Band Edge Recombination in CdSe, CdS and CdSxSe1−x Alloy Nanocrystals Observed by Ultrafast Fluorescence Upconversion: The Effect of Surface Trap States

Abstract

The effect of surface trap states on band edge recombination in CdSe, CdS and CdSxSe1−x alloy nanocrystals has been determined using fluorescence upconversion spectroscopy. These measurements reveal that there is both a size and composition dependence on the short-lived (τ1) and long-lived (τ2) components of fluorescence lifetime at the band edge. An increase in nanocrystal diameter, ranging from 23 to 60 Å, is accompanied by an increase in τ1. This behavior is explained by the decrease in accessible trap sites through a reduction in surface-to-volume ratio. Similarly, τ2 is found to increase with increasing nanocrystal size. However, with increasing sulfur concentration in the alloy nanocrystals, both a reduction in the magnitude of τ1 and a reversal in the trend for τ2 are observed. These changes in lifetimes associated with the addition of sulfur are explained by increased trapping on the nanocrystal surface. These results indicate that carrier dynamics may be controlled not only through size, but also through composition of the nanocrystals. Compositional variation has been shown not only to affect carrier dynamics, but also to affect the optical properties of nanocrystals. An increase in the Stokes shift is observed for CdSxSe1−x alloy nanocrystals as compared to CdSe and CdS nanocrystals. This indicates that the Stokes shift is highly influenced by the nonlinear effects of alloying.