We report on the preparation and structural characterization of CdSe nanocrystals, which are covered by a multishell structure from CdS and ZnS. By using the newly developed successive ion layer adhesion and reaction (SILAR) technique, we could gradually change the shell composition from CdS to ZnS in the radial direction. Because of the stepwise adjustment of the lattice parameters in the radial direction, the resulting nanocrystals show a high crystallinity and are almost perfectly spherical, as was investigated by X-ray diffraction and electron microscopy. Also, due to the radial increase of the respective valence- and conduction-band offsets, the nanocrystals are well electronically passivated. This leads to a high fluorescence quantum yield of 70-85% for the amine terminated multishell particles in organic solvents and a quantum yield of up to 50% for mercapto propionic acid-covered particles in water. Finally, we present experimental results that substantiate the superior photochemical and colloidal stability of the multishell particles.

A method for the synthesis of nearly monodisperse CuInS(2) semiconductor nanocrystals (from <2 to 20 nm) was developed using generic and air-stable chemicals in a non-coordinating solvent. Such "greener" approaches also allowed the reaction temperatures to be below 200 degrees C. By introducing reactivity-controlling ligands for Cu, namely thiols, control of the Cu:In stoichiometric ratio in the nanocrystals was achieved. Amines were identified as catalytic reagents for the rapid oxidation of the CuInS(2) nanocrystals, which could be prevented by the formation of CuInS(2)/ZnS core/shell nanocrystals by a one-pot approach. CuInS(2)/ZnS core/shell nanocrystals also showed greatly improved optical properties, with photoluminescence quantum yield up to about 30% and an emission peak position tunable from 500 to 950 nm. The versatility of the synthetic strategy was demonstrated by extending it to the synthesis of AgInS(2) nanocrystals by simply replacing the copper salt by a silver salt.

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Efficient Cu-doped InP quantum dots (Cu:InP d-dots) emitters were successfully synthesized by epitaxial growth of a ZnSe diffusion barrier for the dopants. The Cu dopant emission of the Cu:InP/ZnSe core/shell d-dots covered the important red and near-infrared (NIR) window for biomedical applicaitons, from 630 to 1100 nm, by varying the size of the InP host nanocrystals. These new d-dots emitters not only compensate for the emission wavelength of the existing noncadmium d-dots emitters, Cu- and Mn-doped ZnSe d-dots (450-610 nm), but also offer a complete series of efficient nanocrystal emitters based on InP nanocrystals. The one-pot synthetic scheme for the formation of Cu:InP/ZnSe core/shell d-dots was successfully established by systematically studying the doping process, the dopant concentration-dependent photophysical properties, and the dopant diffusion during shell epitaxy, etc. Complete elimination of InP bandgap emission and efficient pure dopant emission (with photoluminescence quantum yield as high as between 35-40%) of the core/shell d-dots were achieved by optimizing the final doping level and the diffusion barrier thickness.

Amine ligands were identified to bond on the surface of CdSe nanocrystals in a dynamic fashion under elevated temperatures in the reproducible growth domain of the specific designed growth reactions. The surface ligand dynamics was found to strongly depend on the growth temperature, the ligand concentration, and the ligand chain length. The strong chain-length dependence was originated from the interligand interactions in the ligand monolayer of a nanocrystal, provided fatty amines being weak ligands for CdSe nanocrystals. When the growth reaction was above the boiling point of an amine ligand, the surface ligand dynamics was violent, a quasi-gas-phase state, indicated by strong temperature-dependent and fast growth rates of the nanocrystals. Approximately below its boiling point, a significantly weak temperature dependence of the growth rate of the nanocrystals associated with the quasi-liquid state of the surface ligands was observed. A direct result of studying the surface ligand dynamics of this well-established nanocrystal system was the formation of high-quality CdSe nanocrystals under much reduced temperature, 150 degrees C, in comparison to the standard 250-350 degrees C temperature range. This was achieved by using fatty amines with a short hydrocarbon chain at a low ligand concentration in the solution. Preliminary results indicate that a similar temperature (160 degrees C) also worked for the growth of InP nanocrystals.

Controlled doping is a critical step toward various unique nanostructures. This report shall demonstrate that doping chemistry of colloidal nanocrystals is much more complex than what has been proposed in the existing experimental and theoretical reports. Four individual processes, namely "surface adsorption", "lattice incorporation", "lattice diffusion", and "lattice ejection", will be identified, each of which possesses its own critical temperature. A given type of host nanocrystals can be switched from being impossible to dope to becoming successfully doped. The key is to program the reaction temperature to accommodate all elementary processes.

We reported a simple synthetic method for Cu–Zn–In–S nanocrystals (NCs) using commercially available, low toxic precursors. The various sizes and composition of Cu–Zn–In–S NCs were well controlled by simply changing reaction temperature and the ratio of the precursors, respectively. Accordingly, as produced these particles exhibited tunable photoluminescence (PL) spectra with emission color tuned from visible to the NIR region and had a relatively high QY of over 70% without coating any wide band gap shell materials. Further experimental results revealed that Cu–Zn–In–S NCs showed some advantages in contrast to typical plain II–VI and III–V semiconductor NCs. Preliminary tentative application on the preparation of QD-LEDs verified the feasibility in the lighting field.

The initial formation of semiconductor nanocrystals/nanoclusters, that is, nucleation in the classic literature, was examined both theoretically and experimentally. An experimental method based on determining the initial reaction rate for the formation of nanocrystals/nanoclusters with fixed size and size distribution was developed using InP and CdS nanocrystals/nanoclusters systems, especially the InP one. This experimental strategy relies on the size-dependent absorption spectra of these semiconductor nanoparticles as quantitative probes. The experimental results along with theoretical analysis indicate that the classic nucleation model was unlikely relevant for such crystallization systems, whose bulk crystal solubility in a solution is extremely low. Instead, the formation process was found to match a reaction-controlled kinetics model. The results further imply that understanding of crystallization and development of controlled synthesis of high quality colloidal nanocrystals are both closely related to identifying the molecular mechanism and chemical kinetics.