Working group of Prof. Gauglitz

Quantum Dots (QDs)

Quantum Dots (QDs) are nanoscopic, in the ideal case "zero dimensional semiconductor structures". The size of the QDs is smaller than the de-Broglie-wavelength of the electrons or smaller than the Bohr radius of the exciton. Because of this restriction concerning the electron movement (quantum confinement), the optical, electric, magnetic and partially also the structural characteristics of the semiconductor change considerably.

Above all, the optical characteristics of the QDs are of special interest: QDs behave almost like "ideal fluorophores". They have a high quantum yield, narrow symmetrical emission spectra, a wide absorption spectra and a high photochemical stability.

Moreover, it is quite simple to synthesize QDs of different emission wavelengths, but with the same absorption wavelength by the strategy of high temperature pyrolysis of organometallic components.

Of special interest are CdSe nanoparticles, because for the issue to emit in the visible part of the electromagnetic spectrum, approximately between 480 and 650nm. Simple core CdSe QDs have a very minor photochemical stability and therefore they get covered with an additional ZnS-shell of 1 to 2 monolayers thickness. To transport the core-shell-system in aqueous media it is chemically modified through a further coating. A simple possibility for this coating is the creation of micelles. Their bifunctional molecules bind preferably covalent with one functional group to the ZnS-surface of the QDs, and the other functional group of the molecule makes the micelle water-soluble. A further possibility exists by coating with polymers. The polymer shell surrounds the QDs completely and offers the advantage to create a variety of chemically reactive surfaces through different polymers. These surface-modified QDs can now be used as a alternative for classical organic fluorescent labels.