Nature of the Positron State in CdSe Quantum Dots

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the
electronic structure and surface composition of ligand-capped semiconductor quantum dots (QDs)
embedded in thin films. The nature of the associated positron state, however, whether the positron is
confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-
annihilation lifetime spectroscopy studies of CdSe QDs reveal the presence of a strong lifetime
component in the narrow range of 358–371 ps, indicating abundant trapping and annihilation of
positrons at the surfaces of the QDs. Furthermore, our ab initio calculations of the positron wave
function and lifetime employing a recent formulation of the weighted density approximation
demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental
values. Our study thus resolves the long-standing question regarding the nature of the positron state
in semiconductor QDs and opens the way to extract quantitative information on surface composition
and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-
annihilation techniques.