Cluster morphology, simulation

The cluster morphology may depend on details of colloidal particle interactions, mechanism of particle attachment to the cluster, and dimensionality of the problem. The existing models for cluster morphology simulation account for the trajectory of... 

Figure 9.6 Simulated CVD cluster morphology for coverages of -0.07 and 0.25 monolayer, showing the effect of differential dissociation probability, (a) and (b), versus the effect of a uniform dissociation probability, (c) and (d) the physisorbed precursor molecules are mobile in each case. Note, (c) and (d) are equivalent to the simulated morphology for growth by PVD. (Reproduced with permission from Reference [73].)...

A number of theoretical models and computer simulation approaches were developed for description of the cluster morphology [72], reaction kinetics, and time dependence of the cluster-size distributions [73],... 

The MD simulations provided the necessary thermodynamic information to obtain the equilibrium configurations of the films. Often the deposition process will produce films which are not in the equilibrium configuration, and then the problem is to determine the stablity of these films against changes in morphology. Here simulations can also be helpful, since data on the surface energies and chemical potentials of strained films can be used to calculate the probability of cluster nucleation, using classical nucleation theory. 

The morphologies of the large ionic clusters observed in these simulations rather suggest free chain end folding to produce rudimentary lattice structure as a possible pre transitional mechanism. 

Nanoparticles have different morphologies than flat, bulk surfaces. Perez et al. have considered the activation of water and COads + OHads reactions on Pt and PtRu clusters including the effects of solvation." They found that the presence of under-coordinated Ru adatoms on the Pt cluster surfaces enhances the production of OHads from water compared to Ru alloyed into the nanoparticle surfaces. More significantly, they found that the presence of an aqueous environment simulated by up to six water molecules dramatically stabilized the transition state and products of the reactions. For example, in a gas-phase environment they calculated a water dissociation barrier of 20 kcal/mol whereas in the solvated environment the barrier was reduced to 4.5 kcal/mol on the alloy surface. The barrier for water dissociation on the Ru adatom in the aqueous environment was only 0.9 kcal/mol. Although their results are for an adatom on a near flat (111) surface, they may have significance in describing the catalytic properties of undercoordinated Ru atoms at edge and corner sites on nanoparticles. 

The dynamics of adsorbed species over MgO(OOl) surface was studied by MD method. The migration of the adsorbed species was found to depend on the morphology of MgO and the thermal vibration of surface atoms in MgO lattice. Further, the situation where the supercritical fluid and adsorbed species exist together was simulated. The collision of supercritical fluid with the adsorbed species was identified as the primary cause of extraction. Additionally, the supercritical fluid form clusters around the desorbed species avoiding the readsorption. Thus, clustering is the secondary cause for the increased efficiency of supercritical extraction even above the critical conditions. The details of these simulation studies are given in the following section. 

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