Hexagonal-phase NaYF(4)/NaGdF(4) core/shell nanocrystals were synthesized and investigated by X-ray photoelectron spectroscopy (XPS) using tunable synchrotron radiation. Based on the ratio of the Y(3+) 3d to Gd(3+) 4d core level intensities at varying photoelectron kinetic energies, we conclude that Gd(3+) resides predominantly at the surface of the nanocrystals, proving a core/shell structure. These nanocrystals show potential for use as contrast agents in magnetic resonance imaging (MRI) applications and optical imaging.

The structure and chemical composition of the shell distribution on NaYF 4 /NaGdF 4 core/shell nanocrystals have been investigated with scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS). The core and shell contrast in the high-angle annular dark-field (HAADF) images combined with the EELS and EDS signals indicate that Gd is indeed on the surface, but for many of the particles, the shell growth was anisotropic. SECTION Nanoparticles and Nanostructures

No abstract

Cation-exchange reactions have greatly expanded the types of nanoparticle compositions and structures that can be prepared. For instance, cation-exchange reactions can be utilized for preparation of core/shell quantum dots with improved (photo)stability and photoluminescence quantum yield. Understanding the structure of these nanomaterials is imperative for explaining their observed properties and for their further development. Core/shell quantum dots formed by cation exchange are particularly challenging to characterize because shell growth does not lead to an increase in overall particle size that can easily be characterized by standard transmission electron microscopy (TEM). Here, we report on the direct observation of the PbSe/CdSe core/shell structure (formed by cation exchange) using high-angle annular dark field (HAADF) imaging and energy-filtered TEM (EF-TEM). These results are further confirmed by energy-dependent X-ray photoemission spectroscopy (XPS) data that show increasing Pb/Cd signal with increasing X-ray photon energies. High-resolution XPS at varying X-ray photon energies was also used to examine chemical speciation and reveal greater complexity in both the PbSe core-only and the PbSe/CdSe core/shell structures than previously reported. Finally, small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) methods are combined to provide further inorganic and organic structural information. All experiments agree within error, and the results are summarized as final structural models for the core and core/shell particles.

PbSe/CdSe core/shell quantum dots (QDs) were prepared and investigated as thick films using temperature-dependent photoluminescence. In addition to increased photostability, the CdSe shell leads to a four-fold increase of the activation energy for nonradiative exciton decay for the core/shell QDs compared to that for the bare PbSe QDs. The onset for exponential decay of luminescence is ∼240 K in the core/shell samples. From further analysis of the temperature-dependent photoluminescence shift and emission line width, we find that the cation exchange reaction broadens the QD size distribution and increases the temperature-independent state broadening. However, the temperature-dependent contribution to the line shape of the core/shell QDs is similar to that in the cores.