Size-selective precipitation

A specific example where heterogeneous supports provide nanoparticle size-control is the immobilization of homogeneous silver nanoparticles on polystyrene [366]. This work was extended later to the development of a one-pot method for the size-selective precipitation of silver nanoparticles on PVP-protected thiol-functionalized silica. During the immobilization of very small silver nanoclusters both the size of the silver nanoclusters and the thickness of the silver layer on the support could be controlled directly by the reaction parameters applied (Fi re 16) [367]. 

Anand, M., Odom, L.A. and Roberts, C.B. (2007) Finely controlled size-selective precipitation and separation of CdSe/ZnS semiconductor nanocrystals using C02 gas-expanded liquids. Langmuir, 23 (13), 7338-7343. 

The procedure to fabricate colloidal silver, (Ag°) , spherical nanoparticles is similar to that already described (see Section 9.3.3) The Cu( AOT)2 is replaced by the silver derivative. The relative concentration of Na(AOT), Ag(AOT)2, and the reducing agent remain the same. Control of the particle size is obtained from 2 nm to 6 nm (67). To stabilize the particles and to prevent their growth, 1 p.l/mL of pure dodecanethiol is added to the reverse micellar system containing the particles. This induces a selective reaction at the interface, with covalent attachment, between thio derivatives and silver atoms (68). The micellar solution is evaporated at 60°C, and a solid mixture of dodecanethiol-coated nanoparticles and surfactant is obtained. To remove the AOT and excess dodecanethiol surfactant, a large amount of ethanol is added and the particles are dried and dispersed in heptane. A slight size selection occurs, and the size distribution drops from 43% to 37%. The size distribution is reduced through the size selected precipitation (SSP) technique (38). 

Size-Selective Precipitation from Molecular Precursors... 

Preparation and chemical and photophysical properties of size-quantized HgS/CdS particles were reported [83, 84], Single-crystal germanium quantum wires [85], CdTe [86], and HgTe [86] were prepared in solution. Size and monodispersity control of ultrasmall CdS particles were achieved by thiol capping [87] and by size selective precipitation [88]. Preparation, TEM, and X-ray scattering analysis of a glassy network of CdSe nanocrystallites connected by molecular bridges were reported [89]. 

Impurity ions in wurtzite lattices are described by the same expressions for P2, and P3c, with a numerically insignificant difference in P3o. These expressions are only quantitatively accurate in the dilute limit, but many of the doped nanocrystals discussed in this chapter fall in this limit. The reader is referred to Ref. 42 for a generalized treatment of the problem. Figure 2(b) plots the probabilities calculated from Eq. 4a-d as a function of impurity concentration. The fraction of dopants having at least one nearest-neighbor dopant is quite high even at moderate impurity concentrations (<5%). Needless to say, whereas purification to ensure size uniformity is possible (size-selective precipitation), no purification method has yet been developed for ensuring uniform dopant concentrations in an ensemble of nanocrystals. 

Fig. 8 (a) TEM image of as-prepared cube-like PbTe nanocrystais. Inset shows the SAED pattern, (d) Ordered array consisting of 15 nm cubic-shaped PbSe nanocrystais after size selective precipitation. Reprinted with permission from J. E. Murphy, M. C. Beard, A. G. Norman, S. P. Ahrenkiel, J. C. Johnson, P. Yu, O. 1. Micic, R. J. Ellingson and A. J. Nozik, J. Am. Chem. Soc., 2006, 128, 3241. 2006 American Chemical Society. 

Ag2S was also effectively stabilized by cysteinyl ligands. These clusters are synthesized using a molar ratio of 2 1 cysteine silver ions upon which stoichiometric amounts of inorganic sulfide were added to nucleate the nanoparticle, with subsequent size-selective precipitation. The resultant nanoparticles had an absorbance shoulder at 300 nm. Further analysis using high-resolution transmission electron microscopy (HRTEM) revealed a particle size of approximately 9.00 2.25 run in diameter. Selected area electron diffraction (SAED) analysis also demonstrates the highly crystalline natme of the product. ... 

The size-selective precipitation (SPP) was predominantly developed by Pileni [50c]. One example (SPP) is monodisperse silver particles (2.3 nm, 0= 15%), which are precipitated from a polydisperse silver colloid solution in hexane by the addition of pyridine in three iterative steps. Recently, Schmid [52a] has reported the two-dimensional crystallization of truly monodisperse AU55 clusters. Chromatographic separation methods have thus far proven unsuccessful because the colloid decomposed after the colloidal protecting shell had been stripped off [42a]. The size-selective ultracentrifuge separation of Pt colloids has been developed by Colfen [52b]. Although this elegant separation method gives truly monodisperse metal... 

Solubility (in the molecular sense, rather than in the sense of forming dispersions and sols) opens up a number of possibilities. The first and perhaps most important, is that it allows size-selective precipitation [10], permitting monodisperse nanoparticles to be prepared. It is only when particles are monodisperse that their size-dependent physical properties can be studied in detail [6j. It is also possible to organize these monodisperse nanoparticles via slow evaporation to yield superlattices [11-13]. Superlattices of nanocrystals can rightly be described as a new class of materials, comprising crystals of crystals as opposed to most crystalline solids which are crystals of atoms [14]. In contrast, naturally occurring opals are crystals of amorphous silica spheres [15]. 

Synthesis of Core-Shell Nanocrystals with InAs Cores Preparation of the InAs core-shell nanocrystals is carried out in a two-step process. In the first step, the InAs cores are prepared using the injection method with TOP as solvent. This allowed hundreds of milligrams of nanocrystals per synthesis to be obtained (as detailed above), with a size-selective precipitation being used to improve the size distribution of cores to a 10%. In the second step, the shells of the various materials were grown on the prepared cores. This two-step approach followed methods developed for the synthesis of group II-VI core-shell nanocrystals. 

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