination in the instrument. This 

 type of replica is used almost exclusively in 

 the examination of the surfaces of specimens 

 of large bulk. 



]Many of the plastic replica techniques 

 described in the literature consist of complex 

 variations in handling procedures for the 

 removal of the replica from the specimen 

 surface and its subsequent mounting on a 

 support grid. Different plastics are also used. 

 To illustrate the preparation of a plastic 

 replica a typical routine procedure is briefly 

 described here. 



First, the specimen surface must be 



suitably prepared. For example, in the case 

 of a piece of metal the preparation will be 

 more or less identical with standard metal- 

 lographic procedures required for optical 

 microscopy. The metal is then dipped into a 

 solution of "Formvar" (polyvinyl formal) in 

 dioxane. It is next held vertically in a dry 

 atmosphere with the bottom touching a filter 

 paper, which absorbs the surplus solvent, 

 and the specimen is allowed to dry. The 

 "Formvar" film can be removed from the 

 metal specimen in two ways. If it is a reason- 

 ah\y thick film the following procedure can be 

 safely adopted. A specimen support grid (a 

 ^-in. diameter copper disc perforated 200 

 mesh/in.) is fixed to a length of cellulose ad- 

 hesive tape by its edges only, the center area 

 of the grid being separated from the tape 

 by a small disc of very thin paper. The grid 

 and tape are now^ pressed against the sur- 

 face of the "Formvar" film, care being taken 

 to ensure that the surrounding tape is firmly 

 pressed into position. Next, the tape is pulled 

 away from the metal, the "Formvar" film be- 

 ing removed at the same time. The film 

 should now^ cover the area of the support grid 

 in addition to the surrounding adhesive tape. 

 Because the grid is only adhering to the tape 

 at its edges, it can be removed without diffi- 

 culty from the adhesive tape with the plastic 

 film still mounted on it. If the film is thin, it 

 must be removed from the metal by floating 

 onto water. It can then be picked up on a 

 grid for examination in the electron micro- 

 scope. 



The mechanism of image formation en- 

 countered in plastic replicas is of interest, 

 the contrast being produced by variations 

 in thickness across the film. Ideally, the back 

 surface of the plastic replica should be flat, 

 but in practice much of the surface structure 

 of the specimen is visible on this back surface 

 (2). The contrast mechanism is thus not as 

 simple as it first appears. Because of the 

 thickness of the plastic film required and 

 this mechanism of image formation, the 

 resolving power obtainable with plastic 



230 



