Structures of 2D Meads Phases

Recent in situ applications of different surface analytical techniques such as EXAFS, GKS, STM, AFM, etc., opened a new window to get direct information on the structure of Me UPD adlayers at an atomic level. Structural aspects of UPD adlayers based on both electrochemical and modem surface analytical results will be discussed in detail in the next section. 

Adsorption processes on crystallographically well-defined substrate surfaces lead to the formation of 2D Meads phases with well-ordered structures denoted as overlayers . Generally, three different types of overlayers, depending on the degree of registry between overlayer and substrate, can be distinguished commensurate, higher-order commensurate or incommensurate overlayers, as illustrated schematically in Fig. 3.14. TTie term superlattice stmcture is frequently used for commensurate overlayers which can be characterized by either the Wood or the matrix notation [3.271-3.274]. 

The commensurability of overlayers is characterized by the coincidence of reciprocal lattice vectors of adsorbate and substrate. Low or higher order coincidence leads to commensurate or higher order commensurate overlayer structures (Figs. 3.14a, b, c). K the reciprocal lattice vectors do not coincide, the overlayer is incommensurate (Fig. 3.14d). 

Me-S interactions leading to higher order commensurate and under certain conditions to incommensurate Meads overlayer structures give superstructures and moire pattern which can also be described by the Wood or matrk notation (cf. Figs. 3.17, 3.19). 

For an incommensurate adlayer G g), i.e., if no vectors from the set coincide with vectors belonging to the set ), it follows that 5 = 0, and the second term in 

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Cyclic voltammetric and potentiodynamic measurements in the system Ag(hkl)/Bi, H, CIO4 (+ Cl ) show that the kinetics of 2D Meads phase formation and dissolution depend on the structure of the substrate surface [3.119]. It was suggested that Meads surface diffusion may play an important role in the desorption kinetics. 

Structural properties of the 2D Meads phase and S as well as the lateral interaction energy between Me adatoms, V Meads-Meads other parameters have to be taken into account in order to quantitatively explain UPD phenomena (cf. Section 3.3). 

However, a complete physical Me UPD model does not yet exist. Recently, calculations based on a jellium model with lattices of pseudopotentials for the 2D Meads phase and S were started by Schmickler and Leiva [3.234-3.239]. In addition, local density full potential linearized augmented plane wave calculations were carried out by Neckel [3.240, 3.241). Both approaches are important for a better understanding of Me UPD phenomena on single crystal surfaces taking into account structural aspects. 

The function f(r) can be considered as the activity of the 2D Meads phase in the UPD range compared to the Me activity (aue = 1) of a 3D Me bulk phase (cf. eq. (1.2)). The explicit form of f(r) depends on the Meads-S and Meads-Meads interactions and the crystallographic structure of S, and can be derived using appropriate adsorption isotherm models. 

Analogously to 3D nucleation processes, a supersaturated state of an expanded structure is formed first which is characterized by actual Fep () and 9ep(0 values kinetically controlled by Me oiy bulk diffusion and charge transfer. 2D nucleation and growth start from a supersaturated expanded 2D Meads phase and lead to a condensed 2D Meads phase which is characterized, in a first approximation, by time-independent equilibrium values of Tcd(AEf) and cdfAEf). 

In the case of a strong Me-S interaction, the structure and orientation of a Me deposit on top of Me UPD modified S according to the Frank-van der Merwe (cf. Fig. 1.1b) or Stranski-Krastanov mechanisms (cf. Fig. 1.1c) strongly depend on the substrate structure. Independently of crystallographic Me-S lattice misfit, distinct correlations between the epitaxy of a condensed 2D Meads phase and/or 2D Me-S surface alloy phase and the epitaxy of a 3D Me bulk phase can be expected. 

In presence of significant Me-S lattice misfit, the epitaxy of isolated 3D Me crystallites or compact 3D Me films is strongly determined by the structure of internally strained 2D Meads overlayer and/or 2D Me-S surface alloy formed in the UPD range at high F or low AEi. The misfit between the lattice parameters of the 2D Meads phase and/or 2D Me-S surface alloy phase and the 3D Me bulk phase is mainly removed by misfit dislocations. The initial strain disappears after depositing a certain thickness of the 3D Me bulk phase. Usually, a thickness of n Me monolayers where 2 < < 20 is necessary to adjust the 3D Me bulk lattice parameters [4.58, 4.59]. If an incommensurate structure of a 2D Meads overlayer is formed in the UPD range, this structure will also be reflected epitaxially in 3D Me crystallites and ultrathin 3D Me films. 

In systems with significant Me-S lattice misfit, the 2D Meads overlayers and/or 2D Me-S surface alloys formed in the UPD range have a different structure in comparison with the 3D Me bulk phase, and contain considerable internal strain (cf. Section 3.4). Thus, the nucleation and growth kinetics in the OPD range will be strongly influenced by the internal strain energy of 2D Me UPD phases. 

Structure and orientation of a Me deposit on S in the initial stage of 3D Me bulk phase formation can be either independent of or influenced by the surface structure of S, which can be modified by 2D Meads overlayer formation and/or 2D Me-S surface alloy phase formation in the UPD range. Epitaxial behavior of 2D and 3D Me phases exists if some or all of their lattice parameters coincide with those of the top layer of S. The epitaxy is determined by a minimum of the Gibbs function at constant temperature and pressure. 

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