Further Investigations of Photographic Development 

 by Means of the Electron Microscope 



E. Klein 



Wissemchaftlich Photographisches Lahoiatoviiini, 

 Agfa Aktiengesellschaft fiir Photofabrikation, Leveikiisen 



The object of the present paper is a study of the 

 fine structure of silver halide crystals in photo- 

 graphic emulsions and of developed silver visible 

 by examinations with an electron microscope. 



It is impossible to do this by making direct electron 

 micrographs of silver halide or silver, for the follow- 

 ing two reasons: 



( 1 ) Scattering of the electrons on the crystals 

 (silver halide or silver) is so great that, in the crystal 

 thicknesses d encountered in practice (100 A * d •=- 

 20,000 A) no electrons are able to penetrate the 

 specimen in unscattered form and thus it is impossible 

 to determine surface quality or condition. The entire 

 object is reproduced in uniform high opacity com- 

 pared to the surrounding area and no deviations in 

 contrast are apparent within the object. 



(2) The risk of change in the objects during their 

 exposure to the electron beam is especially great 

 with direct observation of silver halide or silver. For 

 one thing the silver halide undergoes severe pho- 

 tolysis, i.e. decomposition into silver and free halide, 

 and for another the mobility of the lattice units in 

 the silver halide and silver is extremely high in an 

 intense beam of electrons, leading to a process similar 

 to melting and frequently to partial evaporation as 

 well. Very intense radiation and the marked rise in 

 temperature associated with it can force the reac- 

 tions to the point where the crystals are completely 

 evaporated (and partial condensation on cold parts 

 in the environment of the crystals ensues). 



The phenomena mentioned under point 2 have 

 already been discussed in detail in an earlier report, 

 with references to the literature already available in 

 this respect (5). 



For the purpose of the present investigations, it 

 was absolutely necessary to employ a replica method 

 since the actual object is not then exposed to the 

 electron beam. Our experience has shown that the 

 best method is that of direct carbon evaporation 

 on to the objects devised by Bradley (I, 6). 



Exposure of the silver halide crystals during the 

 brief period of evaporation with carbon (a few sec- 

 onds) causes no recognizable change in the struc- 

 ture of the crystal, as has been established by com- 

 parison with much greater exposures (contrary to 

 our earlier assumptions (5)). 



In most cases carbon evaporation is carried out at 

 an angle of 45 with a rotating object which, admit- 

 tedly, results in loss of shadow effect but the carbon 

 replicas are more uniform and stable, i.e. they can 

 also be kept thinner (6). Previous tests by Konig and 



Helwig's (7) carbon replica process and that of 

 Konig and Knoch (8) also produced good results. 

 However, if it is desired to use this process (in which 

 the carbon replica is formed by glow discharge in 

 a hydrocarbon atmosphere) to obtain a carbon rep- 

 lica which is stable without being hardened by 

 electron bombardment, the replica must be kept 

 relatively thick. On the other hand, hardening by 

 the electron beam would produce just the effects 

 mentioned above in the case of silver halide and sil- 

 ver. Thus the Bradley process would appear to be 

 more suitable for examination of photographic 

 grains. 



Fig. 1 illustrates a carbon replica of undeveloped 

 silver bromid crystals. In this case evaluation of 

 the stereo photographs reveals mainly a rounded 

 cube shape of the grains. In fig. 2 the form is that of 

 plate-shaped triangles, hexagons and isolated tetra- 

 hedrons. The latter shapes in particular can only be 

 detected by the replica method. Generally speaking, 

 silver chloride crystals are mainly cube-shaped. On 

 the other hand the silver iodide has a quite different 

 appearance and crystallizes in a hexagonal lattice 

 as /^-AgJ. In general hexagonal pyramids are found 

 (fig. 3). It is not possible to detect any characteristic 

 surface structure or other sub-structures in the 

 crystals — a point mainly of importance with the 

 silver bromide and chloride crystals. It is true that 

 steps are found on individual crystals but in 

 the main the crystals possess a well-formed, smooth 

 surface and any large clefts or fissures are not to be 

 seen. If the angle of evaporation is very flat it is 

 possible for so-called "dune effects" to occur when 

 the object is not rotated. A tine structure can be 

 seen, which, however, only occurs on the outside 

 of the replica, as could be established from evalua- 

 tion of the stereo micrographs. This is, therefore, 

 only an apparent structure. 



In order to determine varying quality of crystal 

 structure within a crystal, different kinds of etching 

 were carried out, of which only one with sulphite 

 solution will be mentioned. In this case a terrace-like 

 structure of the pyramid-shaped crystal (fig. 4) is 

 revealed. 



Thus, by means of etching, it is possible to estab- 

 lish those zones of a crystal which tend to be of 

 better solubility due to their disturbed lattice struc- 

 ture, although no sub-crystallites could be found. 

 Due to the high resolving power obtained by the 

 carbon replica process (approx. 50 A) it must be 

 concluded that the sub-structure discovered by Hed- 



