Synthesis in a Structured Medium

The use of a matrix to define the reaction space is an intrinsically attractive approach to the preparation of large amounts of material which could be deposited from solution or from the vapor phase. A number of matrices have been used including zeolites [80], layered solids [81], molecular sieves [82-84], micelles/ microemulsions [85-89], gels [90-92], polymers [93-97] and glasses [98]. The matrix provides a mesoscopic reaction chamber in which the crystal can only grow to a certain size. 

The properties of the nanocrystallites are determined, not only by the confinements of the host material, but also by the properties of the system, which can include a range of factors including the internal/external surface properties e.g. of the zeolite or the lability of a micelle. The particle size is controlled by the system chosen e.g. in zeolites the nanocrystallite diameter is limited by the pore size of the zeolite (typically smaller than 2 nm). 

Stable, cubic phase, PbS nanoparticles were prepared in a polymeric matrix by exchanging Pb + ions in an ethylene - 15% methacrylic acid copolymer followed by reaction with H2S [91]. The size of the PbS nanoparticles was dependent on the initial concentration of Pb + ions with diameters ranging from 13 to 125 A. The smallest particles (13 A) are reported to be molecular in nature and exhibit discrete absorption bands in their optical spectra. Two theoretical models, which take into account the effect of nonparabolicity, were proposed in order to explain the observed size-dependent optical shifts for PbS nanocrystallites. The authors reported that the effective mass approximation fails for PbS nanocrystallites. 

The vast majority of semiconductor nanoparticles are produced, at present, by batch methods in research laboratories in processes which essentially depend on the separation of nudeation and growth. In the semiconductor area high quality 

TOP tri-n-octylphosphine TOPO tri-n-octylphosphine oxide HDA hexadecylamine ODE octadecene 

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