ABSTRACT
A “quantum dot” (QD) is a particle of a semiconductor material that is comparable in size to the material’s Bohr radius-several nanometers for most materials. Excitation by a photon slightly more energetic than the bandgap leads to the creation of an electron-hole pair, or exciton. Recombination of this exciton leads to emission of a photon (Figure 23.1). The connement energy caused by restricting this exciton to a nanometer-sized particle results in a size-dependent broadening of the bandgap. Thus, when the semiconductor has a bandgap energy comparable to energies of photons of visible light, the particles exhibit size-dependent visible photoluminescence. Many materials, belonging to group II-VI, group IV-VI, or group III-V, can be used to produce QDs, although the most common materials for biological applications are cadmium (Cd) based: cadmium selenide (CdSe), cadmium telluride (CdTe), and cadmium sulde. Emission from these particles spans the ultraviolet to visible range (Figure 23.2a). The precise bandgap energy and energetic positions of the band edges depend on not only QD material and size but also to some extent on solvent and pH.
