Pseudomorphic layer

The structure of the striped overlayer is more difficult to determine and is presently not yet fully understood. High resolution STM images indicate a reduction of the Cu-Cu distance from 0.29 nm in the pseudomorphic layers to 0.26 nm in the ridges of the wavy structure [77], In-situ SXS measurements suggest that the wavy overlayer is composed of alternating orthorhombic and cubic stripes [85],... 

In a series of papers, Kolb and coworkers have presented CV and in situ STM studies on palladium deposition on various gold single-crystal electrodes. They have found [430] that PdCU is adsorbed on Au(lll), forming a distorted hexagonal structure, which plays a crucial role in Pd deposition and dissolution. It has also been found that Pd deposition starts from the formation of a pseudomorphic layer in the underpotential region, followed by the formation of the second Pd monolayer at overpotentials. Pd nucleated... 

To all these intrinsic reasons, one would have to add the expected modifications in the electronic structure of the growing film as it thickens, due to the decreasing influence of the substrate. This can be better judged for a system that is not pseudomorphic, such as Ag/Cu(lll). The large (12%) mismatch between Ag and Cu would provoke such a tremendous compressive stress for a pseudomorphic layer that the Ag layers keep their own lattice parameter from the first monolayer on. For 1 ML of Ag/Cu(lll), the surface state has been found to be 120 meV lower in energy than for bulk Ag(lll) [79], and shifts with increasing Ag coverage to the bulk value. 

Fig. 36. Surface stress on As-precovered Si(lOO) during the co-deposition of Ge and As at 500 °C. At 9 ML Ge coverage a slight kink in the stress curve indicates the end of pseudomorphic layer-by-layer growth and defects are formed for higher coverages. Data from [94Sch].

Electron diffraction investigations showed that epitaxy did indeed exist when one metal was electrodeposited on another, but that it persisted for only tens or hundreds of atomic layers beyond the interface. Thereafter the atomic structure (or lattice) of the deposit altered to one characteristic of the plating conditions. Epitaxy ceased before an electrodeposit is thick enough to see with an optic microscope, and at thicknesses well below those at which pseudomorphism is observed. 

Fig. 12.8 A fairly strongly pseudomorphic bright tin deposit (left) has its brightness impaired by the shattered surface layer produced on steel by cold rolling. When this layer is removed, the deposit is mirror bright (right). Coating S m thick...

Cu(100) top is shown [77], It implies that for both substrates, Au(100) and Ag(100), an (for atomic dimensions) incredibly large number of Cu layers has to be deposited, before the overlayer acquires bulk structure. This behavior is vastly different from that on Au(lll) and Ag(lll), where pseudomorphic growth is restricted to one and two layers, respectively. 

Micro-XRD and micro-XANES analyses showed that the earliest stage of sulfide alteration is marked by the progressive oxidation of sulfide-S to sulfate-S that is then rapidly leached out from the system. Sulfide oxidation starts from particle rims or from intra-grain microfractures and is accompanied by a progressive loss of sulfur sulfides are then pseudomorphically replaced by goethite and minor bemalite. In addition to sulfides, many gangue silicates are efficiently altered and only quartz is preserved within the altered layers. 

The structure of the first monolayer of Cu is pseudomorphic with respect to the Ru(0001) substrate whereas successive Cu layers grow epitaxially with a Cu(lll) structure. 

Adsorbate Substrate Atom density mismatch/ML (7o) Pseudomorphic/epitaxial layers Change in CO desorption T(K)... 

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