Energy-minimized monolayer structure

FIGURE 4.1.7 (a) GIXD pattern (bottom of (a)) for a pentacene monolayer and a diffraction pattern (top of (a)) calculated for an energy-minimized crystal structure model based on the GIXD lattice parameters and the (001) layer motif of huUc pentacene as the starting point (b) Normal views of the ab planes of bulk pentacene and the model monolayer structures (a and d) and the respective side views (b and e, e and f). The z-axis is the normal to the ab plane. (From Fritz, S.E. et al., J. Am. Chem. Soc. 126, 4084, 2004. With permission.)... 

Energy minimizations of the corresponding slabs led to similar adsorption energies for both structures, which were lower than the corresponding associative values for 1/2 monolayer coverage by 3-5 kcal/mol. Thus, these results are in accordance with previous DFT simulations for half-monolayer coverage that predict H2O dissociation on the rutile (110) surface. 

It is important to note that the cases of metal crystallites on a substrate and of a substrate of arbitrary thickness deposited upon a metal foil are not equivalent from a thermodynamic point of view because the constraints to which each of these systems are subjected are different. In the first case, a monolayer of TiOx will cover the metal, the amount being determined by the equilibrium with the surface of the substrate. For the second, the entire deposit of TiC>2 must be located on the surface. Since the coverage by a monolayer leads to the smallest free energy, the excess of TiOx should form in the latter case a tridimensional structure with the least possible surface area over the smallest possible part of the substrate surface, thus minimizing the free energy. There are, however, kinetic difficulties to achieve such a structure. For this reason, if coo<0, it is likely that a metastable state of extended patches... 

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