Nonoxide semiconductor nanoparticles are commonly synthesized by pyrolysis of organometallic precursor(s) dissolved in anhydrate solvents at elevated temperatures in an airless environment in the presence of polymer stabilizer or capping material. In the synthesis of metallic nanoparticles, polymers attached on the surface are commonly termed as polymer stabilizers. However, in the synthesis of semiconductor nanoparticles, polymers on the surface are generally referred to as capping materials. Capping materials are linked to the surface of nanocrystallites via either covalent bonds or other bonds such as dative bonds. Examples are sulfur and transition metal ions and nitrogen lone pair of electrons form dative bond.
Semiconductor nanocrystallites
The formation of monodispersed semiconductor nanocrystallites is generally achieved by the following approaches. First, temporally discrete nucleation is attained by a rapid increase in the reagent concentrations upon injection, resulting in an abrupt supersaturation. Second, Ostwald ripening during aging at increased temperatures promotes the growth of large particles at the expense of small ones, narrowing the size distribution. Third, size selective precipitation is applied to further enhance the size uniformity. Although organic molecules are used to stabilize the colloidal dispersion, similar to that in the formation of metallic colloidal dispersions, the organic monolayers on the surfaces of semiconductor nanoparticles play a relatively less significant role as a diffusion barrier during the subsequent growth of initial nuclei. This is simply because there is a less extent or negligible subsequent growth of initial nuclei due to the depletion of growth species and the drop of temperature at the nucleation stage.
Researchers at Nanjing University have sonochemical methods for the preparation of nanoparticles with controllable morphologies. PbWO4 nanostructures with different morphologies, such as dendritic, flowery, and star-like, have been synthesized via a sonochemical route. It has been proved that the ultrasonic irradiation plays a crucial role in the morphology of the product. The researchers used a microemulsion system to successfully synthesize single-crystalline PbF2 nanorods via a sonochemical route.