Surface Bond Contraction

Huang H., Sun C. Q. and Hing P., Surface bond contraction and its effect on the nanometeric sized lead zirconate titanate, J. Phys. Condens. Matter 12 (2000) pp. L127-132. 

Some important yet often overlooked events, such as non-bonding lone pairs, antibonding dipoles, H-like bond, and surface bond contraction, are crucial to practical applications. 

Surface bond contraction XRD/XPS Crystal field Cohesive energy Hamiltonian Origin for the tunability of nanosolids... 

Fig. 8.2 Elasticity and mechanical strength of nitride films, a GaAIN/Al203 exhibits 100 % elastic recovery under 0.7 mN indentation load, while amorphous carbon shows 65 % elastic recovery under the same scale of load, b TiCrN skin is twice harder than the bulk because of surface bond contraction. The abrupt increase of the Motion coefficient for c C nitride and d Ti nitride in the pin-on-disk test of sliding Mction [14] exhibits the critical load that breaks the surface bond. Diamond thin films show no abruptness of the coefficient at even higher loads (reprint with permission Ifom [14])...

The combination of BOLS and 3B mechanisms form comprehensive somces that determine the unusual behavior of a surface. Bond contraction perturbs the Hamiltonian that defines the entire band stracture and the related properties of a SOM chemical reaction causes a repopulation of valence electrons in the valence band and modifies the binding energy. Chemical reaction changes the bond nature while the coordination reduction changes the bond length and energy. All the physical properties should be derivatives of the Hamiltonian of the system or the DOS distribution in the valence band of the soUd. 

Grains of semiconductors of a few nanometers across emit blue-shifted light but the larger chunks of the same material do not [30, 31]. The band gap (Eq) of CdSe can be tuned from deep red (1.7 eV) to green (2.4 eV) by simply reducing the solid diameter from 20 to 2 nm [32]. The Eg [33, 34] and the core-level shift (AE ) [35] of nanosemiconductors increase whereas the dielectric susceptibiUty (x) decreases when the solid size is reduced. Without triggering the electron-phonon (e-p) interaction or electron-hole (e-h) production, STS/M revealed that at 4 K the Eg expands from 1.1 to 3.5 eV when the diameter of Si nanorod reduces from 7.0 to 1.3 nm associated with some 10 % of Si-Si surface-bond contraction [36]. 

BOLS correlation predicts a 25 % bond contraction and 70 % lower of the for the smallest size. The approximation based on Brook s relation overestimates the bond contraction and underestimates the suppression because of the assumption of size-independent G and yo- Actually, the atomic vacancy should expand instead as the remaining bonds of the surrounding atoms contract. The strain of the entire nanoparticle arises from surface bond contraction and has little to do with the atomic void inside. One may note that Eb varies from site to site due to the difference of atomic CN environment at various locations of the solid. 

H.T. Huang, C.Q. Sun, T.S. Zhang, P. Hing, Grain-size effect on ferroelectric Pb(Zrl-xTix)0-3 solid solutions induced by surface bond contraction. Phys. Rev. B 63(18), 184112 (2001)... 

Surface interlayer contraction creates the low-energy component of the CLS [8]. For example, a 12 % contraction of the first-layer spacing Nb(OOl) shifts the 3d3/2 by 0.50 eV [4] a (10 3) % contraction of the first-layer spacing of Ta(OOl) offsets the 4fs/2(7/2) by 0.75 eV [8]. The surface bond contraction enhanced interlayer charge density, and resonant diffraction of the incident light is supposed to be responsible for the positive energy shift [4, 8]. 

Briefly, nitrogen could enhance the hardness of a metal surface because of the bond nature alteration and surface bond contraction. An N-M bond is shorter than a C-M bond because of the ionic radius of and N . The involvement of lone pairs makes the nitride more elastic but readily broken under a critical load. Such an interpretation may provide a possible mechanism for the atomistic friction and self-lubrication of a nitride specimen. 

In 1972, the Agency for International Development (AID) cooperated with the Sulphur Institute to prepare a program to evaluate the feasibihty of using the sulfur surface-bond technique in developing countries. Afterwards AID awarded a contract to Southwest Research Institute to constmct houses in South America and Africa using this technique. Under the contract, about 60 houses have been completed (62). 

Zincblende structure with top Ga and As atoms rotated into, resp. out of surface, (keeping about constant mutual bond length, rotated by projected angle of 27°) Ga and As back bonds contracted by —2.5% and -3.6%, resp. 

It is very interesting that Lennard-Jones 51) predicted in 1928 that lattice parameters near and perpendicular to the surface should be smaller as compared to those within the crystal lattice, but unfortunately this was not confirmed by experiment (52). Indeed truly clean surfaces are hard to obtain, as the highly reactive surface area tends to react with constituents of the atmosphere such as oxygen or water. Under normal conditions many surfaces are covered by oxides or hydroxides, by which the effect of bond contraction at and beneath the surface area is greatly reduced or cancelled. 

Whereas the holes are considered as fairly small and closed entities within the system, the existence of a network of channels may be proposed which may contain even "sheet-like" areas(3). In analogy to the presence of dislocations in a solid material, channels should not end somewhere within the phase, but rather reach the surface at both ends or form a closed loop(3). The molecules surrounding channels are expected to be under great strain with appropriate bond contractions at the inner-surface areas. 

The H2O-, NH3-, and FH-like molecular stmctures and the concept of CN-imperfection-induced bond contraction can be extended to the chemisorption of a solid surface. This leads to the framework of tetrahedral bond formation and its effects on the valence DOS and the surface potential barrier, as well as their interdependence ... 

This part has verified up two concepts that should be of immediate application. One is the rp-orbit hybridization of the electronegative adsorbate as a charge acceptor and the other is the bond contraction of the host surface. Table 8.1 features functionalities and potential applications of the bonding events. The bond contraction is not limited to an oxide surface, but it happens at any site, where the atomic CN is lower than the bulk standard. 

Essential events in surface adsorption include bond contraction and sp-orbit hybridization. Bond and non-bond formation result in all the observations. 

A chemical bond contracts on a siuface or at sites surrounding defects where the atomic CN reduces. An extension of the concept of atomic size shrinks with its CN reduction, initiated by Goldschmidt and Pauling, to solid surfaces results in the BOLS correlation mechanism, which dictates the tunable properties of a nanosolid of which the portion of siuface atoms increases with reducing particle size. 

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. 

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