· Silver halide crystals
· Controlled size
· Controlled morphology
· Surface deposition
· Core-shell crystals
Silver halide crystals
Conventional photographic materials are based on light-sensitive silver halide crystals. The silver halides which are used (silver chloride, bromide, iodide, and mixtures of these) have very low solubilities, so crystals can be produced simply by mixing solutions of silver nitrate and a suitable halide, such as sodium bromide:
AgNO3 (aq) + NaBr (aq) → AgBr (s) + NaNO3 (aq)
For photographic purposes this reaction takes place in the presence of a protective colloid, usually gelatin, which keeps the crystals in suspension and controls crystal growth and aggregation. (Traditionally, within the photographic industry, such a dispersion of silver halide crystals is known as an emulsion.)
The size, shape, structure and composition of the crystals formed all depend on the precise conditions under which they are made, and long experience in this field has taught HARMAN technology Ltd how to control them.
The traditional way of making a photographic emulsion is to add silver nitrate solution to a solution containing gelatin and a halide, which gives a wide range of crystal sizes. The addition of ammonia, either at the start or after the initial precipitation, makes the larger crystals grow at the expense of the smaller ones, by ‘Ostwald ripening’. At the end, there is still quite a broad range of crystal sizes.
It is possible to make emulsions with a narrow range of crystal sizes – ‘monosized emulsions’ - by adding silver and halide solutions at precisely controlled rates to a gelatin solution. In practice, a “seed” emulsion containing very small crystals is usually made first, and portions of this are used for subsequent growth. The addition rates are adjusted to match the changing rate of deposition of silver halide on the surfaces of the growing crystals. In this way, the emulsion can be grown to a predetermined average crystal size. The picture shows crystals from a relatively ‘monosized’ octahedral emulsion, and a conventional ‘polydisperse’ emulsion.
Although the silver halide crystals in ordinary photographic materials are around one micron across, much smaller crystals can be made where necessary.
Many people have heard of “microdots” – photographic transparencies the size of a full stop, used to transmit documents secretly. These are not a myth; they were actually used by spies during the 20th century. In order to put a readable image of a large document into such a small area, special photographic emulsions were needed, with crystal sizes around 50 nanometres.
The area in which nanosized emulsions are required nowadays is holography. Modern holographic materials contain crystals around 30 nanometres or less in diameter.
To make such crystals, it is necessary to use conditions which allow crystal formation (nucleation) but effectively prevent crystal growth. These may include low concentrations of reactants, low temperatures, and very precise control throughout.
The silver halides used in photography readily form mixed crystals, whose shapes are determined by their growth conditions. Silver bromide crystals formed at low pAg are cubic; when formed at high pAg they are octahedral. (pAg has the same significance in relation to silver ions as pH does in relation to hydrogen ions; so pAg = -log[Ag+].)
If silver and bromide solutions are added under appropriate conditions to a silver iodide “seed” emulsion, silver bromide is deposited on certain faces of each silver iodide crystal. As the growth continues, the silver iodide dissolves and is reincorporated along with the bromide, in “twinned” silver iodobromide crystals. Continued growth at high pAg gives flat hexagonal plates.
In addition to controlling the sizes and shapes of crystals, in some cases it is also possible to deposit additional silver salts selectively in particular positions.
This picture shows tabular crystals with cubes of silver chlorobromide attached to their corners. This was achieved by carrying out the final stages of silver and chloride addition after adding a dye which adsorbed onto the surface of the crystals.
This picture shows tabular crystals with specks of a silver salt scattered over their surfaces. This was achieved by carrying out the final stages of silver and bromide addition after adding sodium thiocyanate to the mix.
During crystal growth, the halide solution can be changed. If the addition rates are adjusted for both the increasing crystal size and the changed identity of the halide, it is possible to form crystals where the inner core has a different composition from the outer shell. Similarly, if the halide is changed twice or more, a layered structure can be created.
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