  Positron annihilation lifetime spectroscopy (PALS) and positron-electron
momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs),
supported by transmission electron microscopy (TEM), show that positrons are
strongly trapped at NC surfaces, where they provide insight into the surface
composition and electronic structure of PbSe NCs. Our analysis indicates
abundant annihilation of positrons with Se electrons at the NC surfaces and
with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces,
which demonstrates that positrons can be used as a sensitive probe to
investigate the surface physics and chemistry of nanocrystals inside
multilayers. Ab-initio electronic structure calculations provide detailed
insight in the valence and semi-core electron contributions to the
positron-electron momentum density of PbSe. Both lifetime and PEMD are found to
correlate with changes in the particle morphology characteristic of partial
ligand removal.
