Band Edge Recombination in CdSe, CdS and CdS_xSe_1−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 CdS x Se 1-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 (τ 1 ) and long-lived (τ 2 ) 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 τ 1 . This behavior is explained by the decrease in accessible trap sites through a reduction in surface-to-volume ratio. Similarly, τ 2 is found to increase with increasing nanocrystal size. However, with increasing sulfur concentration in the alloy nanocrystals, both a reduction in the magnitude of τ 1 and a reversal in the trend for τ 2 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 CdS x Se 1-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.

Design of cadmium-free colloidal II–VI semiconductor quantum dots exhibiting RGB emission

“…37,38 The calculated curves (solid lines) agree well with the experimentally observed data (dots). Differences between the observed and calculated data were less than, or comparable to, ∼100 meV.…”

“…To obtain the confining potentials, V 0e and V 0h , the value of ∆E (LUMO-HOMO) of the capping surfactant is needed. In previous reports of the experimentally determined optical gaps of Cd(Se 1 x S x ) and Cd(Te 1 x Se x ) alloyed QDs, oleic acid 37 and hexadecylamine 39 were used as the capping surfactants, respectively; therefore, the experimentally determined ∆E (LUMO-HOMO) of oleic acid, 4.35 eV, and hexadecylamine, 4.13 eV, were used in the calculations for Cd(Se 1 x S x ) and Cd(Te 1 x Se x ) alloyed QDs, respectively. For the Zn(Te 1 x Se x ) and Zn(Te 1 x S x ) alloyed QDs, we assumed that their surfaces were capped with oleic acid, and the∆E (LUMO-HOMO) value of oleic acid was 4.35 eV in the calculations for both Zn(Te 1 x Se x ) and Zn(Te 1 x S x ) alloyed QDs.…”

“…The experimentally observed energy difference between the absorption corresponding to the optical gap and emission, i.e., Stokes shift, for II-VI semiconductor QDs usually ranges between 50 and 150 meV. 37 In Figs. 4 (a) and (b), we indicated the size and composition necessary to realize RGB emission as a white dotted line, assuming a 100-meV Stokes shift.…”

“…Figure 2(a) shows the size dependent optical gap calculated for Cd(Te 1 x Se x ) alloys (solid line), which exhibits large band gap bowing in the bulk system, together with experimental data (dots) extracted from previous reports. 37,39,40 Although the observed gap for the 2.7-nm alloyed QDs with x=0.34 was approximately 300 meV smaller than the calculated value, the other observations agreed well with the calculated curve within ∼100 meV. Figure 2 (b) shows a comparison of the calculated and observed optical gaps of 5.0-and 6.5-nm diameter of Cd(Te 1 x Se x ) alloyed QDs (green and red lines) as a function of the composition x together with the energy band gap of bulk Cd(Te 1 x Se x ) alloys (black line).…”

“…Thus, we first evaluated the applicability of the finite-depth-well EMA calculation to alloyed QDs based on a comparison of the calculated optical gap with that experimentally observed for Cd(Se 1 x S x ) 37,38 and Cd(Te 1 x Se x ) 39,40 alloy systems; these two systems are chosen as typical systems that exhibit small and large band gap bowing, respectively. The size and composition dependent optical gap of Zn(Te 1 x Se x ) and Zn(Te 1 x S x ) alloy systems were then calculated.…”