Interface Bond Contraction

Evidence in Sect. 12.2 shows that bond contraction takes place at sites surrounding impurities and at the interfaces. For instances, a substitutional As dopant impurity has induced an 8 % bond contraction around the impurity (acceptor dopant As) at 

The Au-Au bond of Au nanocrystals embedded in Si02 was found to contract by 0.04-0.03 nm [49]. The Sb-In bond in the first shell of the Sbln embedded in a Si02 matrix contracted slightly by about 0.002 nm compared with that of the bulk Sbln [50]. Mn ion implantation onto a heated Si substrate to form clusters with 6-8 atoms located in the first coordination sphere in three subsheUs [51]. The first subshell has one atom at a distance of 2.31 A, the second subsheU has three atoms at 2.40 A, the third subshell has three atoms at 2.54 A, and finally the fourth subshell contains six Mn atoms at a distance of 2.80 A. The finding of dopant-induced bond contraction and the interface bond contraction could provide an atomic scale understanding of the bond in a junction interface. 

Theoretical calculations, confirmed by electron microscopy measurement [53], revealed that homojunction dislocations in aluminum have either compact or 

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Asphaltenes stabilize the crude oil emulsion by different modes of action. When asphaltenes disperse on the interface, the film formed at a water/ crude oil interface behaves as a skin whose rigidity can be shown by the formation of crinkles at interface when contracting the droplet to a smaller drop size ( ). They can also aggregate with resin molecules on the interfaces and prevent droplet coalescence by steric interaction (Figure 11). Some authors suggest that asphaltenes stabilize the emulsion by formation of hydrogen bonding between asphaltenes and water molecules (79-so)... 

Pauling [6] and Goldschmidt [7] indicated that, if the CN of an atom is reduced, the ionic and the metallic radius of an atom would shrink spontaneously. Therefore, the CN imperfection will shorten the remaining bonds between undercoordinated atoms, which is independent of the nature of the specific chemical bond [8] or the structural phase. Bond order loss-induced bond contraction applies to a liquid surface, gaseous phase, and homo- and hetero-junction interface in addition to atoms at the vacuum-solid interface. 

Undercoordination-induced global bond contraction in solid and liquid skins, terrace edges, gaseous phases, skins of nanosolids, and associated with impurities and interfaces are in accordance with the BOLS correlation mechanism— bonds between the undercoordinated atoms are shorter and stronger. 

Numerical fitting turns out the a value of 1.8, which indicates that an interfacial bond is 80 % stronger than the bond in the parent bulk. If one considers the bond contraction, 0.90 0.92 [72], as the As and Bi impurities in CdTe compound, the m value is around 5.5-7.0. The high m value indicates that bond nature indeed evolves from a compound with m around four to a value of more covalent nature. Therefore, the deformed and shortened interfacial bond is much stronger. This finding means that electrons at an interface are deeply entrapped and densified. Therefore, it is understandable that twins of nanograins [73] and the multilayered structures [74] are stronger and thermally more stable. 

Excessive energy due to bond contraction and bond nature alteration reinforces a compacted interface, which is applicable to multi-layers, alloys, compounds, and impurities. 

At the mixed interface, the n may not change substantially, so we can introduce the interfacial bond energy as Sint = yEb and the interfacial atomic cohesive energy as Sc,int = yzSb, and then, all the equations for the surface effect are adoptable to the interface properties. A numerical fit of the size dependence of overheating for In/Al [61], Ag/Ni [62], and Pb/Al and Pb/Zn [63] core-shell nanostructures, presented in Fig. 14.3(i) has led to a y value of 1.8, indicating that an interfacial bond is 80 % stronger than a bond in the bulk of the core material [64]. If one took the bond contraction to be 0.90-0.92 as determined from the As- and Bi-doped CdTe compound [46] into consideration, the m value is around 5.5-7.0. 

Interfacial bond contraction and the associated bond strengthening, and the bond nature alteration upon alloy and compound formation at the junction interfaces are responsible for the hardening and overheating of twin grains interfaces, and nanocomposites. 

At the liquid-liquid interface between a hydrocarbon oil and water under mixing, the molecules encounter unbalanced attraction forces, pull inwardly, and contract as other molecules leave the interface for the interior of the bulk liquid. As a result, spherical droplets are formed. Customarily, the boundaries between a liquid and gas and between two liquids are the surface and the interface, respectively. The interfacial tension (or interfacial free energy) is defined as the work required to increase the interfacial area of one liquid phase over the other liquid phase isothermally and reversibly. Moving molecules away from the bulk to the surface or interfacial surface requires work (i.e., an increase in free energy). Water molecules and hydrocarbon oil molecules at the interface are attracted to the bulk water phase as a result of water-water interaction forces (i.e., van der Waals dispersion y and hydrogen bonding y ), to the bulk oil phase due to the oil-oil dispersion forces, y 1, and to the oil-water phase by oil-water interactions, y )W (i.e., dispersion forces). As mentioned in Chapter 3, the oil-water dispersion interactions are related to the geometric mean of the water-water and oil-oil dispersion interactions. The interfacial tension is written as ... 

The most important difference between particles inside the bulk and in the interfacial layer comes from the surrounding environment of the particles the particles inside the bulk are in an isotropic environment, while those in the interface are in an anisotropic environment thus, in the interlayer, the forces between the particles are unbalanced. To reduce the resulting surface pressure, some additional processes occur that must be taken into account. On clean surfaces (for example, on a solid surface in vacuum), these processes are the bond-length contraction or relaxation and reconstruction of the surface particles (Somorjai 1994). It results in significantly reduced spacing between the first and second layers compared to the bulk. The perturbation caused by this movement propagates a few layers into the bulk. The other effect is that the equilibrium position of the particles changes that is the outermost layers can have different crystal structure than the bulk. This phenomenon is the reconstruction. 

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