Epitaxial monolayer

Intermediate mechanism between (1) and (2). At first, an epitaxial monolayer is formed, followed by three-dimensional nucleation. This is called the Stranski-Krastanov mechanism. 

Figure 4.4. Representation of an epitaxial monolayer, i.e. in registry with the adsorbent structure.

Block et al., 1990). Heat capacity measurements have also provided strong evidence for the development of different 2-D solid structures. Epitaxial monolayers have been reported for some systems (see Dash, 1975) in which the adsorbed atoms are arranged in regular patterns in registry with adsorbent structure, as in Figure 4.4. 

Figure 9. The octahedral (III) face of AgBr covered with three possible ordered, epitaxial monolayers of adsorbed dye.

Berben et al. studied iron and chrome oxides supported on alumina and employed for C laus tail-gas clean-up. They found them to deactivate by segregation of the active phase from the support. In this case the active epitaxial monolayer formed a segregated inactive sulfur phase (seen by electron microscopy and identified by X-ray diffraction, XRD) by reaction with the reagent roducL... 

It has been considered up to now that Cu on the ZnO in oxidized or electron deficient state is stabilized by certain chemical or structural causes and then becomes an adsorption site for COj and CO at the initial stage of the reaction. As a structure of the active sites, a two-dimensional epitaxial monolayer of Cu over ZnO and small Cu clusters which are not crystallized have been directly observed by XPS and EXAFS analyses. It has been proposed that the active sites are generated by the mild reoxidation of the surface of Cu-Zn alloy by the feed gas containing COj. The relation between the catalytic activity of various Cu-ZnO catalysts and the Cu surface area is measured and the effects of support oxides are classified in a different way from preceding work. The specific activity is found to be controlled by the oxygen coverage. 

Side-view models for (a) the (6 V3x6 Vs ) R30° reconstruction of SiC(OOOI) ( zero-layer ), and (b) epitaxial monolayer graphene. After hydrogen intercalation, (c) the zerolayer, and (d) monolayer graphene, are decoupled from the substrate. Reprinted with permission from Physical Review Letters, 103, 246804. 2009 American Physical Society (RiedI et a ., 2009). 

Theoretical studies have been devoted to various aspects of the first stages of deposition, such as the electronic structure of supported clusters, the electronic structure of epitaxial monolayers and the modelling of growth modes. 

The growth of a well ordered fullerene monolayer, by means of molecular beam epitaxy, has been used for the controlled nucleation of single crystalline thin films. The quality and stability of molecular thin films has been shown... 

For ultrathin epitaxial films (less than "100 A), Grazingincidence X-ray Diffraction (GrXD) is the preferred method and has been used to characterize monolayer films. Here the incidence angle is small ("0.5°) and the X rays penetrate only "100-200 A into the specimen (see below). The exit angle of the diffracted X rays is also small and structural information is obtained about (hkl) planes perpendicular to the specimen sur e. Thus, GIXD complements those methods where structural information is obtained about planes parallel to the surface (e.g., Bra -Brentano and DCD). 

Rgure 3 Experimental and calculated results (a) for epitaxial Cu on Ni (001). The solid lines represent experimental data at the Cu coverage indicated and the dashed lines represent single-scattering cluster calculations assuming a plane wave final state for the Cu IMM Auger electron A schematic representation lb) of the Ni (010) plane with 1-5 monolayers of Cu on top. The arrows indicate directions in which forward scattering events should produce diffraction peaks in (a). 

Stimulated by these observations, Odelius et al. [73] performed molecular dynamic (MD) simulations of water adsorption at the surface of muscovite mica. They found that at monolayer coverage, water forms a fully connected two-dimensional hydrogen-bonded network in epitaxy with the mica lattice, which is stable at room temperature. A model of the calculated structure is shown in Figure 26. The icelike monolayer (actually a warped molecular bilayer) corresponds to what we have called phase-I. The model is in line with the observed hexagonal shape of the boundaries between phase-I and phase-II. Another result of the MD simulations is that no free OH bonds stick out of the surface and that on average the dipole moment of the water molecules points downward toward the surface, giving a ferroelectric character to the water bilayer. 

In fact, different techniques revealed cadmium segregation and decrease of the Pb/Se ratio near the InP/PbSe interface, indicating that during the first steps of deposition a CdSe layer is formed on InP prior to the PbSe growth. It was suggested that selective adsorption of Cd(0) on the InP surface gives rise to an epitaxial CdSe monolayer, which facilitates an ordered PbSe growth on account of the small lattice mismatch (0.7%) at the CdSe/(rock salt)PbSe interface. Importantly, it was found... 

Zinc sulfide, ZnS, has been epitaxially deposited by the dual bath approach on Au(lll) surface and studied by STM and XPS [48]. The first complete ECALE cycle resulted in the formation of nanocrystallites of ZnS randomly distributed across Au(l 11) terraces, on account of lattice mismatch induced strain between ZnS and Au(lll) - although the mismatch is only 0.13% for ZnS/Au(lll). Atomically resolved STM images showed the ZnS/Au(lll) monolayer to be sixfold symmetric. The average diameter of the crystallites was 10 5 nm and the apparent coverage 0.38. 

Numerous works have been implemented on tellurium electrochemistry and its adsorption at metal surfaces. The morphological structures of electrodeposited Te layers at various stages of deposition (first UPD, second UPD, and bulk deposition) are now well known [88-93]. As discussed in the previous paragraphs, Stickney and co-workers have carried out detailed characterizations of the first Te monolayer on Au single-crystal surfaces in order to establish the method of electrochemical atomic layer epitaxy of CdTe. 

Varazo K, Lay MD, Sorenson TA, Stickney JL (2002) Formation of the first monolayers of CdTe on Au(l 11) by electrochemical atomic layer epitaxy (EC-ALE) studied by LEED, Auger, XPS, and in-situ STM. J Electroanal Chem 522 104-114... 

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