Photography: Making every photon count
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More than 150 years after the invention of photography, the underlying solid-state photochemistry is still being investigated and improved. One aspect of continual importance is the efficiency of the image-recording stage, which strongly affects the sensitivity of the photographic emulsion. Photons falling on a silver halide microcrystal in the emulsion cause a halide ion (such as chloride or bromide) to lose an electron, which is then captured by a silver ion, converting it into a neutral silver atom. The build-up of silver atoms gives rise to silver clusters: the 'latent image', which can be developed after several more steps into a print1. In reality, the halide ions generate pairs of electrons and holes (the positively charged counterparts of electrons), and it is generally accepted that recombination of photogenerated electrons and holes is a major loss process2, 3. On page 865 of this issue, Belloni et al.4 present an ingenious way of overcoming this recombination process by doping the silver halide with formate ions, which leads to an increase in the number of silver atoms generated per photon absorbed.