White-light emission from ultrasmall CdSe nanocrystals offers an alternative approach to the realization of solid-state lighting as an appealing technology for consumers. Unfortunately, their extremely small size limits the feasibility of traditional methods for nanocrystal characterization. This paper reports the first images of their structure, which were obtained using aberration-corrected atomic number contrast scanning transmission electron microscopy (Z-STEM). With subangstrom resolution, Z-STEM is one of the few available methods that can be used to directly image the nanocrystal's structure. The initial images suggest that they are crystalline and approximately four lattice planes in diameter. In addition to the structure, for the first time, the exciton dynamics were measured at different wavelengths of the white-light spectrum using ultrafast fluorescence upconversion spectroscopy. The data suggest that a myriad of trap states are responsible for the broad-spectrum emission. It is hoped that the information presented here will provide a foundation for the future development and improvement of white-light-emitting nanocrystals.

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.

The exciton dynamics of CdSe nanocrystals are intimately linked to the surface morphology. Photo-oxidation of the selenium surfaces of the nanocrystal leads to an increase in radiative decay efficiency from both the band edge and deep trap emission states. The addition of the primary amine hexadecylamine curtails nonradiative excitonic decay attributed to the dangling surface selenium orbitals by passivation of those trap sites by the methylene protons on the amine, leading to enhanced band edge emission and the absence of deep trap emission. Furthermore, CdSeZnSe core/shell nanocrystals are not immune from contributions from surface states because of the alignment of the band structures of the core and shell materials.