Surface Composition and Electronic Structure of Colloidal CdSe and PbSe Nanocrystals Probed by Positron Annihilation

Examining surfaces at the nano-scale is one of the most challenging problems of modern materials science [1]. We have devised an efficient way to probe the surface of CdSe [2,3] and PbSe [4] nanocrystals (NCs) using positrons. A slow positron can be trapped at a surface [5,6] and then annihilates. One can analyze the distribution of photons that are caused by this annihilation to gain unique surface information. Our analysis in PbSe NCs indicates abundant annihilation of positrons with Se electrons and with O electrons of the oleic ligands bound to Pb ad-atoms, which demonstrates that positrons can be used as a sensitive probe to investigate the surface chemistry of NCs inside multilayers. The positron-electron momentum distribution (PEMD) of PbSe nanocrystals is broadened relative to that of a bulk PbSe single crystal. Ab-initio electronic structure calculations provide detailed insight in the Se(4p), Pb(4p), Se(4s), Pb(6s), Pb(5d) and Se(3d) valence electron contributions to the PEMD of PbSe. The momentum density broadening results primarily from a the strong localization of the positron wave function at the surfaces of the ionic rock salt PbSe NCs in sharp contrast to previous findings for wurtzite CdSe NCs where quantum confinement of the valence electrons is the dominant factor. Both positron lifetime and PEMD are found to correlate with particle size and morphology.