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.