Monoatomic layer

Up to this point we have considered the mechanism of deposition of a single monoatomic layer (Section 7.6) and multilayers composed of a few monoatomic layers (Section 7.7). In Sections 7.8 to 7.12 we discuss how coherent electrodeposits develop. 

Propagation of microsteps with a height of 30 to 100 A (15 to 50 monoatomic layers) on a quasi-ideal surface of Ag was observed directly by Bostanov et al. (60) using the Nomarski differential contrast technique. 

A technique frequently used to characterize the pressure state in the high vacuum regime is the calculation of the time required to form a monomolecular or monoatomic layer on a gas-free surface, on the assumption that every molecule will stick to fhe surface. This monolayer formation time is closely related with fhe so-called impingement rate z. With a gas at rest the impingement rate will indicate the number of molecules which collide with the surfece inside the vacuum vessel per unit of time and surface area ... 

Electrodeposition behavior of Pb at Pt(lll) illustrates the influence both of the nature of the supporting electrolyte anion and of the nature of the depositing metal [44, 46, 47]. The Pt(lll) surface was not pretreated with I2 in these studies. At the onset, comparison of the Pb Auger signal with the coulometric data for the amount of Pb deposited revealed that the Pb deposit was not stable at open circuit when the packing density of deposited Pb exceeded about one monoatomic layer. Accordingly, the structural inves-... 

Electrodeposition of Bi onto Ag(lll) from aqueous acetate electroyte yielded an hexagonally close-packed superlattice, Ag(lll)(2N/3 x 2 /3) R30° Bi for which Bi = 7/12. The (2 /3 x 2v/3)/ 30° cells were present in parallel, strip-shaped domains averaging 8.5 silver unit meshes in width and unlimited length. In contrast, electrodeposition of Cu at Ag(lll) yielded no UPD peaks. While the Cu electrodeposit at Ag was stable at open circuit and was present as monoatomic layers as judged by attenuation of the Ag substrate Auger signal, it was somewhat disordered as judged by LEED. 

Another situation appears in methods used for surface analysis, where detectability is given as a percentage relative to the monoatomic layer. 

Under working conditions, thoria is reduced and the released thorium atoms act as an emitter [6.21]. The reason for the formation of a monoatomic layer of metallic thorium is still not yet fully understood. It is assumed that the reduction of thoria by tungsten according to... 

Catalysts based on /-quartz predomonantly decomposed the (+)-enantiomer of butan-2-ol, so the product mixture had a (-)-optical rotation. Considering the mechanism of action of that chiral catalyst the authors concluded that a maximal effect may be attained at small coverings of metal (close to a monoatomic layer) on the chiral surface of the optically active quartz. Increasing the content of metal increased the rate of reaction but decreased the asymmetric effect and as well as the resulting optical rotation of the product (Figure 2.2.). 

Carrying out vapor phase experiments in N2 or N2/air and attaining a polarimetric accuracy of 0.01°, Stankiewicz found that such catalysts were more effective than those containing monoatomic layers, which contradicted the data of Schwab (1934). 

The experimental data indicates that metal-quartz catalysts are more selective if they contain a thin, near monoatomic layer of metal on their surfaces. In accordance with this requirement the complicated net of border zones between metal and quartz must be created. Only on these borders can effective asymmetric reactions take place. Guided by this point of view Klabunovskii and Patrikeev considered the role of quartz as an as mimetric adsorbent that resolved the enantiomers and preferentially accumulated one of them on the border zones between metal and quartz where the reaction proceeds. In the case of the reaction of racemic butan-2-ol on Cu-/-quartz catalyst, (S)-(+)-butan-2-ol adsorbed and decomposed preferentially resulting in (-)-optical activity for the product mixture (Scheme 2.9.). 

If atoms are isolated from each other (a monoatomic layer) after drying the sample, atomization will be a desorption from the graphite surface and will be determined by the adsorption isotherm. If the atoms form small heaps or crystals, the atomization will be a pure volatilization. The formation of small atom heaps can be assumed as probable, and thus the volatilization mechanism is more likely. 

Hydrogenation reactions have been most extensively studied for evaluating the catalytic activity of nanoclusters. Precious monometallic (Pd, Pt, Rh) nanoclusters capped by PVP or polyvinylalcohol have high catalytic activities for hydrogenation of olefins. We applied PVP-capped Pd/Pt and Au/Pd bimetallic nanoclusters prepared by the coreduction method to the selective hydrogenation of 1,3-cyclooctadiene to cyclooctene. In both cases the bimetallic nanoclusters with a Pd content of 80 % showed the highest activity. As shown in the previous section, such bimetallic nanoclusters were found to have Pt- or Au-core/Pd-shell structures and Pt- or Au-cores are completely covered with Pd monoatomic layers. This improvement in catalytic activity can be interpreted only by a ligand effect of the core elements. 

Walgraef, D. Reaction-diffusion approach to nanostructure formation and texture evolution in adsorbed monoatomic layers. Int. J. Quantum Chem. 98(2), 248-260 (2004)... 

Additionally, Enyashin and co-workers have investigated the structure of the imbibided salt and found that Pbl2 exhibits an axial shell-like structure with respect to the tube axes. The outermost layer appears to be a pronounced Pb layer, whereas a more liquid-like structure is observed in the center of the nanotube. In comparison with Pbl2, KI shows a slightly different behaviour upon imbibition. The minisci are not distinct, and no monoatomic layer is found next to the hosting tube s wall. From analysing the radial distribution of KI inside the nanotubes, Enyashin and co-workers conclude that KI has no shell-like structure and shows an unchanged liquid state inside the nanotube. 

Fig. 4. Potential-sweep voltamogram showing the presence of a monoatomic layer of lead on a gold surface.

Note A single sputtering event, i.e., penetration, lattice perturbation by impact cascades, and final particle ejection, takes less than 10 s. Interestingly, for primary ion beam densities < 10" A cm" (equal to 6 x 10 ions s" cm" ) no interference of processes caused by different primary ions will occur, because the maximum cross section of an impact is only in the order of 10 nm. In other words, each impact is independent of prior, subsequent, or nearby synchrone-ous hits [82,83]. In the time domain this means that a given surface area suffers 10-100 hits min" in dynamic SIMS, but only 0.01-0.1 hits min" in static SIMS. For a monoatomic layer, observation times in the order of 1 h can thus be realized in static SIMS, i.e., the attribute static refers to the extended preservation of the surface. 

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