Colloidal nanocrystals
Colloidal nanocrystals are solution-grown, nanometre-sized, inorganic particles that are stabilized by a layer of surfactants attached to their surface. The inorganic cores possess useful properties that are controlled by their composition, size and shape, and the surfactant coating ensures that these structures are easy to fabricate and process further into more complex structures. This combination of features makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices.
Colloidal quantum dot synthesis
The synthesis of colloidal quantum dot nanoparticles is usually an organo- metallic precursor, high temperature, solvent based, airless chemical procedure that begins with the raw materials, a high boiling point ligand, and a Setrioctylphosphine conjugate. The three component system synthesis of colloidal quantum dots composes of precursors, organic surfactants, and solvents. On heating reaction medium to a sufficiently high temperature, the precursors chemically transform into monomers. Once the monomers reach super saturation level, the nanocrystal growth starts with a nucleation process. The temperature during the growth process is one of the critical factors in determining optimal conditions for the nanocrystal growth. Rearrangement and annealing of atoms is allowed during the synthesis process while promoting crystal growth. Monomer concentration has to be stringently controlled during nanocrystal growth.
Growth process
The growth process of nanocrystals can occur in two different regimes, “focusing” and “defocusing”. At high monomer concentrations, the critical size at which nanocrystals neither grow nor shrink is relatively small, resulting in growth of nearly all particles. In this regime, smaller particles grow faster than large ones since larger crystals need more atoms to grow than small crystals resulting in “focusing” of the size distribution to yield nearly mono disperse particles. The size focusing is optimal when the monomer concentration is kept such that the average nanocrystal size present is always slightly larger than the critical size. When the monomer concentration is depleted during growth, the critical size becomes larger than the average size present, and the distribution “defocuses” as a result of Ostwald ripening.
Colloidal semiconductor nanocrystals
Conventional doping by introducing impurity atoms for making colloidal semiconductor nanocrystals has been unsuccessful as impurities tend to be expelled from the small crystalline cores as observed for magnetic impurities, and thermal ionization of the impurities which provides free carriers is hindered by strong confinement.
Colloidal semiconductor nanocrystals combine the physical and chemical properties of molecules with the optoelectronic properties of semiconductors. Their color is highly controllable, a direct consequence of quantum confinement on the electronic states. Such nanocrystals are a form of 'artificial atoms’ that may find applications in optoelectronic systems such as light-emitting diodes and photovoltaic cells, or as components of future nano electronic devices. The ability to control the electron occupation especially in n-type or p-type nanocrystals is important for tailoring the electrical and optical properties, and should lead to a wider range of practical devices.
Formation of Colloidal Semiconductor nanocrystals
Colloidal semiconductor nanocrystals can be synthesized from precursor compounds dissolved in solutions like traditional chemical processes. The synthesis technique for the production of the different samples involves organic solvents and surfactants and reactions at elevated temperatures. The presence of clusters offers a possibility to control the size of the nanocrystals even at very small dimensions. In the case of CdSe, nanocrystals of this size emit a blue fluorescence.