Aqueous Synthesis of Nanocrystalline Semiconductors: Quantum Dots

Five distinguishable emission colors of CdS and (ZnxCd1-x)S QDs excited by a UV lamp. From left to right (red to blue), the emission maxima are located at 660, 620, 550, 515 and 475nm, respectively.

We have developed a direct synthetic method forproducing water soluble quantum dots (QDs) that are ready for bioconjugation. The method can produce aqueous QDs with wavelength varying from 400 nm to 700 nm. Highly luminescent metal sulfide (MS) QDs are produced in an aqueous synthesis route.  MS QDs are capped with thiol-containing charged molecules in a single step. The resultant MS QDs exhibit the distinctive excitonic photoluminescence desired of QDs and can be fabricated to avoid undesirable broadband emissions at higher wavelengths. This provides a significant improvement over the present complex and expensive commercial processes for the production of QDs. The aqueous QDs are stable in biological fluids over a long period of time. In addition, nontoxic ZnS QDs have been produced with good photoluminescence properties by refluxing the ZnS QD suspensions over a period of time.

More recently we have produced a variety of near-infrared (NIR) QDs that can fluoresce in the NIR range. Both the visible and NIR QDs are shown to be brighter and more stable for biomedical imaging applications than commercial fluorescent dyes.

Size Effect in Nanoparticles

The crystalline structure of BaTiO3 nanoparticles has been shown by many authors to depend on the size of the particles. However, the reason for the size dependence is not clear. We showed that the size effect of crystalline structure of BaTiO3 is related to the depolarization effect of the small particles. The large depolarization energy prohibits the small particles from becoming polarized (the tetragonal structure) and causes the particles to remain as unpolarized (the cubic structure). The depolarization effect is demonstrated by coating the BaTiO3 particles with Cu and that the tetragonality of the powders (c/a lattice constant ratio) is enhanced by the metal coating. After oxidation of the metal coating, the tetragonality of BaTiO3 powders decreases. In addition, it is shown that particle clustering can stabilize the tetragonal structure down to a smaller particle size than individual BaTiO3 particles due to the reduction of depolarization energy by clustering.

Size also plays an important role in the coating of particles. We found that nanoparticles were difficult to coat compare to micron-sized particles due to higher solubility of the small particle size. Size effect also plays an important role of the emission spectrum of quantum dots that we synthesized.