Strain energy substrate-induced

Let us examine the instability oi strained thin films. In Fig. 3, thin films of30 ML are coherently bonded to the hard substrates. The film phase has a misfit strain, e = 0.01, relative to the substrate phase, and the periodic length is equal to 200 a. The three interface energies are identical to each other = yiv = y = Y Both phases are elastically isotropic, but the shear modulus of the substrate is twice that of the film (p = 2p). On the left-hand side, an infinite-torque condition is imposed to the substrate-vapor and film-substrate interfaces, whereas torque terms are equal to zero on the right. In the absence of the coherency strain, these films are stable as their thickness is well over 16 ML. With a coherency strain, surface undulations induced by thermal fluctuations become growing waves. By the time of 2M, six waves are definitely seen to have established, and these numbers are in agreement with the continuum linear elasticity prediction [16]. 

A quite different class of adsorbate-induced surface reconstruction is formed by those systems involving pseudo-(lOO) reconstruction of the outermost atomic layer this behaviour has been found to occur on fcc(lll) and (110) surfaces in several metal/adsorbate combinations. The essential driving force for such reconstructions appears to be that adsorption on a (100) surface (typically in a c(2 x 2) arrangement) is so energetically favourable that, even on a surface with a different lateral periodicity (and point-group symmetry), reconstruction of the outermost layer or layers to form this (100)-like geometry is favoured. This must occur despite the introduction of strain energy at the interface between the substrate and the... 

Additionally, substrate induced strains can be accounted for by introduction of the average strain energy per atom, . Hence, a strained cluster of N atoms would give an energy contribution of sN to the formation energy of the cluster. In a real system, the solvent influence must also be accounted for. 

Because of the simplicity of the derivation of eq. (4.53) in the calculation of the binding energy of the cluster, a substrate-induced strain energy term may be added to the excess energy as given in eq. (4.48) (cf. Section 4.3). 

Fig. 19.1 Models of pseudomorphic monolayCTs of Pt on three different substrates inducing compressive strain (Ir(lll) and Pd(lll)) and expansive strain (Au(lll)) and activation energies for O2 dissociation and OH formation on differoit PtML/X surfaces as a function of oxygen binding energy. Figure redrawn by the author with permission based on a diagram provided by R. Adzic, based on diagrams in [17]...

Other forces can arise as a result of elastic strain on the growing film, which can be due to a surface-induced ordering in the first few layers that reverts to the bulk liquid structure at larger distances. This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances [7]. Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors. 

The catalytic specificity of the cycloamyloses has led to their utilization as a model for understanding enzymatic catalysis. It is the authors expectation that the cycloamyloses will continue to serve as an enzyme model as well as a model for designing more efficient catalytic systems. Toward this end, it would seem profitable to pursue the idea that the cycloamyloses may lower the activation energy of a chemical reaction by inducing strain into the substrate. 

Later on12, Koshland proposed the induced fit model of the active site action that considers that during the formation of the enzyme-substrate complex, the enzyme can change its conformation so as to wrap the substrate like it happens when a hand (substrate) fits in a globe (enzyme). This flexing puts the active site and bonds in the substrate under strain, which weakens the bonds and helps to lower the activation energy for the catalyzed reaction. 

Analysis of the activation parameters for the different encapsulated substrates reveals that the source of catalysis is more complex than simply a reduction of the entropy of activation, since different effects are observed for substrates 26,27,30. While the rate acceleration for the encapsulated 26 was exclusively due to lowering the entropic barrier, for 27 and 30 a decrease in the enthalpic barrier for rearrangement is observed in addition. It is possible that, for 27 and 30 binding into the narrow confines of the metal-ligand assembly induces some strain on the bound molecules, thereby raising their ground-state energies compared to those of the unbound... 

Xenobiotic compounds are usually attacked by enzymes whose primary function is to react with other compounds, a process that provides neither carbon nor energy called cometabolism. Cometabolism usually involves relatively small modifications of the substance that is cometabolized (the secondary substrate), compared to the primary substrate. The enzymes that carry out cometabolism tend to be relatively nonspecific. As an environmentally significant example of cometabolism, at least one strain of bacteria degrades trichloroethylene with an enzyme system that acts predominantly on phenol. The enzyme activity can be induced by exposure to phenol, after which it acts on trichloroethylene. 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

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