Nanocrystals cluster aggregation

In this coimection, a cryochemical solid-phase synthesis of metal-polymer systems is of special importance. As a result of such a synthesis, metal clusters and organometallic assemblies formed at low temperatures are buried in a polymer environment, which offers possibilities to stabilize and study these products over a large temperature range. This method was first offered and described in reference 10. The thermal rearrangement of the initial low-temperature system is governed by relaxation processes in polymer matrix. In particular, the aggregation of metal atom clusters to form metal nanocrystals in cryochemically produced metal-polymer systems yields new nanocomposite materials with valuable properties. The study of the mechanism of cluster aggregation, which depends on the characteristics of the polymer matrix, will allow the nanocomposite structure to proceed in the needed direction. Thus, it becomes possible to determine the methods of cryochemical synthesis of metal-polymer materials with predetermined properties. 

The cluster aggregation to nanocrystals depends on the metal content. In the obtained Ag-ClPPX films the relative part of metal, stabilized in cluster form at 293 K, decreases from 90% to 20% while increasing the total metal content from 0.25 to 2 vol. % [44]. The stability of clusters is greatly influenced also by the structure of the polymer matrix [36, 44]. Comparison of spectra in Figures 2.5 and 2.6 shows that in cryochemically synthesized films, the Ag-CNPPX stability of Ag clusters is much higher than that in similar films Ag-ClPPX. The data on DJ D )o (see above) in the spectra of various films show that in cryochemically synthesized Ag-CNPPX films heated to ambient temperature, up to 70% from total incorporated Ag (in amount of about 2 vol. %) is stabilized as small noncrystalline clusters [44]. A similar result has been reported in reference 36 for Ag-PPX (Table 2.1). But in Ag-Cl-PPX containing the same total Ag, the fraction of Ag clusters at 295 K is only about 20% [24, 44]. 

Based on spectral data, it has been concluded that the change of Ag content in the cryochemically synthesized hlms Ag-PPX and Ag CIPPX from 0.3 to 2 vol. % (that is, 4-20 at. %) does not influence d of Ag nanocrystals formed as a result of the cluster aggregation [36, 44]. The same result has been obtained for Pb-PPX Aims with Pb content varied from 0.01 to 1 vol. % [52]. Unlike other similar systems, PbS-PPX nanocomposites were studied at high PbS loading in the range from 4.7 to 10.2 vol. % of PbS. In this case according to X-ray data, d of PbS nanocrystals also does not depend on PbS content [53]. Thus, it may be concluded that the independence of the nanocrystal mean size from the metal or semiconductor content is a general feature of nanocomposites resulted from solid-state synthesis. 

Because Dcr is proportional to the content of Ag nanocrystals and Ai/2 is in inverse proportion to their mean size d [77, 78], one may conclude that the state and amount of Ag nanocrystals were not affected by the cryopolymerization. Sharp growth of DCI at heating of obtained metal-polymer films specifies that the main part of Ag at 77 K is in a form of small noncrystalline Ag clusters, which aggregate with formation of nanocrystals under action of thermal relaxation processes in polymer matrix. According to data in work [79], in UV vis spectra of PPX films on a background of the PPX absorption only absorption bands of Ag with n> 15 could be observed in open range of PPX spectrum at X >320 nm. Because in this spectral range... 

The nanocrystals of such type form in various liquid media, such as organic solution [77, 81] or the softened quasi-liquid glass [82, 83], where there are no steric hindrances for the growth of equilibrium crystals without surface defects. At the same time, barriers for aggregation of clusters or atoms to metal nanocrystals in solid system that arises during the cryochemical solid-state synthesis favor the formation of crystals with structural defects,... 

Nanocrystals are crystals of nanometer dimensions, usually consisting of aggregates of a few hundred to tens of thousands of atoms combined into a cluster. Nanocrystals have typical dimensions of 1 to 50 nanometers (nm). 

Giant clusters of Pd nanocrystals are obtained by the mesoscopic assembly of nanocrystals of uniform size. This observation provides an illustration of the principle of self-similarity. These clusters of metal nanocrystals are perhaps bound by the same laws that endow special stability to magic numbered nanocrystals. It would therefore appear that these laws are invariant under scaling. The giant clusters of Pdssi nanocrystals obtained by us are different from other types of ligand directed nanocrystal aggregates which are formed mostly due to the directional nature of the ligand shell. 

A solid polymer matrix hinders the aggregation of cryochemically prepared Ag clusters so that substantial part of Ag in the prepared Ag-PPX systems remains in noncrystalline form at ambient and even higher temperatures. As was stated in references 24, 36, and 44, all of the Ag introduced into the investigated PPX matrices transforms to nanocrystals after annealing at 373 K. 

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