Nature of the Positron State in CdSe Quantum Dots
Wenqin Shi (TU Delft - RST/Fundamental Aspects of Materials and Energy)
Vincent Callewaert (Universiteit Antwerpen)
Bernardo Barbiellini (Lappeenranta University of Technology, Northeastern University)
Rolando Saniz (Universiteit Antwerpen)
Maik Butterling (TU Delft - RST/Fundamental Aspects of Materials and Energy)
Werner Egger (University of the Federal Armed Forces Munich)
Marcel Dickmann (University of the Federal Armed Forces Munich)
Christoph Hugenschmidt (Technische Universität München)
Behtash Shakeri (University of Maine)
Ekkes Brück (TU Delft - RST/Fundamental Aspects of Materials and Energy)
Stephan W.H. Eijt (TU Delft - RST/Fundamental Aspects of Materials and Energy)
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Abstract
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.