Defects in anisotropic silver halide microcrystals studied with transmission electron microscopy

For more than 100 years silver halide microcrystals, where the halide is chloride or bromide, are used as the light-sensing element in photographic films. The combination of properties, like high sensitivity, high resolution and excellent image quality, leads to its dominant use in the imaging industry. Only with the development of the CCD camera in recent years a valuable alternative was found. Yet the image quality is still inferior to silver halide films. Therefore it is believed that the silver halides will still play an important role in the 21st century. Despite the progress over the last 100 years, there is still room for improvement and therefore the silver halides are still intensively investigated. One part of the research deals with the production of homogeneous populations of silver halide crystals with a specific morphology. This thesis reports the results of an investigation of the defect structure in populations of tabular silver halide crystals. The influence of these defects on the morphology of the crystals and the photographic properties is discussed. The technique used for this investigation is transmission electron microscopy (TEM), which is suitable to visualise, to localise and to characterise the defects completely. The samples have been investigated mainly in plan view, but also in edge view. The samples for the edge-view investigation have been prepared with ultramicrotomy. First the growth mechanisms of tabular AgCl and AgBr crystals with {100} surfaces are studied. It is shown that in both cases a limited number of mixed dislocations with an a/2<110> Burgers vector, lying in the (001) plane induce the tabular growth. The origin of these defects has been determined and the relation with the anisotropic growth is explained. Next, the growth of AgCl crystals with {111} faces is investigated. Using similar growth conditions as for AgBr, a large number of globular crystals are produced and the growth mechanism of these globular crystals is discussed. It is shown that the presence of twins on a {111} type of plane that is not parallel with the tabular plane is responsible for the thickness growth. Further, the introduction of a mixed bromo-iodide shell with high iodide concentration in an AgBr {111} tabular crystal population is investigated. Different incorporation methods are compared and their influence on the defect structure and the thickness of the crystals is discussed and related to the incorporation kinetics of the iodide. The crystals have also been studied in edge view. This showed the location of the parallel twins and the shell with high iodide concentration. The shell with low iodide concentration and the stacking faults, on the other hand, were not recognised in edge view. In the last chapter of this thesis, the formation of silver filaments during development of AgBr {111} tabular crystals is investigated. The evolution of the filament growth and the defect structure are investigated. It is shown that a large number of parallel twins and stacking faults formed on {111} type planes are present in the majority of the filaments. Further the influence of thickness of the tabular crystals on the filament morphology and on the colour of the developed film is discussed. It is argued that the compactness of the filaments is the major difference between the samples.