Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Abstract

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, 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

Magnetic and optical properties of Mn-doped ZnO vertically aligned nanorods synthesized by hydrothermal technique

“…[2][3][4][5][6][7] The Tam et al explained important advanced characterization used for defects in ZnO nanorods based on positron annihilation spectroscopy (PAS). 8 By using slow positron beams the PAS can also uniquely probe the outermost surfaces of nano-structured semiconductor materials as recently demonstrated by Chai et al 9 In addition, magnetic properties can be modified by introducing 3d-transition metal impurities into the ZnO lattice. A few studies have suggested that transition metal doped ZnO nanostructures possess interesting magnetic properties along with optical properties.…”

Ligand-surface interactions and surface oxidation of colloidal PbSe quantum dots revealed by thin-film positron annihilation methods

“…Previous positron lifetime experiments have shown that more than 90% of positrons implanted in a layer of PbSe QDs capped with Oleic Acid (OA) ligands trap at the surface of PbSe QDs, 16 where the positron is bound in a potential well behaving like a image potential at large distances (Figure 1). 20,21 Positrons implanted in the PbSe QD layer primarily lose their high initial kinetic energy in the PbSe cores of the QDs where they thermalize.…”

“…caused mostly by quantum confinement of the Se(4p) valence electron orbitals, 16 positron confinement 28,29 and other small contributions of atoms at the surface connected with the ligands. 16 The confinement of valence electrons and positrons for PbSe QDs capped with OA and OLA ligands leads to a small difference in the 1 a.u. peak area, 16,30 since the QDs have comparable sizes and the ligands have the same large chain length.…”

“…12,13 Clearly, the development of methods to investigate and monitor surfaces of PbSe QDs embedded in a sub-surface layer in such devices is of crucial importance. In this regard, positron annihilation spectroscopy (PAS) offers unique advantages over XAS, OAS and XPS, since it combines a high sensitivity to selectively probe surfaces of nanoparticles [14][15][16] with an established capacity for depth-profiling of films in the range of ~10 nm to a few m. [17][18][19] These unique merits are highly important in studies of photovoltaic devices, where charge carrier separation requires the formation of a p-n junction, 8 and PAS can probe the involved light absorbing and charge separation layers independently.…”

“…The PbSe QD films were examined by 2D-ACAR using the thin-film POSH-ACAR setup at the Reactor Institute Delft. [14][15][16] For comparison, a PbSe single crystal was studied by 2D-ACAR using a 22 Na positron source. The 2D-ACAR distributions were obtained with positron implantation energies of 1 keV for PbSe QDs with EDA and 3.4 keV for PbSe QDs with OLA and OA ligands.…”