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

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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.

A novel method for patterning optically active colloidal PbSe nanocrystals on Si surfaces is reported. Oleate-capped PbSe nanocrystals were found to adhere preferentially to H-terminated Si surfaces over oxide and alkyl-terminated Si surfaces. Scanning probe lithography was used to oxidize locally a dodecyl monolayer on the Si surface of a silicon-on-insulator wafer prepatterned with photonic crystal microcavities. Aqueous HF was then used to remove the oxide and expose H-terminated Si areas, yielding patterned PbSe nanocrystals on the Si surface after exposure to a nanocrystal solution. This patterning technique allows for the selective deposition of PbSe nanocrystals at the main antinode of the silicon-based microcavities. More than a 10-fold photoluminescence enhancement due to the cavity-nanocrystal coupling was observed.

Lead-based quantum dots (QDs) can be tuned to emit in the transparent region of the biological tissue (700 to 1100 nm) which make them a potential candidate for optical bioimaging. However, to employ these QDs as biolabels they have to retain their luminescence and maintain their colloidal stability in water, physiological saline buffers, different pH values, and biological media. To achieve this, four different surface modification strategies were tried: (1) silica coating; (2) ligand exchange with polyvinylpyrrolidone; (3) polyethyleneglycol-oleate (PEG-oleate) intercalation into the oleate ligands on the surface of the QDs; and (4) intercalation of poly(maleicanhydride-alt-1-octadecene) (PMAO) into the oleate ligands on the surface of the QDs and further cross-linking of the PMAO. The first two methods exhibited excellent dispersion stability in water, but did not retain their photoluminescence. On the other hand, the intercalation strategy with PEG-oleate helped the QDs retain their luminescence but with poor colloidal stability in water. The fourth and final strategy involving intercalation and cross-linking of the amphiphilic polymer PMAO provided the QDs with colloidal stability in water but also resulted in the QDs retaining their luminescence as well. This process involved two steps; (1) the intercalation between octadecene chains of PMAO with the oleates on the surface of the QDs with some of the anhydride rings opened with PEG-amine; (2) the anhydride rings were cross-linked with bis(hexamethylene)triamine (BHMT) to avoid detachment of the polymer from the surface of QDs because of the polymer’s dynamic nature in solvents. The presence of PEG molecules potentially improves the biocompatibility of the QDs and the presence of carboxylic acids after reaction with BHMT makes them suitable for further surface functionalization with antibodies, proteins, and so forth. The surface-modified QDs have excellent dispersibility in water, saline buffers, and in various pH conditions for more than 7 months and more than 20 days in serum-supplemented growth media. In addition to the colloidal stability, the QDs retained their photoluminescence even after 7 months in the aforementioned aqueous media. The intercalation and cross-linking process have also made the QDs resistant to oxidation when exposed to ambient atmosphere and aqueous media.

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