About semi conducting nanoparticles

Nanoparticles of semi conducting materials have all three dimensions in the range of 1–20 nm and possess novel electronic, magnetic, catalytical, and optical properties. This is due to their large surface-to-volume ratio and their reduced size. 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 “bandgap” to increase.
Preparation methods
Traditional methods such as chemical vapor deposition and molecular beam epitaxy methods have been used but have limitations as they produce particles that are attached to a substrate or embedded in a matrix, thereby limiting their potential in applications.
Colloidal access
The colloidal access to nanoparticles is achieved by carrying out a precipitation reaction in a homogenous solution in the presence of stabilizers, whose role is to prevent agglomeration and further growth. The colloidal growth stability of the crystals can be improved by using solvents with low dielectric constants or by using stabilizers such as styrene/maleic acid copolymer.
Ostwald ripening
In a process known as Ostwald ripening, small crystals, which are less stable, dissolve and then recrystallize on larger and more stable crystals. For this method to be effective, the nanoparticles must have low solubility, which can be achieved by judicious choice of solvent, pH, and  passivating agent.
The problems associated with the low-temperature colloidal route could be overcome by injecting precursors that undergo pyrolysis at high temperature into a high boiling point coordinating solvent. This route uses a volatile metal alkyl(dimethylcadmium) and a chalcogen source tri-n-octylphosphine selenide (TOPSe), dispersed in tri-n-octylphosphine (TOP) and injected into hot TOPO (tri-n-octylphosphine oxide). The particles produced by this method are  monodispersed and crystalline.
Chemical routes
An alternative chemical route to nanoparticles uses single-molecule precursors in which the metal-chalcogenide bond is available has proven to be a very efficient route to high-quality nanoparticles. The decomposition of the precursor drives the formation of the nanoparticles with termination of growth occurring when the precursor supply is depleted. After the initial injection there is rapid nucleation, followed by controlled growth of the nuclei. When the nanoparticles reach a desired size, the further growth is arrested by quickly cooling the solution. The nanocrystals are isolated from the growth solution by adding another solvent that is miscible with the initial solvent.  The resultant turbid solution is centrifuged, and the nanoparticles are isolated in the form of a powder. Metal complexes of alkylthioureas have also proven to be very good precursors for nanoparticle synthesis.


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