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

Abstract

Positron Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) measurements reveal modifications of the electronic structure and composition at the surfaces of PbSe quantum dots (QDs), deposited as thin films, produced by various ligands containing either oxygen or nitrogen atoms. In particular, the 2D-ACAR measurements on thin films of colloidal PbSe QDs capped with oleic acid ligands yield an increased intensity in the electron momentum density (EMD) at high momenta compared to PbSe quantum dots capped with oleylamine. Moreover, the EMD of PbSe QDs is strongly affected by the small ethylediamine ligands, since these molecules lead to small distances between QDs and favor neck formation between near neighbor QDs, inducing electronic coupling between neighboring QDs. The high sensitivity to the presence of oxygen atoms at the surface can be also exploited to monitor the surface oxidation of PbSe QDs upon exposure to air. Our study clearly demonstrates that positron annihilation spectroscopy applied to thin films can probe surface transformations of colloidal semiconductor QDs embedded in functional layers.

Structural properties of PbTe quantum dots revealed by high-energy x-ray diffraction

“…Thermoelectric properties of PbTe are reviewed by Yana et al [29], and the material has been used for a variety of device applications since the 1960s [30]. PbTe quantum dots have a small carrier-cooling rate, which makes them an interesting material for solar cells [31,32]. PbTe crystallizes in the rocksalt structure (Fm-3m group) under normal temperature and pressure with a lattice parameter of 6.462(1) Å [34].…”

Application of the weighted-density approximation to the accurate description of electron-positron correlation effects in materials

“…In order to restore it, one introduces an effective (weighted) electron density n * e (r p ) at each point, i.e. one determines n * e (r p ) such that {γ(n * e (r p )) − 1} dr e n e (r e )e −a(n * e (rp))|re−rp| = 1, (17) where the coupling constant dependence drops out due to the linear scaling of g h with λ, and as a result the adiabatic integration in the expression for the correlation potential (4) can be performed analytically to obtain:…”

“…3(b) and (c). For this simple model, one can easily solve (17) to find n * ≈ 1.59 n e (0), where we used the enhancement factor parametrized by Boronski and Nieminen 19 , see the blue curves in Figs. 3(b)-(c).…”

“…These advances have been used to demonstrate multi-electron emission in Auger processes 12 to determine the binding energy of positrons to the surface of Bi 2 Te 2 Se 13 , and directly measure Auger emission from holes created in the valence band of graphene 14 . Finally, positrons have been shown to act as self-seeking probes for the surface of colloidal quantum dots 15 to enable a detailed characterization of their surface properties 16,17 .…”

“…In particular, while it is straightforward to construct the corrugated image potential for perfect surfaces, this quickly becomes unfeasible for more complex surfaces. For example, imagine a molecule adsorbed on a surface, a problem relevant to the study of positron states in colloidal quantum dots 17 . In this case, one can expect that, depending on the position of the positron, the induced image charge will be distributed over the molecule and/or the surface.…”