Crystallographic Me-S misfit

In many physically important cases of localized adsorption, each adatom of the compact monolayer covers effectively n > 1 adsorption sites [3.87-3.89, 3.98, 3.122, 3.191, 3.214, 3.261]. Such a multisite or 1/n adsorption can be caused by a crystallographic Me-S misfit, i.e., the adatom diameter exceeds the distance between two neighboring adsorption sites, and/or by a partial charge of adatoms (A < 1 in eq. (3.2)), i.e., a partly ionic character of the Meads-S bond. The theoretical treatment of a /n adsorption differs from the description of the 1/1 adsorption by a simple Ising model. It implies the so-called hard-core lattice gas models with different approximations [3.214, 3.262-3.266]. Generally, these theoretical approaches can only be applied far away from the critical conditions for a first order phase transition. In addition, Monte Carlo simulations are a reliable tool for obtaining valuable information on both the shape of isotherms and the critical conditions of a 1/n adsorption [3.214, 3.265-3.267]. 

In the case of strong Meads-S interaction, expanded commensurate Meads overlayers as well as one or two close-packed commensurate or incommensurate Meads monolayers can be formed in the UPD range depending on AE (Fig. 6.13). Then, metal deposition in the OPD range follows either the Frank-van der Merwe (Fig. 6.13a) or the Stranski-Krastanov (Fig. 6.13b) growth mechanisms in the absence or presence of significant crystallographic Me-S misfit, respectively. In the first case, a... 

Three different growth modes (Volmer-Weber, Frank-van der Merwe, and Stranski-Krastanov) can be distinguished, depending on the vertical binding energy between a metal adatom, Meads> on foreign substrate, S, and on the crystallographic Me-S misfit, as schematically illustrated in Fig. 3. 

In the following, structural aspects of substrate surfaces and Me UPD overlayers obtained by in situ GIXS, STM and AFM are discussed. UPD systems without and with different crystallographic Me-S lattice misfit are presented. 

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. 

Independent of the Me UPD system (chemical nature of Me and S, crystallographic orientation of S, Me-S lattice misfit), thermodynamic and structural results... 

In absence of Me-S lattice misfit, the structure of growing 3D Me layers usually continues that of the condensed and commensurate 2D Meads overlayer and or 2D Me-S surface alloy formed in the UPD range at high For low AEi. The structure of a 3D Me film is usually in complete registry with the structure of the modified substrate surface SQikl) [hkl] II Me(M0 [hkl], where (hkl) and [hkl] are the Miller indices and crystallographic directions, respectively. 

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