ELECTRON MICROSCOPY 



Fig. 22. Carbon replica of a silver bromide par- 

 ticle etched with potassium cyanide. (By courtesy 

 of Dr. J. F. Hamilton) 



Stability of Colloidal Particles in the 

 Electron Beam. One of the difficulties en- 

 countered in electron microscopy is that the 

 amount of energy which is transferred from 

 the beam to electrons in the specimen can 

 often be in excess of the chemical binding 

 energies. Hence, precautions must be taken 

 against possible decomposition of the speci- 

 men in the beam. Stability depends on 

 whether, after excitation of electronic energy 

 levels, the substance reverts to its original 

 structure or to a new configuration. Thermal 

 stability is also important, since consider- 

 able temperature changes of the specimen 

 can occur; temperatures of the order of 

 100°C or more can easily be obtained. In 

 many cases decomposition of the specimen 

 in the beam is rapid, making direct examina- 

 tion impossible; For example, both silver 

 chloride and silver bromide rapidly decom- 

 pose to yield a mass of metallic silver (47, 

 48); thus replica techniques are normally 

 used for examination of these crystals (49). 

 The interaction of the specimen with the 

 beam, however, may frequently be helpful 

 in studying the decomposition of crystals. 

 It was shown by Sawkill (50) that single 

 crystals of silver azide could be decomposed 

 in the beam of the electron microscope and 



that the decomposition could be followed in 

 (l(;1ail by examining micro-diffraction pat- 

 terns and electron micrographs taken at 

 various stages. In a similar type of study 

 Goodman (51) has examined the dehydration 

 of single crystals of magnesium hydroxide to 

 magnesium oxide imder the influence of the 

 electron beam. Electron microscope observa- 

 tions have also proved useful in studies on 

 the motion of electrons and holes in photo- 

 graphic emulsion grains (52) and in studies 

 on latent image formation (53). 



A useful technique for examining dynamic 

 changes in crystals of colloidal dimensions 

 is to employ a cinecamera to record the 

 image obtained on the fluorescent screen of 

 the microscope. The first observations of this 

 type were carried out by von Ardenne (54) 

 using a camera fitted into the microscope, 

 and later by Preuss and Watson (50) using 

 an external cinecamera. 



In recent studies on the movement of dis- 

 locations in thin metal films (38, 39) and 

 dynamic changes in silver iodide particles 

 (40, 41) cine recordings were made of the 

 phenomena taking place. In this work the 

 fluorescent viewing screen was tilted at a 

 suitable angle and recordings were made by 

 photographing directly through one of the 

 observation windows at microscope magnifi- 

 cations of 40,000 X or 80,000 X. A Kodak 

 cine special camera was used and modified to 

 take a 1 in. f/0.95 Angenieux lens at a work- 

 ing distance of 15-20 cm; a speed of 16 

 frames per second was generally used. 



The studies on silver iodide were made 

 directly on colloidal particles. Two types of 

 effects were noticed under the influence of 

 the beam — mobile changes of contrast within 

 the particles and filament growth from the 

 particles. The first effect was obtained with 

 hexagonal plates of silver iodide. Changes 

 of contrast were observed under the in- 

 fluence of the electron beam which were 

 highly mobile and migrated within the par- 

 ticle boundaries at rates dependent on the 

 beam intensity. The nature of these changes 



142 



