Nature of the Positron State in CdSe Quantum Dots.

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.

A Closer Look into the Traditional Purification Process of CdSe Semiconductor Quantum Dots.

This paper describes how the postprocessing procedure for wurtzite CdSe quantum dots (QDs) 4.8 and 6.7 nm in diameter is affected by both the choice of nonsolvent and the number of processing steps. Using a host of analytical techniques (ultraviolet-visible, photoluminescence, nuclear magnetic, X-ray photoelectron, and infrared spectroscopy, as well as thermogravimetric analysis), we find that control over the ligand type and surface density can be achieved simply by the number of washing steps used during the postprocessing procedure. Using multiple washing steps we can achieve colloidally stable solutions of QDs with organic mass fractions as low as 13% by mass. For CdSe QDs passivated with trioctylphosphine oxide (TOPO) and stearic acid (SA), essentially no TOPO is bound to the particle surface after three or four washing steps, with a plateau in the amount of SA being removed. The results can be explained using the L- and X-type ligand classification system for QDs, with L-type ligands (TOPO) removed in the early processing steps but the removal of X-type (SA) ligand stalling at a large number of washing steps due to charging of the QDs. Importantly, very little change is observed in the photoluminescence (PL) properties, suggesting that the choice of nonsolvent during postprocessing will allow the production of QD materials with very low organic content by mass but with good PL quantum yields.