Quantum Dots Preparation
“There are several ways to confine excitons in semiconductors, resulting in different methods to produce quantum dots. In general, quantum wires, wells and dots are grown by advanced epitaxial techniques in nanocrystals produced by chemical methods or by ion implantation, or in nanodevices made by state-of-the-art lithographic techniques.” (Wikipedia, Quantum Dots Preparation, 7/18/2011)
“Colloidal semiconductor nanocrystals are synthesized from precursor compounds dissolved in solutions, much like traditional chemical processes. The synthesis of colloidal quantum dots is based on a three-component system composed of: precursors, organic surfactants, and solvents. When heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into monomers. Once the monomers reach a high enough supersaturation 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. It must be high enough to allow for rearrangement and annealing of atoms during the synthesis process while being low enough to promote crystal growth. Another critical factor that has to be stringently controlled during nanocrystal growth is the monomer concentration. The growth process of nanocrystals can occur in two different regimes, “focusing” and “defocusing”. At high monomer concentrations, the critical size (the size where 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 monodisperse 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.”
“There are colloidal methods to produce many different semiconductors. Typical dots are made of binary alloys such as cadmium selenide, cadmium sulfide, indium arsenide, and indium phosphide. Although, dots may also be made from ternary alloys such as cadmium selenide sulfide. These quantum dots can contain as few as 100 to 100,000 atoms within the quantum dot volume, with a diameter of 10 to 50 atoms. This corresponds to about 2 to 10 nanometers and at 10 nm in diameter, nearly 3 million quantum dots could be lined up end to end and fit within the width of a human thumb.”
“Large batches of quantum dots may be synthesized via colloidal synthesis. Due to this scalability and the convenience of benchtop conditions, colloidal synthetic methods are promising for commercial applications. It is acknowledgedto be the least toxic of all the different forms of synthesis.”
Recent Journal Articles
Copolymer nanosphere encapsulated CdS quantum dots prepared by RAFT copolymerization: synthesis, characterization and mechanism of formation
(1519-1533) Colloid and Polymer Science 289 #14 (2011)
Das, Zhong and Claverie of the University of Quebec, Canada, synthesized cadmium sulfide (CdS) quantum dots (QDs) encapsulated in block copolymer spheres by an aqueous emulsion polymerization process. First, stable dispersions of CdS QDs in water were prepared using a polymer dispersant, either poly(acrylic acid) or a random copolymer having an average of ten acrylic acid and five butyl acrylate units. These polymer dispersants were prepared by reversible addition-fragmentation chain transfer polymerization. Then, the CdS QDs dispersed in water were encapsulated in a polystyrene shell using an emulsion polymerization process. (RDC 8/15/2011)
Facile preparation of cds quantum dots using hyperbranched poly(amidoamine)s with hydrophobic end-groups as nanoreactors
(1077–1083)Journal of Applied Polymer Science 122 #2 (2011)
Shi et al, China, synthesized hyperbranched poly(amidoamine)s with methyl ester terminals (HPAMAM) by one-pot approach and subsequently used as nanoreactors to prepare CdS quantum dots (QDs). HPAMAM could bind Cd2+ through their internal amines, while the external methyl ester groups prevented the aggregation of polymers. After reaction with S2−, CdS QDs sequestered within individual hyperbranched polymers were obtained. Tests confirmed the formation of CdS QDs with small particle size and narrow size-distribution. (RDC 7/13/2011)
A new two-phase route to cadmium sulfide quantum dots using amphiphilic hyperbranched polymers as unimolecular nanoreactors
(991–997)Journal of Applied Polymer Science 120 #2 (2011)
Shi developed a two phase system consisting ofchloroform and water for palmitoyl chloride functionalized hyperbranched polyamidoamine (HPAMAM-PC) and cadmium acetate/sodium sulfide. The amphiphilic HPAMAM-PC, with a hydrophilic dendritic core and hydrophobic arms, formed stable unimolecular micelles in chloroform and was used to encapsulate aqueous Cd2+ ions. After the reaction with S2− ions from the aqueous phase, monodisperse and uniform-sized CdS quantum dots stabilized by HPAMAM-PC unimolecular micelles were obtained. (RDC 1/11/2011)