Nature of the Positron State in CdSe Quantum Dots

Abstract

Previous studies have shown that positron-annihilation spectroscopy is a highly sensitive probe of the electronic structure and surface composition of ligand-capped semiconductor quantum dots (QDs) embedded in thin films. The nature of the associated positron state, however, whether the positron is confined inside the QDs or localized at their surfaces, has so far remained unresolved. Our positron-annihilation lifetime spectroscopy studies of CdSe QDs reveal the presence of a strong lifetime component in the narrow range of 358-371 ps, indicating abundant trapping and annihilation of positrons at the surfaces of the QDs. Furthermore, our ab initio calculations of the positron wave function and lifetime employing a recent formulation of the weighted density approximation demonstrate the presence of a positron surface state and predict positron lifetimes close to experimental values. Our study thus resolves the long-standing question regarding the nature of the positron state in semiconductor QDs and opens the way to extract quantitative information on surface composition and ligand-surface interactions of colloidal semiconductor QDs through highly sensitive positron-annihilation techniques.

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

“…This three-dimensional arrangement of atoms in PbTe NPs revealed by HE-XRD will be difficult to access via other probes [42,62]. Notably, positron-annihilation spectroscopy (PAS) [13] has recently provided information concerning the surface of semiconductor quantum dots [63]. The complementary combination of HE-XRD and PAS techniques can thus provide a more complete characterization of the microstructure of PbTe NPs [64], which show much promise for optoelectronics applications.…”

Photochromic YOxHy Thin Films Examined by <i>in situ</i> Positron Annihilation Spectroscopy

“…PAS detects electronic structure properties associated with the annihilation electrons, and is highly sensitive to the presence of neutral and negatively charged vacancies. Recently, we investigated various types of functional layers for thin film solar cells, including CdSe quantum dot (QD) layers for QD-based solar cells [1], demonstrating that the high sensitivity of positrons to detect surface properties of the QDs stems from the presence of a positron surface state. The degradation of ZnO/Cu(In,Ga)Se 2 [2] and of methyl ammonium lead iodide (MAPbI 3 ) perovskite solar cells [3] was linked to the in-diffusion of water molecules and to the interplay of the subsequently occurring chemical reactions and vacancies in the ZnO and MAPbI 3 layers.…”

Positron Annihilation Spectroscopy as a Diagnostic Tool for the Study of LiCoO2 Cathode of Lithium-Ion Batteries

“…In detail, for a cathode corresponding to a battery state of charge of 50% (x = 0.5), the measured first component of the positron lifetime of 180 ps is in excellent agreement with the calculated value. This finding implies that the bulklike state in the grain [49] becomes very similar to a true positron bulk state. However, in the case of the discharged battery (x = 1.0), the positron spillover increases and the lifetime for the bulklike state is about 30 ps higher than the calculated bulk value.…”