Quantum antidots are subnanometer scale vacancy clusters, the localized electronic structure of which can significantly alter the properties of a nanomaterial. We use positron spectroscopy to study vacancy clusters generated during the formation of gold nanoparticles via ion implantation in an MgO matrix. We observed that quantum antidots are associated with the nanoparticle surfaces after annealing in an O 2 atmosphere, but not after annealing in a H 2 atmosphere. In the former case, the

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... ce of quantum antidots bound to the gold nanoparticles correlates with the redshift of the gold surface plasmon resonance, thus allowing an explanation for the redshift based on the transfer of electrons away from the metal particles.