Nanocluster radius

FIGURE 15.8 The dependences of structural changes specific spatial scale X (1) and 1.19 (2) and nanoclusters radius r, (3) on testing temperature Tfor PC [22],... 

In Fig. 15.8, the dependences of and nanoclusters radius on T are adduced, where was determined according to the Eq. (15.4) and the value was calculated according to the Eq. (4.64). As it follows from the data of Fig. 15.8, the values and r, agree within the whole studied temperatures range. Let us note, that if in Ref [23] the value = 2.61, then for PC the indicated above agreement was obtained at = 1.19 and 1.06. This distinction confirms the thesis about distinction of synergetics laws in reference to... 

PbS has attracted much attention due to its special direct band gap energy (0.4 eV) and a relatively large exciton Bohr radius (18 nm) and their nanoclusters have potential applications in electroluminescent devices such as light-emitting diodes. PbS nanocrystals with rod like structures with diameters of 20-60 nm and lengths of 1-2 pm have been obtained using the sonochemical method and by using PEG-6000 [66]. Addition of PEG and the time of sonication have been found to play a key role in the formation of these rods. 

Primary nanoclusters are formed by heterogeneous nucleation near interfacial boundary of a droplet and a substrate. Therefore parameters of the substrate play a substantial role. For the cluster of g atoms, located at the boundary between the droplet and substrate, free energy of Ad>(g), is (for simplicity we assume that the cluster has a semispherical form with characteristic radius about lnm) ... 

FIGURE 20.3 Dependence of free energy G of a cluster on the radius r of the cluster. (From J. Colloid Interface Sci., 317, Finney, E.E. and Finke, R.G., Nanocluster nucleation and growth kinetic and mechanistic studies A review emphasizing transition-metal nanoclusters, 351-374, Copyright 2008, with permission from Elsevier.)... 

An extremely important characteristic of nanoclusters is their high surface area. If a spherical model of the cluster is used and the bond distances within the cluster are assumed to be the same as in the bulk, the radius a (nm) of the cluster as a function of N can be calculated from the bulk density p (gcm ) and the formula weight FW of the unit (gmol ) ... 

As the size of the clusters becomes smaller and approaches the Bohr radius of the bulk exciton, quantum confinement effects will become apparent. The electronic excitations shift to higher energy, and the oscillator strength will be concentrated in just a few transitions, so changing the density of electronic states and leading to blue shifts of the absorption band edge [4j, That is not sensitive for routine absorption spectra of nanoclusters for surface states and absorption of outside molecules. The fluorescence of nanoclusters is, by contrast, quite sensitive to surface conditions associated with surface localized states. For strong confinement, an analytic approximation obtained by Brus for the lowest excited state for small R is... 

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