Semiconductor nanoparticles
Semiconductor nanoparticles also referred to as Q-dots exhibit unique physical properties that give rise to many potential applications in areas such as nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics. Semiconductor nanoparticles are generally considered to be particles of material with diameters in the range of 1 to 20 nm. Large surface to volume ratio and the second factor is the quantum confinement effect is responsible for these unique properties.
This is because the large relative fraction of surface atoms in nanoparticles determines the size dependences of the melting temperature and pressure at which the crystal lattice of a semiconductor rearranges. Because of the comparable size of nanoparticles and the delocalization radii of charge carriers non-linear optical effects arise.
The explanation for the effect is that as the diameter of the particle approaches the exciton Bohr diameter, the charge carriers become confined in three dimensions with zero degrees of freedom. As a result of the geometrical constraints, the electron feels the particle boundaries and responds to particle sizes by adjusting its energy. This phenomenon, known as the quantum size effect, causes the continuous band of the solid to split into discrete, quantized levels and the “band gap” to increase.
Synthesis
The main method of preparation of semiconductor nanoparticles was, until recently, classical colloid chemistry, involving controlled arrested precipitation from colloidal solutions. Semiconductor Nanoparticles synthesis typically occurs by the rapid reduction of organmetallic precursors in hot organics with surfactants. A recent method for preparing semiconductor nanoparticles is to use organometallic and/or metal organic compounds under anaerobic conditions. This approach involves the use of organometallic main group compounds.
Surface passivation
Semiconductor nanoparticles can have quantum effects and have high emission yields across the visible and near infrared (NIR) spectrum. The quantum yield and emission life-time of the band gap luminescence are determined by the surface of these nanoparticles or quantum dots. High luminescence yields are achieved by the use of surface passivation to reduce the non-radiative surface recombination of charge carriers.
Two types of passivation methods are commonly employed. One is through so-called band gap engineering, whereby a larger band gap semiconductor with good lattice mismatch is epitaxially deposited onto the core surface. The second method is by adsorbing Lewis bases onto the surface. One example of the latter is otylamine used to passivate the surface of CdSe and CdSeZnS quantum dots.
Zinc nanoparticle Synthesis
ZnO is a wide band gap semiconductor that has attracted tremendous interest as blue light emitting materials, gas sensors and transparent conductors in solar cells. Unlike other semiconductor compounds that contain cadmium, arsenic, or other environmental toxins, zinc and oxygen are “environment friendly” elements.
Synthesis of the nanoparticles is out by adding a trioctylphosphine solution of the appropriate zinc complex to a heated (175°C) solution of trioctylphosphineoxide. The mixture is for 2 hrs and then allowed to cool to room temperature. The particles are from solution via the addition of methanol and dispersed in hexane for analysis.
Non-oxide semiconductor nanocrystals
The most studied non-oxide semiconductors are cadmium chalcogenides. These nanocrystals were probably the first material used to demonstrate quantum size effects corresponding to a change in the electronic structure with size, i.e., the increase of the band gap energy with the decrease in size of particles. These semiconductors nanocrystals are commonly synthesized by thermal decomposition of an organometallic precursor dissolved in an anhydrous solvent containing the source of chalcogenide and a stabilizing material such as polymer or capping ligand. Stabilizing molecules bound to the surface of particles control their growth and prevent particle aggregation.
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 super saturation. 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.