Surfaces of nanomaterials for sustainable energy applications: thin-film 2D-ACAR and PALS studies*

Positron (e$^{+}$) annihilation spectroscopy is one of only a few techniques to probe the surfaces of nanoparticles. We investigated thin films of PbSe colloidal semiconductor nanocrystals (NCs) in the range 2-10 nm as prospective highly efficient absorbers for solar cells. We compare and contrast our findings with previous studies on CdSe NCs. Evidence obtained from our e$^{+}$ lifetime spectroscopy study using the PLEPS spectrometer shows that 90-95\% of the implanted positrons are effectively trapped and confined at the surfaces of these NCs. The remaining 5-10\% of the e$^{+}$ annihilate in the relatively large oleic acid ligands, in fair agreement with the estimated positron stopping power of the PbSe nanoparticle ``core'' relative to the ligand ``shell.'' 2D-ACAR measurements on the same set of films using the low-energy e$^{+}$ beam POSH showed that the e$^{+}$ wavefunction at the surfaces of the PbSe NCs is more localized than for the case of CdSe NCs. Comparison with calculated e$^{+}$ - e$^{-}$ momentum densities indicates a Pb deficiency at the surfaces of the PbSe NCs, which correlates with e$^{+}$ lifetime and the NCs morphology.