Compressed overlayer

Sung YE, Chrzanowski T, Wieckowski A, Zolfaghari A, Blais S, Jerkiewicz G. 1998. Coverage evolution of sulfur on Pt(lll) electrodes From compressed overlayers to well-defined islands. Electrochim Acta 44 1019-1030. 

In this equation. A and /i are the 2D Lame coefficients, is the area of Meads in an undistorted 2D Meads overlayer, and Ad/do = (do - d)/do denotes the relative compression of the adlayer where do and d are the atomic nearest-neighbor distances in the uncompressed and compressed overlayer, respectively. Equation (3.23) can be regarded as a first approximation for a hexagonal close-packed (hep) Meads overlayer with a 2D hep lattice. However, it is a very rough approximation for more expanded overlayer structures such as quadratic or rectangular superlattices and cannot be used for anisotropically compressed and strained Meads overlayers. 

This expression indicates that the change in hybridization energy is opposite and proportional to the shift of the d band center. Thus, if the d band shifts upwards the hybridization energy increases and vice versa. Strain and the associated shift of the d band can be brought about by growing the desired metal pseudomorfically on another material with a different lattice constant. The term pseudomorfic means that the overlayer grows with the same lattice constant as the substrate. The overlayer may thereby be strained or compressed depending on the lattice constants of the two materials. 

Evans RW, Attard GA. 1993. The redox behavior of compressed bismuth overlayers irreversibly adsorbed on Pt(lll). J Electroanal Chem 345 337-350. 

Overlaying the bars that give the breakdown of cost to the operation (manufacturing, maintenance, retail) is the total energy cost. For central gaseous it includes an important contribution from delivery (diesel fuel) and retail (electricity for on-site compression). For a more extensive discussion, see the text. 

C 579 Standard Test Method for Compressive Strength of (Method B) Chemical Resistant Mortars and Monolithic Surfacings C 882-87 Standard Test for Bond Strength of Epoxy-Resin Systems Used with Concrete C 884-87 Standard Test Method for Thermal Compatibility between Concrete and an Epoxy-Resin Overlay... 

Stability of lead monolayers [271] and a dependence of the Pb UPD layer structure on surface coverage have also been studied on Au(lOO) face [272]. A phase transition from an expanded Pb overlayer to compressed hep structure has been considered [270]. A coupled process of gold and Pb UPD oxidation process on single crystal Au(llO) has been studied using XPS method [273]. 

Innocenti et al. have studied the kinetics [101] of two-dimensional phase transitions of sulfide and halide ions, as well as electrosorption valency [102] of these ions adsorbed on Ag(lll). The electrode potential was stepped up from the value negative enough to exclude anionic adsorption to the potential range providing stability of either the first or the second, more compressed, ordered overlayer of the anions. The kinetic behavior was interpreted in terms of a model that accounts for diffusion-controlled random adsorption of the anions, followed by the progressive polynucleation and growth. 

Figure 7.11 Overlay of contour plots for apparent modulus of elasticity in compression ( c)> maximum tension force (Ft) and first-cycle relaxed force (Fri). The shaded area represents optimum conditions.

The C—O stretch frequency of 1823 cm"1 observed at low coverages indicates that the molecules are most probably located in threefold coordinated sites (60) as indicated by the structure model of Fig. 4a. Upon further increasing the coverage the overlayer unit cell is continuously compressed, as indicated by the arrows in Fig. 4a, until at 8 = 0.5 a c4 x 2 structure is reached. This compression is connected with an enormous shift of the IR band that suggests that the CO molecules now move into bridge sites as drawn in Fig. 4b (60). The compression continues until 6 — 0,6. Additional... 

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]. 

Most nonmetallic adatoms will not compress into a one-monolayer overlayer on the closest-packed metal substrates. There appears to be a short-range repulsion that keeps adatoms apart by approximately a Van der Waals distance. Attempts to compress the overlayer further by increasing the coverage (which is... 

Owing to this rapid cure, a compressive strength of 40 N/mm is possible within 2 h at temperatures down to 4°C (Shaw, 1993a). The properties of polyester binder depend primarily upon the chemical composition of the polyester resin component. Typical physical properties of cured polyester binders for concrete overlays are given in Table 9.3 (ACI, 1993). 

Figure 3.19 Anisotropically compressed 2D hep Meads overlayer in presence of significant Me-S misfit, (do,Me-do,s)/doS > 0, forming a row-matching superstructure with moir6 pattern S(100)-c(6x2) MeMoiri.

Assuming that, as a consequence of a 2D isotropic compression of the overlayer, the corresponding increase of the specific surface energy is given by the elastic strain energy according to eq, (3,23), one gets ... 

Fig. 3.28 shows an in situ STM image with lateral atomic resolution of a nearly complete Pb monolayer at A = 15 mV on Ag(lll) [3.173-3.175]. The image shows a compressed hep Pb overlayer with d = 0.34 0.01 nm. Additionally, a superstructure with a moir6 pattern is observed. The mean distance between the moire spots is moir6 = 1-65 0.07 nm. This overlayer is rotated by a = 4.5 0.2° with respect to the Ag(lll) substrate. These results are in good agreement to those obtained from GKS and EXAFS measurements [3.137-3.139, 3.140, 3.141, 3.143, 3.145]. 

Figure 3.28 In situ STM image of a compressed 2D hep Pbads overlayer showing a superstructure with moird pattern in the system Ag(lll)/5 x 10 M Pb(C104)2 + 10 M HCIO4 at Af = 30 mV and...

Figure 3.31 In situ STM images with differently orientated domains (a) and (b) of an anisotropically compressed 2D hep PT>ads overlayer showing a superstructure with moir6 pattern (cf. Fig. 3.19) in the system Ag(100)/5 x lO M Pb(C104)2 + lO M HCIO4 at T = 298 K [3.87]. (a) = 80 mV,...

In order to explain the moird structure, one can consider the following two cases (i) isotropic compression or (ii) anisotropic compression of the Pb overlayer. 

In case (i), the overlayer has a perfect hexagonal symmetry. However, the assumption of row by row matching in the Ag[liO] direction requires an interatomic Pb distance oi d = 0.334 nm, corresponding to a relatively high compression of about 5 percent. 

---